THE MODIFICATIONS OF THE CHROMOSOME CYCLE 6l 



offspring whose genetic characters are identical with those of their 

 parent. An example may be taken from the work of Agar^ on 

 Daphnia, in which hybrids between D, ohtusa and D. pulex bred 

 true through ten generations of diploid parthenogenesis. Obligatory 

 diploid apomictic plants form closed series of plants with almost per- 

 fectly constant characters; they may be compared with clones of 

 vegetatively propagated organisms. It is surprising, then, to find that 

 groups of plants reproducing in this way are often extremely poly- 

 morphic (e.g. the group of Eu-hieracium). How is this divergence in 

 evolution to be explained if segregation and recombination have been 

 abolished? The answer appears to be that segregation, although hin- 

 dered, is not entirely abolished; it is still possible owing to crossing 

 over, while on the other hand the suppression of hybridization prevents 

 the blending of slightly different races into a uniform population. 

 (Fig. 23.) 



lb. Haploid Parthenogenesis 



The development of haploid organisms from nuclei with the reduced 

 number of chromosomes can occur only by haploid parthenogenesis or 

 rarely by apogamy. Haploid parthenogenesis, may, however, yield 

 diploid organisms, since in some forms the development begins by 

 doubling of the original reduced chromosome number; such organisms 

 will clearly be homozygous. 



Facultative haploid parthenogenesis occurs in many animals. The 



offspring show segregation of the factors present in the parents, and 



are homozygous for factors for which the parents were heterozygous. 



Thus Fryer^ isolated a grasshopper {Clitumnus) heterozygous for a 



dominant factor producing a process on the head, and from one female 



obtained twelve homed females and ten unhomed females. Similarly 



Nabours^ describes experiments with the grouse locust Apotettix, in 



YZk 



which an isolated female of the constitution (colour pattern 



yzK 



factors) gave offspring <)YZk : i%yzK : lyZk : 1 YzK by haploid par- 

 thenogenesis with subsequent doubling of the chromosome number. 

 This case shows coupling as well as segregation. Similar haploid 

 parthenogenesis, with segregation, may occur in hybrids in Lepidop- 

 tera even between strongly sexual races.* 

 The most important examples of haploid parthenogenesis in animals 



^ Agar 1920. 2 Fryer 1913. ^ Nabours 1919. * Peacock 1925. 



