A NOTE ON REDUCTION IN THE MATURATION 

 OF MALE EGGS IN APHIS. 



N. M. STEVENS. ' 



In discussing the unpaired heterochromosome in aphids, in 

 1908/ I expressed the opinion that the two heterochromosomes of 

 the parthenogenetic generations must pair before maturation of 

 the male-producing eggs, and separate in the maturation mitosis, 

 one undivided heterochomosome going into the polar body while 

 its mate remains in the egg. The only evidence which I could 

 give in favor of this surmise was two equatorial plates where seven 

 chromosomes appeared in the maturation of parthenogenetic 

 eggs, instead of the eight chromosomes characteristic of the 

 species (Plate II., Figs. 52 and 53). The largest of the seven 

 chromosomes was evidently equal in volume to the sum of the 

 two largest in the plate containing eight. I have never found 

 the males of this species, the parthenogenetic generations con- 

 tinuing up to the time when the host plants are killed by frost. 

 The probability is that a few scattered sexual forms occur among 

 the parthenogenetic, and that the eggs with seven chromosomes 

 were male-producing eggs. 



In many species of aphids the same individual may produce 

 both males and females and often parthenogenetic offspring also, 

 making it very difficult to be certain that one has the male eggs. 

 In the dimorphic red and green aphis found on OEnother a biennis, 

 however, the rule is, that in October parthenogenetic young cease 

 to appear ; apterous mothers produce only male, and winged 

 mothers only female offspring. Only two exceptions to this rule 

 have been observed. Two years ago I had one brood of males 

 produced by a winged mother, and recently, in examining sec- 

 tions of an apterous specimen, I found four large parthenogenetic 

 embryos, while all of the smaller embryos were male. The 

 change here from parthenogenetic to sexual reproduction came 

 during a generation instead of between generations. 



1 " An Unpaired Heterochromosome in the Aphids," Joiirn. Exp. ZooL, Vol. 

 VI., No. I, Jan., 1909. 



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