A. 8 females that are like F₁; 7 others resemble them below, but have stippled back and rump feathers (4 of these have yellow necks, and 3 black necks like those of F₁). Here, then, are two or three subdivisions, or perhaps main classes.
B. 1 female is very close to game, having the characteristic stippling above and salmon breast below. She is darker colored than the game females, therefore more like the Leghorn female.
C. 3 birds resemble the Sebright in plumage, but would not pass muster for real Sebrights.
D. 3 others have spangled breast feathers like the Sebright, but a great amount of stippling on the back.
E. 6 birds are yellow on breast, with stippled back.
F. 6 others are yellow, stippled birds with a little yellow penciling on the breast.
G. 11 black birds with some stippling on the back of the wing, and sometimes with traces of yellow in the hackle.
H. 6 other birds are dark, but not as black as the last. There is some stippling, especially on breast. The hackle is always striped. (4 of these have yellow necks above and below; 2 have dark necks.) The series of feathers photographed in plate 10, figure 4, from a bird of class H, show all gradations between a spangled and a barred condition. It is practically certain that the barring seen here (as well as that under class M) is quite different from that of the Barred Plymouth Rock.
I. 1 yellow hen with a little black (as a band) on wings and tail.
J. 2 other yellow birds with a little black penciling on the back, the tail, and with long wing feathers.
F₂ Males.
K. 2 intermediate males with a black-splotched red breast and black tail. The saddle is coarsely stippled. The corresponding male-feathered bird is red above.
L. A Sebright-like bird with black stippled feathers on back; the rest of the plumage heavily laced. Posterior part of breast and thigh black. The tail stippled.
M. Yellow neck, the back, tail, and lower half of each secondary coarsely stippled. Posterior part of breast barred; cape and anterior breast penciled. The series of feathers photographed in plate 10, figure 5, from the breast of a bird in class M, shows all possible gradations from a penciled to a barred condition.
O. 1 cock-feathered, Red-Breasted game with somewhat stippled feathers. The lower half of each secondary is penciled (Hamburg type).
P. 1 intermediate male with yellow hackle that is black striped. He has a peculiar saddle, the general color of which is reddish brown. Each feather has a faint black edge, and is clear yellow along shaft; the rest of the feather is finely dusted on a yellow background (plate 10, fig. 3). Breast feathers (in front part) are laced with an outer black band edged with yellow. In the posterior region of the breast the feathers are broadly laced. Wing-bow and coverts red, laced with black. The exposed edges of primaries and secondaries are red-brown, the covered parts black. The tail is black.
Back-Cross Hens.
A. Two dark F₁ types. The breast is between stippled and penciled, the head is black.
C. (1) Sebright type. Very dark with much stippling. Some penciling on back. Breast dark; neck like that of Sebright.
(2) Sebright type like (1), but not such clear yellow. Secondaries and tail feathers and coverts stippled (with black tips).
(3) Yellow Sebright. Neck and breast yellow with black base and tip to feathers. Cape, breast, and wings (except bow) penciled to barred.
C. to D. (1) Pale yellow, breast spangled, back lightly penciled, tail same. Secondaries yellow and little stippled. Upper web of primaries stippled.
(2) Breast spangled, rest as in (1).
(3) Same as (2).
L. Dark Sebright. Back-feathers broadly laced and a little penciled. Neck black with yellow centers to feathers and yellow edges (reversed Sebright).
All of the preceding hens except A are in general Sebrights. The last three are pale stippled Sebrights.
Back-Cross Hen-Feathered Cocks.
A. One cock like F₁ male, but rather paler on back.
C. Four cocks. Light Sebrights, but spangled, in general, instead of laced. Feathers clear, not stippled.
G. One black.
L. Dark Sebright. Back and rump black. Feathers with narrow center, not stippled.
Q. Dark Sebright nearer to hen C (1). Thoroughly stippled with game tail. Neck and breast dark Sebright. Probably a new class nearer to (C).
R. Two cocks. Pale yellow instead of reddish, and much less black than are other yellows. No class of hens to match.[2]
In regard to color inheritance the preceding 19 birds are too few to add anything of significance to the other results, except that they serve to emphasize the dominance of the factors making for Sebright coloration. The hen-feathered cocks confirm the other results as to the dominance of the factor or factors in question.
There can be little doubt that some of these classes are complex. They almost merge into each other and in one part of the body individuals may grade off into one class, in other parts into other classes. An almost continuous series of types might be arranged from black to pale yellow.
The difficulty of matching the hen-feathered males to their genetic mates is almost insuperable. In table 1 an attempt was made to put these males with their respective females. The difficulty is, of course, greater for the cock-feathered birds, even with the castration evidence (that is too meager at present for the purpose), but a few of the males may be placed with certainty, and the rest guessed at.
One bird appears to be a hen-feathered game male resembling in many respects the female game, but darker and redder. There is more shafting on cape and wing-bow. The breast is unusually dark-salmon. The hackle is darker than is the game female. Upper wing-coverts broadly laced with black. (Plate 10, fig. 3.)
The occurrence of this hen-feathered jungle-fowl is so unique and the coloration of the bird so interesting that I have added to the plates three feathers of such a bird, viz., a stippled saddle feather, a feather from the back, a hackle feather, and a wing covert with stippled center and a black border. The neck hackle departs somewhat from the hackle of the jungle-fowl hen, but in the same direction as does the neck hackle of the Sebright cock from his hen.
Looking over the F₂ group, the most noticeable thing is the large number of blacks (E and G), all of which are stippled. Probably the factor came from the game, because group E was present in the back-cross as well as in F₂, and because these black birds are always stippled. The yellow color (I and J) may have come from both, each breed having then a black factor that, as a pattern, covers over most of the yellow. It is difficult to distinguish penciling from stippling in the F₂ yellows. Without figuring each of these types, their description in detail is not of much value. The skins will be deposited for reference in the Zoological Laboratory of Columbia University.
C. Back-Cross of F₁ to Game.
As the back-cross of the F₁ to the game might appear more likely to reveal the kinds of germ-cells present in the individual, the results from such a cross may be given before discussing the genetic data. If it were certain that the “game” contained all of the recessive factors that are involved in the experiment, this method of testing the result would be ideal, but there is no way of determining a priori whether this is the case. The question will be taken up later. The presence of two kinds of males with corresponding but largely uncorrelated differences in their plumage makes their classification as a group impossible. It is simpler, therefore, to put the females into their classes first, after which the hen-feathered males may be expected to fall into the same groups (or nearly so), while the identity of the cock-feathered males, i. e., their class relationship can only be determined for the classes that resemble the F₁ and the P₁ birds. The F₂ hen-feathered males can in part be further identified by means of the evidence that castration of these types affords.
Two of the F₂ classes of hens can be identified in this back-cross, viz, (a) 4 hens like the F₁ birds, (b) 3 hens like the game; (c) there were 3 other hens with plain yellow, i. e., not stippled backs. The upper surface was like that of the game female, but much lighter. The first two classes (a), (b) might be again split into two types. There were only two hen-feathered males, one nearly like the F₁ male, the other blacker; they probably belong to different classes.
Of the 7 cock-feathered males, one was like the F₁ castrated males; another had a similar back, but a darker and differently marked breast; 2 were game-cock type; 3 were odd birds much like the game cock above except for absence of black, with reddish heads without any black. The males may be approximately classified as follows:
| Back-cross F₁ ♀ by game ♂. | ||||
| Hen-feathered | Intermediate | Cock-feathered | ||
| ♂. | ♂. | ♂. | ||
| F (or K) | 1 | 1 | ||
| (C) | 1 | 2 | ||
| B | 2 | |||
| A | 2 | |||
| 7 | ||||
Four or five types may then be recognized in this rough grouping. None of the groups seem uniform and probably might be split again.
The theoretical expectation for two pairs of factors calls for 4 classes in the back-cross, but this assumes that the parent type used for back-crossing contains all (here 2) recessives. But this simple assumption can not be true in this case, for the F₁ bird would have been like the Sebright. On the 3-factor assumption the expectation for the back-cross is 8 classes, but this would apply only if the game were the triple-recessive form, which, again, it is not, as shown by the F₁ cross. But if the dominance of one or more of the Sebright color factors is incomplete, then either a 2 or 3 factor assumption might apply to the back-cross.
If only 2 pairs of factors are present we should expect to recover the game type once in 16 cases in F₂. But, as will be shown, only 1 game was recovered out of the 49 F₂ females. This result fits better with a 3-factor assumption, for even with the small number in the back-cross the indications are that more than 4 classes are present.
In the F₂ birds at least 11 classes may be distinguished, and some of these appear composite. For 3 factors the maximum number of possible classes (including heterozygotes) is 27. We can recognize at least 11 F₂ classes amongst the females alone, and a few others are doubtfully present in the males.
In favor of the view that the heterozygous classes are here different from the homozygous, the following evidence may be utilized:
(1) The F₁ birds are entirely different from either parent and they are heterozygous for all the factor differences between the two types. The only alternative explanation for the intermediate condition of F₁ would be that each race carries one or more completely dominant factors. But the latter view is improbable because more of each parent type would then be expected in the F₂ generation.
(2) In the F₂ generation the F₁ type is not as frequent as would be expected on the view that the heterozygotes could not be distinguished.
It is possible to add, now, while this paper is passing through the press (June 1919), the results of a back-cross of 4 F₁ females to a Sebright male carried out during the summer of 1918. The birds being now mature their permanent colors are evident. Making the back-cross in this direction is much less advantageous than the reciprocal described above, because the Sebright contains most of the dominant color factors. The group of birds obtained appeared to be less variable in color than those from the other back-cross, and one can see at a glance that more of them approach the Sebright type; some quite closely.
All of the males are hen-feathered, as expected. No evidence was found that two types of males exist, which would have been expected if the two types noted in F₂ had any hereditary significance. If, then, as the F₂ results suggest, two factors for hen-feathering are present both are dominant, and no genetic distinction is found between individuals in which one or both of the dominant factors are duplex or simplex.
There were 9 adult hens and 10 hen-feathered cocks. An attempt is made below to refer them to their corresponding F₂ classes.
In poultry there are perhaps more different colors and color-patterns than in any other species of domesticated animals. The genetic work has advanced far enough to show that many of the differences depend on Mendelian factors. It is probable that, in addition to the main factors, there are many contributory, minor, or modifying factors that give the finer details to “show birds.”
It is generally supposed that the wild bird from which some at least of the domesticated races have come is Gallus bankiva of India and Indo-China, or else one or another of its subspecies. In any case, the wild type of coloration is approximately known, since the known wild races are colored alike in all essential respects. Even were the color of the wild type not known, the original plumage could be deduced with some degree of probability from the atavism that appears when some of the races are hybridized. It is interesting to find that many of the new plumage characters are dominant to the wild type. The same relation also holds rather generally for other characters of poultry, such as the comb, etc.
Amongst the uniform or single-colored races, the whites, blacks, reds, and buffs have been studied. Bateson and Punnett were the first to show that the white of the White Leghorn is dominant. They also showed that the white of the White Rose Comb bantams is recessive. Another white, that of the White Silky, is also recessive, but due to a different factor from the white factor of the Rose Comb bantams; for, when these two whites are bred together they give colored birds in the first generation. Hurst showed later that the white of the Leghorn is dominant over the black of the Hamburg and the buff of the Cochin. The dominance is often not complete, since tints of black or of buff or even patches of these colors may occur. The latter may be confined to the head, neck, and breast. The black plumage of the Hamburg is dominant over the buff of the Cochins, but incompletely so, as the black background may be marked and shaded with brown. Whether we are dealing here with one pair of factors, or two pairs, could only be determined by an F₂ ratio; whether it is 3:1 or 9:3:3:1.
The blue color of the Andalusian is known not to be a simple color, but to be a fine mosaic of splashed white and black. The color is produced in birds that are heterozygous for splashed white and black, or at least for certain kinds of white and black. This relation was first demonstrated by Bateson and Punnett (1902 and 1905) and later Saunders (1906). It appears also from certain crosses made by Davenport that some of the whites (such as that of the Leghorn) and black (such as that of the Minorca) may at times also give some blue birds when crossed. Whether there are also other races with dominant white color different from that of the Andalusian white (and the same holds for black races also) or whether a special (recessive) white was present in this cross when the blue appeared, was not made out by Davenport.
Lippincott has recently studied the Andalusian cross and obtained essentially the same results as his predecessors. He calls attention to an interesting fact in the splashed whites, namely, that the color splashes are blue when they are found in those parts of the body where the color is blue in the Andalusian. Although the Andalusian is always spoken of as a blue bird, the hen only is entirely blue, while the male is black above and blue below. The splashes on a white male correspond to the black and blue of the Andalusian male, and are black if above and blue if below.
Lippincott found also that the blue birds differ from the black in two characteristics, viz, in the blues the pigment is in larger masses, i. e., it is more clumped, leaving more white between the clumps than in the blacks, and in the blues the pigment is absent in the extremities of the barbules. If the clumping and the condition of the barbules are treated as separate entities, each gives a 3:1 ratio. Lippincott concludes, therefore, that the Andalusian cross is a 2-factor case. If each of these characteristics was independent of the other in the sense that some birds had clumped pigment and others deficiencies in the barbules, then one might conclude that he was dealing with a 2-factor case; but if these two characters are only different aspects of the same gene, and when one is present the other is also, the situation is not different from those that are very common, viz, two or more effects produced by the same genetic factor.
Davenport has recorded results of crossing several breeds of different colors (1906 and 1909). The white of the Leghorn was found dominant to the black of the Minorca breed, although the hybrids, “at least the females,” had some black feathers. This white was also found to be dominant to the mottled Houdan and to the “Red-backed game.” On the other hand, a male Tosa with wild-type plumage by recessive White Cochin female gave “barred” males in F₁; the barring coming in, no doubt, from the Cochin and although not at the time recognized by Davenport as sex-linked inheritance, the statement that barring is “associated with maleness” (as already pointed out by Darwin) indicated that the barring that appeared within the cross was probably the sex-linked barring shown by other breeds.
In Davenport’s cross of White Leghorn by Minorca two blues appeared (as stated above), indicating that the same factors were here present that in the Andalusian white and black strain gives the same result,[3] but why only some of the F₁ appear as blue, while others are not blue, is not yet made clear, unless two factors for white were present. White of the Leghorn breed was found not to be as completely dominant over buff as over black. Black was found dominant over the wild-type (Black-Breasted game), but red is present in F₁ birds also to some extent in those places where red is found in the game. Lacing, as shown by the Dark Brahma, is dominant to the plumage of the Tosa. Penciling also is said to be dominant, as shown in females of the cross between the Dark Brahma and Tosa fowl.
In his later paper (1909) Davenport gives fuller information in regard to some of the F₁ cases reported in his first paper, as well as the F₂ results. Thus, in the cross of Silky to Minorca, that gives black F₁ birds, the F₂ count gave 210 black, 57 game, and 95 white—approximately the expectation for two pairs of factors, one of them giving white (9:3:4). Silky by White Leghorn gave white F₁’s, but the males developed red on the wing bow and saddle when they became mature, and the female a faint blush of salmon (“red”) on the breast. In F₂ there were whites, games, and blacks, approximating to expectation for three pairs of factors, one being a dominant white (52:9:3). Silky by Buff Cochin gave a washed-out buff, but with the jungle coloration partly developed in the tail (black) and hackles and wing bow (redder buff). Davenport represents the Buff Cochin as having lost the jungle patterns and coloration, while the Silky retains it. The heterozygous condition of the genes for the wild-type color in F₁ is made responsible for the part development of color. The White Silky is represented as carrying the factor for black (N), hence in F₂ both black and game-colored birds are expected and they were obtained. When Black Cochin is crossed to Buff Cochin, the F₁ males are in general like the game (black and red) while the females are black (except for some red on the hackle). In this case Davenport represents the Black Cochin as showing a factor for jungle-fowl pattern, but lacking the color that is assumed in his other formulæ to go with this pattern. What is meant by this change is not quite clear to me, unless Davenport supposes there is an independent factor for the jungle-fowl pattern which may be filled in by other colors determined by other factors. But were there enough F₁ birds to exclude the possibility that jungle-fowl birds would not appear in this cross?
Davenport has reported a cross between a female White Cochin and a male Tosa (wild type) from which the daughters were Tosa, except that the shafting was broadened, and the saddle feathers and proximal secondaries were obscurely barred (black and buff); the sons were also like the Tosa, but every feather was repeatedly barred (see above). In F₂ there were 15 white, 25 game, and 16 barred birds. Davenport concludes that “barring is clearly heterozygous and confined to the male sex,” and in a footnote he adds that the sex-linked barring factor of the Plymouth Rock is different from that of this Cochin-Tosa cross, but Goodale informs me that the barring that appeared in this cross is probably the same as that in Barred Rocks.
As pointed out, an interesting feature of color inheritance in poultry is the large number of cases of sex-linked inheritance. It might seem probable here, as in the case of Drosophila, that this is due to a well-recognized difference between sex-linked and autosomal characters, namely, that a recessive mutation in one of the sex chromosomes of a sperm-cell of the male bird will have a chance of showing its effect immediately if that sperm-cell unites with an egg without a Z to form a daughter, whereas it would not immediately show up in the offspring if the mutation were autosomal.[4] In consequence the recessive mutant would have a greater chance of being observed and selected if it appeared in a sex chromosome. But dominant sex-linked characters, however, have the same chance as dominant autosomal ones and the question turns therefore on the kinds of characters shown in the cross.
The first indication of sex-linkage in fowls was furnished by evidence that Spillman published in 1903 on information supplied by poultry-men—information that has been proven subsequently to have been accurate. Spillman pointed out clearly the similarity between the facts he quoted and the then known cases of sex-linkage in the canary and in the currant moth. The case referred to by Spillman was a cross between Barred Plymouth Rock and Black Langshan. Goodale and I repeated the cross, using both Plymouth Rock and American Dominques, publishing the results in 1912. In addition to the F₁ results evidence was obtained for the F₂ generation. The theory was also tested by back-crossing. The results of such a cross that are typical for all cases of the sort are briefly as follows: Plymouth Rock cock by Langshan hen gives F₁ barred sons and barred daughters. These inbred give F₂ barred cocks and barred and black hens (2:1:1).
In the following schemes the sex chromosomes are represented by Z and W, while the exponents stand for the factors involved, viz, B for barred and b for not-barred, which here means a black bird.
In the reciprocal cross, a black cock was mated to a barred hen. The sons were barred, the daughters black (F₁). These inbred gave (F₂) barred males and females, black males and females in the ratio of 1:1:1:1. The chromosome scheme of inheritance is as follows:
One back-cross test consists in mating the F₁ barred males ZᴮZᵇ (from both crosses) to a pure black female. The expectation is for equal numbers of barred and black males and females, and the result was realized. The F₁ barred hen of the first cross (ZᴮW) back-crossed to a black cock is expected to give only barred males and black females, and this result also was obtained. The explanation of the last cross, based on the sex chromosomes, is as follows:
Before these experiments were finished Goodale had made other crosses involving the barring factor, and had obtained results that showed the sex-linked inheritance of this factor (1909). For example, he crossed Buff Rock male (not barred) to white Plymouth Rock females. The sons were barred and the daughters not barred. The reciprocal cross gave barred sons and daughters. A White Rock male (carrying barring) mated to a Brown Leghorn female gave barred sons and daughters. Reciprocally, the chicks were of two kinds as to their down, viz, black chicks and chicks with the down pattern of the barred rock. All these results with Barred Plymouth Rocks show that they carry a sex-linked dominant factor for barring. Its wild-type allelomorph would be game-color (jungle-fowl), but since, when the dominant barring is absent in some of the individuals in these crosses, they are black, it would seem to follow that another dominant factor, one for black, that is not sex-linked, is also present.
Pearl and Surface have also carried out crosses with Plymouth Rocks on a much larger scale. Their results conformed in every way to the foregoing. They crossed Barred Plymouth Rocks and Cornish Indian games. The plumage of the male of the latter race is black with dark red on the back and wing-bows; the females are also black laced with mahogany ground-color on back, breast, wing, and tail coverts. When the male game is mated to the barred hen the sons are barred and the daughters are black. In the reciprocal cross both sons and daughters are barred. The back-cross tests conformed to expectation. The results were the same as those already stated above for the Langshan-Rock cross.
Sturtevant crossed Columbian Wyandottes and Brown Leghorns. The F₁ sons were alike, whichever way the cross was made. They were fairly typical Wyandottes, which race carries therefore more of the dominant plumage characters (two or three?). There were two types of daughters, depending on the direction in which the cross was made. When the father is Wyandotte, the daughters are like him (except for stippling of the Leghorn type). When the father is Brown Leghorn the daughters are somewhat stippled red birds. In the former case the daughters getting their Z chromosome from their Wyandotte father resemble him; in the latter case the daughters getting their Z chromosome from their Leghorn father look more like him. Their failure to look exactly like him must be due to autosomal factors derived from the Wyandotte mother that dominate other autosomal factors from the father.
Hagedoorn crossed Black Breasted Game bantams (like those used in my Sebright crosses) to Brown-Breasted bantams. In the latter the black breast feathers of the male are bordered by lemon; the hens are nearly black. Black-breasted male to “brown-red” female gave both black-breasted sons and daughters. In the reciprocal cross all the sons were black-breasted (like the mother) and all the daughters were brown red like the father. Evidently the factor here for Brown Breasted game is sex-linked and recessive. In this case the new mutant sex-linked character is recessive to the wild type.
Davenport (1912) crossed Brown Leghorns to Dark Brahmas. In the cross and its reciprocal all the sons are alike. Two dominant sex-linked factors were found,[5] viz, the white background characteristic of the Dark Brahmas and the red upper wing-coverts (and back) characteristic of the Brown Leghorns. On the other hand, the daughters differ in the two crosses, in each case resembling their father in their hackle color.
When two sex-linked characters are involved in a cross it is possible to determine by suitable matings whether an interchange between the chromosomes that bear them has taken place. In the case of the sex chromosomes only one sex, the male, has both like chromosomes, viz, ZZ, and we expect from analogy with the Drosophila work that crossing-over would be found between the sex chromosomes only in the male. Goodale has recently (1917) made the important discovery that in poultry crossing-over takes place between the sex chromosomes (ZZ) in the male, but not in the female (ZW or ZO). This relation, therefore, is the reverse in birds and flies, for, in the one, crossing-over takes place in the female and in the other in the male. Whether this difference extends also to the other chromosomes in birds as it does in flies is as yet not known.
Several years ago some crosses between gold and silver Campines were reported by Rev. E. Lewis Jones. The results are consistent with the view that a sex-linked factor pair is responsible for this difference in color, although the author does not apply this view to his results. The results may be seen in the table on page 16, to which Jones has prefixed the number of individuals. The cross also involved hen-feathering versus cock-feathering, which appears here (as in other cases) to be a non-sex-linked dominant factor. As stated above there are in the results a few apparent inconsistencies with this interpretation, due possibly to heterozygous females having been used in the crosses.
Lefevre crossed Silver Spangled Hamburgs and Brown Leghorns. The spangling was found to be a sex-linked dominant factor. A spangled cock bred to a Leghorn hen gives spangled sons and daughters; a spangled hen by a Leghorn male gave spangled sons and not spangled daughters. The daughters do not transmit spangling. Other factors may obscure the results, especially factors for black, or the localization of the pattern. Lefevre says “it would seem probable that multiple factors for black, introduced by the Brown Leghorns, are present, and that these factors may have a cumulative effect, with the result that pigmentation is developed to varying degrees of extension.” Whether the factors for black spoken of as coming from the Leghorns are dominant wild-type factors that have mutant allelomorphs in the Silver Spangled Hamburg is not entirely clear from the quotation.
Baur gives in his Introduction to the Study of Heredity (1914, pp. 202-203) some results (unpublished) that Hagedoorn had obtained by crossing gold and silver races of Assendelver birds. The factor is sex-linked and is no doubt the same factor reported by Jones for gold and silver Campines and by Sturtevant for Columbian Wyandottes. Silver dominates gold and the sex relations are the same as those already reported by others for poultry, viz, the male is ZZ, the female ZW. Gold hens by a heterozygous silver[6] gave 162 silver cocks, 163 silver hens, 168 gold cocks, 160 gold hens, expressed graphically (g for gold, s for silver):
| Zᵍ—W♀ × Z—Zᵍ♂ | |||
| ZˢZᵍ—ZᵍZᵍ—ZˢW—ZᵍW | |||
| Silver | Gold | Silver | Gold |
| male | male | female | female |
When a silver hen was united to a gold cock there were 246 silver cocks and 243 gold hens—crisscross inheritance.
Summary.
From the standpoint of the Brown Leghorn type representing the wild type, the following colors and patterns represent dominant mutations from that type:
| Dominants. | |
| White of White Leghorn. Silver of Dark Brahma. Black of Minorca. Lacing of Brahma. |
Barring of Plymouth Rock. Black (?) of Plymouth Rock. Buff (or red). |
Each of these (in heterozygous condition of course) is dominant; in some cases completely so, in others incompletely dominant. At three different loci in the sex chromosome a dominant mutation has occurred; at three loci in other chromosomes dominant mutant changes have also occurred.
| Recessives. | |
| White of Rose Comb bantam. White of Silky. White of White Rock. |
Brown of Brown-breasted game. Penciling. |
Whether the recessive white that is sometimes found in dominant White Rock stock is different from both of the other recessive whites is not known. There are, then, 5 or 6 recessive characters that are not sex-linked and 1 recessive sex-linked character.
Owing to the relatively large number of color dominants in poultry, some unnecessary confusion has arisen concerning the relation of the dominants to the wild type, and especially to other mutant characters to which they are said to be dominant, in the sense, however, of being epistatic. An imaginary example will illustrate this. For example, if at some locus in the wild type a mutation occurred that gave a dominant black (i. e., a black that shows up when one gene for it is present) and at the same time this black also showed up even when other recessive mutant characters were present in homozygous form, then F₁ birds would be black when black is crossed to such pure recessive stocks. Such cases have indeed been described as dominant, but a knowledge of F₂ would have shown at once the error of such a system. For, if black had been a real dominant, the F₂ would have given 3 blacks to 1 of the other type (such as the wild type), but if the case were one of epistasis, then there would have been 9:3:3:1 classes in F₂ (or some modification of that ratio). In this sense, then, epistasis may be defined as a result that appears when one member of the pair of genes produces its effect regardless of the constitution of the individual with respect to another gene (or other pairs of genes). It is curious at least to note that in the case of dominant white the term epistatic has been much less often used than in the case of black. Theoretically the two situations are exactly alike, but because black could so obviously conceal things beneath it, while white is not thought of as doing so, it seemed “natural” to make such a distinction. In reality it is not a question of covering up at all, but a case of a dominant character (white or black) preventing other colors from appearing.
In the case of recessive white the situation is somewhat different and no one, so far as I know, has gone so far as to speak of such a white as epistatic, although when the animal is white it certainly hides, when completely effective, all the other effects of color-producing factors, but allows them to “show through” in some of the cases. This means not that they do “show through,” but that they only develop to a “lower” degree. The difference between dominant and recessive whites rests on the fact that in one case one member of a pair of factors gives white and in the other both members are necessary. But obviously such a distinction is not important, and if it were worth while the case might be argued for recessive whites being also epistatic. The whole tangle goes back to a false interpretation of presence and absence of characters and presence and absence of factors. As I have gone over this ground recently in my paper on the Theory of the Gene, I need not repeat here what I tried to make clear there.
The occurrence of gland-like cells with an internal secretion in the ovary and testes of fowls has been described by a number of writers and denied, at least for the testes, by others. The work of Boring and Pearl has done much to bring this question to a satisfactory solution, for they have tested out and made use of the best reagents that their predecessors had discovered and have used a much greater amount of material. As they have reviewed very fully the literature of the subject, it will not be necessary to go over the ground again in detail.
In the follicles of the ovary there are present, according to Boring and Pearl, groups or nests of cells lying among the connective tissue of the inner theca. The cells are about three times as large as the ordinary connective-tissue cells of the ovary. The cytoplasm is clear and vacuolated, “only occasionally containing a few acidophile granules which stain with the fuchsin in Mallory’s stain or the eosin of Mann’s stain, while the real interstitial cells are crowded with granules.”
When the egg is set free from its follicle, the latter collapses and the rupture becomes closed. A mass of cells collects in the center of the collapsed structure which develop yellow pigment. The cells, lying in the puckered edge of the follicle, may also develop such yellow color. The cells that produce the yellow pigment come from the nests of cells that lay originally mainly in the theca interna. Either by migration or by division they come to fill up the central cavity. The yellow substance in the cells is not fat, since it does not dissolve in the clearing oils, nor can it be protein, for it does not take acid stains as normal secretion granules of protein. It does not dissolve in HCl, HNO₃, or H₂SO₄, nor in strong KOH, although the latter turns the pigment a bright red color. Many other substances were also tried by Boring and Pearl, but none of them dissolved the yellow pigment, which reacts in this respect in the same way as does the yellow pigment in the luteal cells of the mammal. The similarity in the nature of the pigments in the two cases is an argument in favor of the view that the cells that produce the pigment are the same in both groups. In the mammal the yellow corpus luteum is a large, gland-like organ that develops after the ovum is discharged; in the bird there is also a yellow spot on the ovary, due to the pigment in the collapsed follicle, but it is smaller and much less conspicuous than in the mammal. The evidence concerning luteal cells in the testes of the bird is conflicting. One of the difficulties in the situation is the identification of the cells, which are sometimes regarded merely as the general connective-tissue stroma of the testis that is undoubtedly present; at other times special secretory cells are discerned embedded in the connective tissue, as individual cells or in islands. Boring states (1912) that in newly hatched chicks about half of the tissue of the testes is interstitial connective tissue; the other half consists of tubes or cords whose principal function is the development of the germ-cells. In the paper of 1912 Boring reached the conclusion that there are no “interstitial cells in the testes of the domesticated chicken in the sense that this term has been previously used,” and states that no evidence has been found that an internal secretion of any kind is formed by any cells of the interstitial tissue.
It is not necessary to discuss whether or not connective-tissue cells are present in the testes of birds, for is it generally conceded that they are found at least in certain stages, but it is important to look into the question as to whether among these interstitial cells there are others that have an endocrine function. Mazzetti gives pictures of such gland-cells between the seminal tubules of the cock bird, but says that they are rare, “even though this bird has very marked secondary sexual characters” (Boring and Pearl). It may be remarked parenthetically that if they had been more abundant the bird might have had no secondary sexual plumage since it will be pointed out below that such glandular cells may have as their special function the suppression of these characters.
According to Des Cilleuls, interstitial cells are first found in males about 30 days old and at this time the secondary sexual characters put in their appearance. If, as will be shown in the sequel, he means by interstitial cells the endocrine cells that suppress the development of the male plumage in the female, the appearance of these cells at this time would be significant; but if he implies that their occurrence in the male incites the development of the secondary sexual characters, his interpretation is open to serious doubt. Reeves found interstitial cells in testes of cocks 3, 5½, 9, and 18 months—more in the earlier stages.
In a later communication by Boring and Pearl the whole question is taken up again with improved methods, etc. Previously 21 male birds had been studied, just hatched to 12 months old. More sections of this same material were made which were stained according to Mann’s and Mallory’s methods. In addition, a whole new series of preparations was made. A few interstitial cells, i. e., granule containing-cells were found in newly hatched chicks, but not in any of the 60 mature birds examined.
Finding that the testes of F₂ hen-feathered birds were often flat and pear-shaped instead of rounded and cylindrical, as in ordinary cocks, and that they were often black in color, suggested, as already stated, that the testes of the Sebright might be hermaphrodite in some element. It seemed not impossible that egg-cells might be found. I made a considerable number of sections of the testes of these birds and examined them under the microscope; not finding any egg or egg-like bodies, the slides were laid aside, but the idea that in some other way the Sebright’s testes might correspond to the ovary of the female next recurred to my mind. Consequently, when in the summer of 1918 I had some new material derived from a castrated Sebright male that had partly regenerated its testes and was again going back to hen-feathering, and pieces from one of the old testes of a castrated bird, I asked Miss Boring, who was then in Woods Hole, to make some preparations and examine them to see if she could detect any such elements in them as she had found in the female. Miss Boring reported the occurrence of luteal cells in the testes from hen-feathered males, and the results have been published in a brief preliminary paper (1918). The abundance of these clear cells, supposedly gland-cells with endocrine influences, in the testes of hen-feathered birds is in sharp contrast to their absence in the normal adult cock birds. It seems to follow, therefore, that the hen-feathering in the Sebrights is due to the presence of these cells, whose function is the same as of the similar cells in the female, i. e., the suppression in both of cock-feathering. Castrating the Sebright produces its effect by the removal of these cells that are responsible for the suppression of cock-feathering.
The occurrence of luteal cells in young stages of other races of poultry raises the question as to whether in these races the first or juvenile plumage, that resembles that of the hen rather than that of the cock, may not also be due to an internal secretion from these cells, or whether this juvenile plumage is only the plumage of a characteristic stage in development. Castration of young chicks ought to settle this point. Such castration experiments have been made by Goodale. The absence of any reference to any effect on the juvenile plumage in these early castrated birds probably meant that they did not develop precociously cock-feathering, and he writes me that he examined them carefully and that their plumage is like that of the normal chicks. Geoffrey Smith has reported the occurrence of two kinds of males in a race of Leghorns, the males of one of which become cock-feathered before the other. May not this difference depend on the length of time endocrine cells remain or begin to develop? A histological study of the two types would be of the greatest interest.
In man and other mammals it has long been recognized that in addition to the germinal cells of the testis there are also present other cells, sometimes called interstitial cells, that, so far as known, have no immediate function in connection with the germ-cells, or at least that have other important functions outside the relation to the reproductive organ. That some internal secretion from these cells has an important influence on the secondary sexual characters rather than anything done by or produced by the germinal cells has been very clearly shown by evidence derived from three separate sources, namely, from the operation known as vasectomy, from an exceptional condition known as cryptorchidism, and more indirectly from X-ray treatment. Vasectomy involves either cutting the vasa deferentia in such a way that the cut ends do not reunite. In consequence of the closure of the outlet of the testis the germinal cells slowly degenerate, and finally completely disappear. How such an effect is produced we do not know. That this result does take place is borne out by the unanimous testimony of all those who have successfully performed the operation. Ancel and Bouin showed (1903) that breaking the continuity of the vas deferens suppressed spermatogenesis in 8 to 12 months. Both the Sertoli cells (the nourishing cells of the germinal epithelium) and the interstitial cells persist. Such animals remain sexually active and their secondary sexual characters are not affected. Marshall states that in the hedgehog the remarkable periodic enlargement of the testis takes place even after vasectomy, although the germ-cells have disappeared.
In mammals the testes fail at times to pass through the inguinal canal, and, in consequence of their retention in the body-cavity, the germ-cells fail to develop. On the other hand, the interstitial cells of the testis develop normally. Cryptorchid individuals show the normal secondary sexual characters of their species. How retention of the sperm should give rise to the same result as cutting the duct, viz, absorption of the germinal cells, is not known. A possible solution may be found in the pressure exerted on the testes, both when retained in the abdomen and when their outlets are stopped by tying or cutting the ducts.
Finally, it has been long known that continued or repeated exposure to X-rays or to radium causes the destruction of the germ-cells, but leaves the interstitial cells intact and presumably functional. Destruction of the germ-cell by X-rays has no effect on the secondary sexual characters.
This threefold evidence demonstrates that in the male of the mammalia most, perhaps all, of the secondary sexual characters that are affected by castration are not affected by the destruction of the germ-cells. This conclusion supports very strongly the view that the interstitial cells are the cellular element in the testes that influence through internal secretion the development of the secondary sexual characters of the male.
Equally important are the results that relate to the accessory organs of reproduction, such as the glands that open into the vas deferens (prostate, Cowper’s gland, etc.) and the copulatory organs also. In the castrated mammals these organs diminish in size. On the other hand, after destruction of the germ-cells in the testes (or even when they fail to develop as in cryptorchid individuals) these accessory parts are unaffected. In birds, as will be shown, the situation is entirely different.
The changes that take place in the interstitial cells in mammals that hibernate and in which there is a definite rutting season following hibernation have been examined by several workers. The mole has been studied by Regaud (1904), Lécaillon (1909), Tandler and Grosz (1911); the marmot by Hauseman (1895) and Gaugini (1903); the hedgehog by Marshall (1911); and the woodchuck by Rasmussan (1917). In the mole the interstitial cells are most abundant when the tubules in which the spermatogenesis is taking place are least developed, and vice versa. In the hedgehog the increase in both tissues takes place at the same time. In the woodchuck both tissues increase rapidly after hibernation (during March and April), after which the spermatogenesis continues actively for the two following months (May and June), while the interstitial cells retrograde rapidly during April and remain at a low level for the rest of the year. Retrogression in the germinal epithelium begins in July, after the rutting season is past. It appears from this evidence that the activity of the two tissues does not always run the same course. Since the secondary sexual characters of the male, which are not well developed in these animals, are not so far as known affected by the condition of the testes, the evidence does not have any very direct bearing on our present topic. How far the sexual behavior of these mammals is determined by the quantity or by the activity of the interstitial cells is not very clear from the evidence, although there is a very noticeable increase in the amount of this tissue just before and during the rutting season. In the mole also the interstitial cells begin to increase just before the mating season, and the increase continues for several months after mating has taken place. It is difficult to judge how great or how little the change amounts to unless the whole organ is considered, for the relative volumes of the seminal tubes and the interstitial tissues does not give a measure of the total volume of these tissues, since the testes may decrease greatly in size when the seminal tubes retrograde, and the apparent increase of the interstitial cells at the time may not increase the total amount of that tissue present.
Probably more important than the ratio of interstitial tissue to tubules is the activity of the former. Rasmussan states that in the woodchuck the interstitial cells not only increase in number immediately after hibernation, but the increase in amount of this tissue is largely due to increase in the cytoplasm, in which there appears an accumulation of fatty globules in the more peripheral parts of the cells. In the central cytoplasm an abundance of fine lipoid granules develops.
Marshall has made some interesting experiments on the hedgehog at different seasons. Castration in March prior to the breeding-season has an influence on the accessory generative organs (vesiculæ seminales, prostates, and Cowper’s glands). They remain in the same undeveloped stage in which they were at the time of operation. If castration is carried out very early in the breeding-season, when the accessory reproductive organs are about half developed, their further enlargement is prevented. In so far as the accessory organs rank as secondary sexual organs, their complete development is thus shown to depend on the testes. Transection of the vasa deferentia before the beginning of the breeding-season affects somewhat the enlargement of the testes, but produces no effect on the accessory organs.
Several hermaphrodite birds have been described (Brandt, 1889; Shattock and Seligman, 1906; Pearl and Curtis, 1909; Smith and Thomas, 1913; Bond, 1914; etc.). The most recent and complete account of such birds is that by Boring and Pearl. They examined in all 8 hermaphrodites, or at least 8 birds that showed in their plumage, or other secondary sexual characters, peculiarities of both sexes. Five of the birds came from Herr Houwink in Meppel, Holland, who had a stock in which there appeared, in 1911, two hermaphrodites out of 80 birds, and in 1912, three out of 80 birds. These were the birds studied by Boring and Pearl. In addition, when Pearl saw Herr Houwink’s birds in 1910, “there were then on hand a considerable number of these supposed hermaphrodite birds.” An anatomical study of the Holland birds showed that one of them was nearly a normal female; three, the authors say, were “evidently undeveloped females. They have infantile oviducts and embryonic ovaries.” It should be added that there was a tumor more than twice the size of the ovary attached to or part of the ovary. If the ovary itself was affected by the tumor, or the tumor was a part of the ovary, the slightly unusual condition of the birds might be accounted for. Of the other 3 birds, 2 are also suspected to have ovarian tumors, while in the third bird streaks of a secretion which resembles the substance of the tumor of the other two were found. The change towards male plumage in these 5 birds is probably due either to the incomplete development of ovary or to the effect of the tumor on the ovary. Although luteal cells are described as present, it seems probable that their total number might be less than in a normal bird, and hence their insufficient secretion would fail to suppress the development of male plumage. From this point of view these birds are no more hermaphrodites than is a hen with her ovary taken out.
The remaining Holland birds were entirely different. On the left side there was an ovary in an inactive condition; on the right side there was a testis, producing spermatozoa. Sections of the testis show that it is normal, consisting of a mass of tubules with very little connective tissue between them. In both ovary and testis there are “a few nests of luteal cells near the surface. The ovary contains eggs, but is abnormal to some extent.” The authors state: