Sex Studies of Schistosoma japonicum

ABSTRACT These investigations have been an attempt to throw some light upon the problem of sex in the family Schistosomidae of the Trematoda by studying all the stages in the complicated life-cycle in Schistosoma japonicum through experimental laboratory infections. Sexual dimorphism in the adult flukes, seen only in this family of the trematodes, arises from a series of stages (miracidium, primary sporocyst, secondary sporocyst, cercaria) in which no dimorphism is apparent. The question arises as to when sex is determined, and how ? Working on the assumption that sex is determined in the fertilized egg, it was the purpose of these experiments to show a sexual dimorphism in earlier stages of the life-cycle, and failing that, to demonstrate that these forms are nevertheless potentially sexed. The experiments have brought out the following facts : From preliminary experiments and from the earlier experiments of others, we chose the hamster as best adapted for mammalian infection with the cercariae of Schistosoma japonicum. Directions for the care of these animals are included. Unfortunately the hamster is not obtainable in America.The incidence of infection of snails with the miracidia of Schistosoma japonicum in the Soochow region was found to be about 2 per cent. With this low incidence it was possible to plan mammalian infections by cercariae from a single snail, and to be reasonably sure that practically all such snails would have been infected by a single miracidium only.Exactly half of the snails harboured cercariae which produced female flukes, while the other half produced male flukes.When the cercariae from a single snail were used to infect several mammals (usually six) all the flukes recovered from these animals were of the same sex.When the male flukes are allowed to develop in the mammalian host in the absence of females, normal and unretarded development takes place both as regards their size and form, the development of the reproductive organs, and the production of normal mature germ-cells.When the female flukes are allowed to develop in the mammalian host in the absence of the males, they fail to develop both as regards somatic and germ structures. Maximum bodylength in these flukes is less than one-fifth the normal length. Unlike the normal female, the diameter of the body is constant throughout their length and less than one-quarter the normal diameter. The female reproductive organs fail to develop, with the exception of a blind tube which is topographically the uterus, and an aggregate of germinal cells in the normal region of the ovary which resembles the earliest oogonial cells.In these undeveloped females the mesenchyme cells per unit area are about four times the number found in the normal female. This would indicate that the cells have increased at a normal rate, but that the mesenchyme has failed to increase normally in size.These females have been found to maintain this same undeveloped state for a period of 268 days without change in any of the structures.If at any time male-producing cercariae are introduced into a hamster carrying these immature females, the latter rapidly go to complete normal development as regards size, form, and the production of germ-cells. The most striking example of such a case was the introduction of male cercariae when the undifferentiated but potential females were 128 days old. Nineteen days later these females, though still somewhat smaller than normal, had increased enormously in size and had fully developed reproductive organs. The necessary presence of the male to ensure female development has led to the assumption that the male produces certain hormones without which the female will not develop. The continual copulation of the male and female worms has been referred to in numerous texts as an adaptation to ensure fertilization of the egg in the female where the seminal receptacle was supposed to be absent. Inasmuch as a potential seminal receptacle has been found (see 11) in the female, the writer suggests that this adaptation may more likely be a means of holding the female in closest relation to these male hormones ensuring most rapid development of the females. Some evidence for this suggestion is found in the case of mixed infections where the number of females greatly exceeds the number of males. In such cases it is not uncommon to find the unattached females much smaller and less developed than the copulating females.The male reproductive system differs from the several descriptions found in the literature. The vas deferens which receives in order the vasa efferentia from the ventral side of each of the seven testes, leads forward along the ventral side of the body to a lateral opening in the large seminal vesicle. From the ventral wall of the seminal vesicle the ejaculatory duct extends to the ventral body-wall just in front of the gynocophoral canal and opens to the outside through a definite cirrus within a cirrus sac.The female reproductive system contains a large and definite chamber in the oviduct just as it emerges from the ovary. This chamber is invariably filled with spermatozoa which have reached it by way of the uterus and oviduct. It is without question a fertilization chamber, and is functionally comparable to the seminal receptacle as described in many trematodes. Great variation in structure is found, giving a series of forms in which the most atypical is a definite sac-like vesicle filled with sperms arising from the oviduct near the base of the ovary.The process of spermatogenesis has been followed through stage by stage and fully described. The male is demonstrated to be heterozygous, one-half of the spermatids receiving eight chromosomes, the other half receiving six chromosomes. The diploid number of chromosomes in the male is fourteen. There are two x-chromosomes. This substantiates the evidence obtained experimentally that sex is determined in the fertilized egg. The life-cycle contains the interesting condition of male-determined individuals producing parthenogenetic eggs at one stage and spermatozoa at another.The oogonial cells have sixteen chromosomes as the diploid number. Although the maturation stages were not discovered, there is now little doubt that the female is homozygous and that the reduced number of chromosomes in each egg is eight.