Rare and living alone - do edaphic factors cause allopatry of Polhillia species?

Abstract

• First broad-scale study of soil conditions of all Polhillia species. • First broad-scale investigation of rhizobial symbionts involved with Polhillia species. • Rhizobial investigation finds multiple lineages nodulating with Polhillia species. • Soil characters do not appear to cause allopatry in the Overberg Region. Polhillia C.H.Stirt. is a small recently monographed genus of legumes, comprising eleven species, and is endemic to Renosterveld vegetation within the Greater Cape Floristic Region (GCFR). Most of these species are geographically highly localized, with six species restricted to the Overberg region of the Western Cape. Paradoxically, none of these species have been found to occur in sympatry, even where species distributions overlap. Little is known about the edaphic conditions in which Polhillia species grow, but field observations highlighted that edaphic heterogeneity may influence the distributions of species, especially within the Overberg region. Through observing different habitat parameters, soil analyses and identifying rhizobial symbionts of eight of the eleven Polhillia species, we gained novel insights into the ecology of these previously poorly studied species. We found that environmental heterogeneity in the form of different landscape topological features and differences in soil parameters are important factors that may inhibit sympatry where distributions overlap. We speculate that such differences may have led to the divergence of species even in cases where distances are short enough to allow for gene flow between populations. Although rhizobial symbionts do not appear to influence Polhillia species distributions, it was interesting to find Polhillia nodulating with Rhizobium and three distinct lineages of Mesorhizobium .

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