Diverged subpopulations in tropical Urochloa (Brachiaria) forage species indicate a role for facultative apomixis and varying ploidy in their population structure and evolution

A - Papers appearing in refereed journals

Higgins, J., Tomaszewska, P., Pellny, T. K., Castiblanco, V., Arango, J., Tohme, J., Schwarzacher, T., Mitchell, R. A. C., Heslop-Harrison, J. S. and Vega, J. J. D. 2022. Diverged subpopulations in tropical Urochloa (Brachiaria) forage species indicate a role for facultative apomixis and varying ploidy in their population structure and evolution. Annals of Botany. p. mcac115. https://doi.org/10.1093/aob/mcac115

AuthorsHiggins, J., Tomaszewska, P., Pellny, T. K., Castiblanco, V., Arango, J., Tohme, J., Schwarzacher, T., Mitchell, R. A. C., Heslop-Harrison, J. S. and Vega, J. J. D.
Abstract

Abstract
Background
Urochloa (syn. Brachiaria) is a genus of tropical grasses sown as forage feedstock, particularly in marginal soils. Here we aimed to clarify the genetic diversity and population structure in Urochloa species to understand better how population evolution relates to ploidy level and occurrence of apomictic reproduction.

Methods
We explored the genetic diversity of 111 accessions from the five Urochloa species used to develop commercial cultivars. These accessions were conserved from wild materials collected at their centre of origin in Africa, and they tentatively represent the complete Urochloa gene pool used in breeding programmes. We used RNA-sequencing to generate 1.1 million single nucleotide polymorphism loci. We employed genetic admixture, principal component and phylogenetic analyses to define subpopulations.

Results
We observed three highly differentiated subpopulations in U. brizantha, which were unrelated to ploidy: one intermixed with U. decumbens, and two diverged from the former and the other species in the complex. We also observed two subpopulations in U. humidicola, unrelated to ploidy; one subpopulation had fewer accessions but included the only characterized sexual accession in the species. Our results also supported a division of U. decumbens between diploids and polyploids, and no subpopulations within U. ruziziensis and U. maxima.

Conclusions
Polyploid U. decumbens are more closely related to polyploid U. brizantha than to diploid U. decumbens, which supports the divergence of both polyploid groups from a common tetraploid ancestor and provides evidence for the hybridization barrier of ploidy. The three differentiated subpopulations of apomictic polyploid U. brizantha accessions constitute diverged ecotypes, which can probably be utilized in hybrid breeding. Subpopulations were not observed in non-apomictic U. ruziziensis. Sexual Urochloa polyploids were not found (U. brizantha, U. decumbens) or were limited to small subpopulations (U. humidicola). The subpopulation structure observed in the Urochloa sexual–apomictic multiploidy complexes supports geographical parthenogenesis, where the polyploid genotypes exploit the evolutionary advantage of apomixis, i.e. uniparental reproduction and clonality, to occupy extensive geographical areas.

KeywordsGrassland; Breeding; Forage; RNA-seq; Apoximis; Parthenogenesis; Urochloa; Brachiaria; Panicieae; Megathyrsus; Brizantha; Polyploidy; Humidicola
Year of Publication2022
JournalAnnals of Botany
Journal citationp. mcac115
Digital Object Identifier (DOI)https://doi.org/10.1093/aob/mcac115
Open accessPublished as ‘gold’ (paid) open access
FunderBiotechnology and Biological Sciences Research Council
Funder project or codeBB/R022828/1
BB/P028098/1
BBS/E/T/000PR9818
Publisher's version
Output statusPublished
Publication dates
Online16 Sep 2022
Publication process dates
Accepted13 Sep 2022
PublisherOxford University Press (OUP)
ISSN0305-7364

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