Evolution and hybrid speciation in African Urochloa grasses: applications in sustainable tropical forage systems

<p dir="ltr">African Urochloa grasses are among the most important forage grasses across the tropics, particularly the species within the brizantha complex (i.e., U. brizantha, U. decumbens and U. ruziziensis). Breeding for improved Urochloa forage cultivars is technically challenging due to the range of ploidy levels, sexual and asexual reproductive systems, frequent hybridization, and ambiguous morphologies these grasses display. Breeding for improved Urochloa forages is important as cattle farming contributes substantially to biodiversity loss and methane emissions. A species level phylogeny for Urochloa, focusing on the important African forages, was inferred to clarify evolutionary relationships, identify crop wild relatives (CWR), and model the emergence of forage traits. Focusing on the brizantha complex, repetitive element (RE) composition was analysed and candidate sequences were identified for fluorescent in-situ hybridization (FISH) to determine the pattern of sub-genome inheritance in the allopolyploids. Hybridization events were inferred within brizantha complex allopolyploids using chloroplast and nuclear species tree topologies, phylogenomic networks, and morphological assessments of herbarium specimens. Five forage clades were identified across the genus. Urochloa eminii and U. oligobrachiata were identified as CWRs associated with the brizantha complex. Flow cytometry analysis and chromosome squashes confirmed that U. eminii is a diploid species. FISH and RE analyses determined that U. eminii genome is highly homologous to diploid U. decumbens and U. ruziziensis, confirming that these taxa should be combined into a single species. Allotetraploid U. decumbens should be recognised as a separate species, as FISH, phylogenomic networks, and morphological analyses confirmed its allopolyploid origins. The origins of allopolyploid U. brizantha remain unclear and further sampling of this species across its natural distribution is required. These results clarify the evolutionary history of the brizantha complex and provide breeders with a more informative framework for choosing accessions in cultivar development. African grasslands remain important sources of genetic diversity for tropical forage grass breeding and must be conserved to ensure food security goals are met.</p>