Segmental Allopolyploidy and Diagnostic Markers for Apomixis in Brachiaria

Breeders of Brachiaria and other apomictic forage grasses must efficiently and reliably discriminate between apomictic and sexual offspring in large segregating populations to identify apomict genotypes that can be released as true-breeding cultivars. Reproductive mode can be assessed with progeny tests and embryo sac analysis, but both methods are costly and time consuming. We developed the first saturated genetic linkage maps of interspecific B. ruziziensis x B. decumbens and B. humidicola mapping populations in order to identify diagnostic markers linked to apomixis. Comparative genomics with Setaria italica and high resolution molecular karyotyping provided conclusive evidence for segmental allopolyploidy in B. decumbens (2n = 4x = 36) and a base chromosome number of six in B. humidicola (2n = 6x = 36). The apospory specific genetic region (ASGR) was mapped to B. decumbens chromosome 5 and a region syntenous with S. italica chromosome 1 in B. humidicola. The Pennisetum squamulatum ASGR-BABY BOOM-like (ASGR-BBML) specific primer pair p779/p780 was perfectly predictive for reproductive mode in both mapping populations and a diversity panel of known sexual and apomictic Brachiaria germplasm accessions and cultivars. These results suggest that ASGR-BBML gene sequences are highly conserved across the Paniceae and provide insight on the role of apomixis in polyploid evolution.