WHAT SETTLES SEX?

Otago Witness, Issue 3867, 24 April 1928, Page 76

WHAT SETTLES SEX?

By Professor J. Arthur Thomson. Tne philanthropic fowl laid an egg yesteruay and it may develop into a cock; it lays another egg to-day and it may develop into a hen. What makes the difference? Out of 1000 eggs liberated in the pond by a female frog and fertilised by a ’male frog, #7O will develop into females and 430 into males, unless, of course., they are eaten up before they reach maturity. What makes the difference? There have been scores of answers to this question, and the probability is that the answers should be many, not one. What determines sex in a frog may not apply in the case of a fowl; what is true for a worm may not be relevant for man. Announcing his new theory, an enthusiast declared that it superseded 140, which were erroneous; and to this a critic immediately replied that nothing was more certain than that the new theory was the 141st, which it was necessary to reject. In some animals, like frogs, sex is slow in declaring itself; and it has often been suggested that during the early stages of development the bias may be given towards maleness or towards femaleness by differences in nutrition and environment. When Yung fed his tadpoles oh the minced flesh of ox, fish, and frog, he got the respective percentages of females, 78, 81, and 92. But the snag here is that the experimenter did not take account of the deaths. This is a fatal fallacy, for there might be differential mortality. It is necessary to discover, for instance, whether the males are not more susceptible than the females to any departure from the normal nurture.

Boneilia is a green marine worm whose body, in the female, is about the size of a greengage and bears a retractile proboscis which may be over 6in in length. But the male is a microscopic, mouthless pigmy. To begin with, the free-swim-ming stages of boneilia look all alike, but those that settle down in the mud develop into large females, while those that fix themselves to the proboscis of a fullgrown female and begin to absorb the skin-secretion are arrested and develop into the dwarf parasitic males. This looks like sex-determination by nurture.

'There are certain facts which point to the theory, favoured by many breeders, that decision may depend on the relative ripeness of the egg-cells and sperm-cells at the time of fertilisation. Thus Kuschakewitseh found that out of a large number of over-ripe frogs’ eggs, whose fertilisation had been artificially delayed, all the survivors developed into females. Some animals have two visibly different kinds of eggs. Thus in the common Wheel Animalcule Hydatina and in the vine-pest Phylloxera there are large ova which always develop into females and small ones which always develop into males, no fertilisation being required in either case. Similarly, in the mite Pediculopsis, and in a primitive worm called Dinophilus, where fertilisation occurs as usual, there are large eggs which develop into females and small eggs which develop into males. An interesting point has been noticed in regard to “ identical twins ” — developing from one egg-cell—that they are invariably of the same sex, which may or may not be the case when the twins arise, as is usually the case. from two separate egg-cells developing simultaneously.

In one of the armadillos there are normally “ identical quadruplets,” which* are always of the same sex. In some of the strange parasitic Hymenontera. such as Encyrtus, a single egg-cell develops into a group of embryos, all of; the same sex, but with. .this peculiarity., that all are female if the eig is fertilised and male if not. Everyone, knows that” drone bees develon from unfertilised eggs. Drones .have a mother biff no father; yet they have a grandfather! There i s considerable evidence that the ovary of a pigeon produces two kinds of; eggs, differing in their yolk-forming capa-' city and in other features : and that those predisposed to more abundant assimilation 1 and storage will develop into females. Now, it is probably impossible to change the bias of an egg predisposed to develop ing into a female or, of another predisposed to developing into’a male. But’it is possible that the diet’ and the habits of the parent bird ma’v affect the proportion of the two kinds of ova produced • in the ovary. Thus a’prolonged dieting with Very nutritious food might result in a disproportionate number of female-pro-ducing ova. It is well-known that greenflies produce females onlv during the abundance and prosperity of the summer months, but suddenly produce males when autumn sets in with its short commons and cold. Experiments in prolonged dieting of mammals have not disclosed any influence on the proportion of the sexes.

Many biologists believe that sex behaves as a Mendelian character. Thus in the common currant moth there is strong evidence that the females are “ “ heterozygous.” that is to sav, they have “ maleness ” as a latent or recessive character. They will, on this view, gi ,r e rise to equal contingents of male-producing and female-producing ova. The male moths, on the other hand, are “ homnzvgons ” as regards sex. being without the “ femaleness factor ” ; and, on this view, they will give rise to only male-producing spermatozoa. When a male-producing spermatozoon fertilises a male-producing ovum, the result is, of course, a male. But if a male-producing spermatozoon fertilises a female-nroducing ovum, the result will be a female offspring, femaleness being by hypothesis dominant over maleness. Reconcilable with other views is the theorv expounded bv Geddes and Thomson in “ The. Evolution of Sex ” (18JWI, that what primarily settles sex is a dif-

holism in rb he ra i te ° r r M hm o£ meta ' oo ism. The male organism or snermorr°katab r r?1 4 a . fcive > the n disruptive or katabo ic; the female or egg-producer boHe atl p y h t le T- re Constru ctive or anabol c. Perhaps th ls is the nearest that 0 laa £o t'h o true inwardness of sex.—John o’ London’s Weekly.