Higher Allopolyploids 467 



number of 48. The related species P. laetus has 2n = 16, which 

 would indicate that the species with the higher number is a hexa- 

 ploid. Apparently these two species hybridize in one particular 

 region of California to form P. neotericus Keck, which has 32 

 as the haploid number and would therefore be an octoploid hy- 

 brid. Because of the high chromosome number the loss of a 

 chromosome or two does not greatly disturb the balance, and 

 forms of P. azureus subsp. angustissimus ^d of P. neotericus 

 are known which are respectively hypohexaploid or hypoocto- 

 ploid. The hybrid origin of P. neotericus is inferred from a 

 number of different lines of evidence. It resembles both laetus 

 and azureus and has often been classified as one or the other; 

 the chromosome number represents the sum of the numbers of 

 the two other species; the chromosomes in both P. asureus and 

 P. neotericus behave as if they were high polyploids; P. laetus 

 and P. azureus overlap in their geographic distribution, and P. 

 neotericus occupies an area between the regions of the other two 

 near a place where they overlap. When several different kinds 

 of evidence point to the same conclusion, the argument is very 

 convincing. 



Interesting higher polyploids have been reported by Stebbins 

 and his co-workers in the genus Bromus. B. carinatus has 56 

 chromosomes and always shows 28 bivalents. Since the basic 

 chromosome number is believed to be 7, these plants are octo- 

 ploids. A related species from Arizona, formerly classified as B. 

 carinatus var. anzonicus but better regarded as B. arizonicus, 

 has 84 chromosomes in sporophyte tissue and is a duodecaploid; 

 it always forms 42 bivalents. A '^diploid" hybrid between these 

 species has 2n = 70, receiving 42 medium-sized chromosomes 

 from B. arizonicus and 21 medium and 7 large ones from B. 

 carinatus. In the hybrid the 7 large chromosomes behave as uni- 

 valents, and at least two sets of 7 medium chromosomes from B. 

 arizonicus appear as univalents. Two sets of 7 medium chromo- 

 somes appear to be common to both species, for 14 bivalents are 

 present. Up to 7 trivalents are present, resulting from the pair- 

 ing of two sets of B. arizonicus and one set of B. carinatus. The 

 hybrid is completely sterile. A large number of inversion bridges 

 in the two meiotic divisions indicates that many of the homolo- 

 gous chromosomes are not identical with one another but differ 

 at least by inverted segments. The manner of pairing in the 



