2»s 



CHAPTER 18 



DIPLOID 

 HYBRID 



AMPHIPLOID 



figure 18-3 {above). Seed pods of cabbage 

 and radish, of their hybrid and amphiploid. 

 {After G. D. Karpechenko.) 



figure 18-4 (below). Distribution of Del- 

 phinium species in California. Each species 

 has a unique habitat. 



D. GYPSOPHILUM 



D. RECURVATUM 

 D. HESPERIUM 



accidentally transported by ship to France 

 and England and became established along- 

 side of the European marsh grass, 5". Stricta 

 (2n 56). By the early 1900's a new 

 marsh grass, S. townsendii (2n = 126), ap- 

 peared and largely erowded out the two 

 older species. Since S. townsendii has a 

 chromosome number equal to the sum of 

 the diploid numbers of the older species, is 

 fertile, breeds true, and has an appearance 

 intermediate between the two older forms, 

 this species is undoubtedly an amphiploid 

 of S. alterniflora and S. stricta. S. town- 

 sendii is so hardy that it has been purposely 

 introduced into Holland (to support the 

 dikes) and to other localities. 



Amphiploidy also can be produced arti- 

 ficially. For example, in the greenhouse 

 it is possible to cross radish (2n = 18) with 

 cabbage (2n = 18) (Figure 18-3), thus 

 producing an F] hybrid with 18 unpaired 

 chromosomes at meiosis. If, however, the 

 chromosome number of the hybrid doubles 

 early enough in development, it can produce 

 amphiploid progeny with 2n = 36 chromo- 

 somes (containing nine pairs each from 

 radish and cabbage). Since the amphiploid 

 is fertile and genetically isolated from both 

 radish and cabbage, it constitutes a new 

 species. 



If each chromosome contributed to an in- 

 terspecific hybrid is different and the hy- 

 brid's chromosome number doubles, then 

 each chromosome would have just one part- 

 ner at meiosis, and segregation would be 

 normal. Therefore, the breeding success of 

 the amphiploid is enhanced by greater dif- 

 ferences between the chromosomes of the 

 two species that contributed haploid ge- 

 nomes to the interspecific hybrid. It is not 

 surprising, then, in hybridizing two chromo- 

 somally similar species, that at meiosis their 

 amphiploid produces trivalents and quad- 

 rivalents leading to abnormal segregation 

 and sterility. 



Although amphiploidy is not successful 



