Genetic Systems 11 | 201 



in aphids, gall wasps, cladocerans, many parasitic worms, and 

 rotifers. 



As in plants, reduction division may be entirely suppressed, and 

 the divisions resulting in the egg are mitotic. The egg is then diploid. 

 On the other hand, meiosis may occur essentially as normally, but 

 doubling takes place at a later stage, restoring the diploid condition 

 (Fig. 9.9). This is comparable to the results of diplospory in plants. 

 The usual products of meiosis are four haploid cells. In the higher 

 plants these would be megaspores, and in animals an egg and two 

 (or three) polar bodies. If the first division of meiosis occurs, cross- 

 ing-over may take place and the basis for recombination established. 

 Should the daughter cells, following the second division, fuse in 

 pairs, then only two cells are formed, and meiosis and recombina- 

 tion are partially suppressed. In animals this may be the result of 

 the fusion of the egg nucleus with that of the second polar body. 

 The resulting cells are diploid, and a limited amount of recombina- 

 tion may have occurred. 



White has pointed out that, since no segregation can occur in 

 ameiotic thelytoky, recessive mutations and structural rearrange- 

 ments of the chromosomes tend to accumulate. Daughters will re- 

 semble their mothers, but the line will tend to become more and 

 more heterozygous. This may account for the vigor with which some 

 apomictic organisms are endowed. On the other hand, in partially 

 meiotic thelytoky where segregation can occur, existing heterozy- 

 gosity will be reduced in time, without the fusion of gametes to 

 restore it. 



The effects of apomixis on the organism are varied and complex. 

 In both plants and animals, apomixis commonly is associated with 

 both hybridization and polyploidy, although there is no reason to 

 infer a causal relationship among the three. If a major factor in the 

 relative absence of polyploidy in animals is the sex-determination 

 mechanism, it is clear that thelytokous animals are free to become 

 polyploid. Also, the various cytological components of apomixis are 

 controlled by a large number of genes that are largely of negative 

 selective value when separated. For example, mutations leading to 

 upsets in spindle formation would be deleterious unless combined 

 with mutations leading to the formation of restitution nuclei and to 

 the development of eggs without fertilization. Hybridization may 

 serve to bring these genes together in a functional system. It has 

 been suggested also that some sort of buffering may take place if the 

 organism is polyploid. 



In those plants in which apomixis has been adopted as a genetic 

 system, a common effect is the origin of what are called agamic 



