62 AN INTRODUCTION TO MODERN GENETICS 



are in those organisms which show the so-called Hymenopteran 

 method of sex determination. In the bees and wasps themselves fer- 

 tihzed eggs develop into females, unfertilized eggs develop parthene- 

 getically into males. A similar t}'pe of sex determination is combined 

 with a series of diploid parthenogetic generations in Rotifers and 

 alternates with one such generation in Gall-flies. The inheritance of 

 factors in Hymenoptera has been extensively studied in the parasitic 

 wasp Habrobracon by Whiting.^ He has shown that, as might be 

 expected, the characters of a male are dependent solely on those of 

 his mother, and that he is homozygous or "pure"^ for characters for 

 which she is heterozygous. A father's genes pass only into his daughters, 

 in fact his sons can only be considered his by courtesy. Rare exceptions 

 to this behaviour occur, since diploid males have been found (p. 230). 

 Haploid parthenogenesis in plants is known only in a few flowering 

 plants (e.g. Solanum), although the germination of haploid spores 

 into the gameteophyte generation may be regarded as a somewhat 

 similar phenomenon. 



2. Pseudogamy and Male Parthenogenesis 



The parthenogenetically developing eggs of an animal or plant, 

 whether diploid or haploid, may develop spontaneously or may require 

 activation by a definite stimulus. This stimulus may be artificial,^ as in 

 the haploid parthenogenesis of animal eggs which has been performed 

 by many authors (e.g. pricking frogs' eggs, treatment with chemicals, 

 etc.) or in the apospory provoked by pricking in the alga Vaucheria.* 

 On the other hand, the stimulus required may be that of fertihzation by 

 a sperm, which may be of the same or another species, but which in 

 either case may play no further part in development after activating 

 the egg. Such cases are spoken of as pseudogamy. They are well known 

 in animals (echinoderm by annelid crosses); and in the higher plants 

 haploid parthenogenesis is probably impossible without the stimulus 

 of homologous or foreign pollen, while the phenomenon is also com- 

 mon in diploid apomixis. The offspring of such pseudogamous modes 

 of reproduction are of course entirely maternal in character since the 

 male gamete is responsible only for activation of the egg. 



In some plants it has been found that haploid organisms derived 



^ Whiting 1932. 



2 Muller has suggested that it would be better to speak of factors in haploid 

 organisms such as male bees as hemizygous; and this word can also be used for 

 factors (e.g. in an unpaired X chromosome) for which there is no allelomorph 

 in a normal diploid. ^ Rev. Just 1937. * Wettstein 1920. 



