Chromosome pairing affinities between Old and New World species of Bromus section Pnigma

Abstract

A low frequency of allosyndetic chromosome pairing occurred in interspecific hybrids between species having large chromosomes (Bromus benekenii (Lange) Trimen, 2n = 4x = 28; B. ramosus Huds., 2n = 4x = 28; B. pacificus Shear, 2n = 4x = 28) and species having small chromosomes (B. erectus Huds., 2n = 4x = 28; B. inermis Leyss, 2n = 4x = 28; B. pumpellianus Scribn., 2n = 4x = 28; B. riparius Rehm, 2n = 10x = 70; and B. variegatus, 2n = 2x = 14). These results are consistent with the suggestion that the large (predominantly Eurasion) and small (predominantly American) chromosome species of section Pnigma have followed different evolutionary pathways. Chromosome pairing in interspecific hybrids revealed low affinities between the chromosomes of the Eurasian species B. ramosus and the North American species B. pacificus. Thus there was no evidence that B. ramosus contained a form of the L genome found in American species. However genetic factors suppressing homoeologous pairing could be present in the allotetraploid parents.