POLYEMBRYONY AND SEX' 



A Study of the Origin and Development of Mixed Broods in Polyembryonic 

 Hymenoptera, and the Ratio in Production of Males to Females. 



J. T. Patterson 

 Professor of Zoology, University of Texas 



INTRODUCTION 



IN ANIMALS the term polyembryony 

 has come to include all cases in 

 which two or more individuals arise 

 from a single egg during the course 

 of its early development. There are 

 two types of polyembryony: (1) Spe- 

 cific polyembryony or the habitual pro- 

 duction of multiple embryos in a given 

 species; (2) sporadic polyembryony. or 

 the occasional production of multiple 

 embryos in a species in which the egg 

 typically differentiates into a single in- 

 dividual. 



Sporadic polyembryony occurs in 

 many different species among the higher 

 forms. In fact, cases have been re- 

 corded for every class of vertebrates. 

 To this type belongs the well-known 

 uniovular, or identical twins of the hu- 

 man species. Such twins are enclosed 

 in a single chorion and are therefore 

 often referred to as monochprial or 

 monozygotic. They are invariably 

 homosexual. In contrast with these are 

 biovular or dizygotic twins, which may 

 be heterosexual. In such cases the two 

 individuals are no more similar than 

 brothers or sisters in a family born at 

 different times, or than the individuals 

 in a litter of a multiparous species. 

 They are often called fraternal twins. 

 The interesting fact concerning mono- 

 zygotic twins is their homosexual con- 

 dition which is explained on_ the basis 

 of the chromosome hypothesis of sex- 

 determination. According to this 

 hypothesis, if the egg is fertilized by a 

 male-determining sperm the twins will 

 both be male, but if fertilized by a 

 female-determining sperm thev will 

 both be female. It is supposed that, 

 after the egg has once been fertilized. 



sexuality is irrevocably fixed, and, no 

 matter how many individuals may later 

 develop from it, such individuals will be 

 homosexual. 



The early data collected on sexes 

 among species exhibiting specific poly- 

 embryony also support the chromosome 

 hypothesis. Thus the four embryos 

 which develop from a single egg of the 

 Texas armadillo (Tatiisia novemcincta) 

 are invariably of the same sex. The 

 same is true for the larger polyembry- 

 onic litters of one of the South Amer- 

 ican armadillos (T. Jiybrida). 



The other groups of dioecious species 

 which show specific polyembryony be- 

 long to the parasitic hymenoptera. 

 Because of the large number of individ- 

 uals which develop from a single egg in 

 certain of these insects, data on their 

 sex ratios are of especial interest. Un- 

 fortunately, we have but very few exact 

 data on the sex ratios of these insects. 

 Bugnioh ('91) was the first investiga- 

 tor to record such data. He reared 

 and studied twenty-one polyembryonic 

 broods of Acicniaspis (Encyrtus) fus- 

 cicoUis, and found nine female broods 

 and five male broods, and seven broods 

 that contained both males and females. 

 Of the seven mixed broods, three had 

 males and females in about equal num- 

 bers, three had a large majority of 

 males, and one had a large majority of 

 females. Marchal ('04) studied six- 

 teen broods of Polyqnotits minutiis. 

 He found eight female broods, six male, 

 and two mixed, one with three females 

 and three males, and the other with 

 three females and one male. Silvestri 

 ('06) found, in one hundred and sixty 

 broods of Litoniastix, sixty-three fe- 

 male, ninety-two male, and five mixed 



1 Contributions from the Zoological Laboratories, University of Texas, No. 142. 



344 



