Apomixis and sexuality in diploid and tetraploid accessions of Brachiaria decumbens

Meiotic and aposporous embryo sacs and the initial steps of parthenogenetic embryogenesis and endosperm formation were investigated in diploid and tet raploid accessions of Brachiaria decumbens in two environments, differing m ainly in day length: early summer and late autumn. Both diploid and tetrapl oid accessions were facultative apomicts. Di(ha)ploids showed a much lower level of apomixis (10% to 15%) than tetraploids (80% to 95%). No obligate s exual diploids were found; thus, their occurrence in natural populations is obscure. It is suggested that reproduction in B. decumbens, as in other ag amic complexes of the Paniceae tribe, in general, approximates a diploid-te traproid-(di)haploid reproductive cycle which does not involve triploids. T he dihaploids were fertile and survived in nature. Development of the repro ductive structures depended on the environment. In autumn, in contrast to e arly summer, many meiotic and aposporous embryo sacs degenerated during dev elopment, leading to a significant reduction in the proportion of parthenog enetic embryos. Whether this effect can be attributed to day length or simp ly to age remains to be investigated. The ratio of aposporous to sexual emb ryo sacs was relatively stable over the two seasons.