SEX DETERMINATION 



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5. The Hymenopteran Type of Sex Determination 



In this type of sex determination, which is found in Rotifera, Acarina, 

 Thysanoptera, and some Hemiptera as well as in most Hymenoptera,^ 

 the females are diploid and those of their eggs which are fertilized 

 develop into diploid females, while the other eggs develop partheno- 

 genetically into haploid males, which form haploid sperm by a failure 

 of the reduction divisions. It has always been very difficult to bring 

 this method of sex determination into line with the theory of balance 



Fig. 106. Sex Determination in the Wasp Habrobracon. — The female complex 

 is in two parts, carried on the X and Y chromosomes. The upper two figures 

 show the separation of X and Y in the formation of the second polar body, leaving 

 haploid eggs with either X or / which develop by parthenogenesis as haploid 

 males. Fertilization, if it occurs, happens before the formation of the second polar 

 body. If the fertilizing sperm carries the X chromosome, the egg passes its own X 

 out into the polar body; similarly, if the sperm carries V, it is the Y which is lost 

 in the polar body (lower two figures). 



between male and female factors such as we have discussed up to the 

 present. Darlington^ has suggested that the system may have evolved 

 from a protandric hermaphrodite form in which the development of 

 the haploid eggs was affected by the slowing up which is often seen in 

 haploid and in which the whole life therefore came to be passed in the 

 first (male) stage of the hermaphrodite succession. The need for this 

 and other similar ingenious theories may perhaps be removed if certain 

 facts discovered by Whiting^ in the parasitic wasp Habrobracon turn 

 out to have general importance. In this form, the female factors are 

 located in two chromosomes, which we may call X and Y. The haploid 

 eggs are either X or F and develop into males. Fertilization takes place 



^ Rev. Schrader and Schrader 193 1 — For sex determination in other 

 parthenogenetic organisms, see Morgan 1915, 1926. 



- Darlington 1932a. ^ Whiting 1935. 



