A AND B GENOME HOMOEOLOGIES IN TETRAPLOID AND OCTOPLOID CYTOTYPES OF BROMUS INERMIS

Abstract

Homoeology between the A and B genomes of allotetraploid (2n = 4x = 28) AiAiBiBi and autoallooctoploid (2n = 8x = 56) AIAIAIAIBIBIBIBI cytotypes of B. inermis Leyss. was studied in a tetraploid F1 hybrid (AeAeAiBi) from 4x B. erectus × 4x B. inermis and in a haplo-triploid (AIeAIeBI) which occurred spontaneously in the F2 from open-pollination among plants of the hexaploid F1 hybrid (AeAeAIAIBIBI) from 4x B. erectus × 8x B. inermis. Chromosome pairing at metaphase I in both the tetraploid (AeAeAiBi) and haplo-triploid (AIeAIeBI) indicated that for each A genome chromosome there was a corresponding B genome homoeologue. There was no convincing evidence of gross structural differences between the two homoeologous genomes. The frequency of trivalent formation in the haplo-triploid was approximately one-half that found in two pentaploids (2n = 5x = 35) AIeAIeAIBIBI. This indicates that the pairing affinity between the A and B genomes is one-half that between homologues as expressed by trivalent formation in triploids of the type AAB and AAA. Homoeologous chromosome pairing (A with B) may be controlled by a gene which is hemizygous ineffective.