Animal Breeding And Determination Of Sex
( Originally Published 1912 )
Probably no other one thing has occasioned so much speculation regarding the wonderful processes of reproduction as has the desire to control sex. The power to do so would be temporarily at least very profitable to breeders. Their general desire to have the increase of their herds consist mainly of males or females, according to which would be most profitable at the time, is responsible for a multiplicity of directions for ensuring the production of male or female at will.
In the human race the sex of the foetus is distinguish-able at about the eighth week of pregnancy. It is not known whether the sex is determined when the spermatozoon enters the ovum or whether ensuing conditions are responsible for the development of male or female organs. The fact that sex is discernible only at the eighth week by no means indicates that it was previously undetermined.
The commonest idea about sex determination is that females bred at the beginning of thé period of heat produce male offspring. Other no tions are based on the same supposed principle, namely, that an ovum fertilized while immature produces a male; maturity is supposed to be in proportion to the age of the ovum and the nutritive condition of the dam.
Fertilization ordinarily occurs in the Fallopian tubes, the ovum descending from the ovaries when fully prepared and separated from its containing sac through which alone nutrition can be supplied. Early or late service can therefore have no connection with the completeness of the nourishment of the ovum. As to the possibility of sex being attributable to a fresh or stale condition of the ovum, it is inconceivable that anything so fundamental as the production of male or female organs could be controlled by any change resulting from a few hours' residence outside the ovaries but still subject to internal body conditions. Furthermore, if females were the result of service late in the period of heat, then in herds and flocks continuously accompanied by males, and where service ordinarily occurs at the first indication of heat, we should find all male offspring. The experience and observations of managers of large cattle and sheep ranches do not substantiate this idea. Evidence of heat and the time of service are at best crude indications of the real time of fertilization.
Considerable publicity has been given the theory of Dr. Schenck, whose advice it is purported has been sought by royal families of Europe. Schenck supposes sex to be influenced by the condition of the ovum at fertilization. When the urine shows a large proportion of sugar he argues a lower nutritive state of the body and therefore the unripeness of the ova because of the' less amount retained, and conception occurring at such time must result in a male. No extensive statistics covering tests of Schenck's theory are available; inasmuch as in any case there are equal or slightly greater probabilities of the production of male offspring, no surprise need be occasioned by isolated instances of the appearance of males succeeding endeavors for their production. The long-continued practice of flushing ewes at mating time has never been claimed to influence the sex of the lambs as would seem to be the case if this idea were correct, though it is fair to state that Schenck emphasizes the composition rather than the amount of food.
It was once held that the right ovary produces germ cells that always result in females, while those from the left are male, but instances are now known where females with one ovary removed still continue to produce offspring of both sexes. A similar claim was made with regard to male parents, but experiments conducted by James Buckingham of Zanesville, O., disproved this. Mr. Buckingham used nine sows divided into three similar lots. In each lot the first sow had the right ovary removed, the second the left, and the third was normal. One lot was bred to a boar whose right testicle was removed, the other two to boars with the left one removed. The litters had from seven to nine pigs each. In no litter was there less than three males or more than five females. This experiment, reported by Mr. Buckingham in the "Country Gentleman" of 1865, shows clearly that neither ovary or testicle produces either sex exclusively. An idea discussed in the fore-going is the basis for the notion which sometimes finds expression in the direction to breed an animal in the first, third or fifth period of heat after the delivery of a female if a male is desired. The ovaries may act alternately but there is no reasonable ground for supposing that they differ in the sexual possibilities of their ova.
Some believe that the older or more vigorous parent will control the sex. This would seem to suppose that the hereditary material represents the sex of the parent from which derived and that in development the supposed greater activity of the stronger parent's germ-plasm dominates that of the other and directs the production of the organs of the corresponding sex. Why this should be so only in regard to the formation of the sexual organs is hard to see. Neither have we any good grounds for supposing that the germ plasm represents or seeks to produce the sex of the animal from which it was derived. That such is not so in bees has been shown. The unreasonableness of this theory and the lack of data to support it render it untenable.
The nutrition of the pregnant dam has also been thought to have a bearing on the sex of the offspring. As was previously stated, the fact that in higher animals the sex of the foetus can be observed only toward the close of the first quarter of the period of gestation by no means indicates that conditions prevailing subsequent to fertilization control the sex. On the other and it is not proved that such is not the case. The general assumption is that the development of female young demands greater amounts of food and more favorable conditions than does the production of males. The claim is made that statistics reveal the fact that in countries that have been ravaged by war, and the food supply of the inhabitants diminished, an increased proportion of male children is noticeable. This is presented as a natural provision for the restoration of the proportion of males depleted by the conflicts. Data compiled by other students of the subject do not show any disproportion of sexes attributable to nutrition. Considerable experimental evidence is available on this point. Although relating to lower forms of life this evidence may fairly be considered as bearing upon the principles that govern sex in domestic animals.
Born (1881) reared 1,443 tadpoles on a highly nitrogenous diet, and secured 95 per cent females; others on ordinary diet gave 62 per cent females. While the tadpole is an independent animal its transition to the frog stage corresponds closely to foetal development of larger animals. Born's method of observing the sex is now claimed to have been inexact. Yung (1883) reported tests in each of which a liberal nitrogenous diet was furnished developing tad-poles, giving in, each case over 70 per cent females. More recent repetitions of the same test have failed to make it appear that the food influences the sex of frogs. Cuenot found great irregularity in the proportion of females in separate tests, sometimes males predominating and sometimes females. Much care was exercised in determining the sex, although as in earlier cases the sex of those dying during metamorphosis could not be ascertained. On the whole he considered that his results did not indicate any influence of nutrition upon sex. Cuenot also experimented with rats, one lot being liberally fed with a variety of food materials while the other received only limited quantities of bread. The first group produced forty-three males and forty-nine females. In small litters it would seem that each individual would be fully nourished and according to the nutrition theory an excess of females should appear. The total for all litters comprising less than nine young were, seventy-one males and sixty-two females.
From 1867 to 1873 three investigators reported results favoring the idea of supposing that nutrition has an influence upon sex in butterflies. In Experiments with one case Mrs. Treat reported that a starved lot produced thirty-four males and one female while a well-fed lot gave four males and sixty-eight females. Five other experimenters reporting between 1868 and 1874 found no such influence. It was pointed out by Riley that since females are weaker a greater number of that sex would succumb under adverse food conditions, thus showing a preponderance of males among the living ones produced, not through the influence of nutrition upon sex but because of the elimination of the females before emerging as adults to be counted.
Kellog and Bell of California have made an exhaustive study of this subject. Their results have to do not only with the effect of nutrition upon the developing silkworms with which they worked but also show the influence upon sex of liberal and scant, or as they term it, optimum and minimum food supply, furnished to parents and grandparents. In their table which follows M indicates minimum and O optimum food supply. The number of deaths before maturity are separately listed. An examination of the table will show that the five lots receiving a minimum diet produced fifty males and forty-six females while the three lots on optimum diet produced twenty-three males and twenty-seven females.
Argument in favor of nutrition as a determinant of sex is sometimes based on the development of the queen bee. It is known that when a colony loses its queen the workers by furnishing liberal amounts of the royal jelly to a larva develop for themselves a new queen. This larva would otherwise have become an ordinary worker. If we could properly assume the worker to be of the male or neuter sex it might seem that the extra food was responsible, for a change in sex ; the worker, however, is in reality an undeveloped female, and the effect of the extra food utilized by the queen was to develop in her the egg-laying organs and make her a functional female. The nutrition effects no change in sex for femaleness was already produced. The nutrition permitted the completion of the egg-laying organs. Apparent evidences of the effect of nutrition upon sex, as is seen, lend themselves to other interpretations and cannot be entertained by a fair-minded practical breeder. In the interesting case of the queen already referred to it is well known that the eggs she lays prior to her maiden flight and impregnation produce males exclusively. Subsequent to impregnation all offspring are workers, or undeveloped females, any one of which presumably might become a queen if properly fed during development following hatching from the egg. In this case then it seems apparent that sex was determined in the fertilized egg. The fact that the unfertilized eggs gave males and those fertilized gave females would go to indicate that here the parent transmits its opposite sexual qualities. If it be really true that the matter of sex is settled at fertilization and that this applies in domestic animals, it must dissipate all hopes of our ever being able to control sex.
The production of twins constitutes the only piece of evidence discussed as bearing directly on the point in larger animals. In the human race and in cattle the bearing of twins is the exception to the rule and twins are sometimes no more alike than children of the same parents born at separate times. In 'other cases the re-semblance is so great as to render distinction very difficult. Like or identical twins are believed to be the result of the separation of. the two cells produced by the first division of the fertilized ovum, the two halves developing separately producing two individuals with exactly similar inheritance. Unlike twins are considered to be produced by the fertilization and development of two distinct ova. As will be readily recognized in this case the inheritances may differ very widely. Since the like twins, those produced by a single fertilized cell, always have the same sex, it seems fully probable that the germ cells them-selves contain the determinant of sex and that it is not dependent upon conditions governing gestation. We have also dismissed theories of control of sex based on ideas presented in the first section of this chapter. We have more scientific and more reasonable grounds for considering sex to be determined when the reproductive cells unite.
Further evidence to support the thought that sex is determined at fertilization is drawn from recent investigations of the germ cells of a number of species of insects. Recently several investigators have located extra chromosome-like bodies in germ cells of numerous kinds of insects. These extra bodies, or accessory chromosomes as they are now improperly called, were not at first regarded as chromosomes because of their unusual size, being sometimes larger and sometimes smaller than the ordinary chromosomes among which they occur. Further difficulty was also afforded by their having no mates.
The accessory chromosome was first noted by von Siebold in 1836; he found it only in the spermatozoa of a particular snail. Following this there were found several instances in which spermatozoa were of two equally numerous kinds, those having a certain number of chromosomes and those with one more than that number. In the early stages of male germ cells this accessory chromosome splits and divides in the ordinary manner. In one of the reducing division stages it fails to split, however, thus producing from each spermatocyte two ordinary spermatozoa and two with an accessory chromosome. It was suggested by McClung that when the female egg is fertilized by a spermatozoon containing the accessory chromosome the resulting offspring would be of one sex while the union of the ovum with an ordinary spermatozoon would produce the opposite sex. Shortly afterward however a similar irregularity was found to exist in the eggs of some females.
It is clearly established that in the case of the common squash bug (anasa tristis) the body cells of the female have twenty-two chromosomes and those of the male twenty-one. The same is true of the primitive germ cells. In the reduction of the germ cells with twenty-two chromosomes all ova will of course have eleven chromosomes each. In the case of the male germ cells, however, since the twenty-first chromosome does not divide, one-half the spermatozoa will have ten and the other half eleven chromosomes. If, then, a eleven chromosome egg be fertilized by a ten chromosome spermatozoon the off-spring will have twenty-one, the number occurring in males. If it be the eleven chromosome spermatozoon that fertilizes the ovum, the resulting number will be twenty-two or female.
The known facts concerning the accessory chromosomes by no means dispose of the problem of the determination of sex. The fact, however, that the male and female adults in the case referred to have different numbers of chromosomes in their body cells is practical proof that the accessory chromosome is associated with sex, and this being true, the happenings presented .in the diagram are highly probable. This is additional strong evidence that sex is determined at fertilization, and that it is beyond human influence before so determined. In view of the present study of the subject it seems quite likely that we may soon have a more intelligent idea of the basis of sex. That sex of farm animals should generally be under the control of man seems hardly desirable. The present nearness to equality of numbers of the two sexes is of great importance in the preservation and improvement of types by affording a large number of males from which to select the best ones for service. Full control over sex would seem to give man a power that would not be exercised to his own best advantage or to that of the races of his animals.