Races and the Origin of Species 251 



18.14. H. Kihara and co-workers have produced triploid (33 chromosomes) water- 

 melons with no seeds, and tetraploid (44 chromosomes) watermelons with seeds 

 but larger than the diploid. How do you suppose this was accomplished? How 

 do you suppose these types are maintained? 



18.15. In each of the following cases, an interspecific hybrid can be formed experi- 

 mentally. State whether or not you would expect each of the hybrids produced 

 from the parents described below to become established in nature. 



(a) In California, the Monterey cypress grows along the coast on the rocks. 

 whereas the allopatric Gowen cypress grows two miles inland in the sand 

 barrens. 



(b) Two sympatric species of pine occur in California. One of these, the 

 Monterey pine, sheds its pollen before March; the other, the bishop pine, 

 some time later. 



(c) The hybrid between Crepis neglecta (n = 4) and C. fuliginosa (n = 3) 

 shows unpaired and paired chromosomes as well as multivalents during 

 meiosis. 



18.16. Drosophila pseudoobscura crossed with D. persimilis produces sterile males, but 

 partially fertile females. Using marked chromosomes, the interspecific hybrid 

 female can be backcrossed to pseudoobscura, and their progeny can have various 

 combinations of the chromosomes of the two species. When male progeny of 

 the backcross are examined for the length of the testis, it is found that the testis 

 is essentially normal when the X chromosome is from pseudoobscura; when the 

 X is from persimilis, the testis is shorter — increasing in abnormality as the num- 

 ber of autosomes coming from pseudoobscura increases. What can you con- 

 clude about reproductive barriers from these results? 



18.17. The cotton species Gossypium hirsutum and G. barbadense are tetraploids 

 (2n = 52) and are phenotypically intermediate between the diploid species G. 

 herbaceum and G. raimondii. (Each is 2n = 26.) If cytological examination 

 is made of meiosis in various hybrids, what would the following results reveal 

 about the origin of these species? 



(a) barbadense X raimondii shows 13 pairs and 13 singles. 



(b) barbadense X herbaceum shows 13 pairs and 13 singles. 



(c) raimondii X herbaceum shows 26 singles. 



18.18. What do the following two cases have in common? 



(a) Through artificial selection the evolution of corn, Zea mays, was aided by 

 genes incorporated from teosinte, Zea mexicana. 



(b) Commercial wheat contains genes for rust resistance obtained from goat 

 grass chromosomes. 



