204 I The Process of Evolution 



Agamic complexes in plants other than in Crepis are less well 

 known. They may be quite small or exceedingly large and complex. 

 The chromosome numbers may become very high and unbalanced. 

 Study of other genera has proved to be quite difficult where the 

 ancestral sexual diploids have become extinct (Riibiis) or where 

 there may be several agamic complexes in one genus (Poa). 



Large agamic complexes are less common in animals. The case of 

 Artemia salina ( brine shrimp ) with sexual diploids and thelytokous 

 triploids, tetraploids, pentaploids, octaploids, and decaploids is well 

 known. In the Curculionidae (weevils) and Lumbricidae (earth- 

 worms) larger complexes have been found; these may include 

 diploids, triploids, tetraploids, and pentaploids, or even hexaploids 

 and decaploids. In the weevils there is reason to believe that occa- 

 sional fertilization of parthenogenetically developing eggs by sperm 

 from bisexual races or species takes place. 



Thirteen species of thelytokous earthworms have been studied 

 cytologically; all are polyploids ranging from triploids to a possible 

 decaploid. Some sexual species are polyploid as well. Since the Lum- 

 bricidae generally are hermaphroditic, thelytoky involves modifica- 

 tion or loss of the male organs. Oogenesis is complex, there being 

 chiasma formation and bivalents even in odd-numbered polyploids. 

 The apparent success of these forms, if their wide geographic dis- 

 tribution is to be a criterion, may be accounted for by postulating 

 heterosis as a result of allopolyploidy. 



Apomixis has been reported in the vertebrates, but its extent and 

 evolutionary importance are virtually unknown. Some subspecies of 

 the European lizard Lacerta saxicola are parthenogenetic. Several 

 species of the American lizard Cnemidophorus may consist only of 

 females, since no males have ever been collected; this suggests that 

 parthenogenesis may occur. An interesting situation has been studied 

 in some detail in the fish genus Mollienesia. Several populations of 

 M. formosa have been sampled in southern Texas, where they occur 

 in streams and drainage ditches. The fishes have also been raised 

 in the laboratory and their genetic similarity studied by means of 

 tissue transplants. Grafts of donor tissue are rejected by the host 

 fish (because of the presence of tissue antigens produced by the 

 host genes) in a period of time roughly proportional to the degree 

 of genotypic similarity between the host and donor. 



Mollienesia formosa is parthenogenetic, but eggs do not develop 

 without the stimulation of sperm. Since males of M. formosa are 

 exceedingly rare in nature, sperm from related species (in this in- 

 stance M. latipinna ) is necessary to initiate development. This mode 

 of reproduction, in which the genetic information of the sperm is 



