Genetic Systems II | 205 



not incorporated into the zygote, is known as gynogenesis. As in 

 other instances of apomixis, the genetically identical progeny of a 

 female form a clone. Tissue-transplant studies have shown that two 

 clones of M. formosa make up about 80 percent of the population in 

 one drainage ditch near Olmito in the valley of the Rio Grande. 

 The remainder belong to a third clone or cannot be identified. 

 Clones sampled in 1961 were the same as those found in 1960. 



In 1954, several dozen Mollienesia formosa were taken from the 

 Olmito ditch and released in the San Marcos River some 250 miles 

 to the north. The species has become established, as has M. htipinna 

 which was introduced into the area many years earlier. The San 

 Marcos population thus was available for comparison with the 

 Olmito fishes; it was also sampled in 1960 and 1961. Only two clones 

 were found; these were the common clones at Olmito. Thus the 

 clones of these Mollienesias probably have remained relatively un- 

 changed (within the limits of tissue-transplant discrimination) for 

 at least a decade and possibly for much longer. 



There remains to discuss only cyclical parthenogenesis, a peculiar 

 genetic system found in aphids, gall wasps, Cladocera, and rotifers. 

 The cytological mechanisms differ from group to group, but we may 

 single out a species of aphid as an example. In Tetraneura idmi, 

 which produces galls on elm leaves, there is a sequence of genera- 

 tions which have been given names descriptive of their behavior: 

 fundatrices, emigrantes, exules, sexuparae, and sexuales. In the 

 spring, females of the fundafrix generation become adults within 

 the elm leaf galls. There each produces, parthenogenetically, female 

 offspring which later develop wings and fly away to feed on the 

 roots of grasses. They are the emigrantes, which produce, also par- 

 thenogenetically, several generations ( females ) of exules. Eventually 

 the exules give rise to the sexuparae, winged females which fly back 

 to the elm and there parthenogenetically produce both males and 

 females called sexuales. The latter pair, and from fertilized eggs 

 appear once more the female fundatrices, the gall-making genera- 

 tion. 



Cytological investigation shows that female sexuales have 2n = 14, 

 while males have 2n = 13; there is evidently an XX:XO sex-chro- 

 mosome system. The fundatrices, emigrantes, and exules types of 

 thelytokous females have a diploid set of 14 chromosomes. There is 

 a single maturation division in oogenesis. The eggs produced are 

 diploid because the division is not reductional and they develop into 

 females that are identical genetically except for mutation. The 

 sexuparae produce eggs of two kinds. In those eggs that will be 

 female-determining, all the chromosomes split, as in mitosis. In those 



