200 I The Process of Evolution 



of the integument or nucellus of the ovule produce the new sporo- 

 phyte by mitotic divisions. Thus sporophytic tissue gives rise directly 

 to a new sporophyte that is enclosed in the usual seed coats and is 

 superficially indistinguishable from the usual situation. Adventitious 

 embryony also may be pseudogamous. 



It will be seen from the above that apospory and adventitious em- 

 bryony produce plants genetically identical with their parent. Since 

 diplospory may involve divisions that are partly meiotic, the possibil- 

 ity of genetic recombination exists. The nature and possible results 

 of partial recombination of this sort are discussed below under 

 thelytoky in animals. 



Just as apomixis in the higher plants is complicated by the exist- 

 ence of alternation of generations, so is it complicated in higher 

 animals by sex. In a number of groups of animals, males arise from 

 unfertilized eggs and are haploid. Females are produced from fer- 

 tilized eggs and are diploid. This type of apomixis, known as haplo- 

 diploidij, haploid parthenogenesis, or arrhenotoky, obviously may be 

 a means of sex determination. Apparently haplo-diploidy has arisen 

 about seven times in the Metazoa (among the insects, arachnids, 

 and rotifers). In the insects, the Hymenoptera as an order are char- 

 acterized by this form of apomixis and sex determination, which also 

 is known in the Homoptera, Coleoptera, and Thysanoptera. 



In haplo-diploidy, no true meiotic division occurs in the male, the 

 sperm being produced by a mitosis or a simulated meiosis and thus 

 genetically identical (except for mutation) with the parental geno- 

 type. The evolutionary genetics of haplo-diploid organisms is by no 

 means well understood. For example, it can be seen that recessive 

 genes must be immediately expressed in males of species with this 

 genetic system, unless some special mechanism prevents this. It has 

 been suggested that, by virtue of some sort of "repeat" mechanism of 

 gene duplication, the organisms may be functionally diploid. It is 

 well known that most highly diflFerentiated tissues of an organism 

 are endopolyploid (often to a high degree); this might protect the 

 soma from harmful mutations. On the other hand, haploid males 

 could act as a sort of screen for recessive lethals. 



A rather different mode of apomixis in animals is called thelytoky; 

 it is a kind of parthenogenesis in which females are produced from 

 unfertilized eggs. It appears to have arisen in most major animal 

 groups, including the vertebrates, under natural conditions. In some 

 organisms thelytoky is complete: Males are very rare or unknown 

 and every individual arises from unfertilized eggs. More common 

 is cyclical thelytoky where there is an alternation, usually in an 

 annual cycle, of sexual and asexual generations. This mode is found 



