Polymorphism, outcrossing and polyploidy in Bromus diandrus and B. rigidus.

The genetic systems of Brornus diandrus Roth (great brome) and B. rigidus Roth (rigid brome) were studied in 30 populations sampled from Western Australia. Populations of B. diandrus were polymorphic and the dominant phenotype had scabrid lemmas and glabrous paleas. Polymorphs of B. rigidus had short and long awns with flat seeds and long awns with round seeds: all these phenotypes had scabrid lemmas and hairy paleas. Frequencies of the polymorphs varied between sites and years. Palea hairiness of B. diandrus is controlled by a single, dominant gene and was used as a marker gene for estimations of outcrossing frequencies and population homozygosis. B. diandrus is self-compatible and inbreeding with outcrossing frequencies less than 1%. Florets of both species were facultatively cleistogamous; there were high levels of chasmogamy in irrigated populations but most anthers had dehisced prior to exsertion. The high proportion of seed set in cleistogamous florets and similar floral biology suggested that the breeding system of B. rigidus was similar to that of B. diandrus. There was no evidence of species hybrids in the field and attempts at hybridisation failed. Populations of these species were homozygotic for a few major genotypes. Meiotic chromosomes of both species and all biotypes formed mainly bivalents and a few quadrivalents at metaphase I. This, together with the high proportion (Ͱ5 98%) of fertile pollen grains and the 3 : 1 segregation of hairy: glabrous paleas in selfed F2 progenies, indicated that inheritance in these species was disomic. The genomic formula proposed for B. diandrus is either AABBCCDD or AAAABBCC. The properties of polymorphism, autogamy and disomic inheritance within polyploidy would favour the development of tolerance to herbicides within populations, provided selected genotypes were competitive.