Microsatellite-assisted identification and comparative population genetics of Malagasy olive species (Noronhia spp., Oleaceae)

An exhaustive inventory of Madagascar's native flora is necessary to optimize its conservation, particularly for highly diversified endemic groups of plants whose many habitats are threatened with short-term loss. The Malagasy olive (Noronhia spp., Oleaceae), with at least 80 species, is recognized as one of the highly diversified tree genera of the SW Indian Ocean Flora. Field identification of Noronhia species is, however, often a challenge and numerous sterile samples need posterior identification. As molecular tools may assist this task, we tested cross-species amplification of 13 nuclear microsatellite loci (ie SSRs) among a large sample of Noronhia taxa (60 of the 82 described species in Madagascar). Eight species were also investigated at the population level with 13 nuclear and 15 plastid SSRs. A high cross-species transferability of markers was demonstrated, although three nuclear loci showed evidence of null alleles in some species. Based on the amount of genetic diversity observed within eight diploid species, we showed that putative polyploids could be efficiently detected using nuclear SSRs. Our markers thus allowed revealing one polyploid taxon (N. louvelii), but their relatively low frequency suggests that whole-genome duplication was not a major process involved in the high diversification of Noronhia as previously hypothesized. Finally, plastid lineages as well as genetic clusters identified with nuclear SSRs allowed distinguishing all six species collected at Andohahela and Marojejy. These results demonstrate that microsatellites can assist Noronhia identification in their diversity hotspots, as well as reveal contrasting genetic diversity patterns (such as isolation by distance) between sympatric taxa.