Popular Science Monthly/Volume 74/June 1909/Facts Concerning the Determination and Inheritance of Sex
|FACTS CONCERNING THE DETERMINATION AND INHERITANCE OF SEX|
ADJUNCT PROFESSOR OF ANATOMY, UNIVERSITY OF VIRGINIA
SINCE the time of Aristotle numerous observations have been recorded concerning the phenomenon of sex. Long prior to this period, undoubtedly, men were vexed by such questions as: Why in the same litter of animals are some male and others female? Why in the same brood of chickens do some develop into hens and others into cocks? Why in the same family are some of the children girls and others boys? What determines that one animal or plant shall be male and another female? When does the sex of thee organism become unalterably fixed? What is the nature of sex? A great deal of light has been recently thrown upon all these questions. In modern times a solution is being attempted by refined scientific means. Statistical, experimental and cytological methods of research are being employed. The fogs of mystery enshrouding the phenomenon of sex are becoming more and more attenuated and we may confidently hope soon to see them cleared away. Much has lately been discovered that strongly indicates the probable answer to the foregoing questions. Moreover, in their attempts to elucidate the enigmas of sex, men have been actuated as much by a pure scientific motive of love of truth as by the practical bearing of completer knowledge respecting sex-determination on the matter of the control and regulation of sex. The two main problems involved concern the time when sex is determined and the means by which such determination is established.
Aristotle had noted in the case of pigeons that of two eggs laid in each batch, one generally produced a male and the other a female. He further reported that the first gave rise to the male and the second to the female. Flourens has confirmed this fact for eleven sets of eggs and Cuénot has more recently obtained the same result. What is the meaning of this fact? Aristotle speculated and men are still speculating. When Drelincourt wrote regarding the matter of sex in the eighteenth century he gathered together two hundred and sixty-two theories and hypotheses concerning the nature and cause of sex and declared them all "groundless." Blumenbach subsequently contributed another and reported two hundred and sixty-three worthless theories, having added Drelincourt's to the list. Since then the number of theories not well founded on actual facts has increased to more than five hundred. The great majority of these are largely unscientific, the result mostly of philosophical speculation or a priori opinion. As examples of such it was supposed that an egg from the right ovary gave rise to a male and an egg from the left ovary to a female; that the sex of the offspring was like that of the younger or more superior parent; the exact opposite of the latter, that the sex was that of the weaker or older parent; or that the younger or more vigorous parent determined the opposite sex in the offspring; and many others involving equally off-hand assumptions. That the sex of the offspring is that of the weaker parent has been recently advocated again and statistically apparently well supported by Dr. Romme, a well-known physiologist of London. In further support of his theory he cites the fact that among barbarous nations continually at war there is always a preponderance of boys over girls.
Professor Schenck's famous sex-theory is in essence the same. He proposes to increase the physical vigor and the number of the red blood corpuscles in a person when it is the wish to beget a child of the opposite sex. Renewed interest has been aroused in this theory, due to Professor Schenck's connection with the royal family of Russia, where he put his principle to the test with an apparently successful result when the Czarina gave birth to the desired son. But Professor Simon Newcomb, as the result of a very extensive statistical investigation, concludes that "it seems in the highest degree unlikely that there is any way by which a parent can affect the sex of his or her offspring."
Within the last decade students of the problem of sex have become very generally agreed that sex is inherited. In other words, they believe that the same mechanism that provides that an offspring shall have one or the other of a pair of contrasting characters represented in the two parents (a long sharp nose or a short stubby nose, for example) provides also that the animal shall have either the sex of the mother or that of the father. The problem of sex seems to be part and parcel with the general problem of inheritance. Furthermore, characters of the parents are believed to be inherited by the offspring according to Mendelian principles. And it can be shown, as was done by Dr. Castle, of Harvard University, in 1903, that sex may be regarded as the result of a Mendelian segregation, dominance and inheritance of sexual characters. And now it becomes incumbent to explain Mendelian inheritance before continuing the discussion of the problem of sex, seeing that sex appears to be inherited in Mendelian fashion.
Mendelism is the term employed to designate a set of phenomena that appear when animals or plants with sharply contrasting characters (white flower petals and colored petals; gray fur and white fur; short stature and long stature; sagacity and stupidity; etc.) are crossed. The principles included under the term Mendelism were first discovered by Gregor Mendel, abbot of Brünn, in 1866. Since then our knowledge of Mendelian inheritance has been greatly extended through the researches notably of Professor de Vries, of Amsterdam; Professor Bateson and Professor Punnett, of Cambridge University, England; Professor Cuénot, of Paris; Professor Castle, of Harvard University; Dr. Davenport, of the Station of Experimental Evolution of the Carnegie Institution at Cold Spring Harbor, Long Island, and Professor Morgan, of Columbia University. For the purpose of elucidating this subject in respect to its essential principles and its peculiar nomenclature we will take the simple case of two varieties of pea, one with seed colored yellow and the other with seed colored green. These are said to be "pure" in the sense that from green seed-bearing individuals when interbred only green seed-bearing plants will arise, and from yellow peas only yellow seed-bearing plants. In order that plants or animals may be successfully crossed it is requisite that they be closely related species or varieties.
Mendel found at the beginning of his work that when a cross is made between two such varieties of peas, the first succeeding generation yields peas that are all colored yellow. When flowers from plants of the latter are self-pollinated, i. e., crossed with their own kind, there result peas of two kinds, yellow and green, and the proportion of such seed is three of the former to one of the latter. This means that the yellow color is "dominant" over the green color, which is said to be "recessive." Mendel further discovered that when the green recessives were self-pollinated their seed always produced only green seed-bearing plants. Such were called "pure recessives" or "extracted recessives."
He found also that among the 75 per cent, of yellow seed 25 per cent, always bred true, i. e., they gave rise to only yellow seed-bearing plants. Such were designated "pure" or "extracted dominants." But the remaining 50 per cent, on self-fertilization again yielded peas in the proportion of 75 per cent, yellows to 25 per cent, pure recessive greens. The Mendelian proportion then in the case where a single set of contrasting characters ("unit characters" or pair of "allelomorphs") are crossed is 25 per cent, pure dominants (yellow in color); 50 per cent, color hybrids (also yellow in color, since yellow dominates over green); and 25 per cent, pure recessives (green in color).
Mendel explained the foregoing facts on the assumption that during the process of maturation, when the unripe cells divide each into four mature germ-cells, eggs or pollen grains (ova or spermatozoa in animals), the carriers of the qualities yellow and green are segregated into different cells. In other words, no eggs or pollen grains (sperms) carry both characters, but only one or the other character. Accordingly, among the eggs 50 per cent, carry the quality yellow and 50 per cent, carry the quality green; among the pollen grains 50 per cent, again carry only the yellow color and 50 per cent, the green. At fertilization eggs and pollen grains meet fortuitously and according to the laws of chance there are twice as many possibilities that an egg with a green color-determinant (G) shall meet a pollen grain with a yellow color-determinant (Y), and vice versa, as that an egg with a green determinant shall meet a pollen grain with a green determinant, or that an egg with a yellow determinant shall meet a pollen grain with a yellow determinant. Hence the combinations produced may be expressed thus: 1YY: 2YG: 1GG. Among the seed 50 per cent, are hybrids in respect to color, but since the yellow color dominates over the green all appear yellow, giving a total of 75 per cent, yellow peas and 25 per cent, green peas.
When pure strains of sweet peas with white flowers are crossed with pure-bred peas with red flowers similar results follow. All the plants of the first generation bear red flowers, showing the dominance of red color over white color. When these are interbred the plants of the second generation split up in the proportion of 75 per cent, red-flower-bearing to 25 per cent, white-flower-bearing; or 25 per cent, will be red pure dominants, 50 per cent, red hybrids and 25 per cent, white pure recessives. Also when white mice (albinos) are crossed with gray mice (or pigmented mice) the first generation are all gray, when the latter are bred among themselves the second generation includes albinos and gray individuals in the proportion of 1 pure gray: 2 gray hybrids: 1 pure white.
In applying Mendelian principles to sex, maleness and femaleness are regarded as unit characters and during the maturation of the germ-cells the carriers (chromosomal elements) of the male and female qualities are believed to be segregated in different cells, both ova and spermatozoa, so that one half of the ova contain the male-determining factor and the other half the female; and likewise the spermatozoa. The result of a fortuitous intermingling of ova and sperm, according to strict Mendelian laws, should produce male and female individuals in the proportion of 3:1 or the reverse. Suppose femaleness dominated, then there would be 75 females in every 100 of population or of any particular species. No such disproportion of sexes obtains. In fact, barring a few exceptions which may be explained as adaptations or as the result of a selective mortality, all species are approximately equally divided into two classes with respect to sex. Further assumptions must be made and pure Mendelism modified. The obvious and necessary assumptions are that there are no individuals pure in sex; all are hybrids; and the sex that the organism attains is the result of a struggle between the mingled male and female tendencies and the dominance, now of one tendency, now of the other. The dominance may be due, as Dr. Thomson, of the University of Aberdeen, suggests, to slight metabolic fluctuations, now in favor of maleness (katabolic), now in favor of femaleness (anabolic), the net result being an approximately equal proportion of males to females. In other words, there must be selective fertilization, that is, a female ovum must be fertilized by a male sperm, and vice versa. The various assumptions made are not pure speculations, but rest upon many facts. Some animals do possess two kinds of eggs, of which one (the larger) develops into females and the other kind (the smaller) into males; there are many instances of selective fertilization; and many animals do produce two kinds of spermatozoa.
If there is only one kind of egg, as Correns suggests—as may well be the case, since of the four potential ova, three (the polar bodies) degenerate during maturation and only one becomes capable of fertilization—and two kinds of spermatozoa, the explanation of sex becomes very much simpler. One sex (female) must then be pure in respect to sex (a homozygote) and the other must be a sex-hybrid (heterozygote). If the egg is female in tendency, in order that there should appear 50 per cent, males, maleness must dominate over femaleness. In any case, an interpretation of the facts involves the application of some phase of Mendelism. As will appear farther on, the case of the honey-bee, ants and plant-lice offer serious obstacles to the universal application of Correns's interpretation, and even of the whole Mendelian scheme.
Within the last decade three main positions have been advocated by various investigators in regard to the cause of sex. The position held by Beard and his school is to the effect that there are two kinds of eggs, male and female, and one kind of spermatozoa; and that sex is determined exclusively by the egg, the spermatozoon having no role in sex production. This position can be supported by various facts. A certain worm (Dinophilus apatris) carefully studied by Korschelt, is known to produce two kinds of eggs, large and small, the former developing into females, the latter into males. Of course it might be urged that in consequence of selective fertilization only the small eggs are impregnated by a male-producing spermatozoan and the large eggs by a female-producing, and an entirely different interpretation of the facts would be necessitated. But in the case of Hydatina senta, a rotifer or "wheel animalcule," where large and small eggs are also laid, the former without fertilization develop into females and the latter into males. However, both kinds of females, the large-egglaying kind and the small-egg-laying kind, came originally from fertilized eggs, and it may be that a difference in metabolic activity of the several females kept the eggs of the one small and allowed those of another to grow large and so gave the victory to the female determinant in the well-nourished egg. This assumption is supported by the fact that the amount of nourishment taken by the young female between the time of emergence from the egg and deposition of her first egg determines which kind of egg she shall subsequently produce. The well-nourished females produce female eggs, the poorly nourished produce male eggs. Sex, however, is determined while the ovum is still in the ovary. When the eggs have once been formed no subsequent change of food or temperature can alter the kind of eggs that are produced. Phylloxera vitifola, the plant louse that lives on the roots of the grapevine, also produces large and small eggs, developing parthenogenetically (without fertilization) into females and males, respectively.
The second position is that taken by Strasburger, Castle, Wilson and others who have derived their facts largely from the group of insects. These investigators are inclined to believe that there are two kinds of eggs as there are two kinds of spermatozoa, and that sex is the outcome of an interplay or struggle of the sex determinants of these elements and the dominance of one or the other sex. According to these biologists all animals are sex-hybrids; either sex is potentially present originally, but by reason of the dominance of one or the other sex-determinant the particular sex becomes patent and makes the animal definitively male or female. Dominance may be due, in many cases, as is indicated by some of the Hemiptera, to the presence of one or several extra and specific chromosomes—"idiochromosomes," "X-element"
(Wilson). According to this view fertilization is selective, i. e., a female egg is fertilized only by a male sperm, and vice versa. There are many observations and ascertained facts to support this position.
The case of the large "walking-stick" insect (Aplopus mayeri) or "devil's riding horse" of Loggerhead Key, Dry Tortugas, Florida, which I studied in 1907, illustrates the position under discussion. Similar facts had been reported previously for many insects by various American cytologists, notably Professor McClung, of the University of Kansas; Professor Montgomery, of the University of Pennsylvania, and Professor E. B. Wilson, of Columbia University. Aplopus mayeri (named after Dr. A. G. Mayer, director of the Biological Laboratory of the Carnegie Institution of Washington at Dry Tortugas, Florida) produces two kinds of spermatozoa differing in the number of chromosomes (rod-like bodies supposed to be the vehicles of the hereditary qualities) that the two classes possess. One half of the spermatozoa hold 17 chromosomes and the other half 18, the additional one being large and V-shaped (called by McClung the "accessory chromosome"). The somatic cells of the male contain 35 chromosomes, the somatic cells of the female, 36 chromosomes. When the eggs mature, the 36 chromosomes are reduced, by fusion in pairs and a subsequent double division, to 18 chromosomes. Now at fertilization when an egg of 18 chromosomes unites with a spermatozoan of 18 chromosomes an organism whose cells contain 36 chromosomes results, and this is a female; when the other combination occurs, 18 chromosomes from egg and 17 chromosomes from sperm, an organism results that has only 35 chromosomes, and this is a male. It seems, therefore, as if the accessory chromosome was a sex-determinant in some sense—probably only in the sense of a visible accompaniment of some hidden underlying physiological cause of sex—perhaps as the carrier of a specific enzyme or "hormone."
To bring these facts into line with Mendelian principles it becomes necessary to postulate (1) two kinds of eggs, just as there are two kinds of spermatozoa, (2) selective fertilization, and (3) dominance of femaleness, and the observed facts can be explained. If the accessory chromosome is a sex-determinant, then when an egg is fertilized by a sperm lacking this element, the egg itself must carry the factor that determines that the sex of the resulting organism of 35 chromosomes shall be a male. In the event of the other possible combination the egg selected must have been one carrying the female determinant and, since the accessory chromosome is a male-determinant and the union of the two produces a sex-hybrid, femaleness must dominate in order to yield a female organism of 36 chromosomes.
The most recent position is that of Dr. Correns, professor of botany in the University of Leipsig, to the effect that there is only one kind of egg (always female) and two kinds of spermatozoa. He has brought forth facts to amply support this view in flowering plants. He shows that the spermatozoa determine the sex and that the time of this determination is the instant of fertilization. Correns attempts to bring the second position above mentioned into harmony with his own, and the facts upon which Beard's position is based are given a reasonable interpretation. We shall consider these various positions from Correns's standpoint, and will note how he disposes of the obstacles above referred to.
There are now known about one hundred cases, mostly among the air-breathing arthropods, where there is a dimorphism of spermatozoa. But if fertilization determines the sex, what about such cases where eggs develop without fertilization, as in the drone honey-bee, ants and plant lice (aphids and phylloxerans)? These several examples until recently had been serious stumbling-blocks to the theory that sex is dependent on a dimorphism of spermatozoa. Nor is the matter yet as clear as we could wish it to be. All the fertilized eggs of the honeybee develop into females (workers or queens, depending upon the kind and amount of food they receive), the unfertilized eggs develop into males or drones. Meves has recently found that while the drone honey-bee produces two kinds of spermatozoa, one set, the male-producing, degenerates and becomes non-functional. Hence all fertilized eggs must develop into females. The facts become intelligible if we assume that all the eggs possess a predominating male tendency. There appears to be Justification for this assumption since two polar bodies are formed and a reduction division takes place which may be thought of as partially eliminating the female tendency. Due to the preponderance of female elements, fertilized eggs become females. Unfertilized eggs become males, since the male tendency is undominated.
Aphids produce brood after brood of parthenogenetic females while food and climatic conditions remain favorable. When environmental conditions become adverse both males and females appear between whom eggs are fertilized, which as "winter eggs" resist the elements until the following summer, when they develop into parthenogenetic females. These facts may be thus explained: the original ancestor (stem-mother) of a parthenogenetic series arises from a fertilized egg; the eggs of the parthenogenetic females are all hybrids as to sex. No reduction is thought to take place when the single polar-body is formed. When conditions are favorable, metabolic activities give the ascendency to the female elements; when these become adverse, the male element gains the ascendency in some of the eggs. When males and females appear the sex determinants are segregated in the ova and spermatozoa in equal proportions in each. During maturation in the male aphid, however, Dr. N. M. Stevens, of Bryn Mawr College, and also Dr. W. B. von Baehr, of Germany, have discovered that half the sperm degenerate (these lack the accessory chromosome) and that these are male-producing kind. Hence, since only female-producing sperm remain, all fertilized eggs (sex-hybrids) must develop into females.
In the case of Phylloxera, where many generations of wingless parthenogenetic females appear while environmental conditions remain favorable, and when these become adverse, give way to winged males and females. Dr. T. H. Morgan has discovered a similar degeneration of the male-producing spermatozoa and the loss of a chromosome in the parthenogenetic males. In these cases we must assume that the eggs are intrinsically different (male and female), as they really appear externally, or else, as Correns proposes, that the eggs are potentially male and that in the fertilized egg where both sex tendencies are present femaleness dominates when environmental conditions are favorable to constructive metabolism. But some mechanism or condition must remain whereby an apparently homozygous male may produce spermatozoa bearing a female tendency. It is by no means proved that the whole quota of female tendencies (elements) is eliminated at maturation. The male and female (paternal and maternal) chromosomes are probably promiscuously arranged on the spindle. The persistence of chromosomes bearing female characters may supply the demands for the production of female-producing spermatozoa.
The sex of the offspring then appears to be the result of the interaction of two factors, both unknown quantities. By the union of an egg and a sperm an organism is produced which has a certain sex. If we let X represent the unknown sex tendency of the ova, and Y the unknown sex tendency of the spermatozoon, then we may state the process of sex production thus: X + Y = sex of offspring. If we could discover the value of X or Y we could solve the equation and discover the secret of sex. Correns, as the result of experimentation with certain flowering plants since 1900, has found the value of X in these particular forms and has thus contributed invaluable facts for the reinterpretation of former observations and for the formulation of a wider generalization regarding sex phenomena.
Many facts are known which leave little doubt that in most animals sex is absolutely fixed in the fertilized ovum. A real exception is the case of the frog, where the embryo or even the larval tadpole is hermaphroditic, i. e., it contains both ovaries and testes. In a later stage of development one or the other of these pairs of organs degenerates when the frog becomes a definitive male or a definitive female.
In the human species twins are frequently of the same sex, either male or female. Such twins are known as "identical twins" when enveloped in a common chorion or fœtal membrane. They are the result of an independent development of accidentally separated cells at the two-cell stage of development. Double monsters likewise are always of the same sex. "Ordinary twins" are as frequently of opposite sex as of the same sex. In this case each fœtus is enveloped in its own chorionic membrane. Such twins are the result of the synchronous development of two ova simultaneously successfully fertilized. These facts show that sex is already determined in the fertilized egg and before the first segmentation. Similar evidence is contributed by Jehring, who studied poly-embryony in a species of armadillo (Tatusa hyhrida) found in Paraguay. Here as many as eight offspring appear at a single birth. Jehring reports that all eight fetuses are enclosed in a common fetal envelope. Hence the eight offspring must result from the development of the products of division as the eight-cell stage of segmentation. Since the offspring are all of the same sex, sex must have been already determined in the fertilized egg prior to the first cleavage. Again, Professor Silvestri, of Naples, has quite recently contributed new facts which lead to the same positive conclusion. He has discovered that Litomastix, a kind of bee (Chalcidæ), lays its eggs in the egg of a moth, Plusia. As the latter develops into a larval caterpillar, the egg of Litomastix segments into a chain of many eggs, each of which gives rise to an embryo bee. The caterpillar may contain a hundred such embryos and they are all of the same sex—female if the egg was fertilized; male if unfertilized. Sex must have been already fixed in the fertilized egg. But this is an entirely different matter from the position that Correns takes that sex is fixed at the time of fertilization and determined by the spermatozoa. Correns made his experiments with two species of Cucurbitaceæ, the family to which our pumpkin and squash belong, growing wild in central Europe. These two species are Bryonia alba and Bryonia dioica. The former is the plant which supplies the root employed in pharmacology for the treatment of dropsy. The especial value of these species for experiments relative to the problem of sex, lies in the fact that the former is hermaphroditic or monoecious, i. e., mature male and female flowers appear on the same plant; and the latter is diecious, i. e., male and female flowers appear on different plants. These two species can be crossed in either direction. Correns made three sets of experiments, the results of which have led him to a unique and simple explanation of the cause of sex, in the light of which former theories must undoubtedly be more or less extensively modified. Correns first crossed Bryonia dioica with Bryonia alba, using the eggs of the former and the pollen grains of the latter. Sterile hybrid-offspring appeared from the cross and they were all females. Correns explains that since germ-cells of the hermaphrodite forms carry only hermaphrodite determinants, the male gametes (pollen grains) can have had no influence on the sex of the offspring of this cross. In other words, hermaphroditism is recessive to dieciousness. And since the offspring were all female, all the eggs of Bryonia dioica must have had female tendencies.
Correns next pollinated the pistils of Bryonia dioica with the pollen of the same species. Forty-one pure dioica offspring were obtained from this cross, twenty-one of which were female and twenty male, or in round numbers each sex appeared in 50 per cent, of the individuals. Since the eggs were all female, as shown by the first experiment, this result must mean that the pollen grains determine the sex and that they must be of two kinds, male and female in tendency, and approximately equal in number. It must also mean that the male tendency dominated over the female, else no males could have appeared.
In a third set of experiments the pistils of Bryonia alba were pollinated by Bryonia dioica. The offspring in this case were sterile hybrids, and of the seventy-six which came to maturity thirty-eight were male and thirty-eight female. This again shows that hermaphroditism is recessive to dieciousness since no hermaphrodites appeared. It shows also again that there must be two kinds of male gametes with male and female tendencies, respectively, of equal number, and that they determine the sex. It shows, moreover, that sex is determined at the time of fertilization, for before that instant all the female gametes (ovules) had the tendency to develop into hermaphrodite forms. It appears also in the light of this experiment that hermaphroditism and dieciousness must be regarded as unit characters, since they behave in crossing according to Mendelian laws. Hermaphroditism probably has its chromosome determinant as also dieciousness, hence the cytological study of the germ cells of hermaphrodite forms will hardly supply the clue to the cause of sex as it was until recently believed to do.
In those insects where there is a dimorphism of spermatozoa, consisting in the presence of an accessory chromosome in one half of the gametes, no obvious difference appears among the eggs. There are no real grounds for a division of the ova into those with male character and those with female character. In the light of Correns's recent observations, the eggs had better all be regarded as possessing the same sex tendency, and this probably female in nature. The spermatozoon with the accessory chromosome need then only be thought of as possessing the female tendency, and the one that lacks it, as possessing the male tendency, and the facts are brought into line with the results of Correns and admit of the same interpretation. In the case where large and small eggs are produced, as by Dinophilus apatris, it is conceivable that there is selective fertilization, i. e., while both tvpes of egg may be female in tendency, only the larger admit of fertilization by a female-producing spermatozoon.
The present status of the case concerning the determination of sex, as well supported for a large class of plants and animals, appears to be that sex is determined by the spermatozoa (or pollen grains)—which are male and female in the proportion of 1:1—and at the instant of fertilization. But surely it would be the utmost folly to hold on the basis of so comparatively few facts, that this explanation applies universally. Nature arrives at similar ends by devious and divers ways and it is not inconceivable that sex has been attained by several paths, and is now determined in different modes and at different times in the different groups of animals and plants.