CHAPTER 4 



SEX-DETERMINATION IN HABROBRACON, 

 SCIARA AND LYMANTRIA DISPAR 



Habrobracon. As long ago as 1845 Dzierzon observed that 

 in the honey-bee the sex of the individual was determined 

 by the occurrence or non-occurrence of fertilization; the 

 egg being fertilized gave rise to a female; the egg not being 

 fertilized and developing parthenogenetically yielded a male. 

 This difference was later interpreted as a difference between 

 diploidy (in respect of the chromosomes) and haploidy and 

 was found to be characteristic of the hymenoptera generally. 

 The possible evolutionary origin of this haplo-diploid sex 

 determining mechanism has been reviewed by White (1945) 

 and Whiting (1945). 



When Bridges formulated his concept of genie balance it 

 became necessary to discover whether or not the facts relat- 

 ing to parthenogenesis could be newly interpreted. Accord- 

 ing to this concept the haploid stated yielded maleness as 

 did also this state in duplicate. If diploidy was to yield 

 femaleness, then the two sets of chromosomes had to 

 be qualitatively different so that where iN=maleness, 

 N+N'=femaleness. 



That this is so has been shown by Whiting and his school 

 working with the wasp Habrobracon juglandis which is 

 parasitic on larvae of the meal moth Ephestia. Torvik-Greb 

 (1935) showed that in Habrobracon the female is diploid 

 with 20 chromosomes and the male haploid with 10. The 

 reduced egg has a set of 10 and the sperm, through an 

 aborted reduction, retains 10. Fertilized eggs have 10+10 

 and give rise to females; the same eggs unfertilized develop 

 parthenogenetically into males with a set of 10 maternal 

 chromosomes. 



These cytological findings are in harmony with the sex- 

 linked mode of inheritance displayed by certain mutant 

 forms. Thus when a female with the recessive mutant orange 



30 



