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

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.

Transgene expression driven by heterologous ribulose-1, 5-bisphosphate carboxylase/oxygenase small-subunit gene promoters in the vegetative tissues of apple (Malus pumila mill.).

It is desirable that the expression of transgenes in genetically modified crops is restricted to the tissues requiring the encoded activity. To this end, we have studied the ability of the heterologous ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small-subunit (SSU) gene promoters, RBCS3CP (0.8 kbp) from tomato (hycopersion esculentum Mill.) and SRS1P (1.5 kbp) from soybean (Glycine max [h.] Mers.), to drive expression of the beta-glucuronidase (gusA) marker gene in apple (Malus pumila Mill.). Transgenic lines of cultivar Greensleeves were produced by Agrobacterium-mediated transformation and the level of gusA expression in the vegetative tissues of young plants was compared with that produced using the cauliflower mosaic virus (CaMV) 35S promoter. These quantitative GUS data were assessed for their relationship to the copy number of transgene loci. The precise location of GUS activity in leaves was identified histochemically. The heterologous SSU promoters were active primarily in the green vegetative tissues of apple, although activity in the roots was noticeably higher with the RBCS3C promoter than with the SRS1 promoter. The mean GUS activity in leaf tissue of the SSU promoter transgenics was approximately half that of plants containing the CaMV 35S promoter. Histochemical analysis demonstrated that GUS activity was localised to the mesophyll and palisade cells of the leaf. The influence of light on expression was also determined. The activity of the SRS1 promoter was strictly dependent on light, whereas that of the RBCS3C promoter appeared not to be. Both SSU promoters would be suitable for the expression of transgenes in green photosynthetic tissues of apple.