J.B. HACKER

CSIRO Division of Tropical Crops and Pastures, 306 Carmody Road, St Lucia, Queensland 4067.

Abstract

Brachiaria decumbens, a widely-sown tropical pasture grass, is an obligate apomict and genetic improvement by breeding is not possible in the absence of a sexual form. Forty accessions of the genus Brachiaria, including 14 species, were grown as spaced plants in afield experiment. Plots were assessed for morphological variation which might be associated with sexuality.
A single accession of B. decumbens exhibited morphological variation and genetic variation was further demonstrated using starch gel electrophoresis. The accession proved to be a diploid. When pollinated with pollen from a naturalised population of tetraploid B. decumbens a triploid hybrid was produced. Chromosome pairing at meiosis showed the genome from the diploid was homologous with 1 genome of the male parent. Chromosomes of the other genome failed to pair, resulting in total male and female sterility.
Successful utilisation of this novel source of sexuality in a breeding program will require chromosome doubling of the hybrid or of the diploid sexual parent prior to hybridisation