Breeding system, evolution and taxonomy of Bromus arvensis, B. japonicus and B. squarrosus (Poaceae)

Bromus arvensis, B. japonicus and B. squarrosus are morphologically similar diploid annual grasses with largely overlapping species descriptions and inconsistent taxonomy. To evaluate the breeding system, genetic diversity and differentiation in the species complex, 70 seed accessions of different geographic origin were analyzed for isozymes. Estimates of outcrossing rate (t) in two accessions of B. arvensis equalled 0.88 and 1.09, indicating nearly complete allogamy with random mating, whereas B. japonicus and B. squarrosus (t=0.00) are extreme selfers. It is suggested that B. japonicus and B. squarrosus are autogamous relatives of the outcrosser B. arvensis which have been diverged and are distinguishable from the latter by two species-specific allozymes, MDH-A2 and PRX-F4. In total, 24 multilocus isozyme lineages (MLILs) were detected among the 42 accessions of B. japonicus and B. squarrosus, with some MLILs containing accessions of both taxa. The neighbor-joining and UPGMA cluster analyses of the isozyme data showed that accessions of B. japonicus and B. squarrosus do not form separate clusters, but were intermingled in several clusters, supporting morphometric evidence that they actually belong to the same species B. squarrosus. No geographic pattern was found among the accessions or MLILs of B. japonicus and B. squarrosus.