Host specificity is a driver of speciation in parasitic plants

Broomrapes (Orobanche spp.) are a cosmopolitan genus of root holoparasites–non-photosynthetic angiosperms that obtain nutrients and energy by connecting to the roots of other plants. Using a phylogenetic study, based on mitrochondrial and ribosomal nuclear DNA markers, of western-hemisphere broomrapes, Schneider et al. demonstrate that extensive cryptic diversity is congruent with host association.

 Range map of host-specific clades of Orobanche sect. Gymnocaulis.
Range map of host-specific clades of Orobanche sect. Gymnocaulis. Coloured circles represent individuals sampled in the phylogeny (Figs 1–3). Coloured lines show the approximate range of each clade. Further study is needed to determine the range of each of the three host-specific lineages of O. uniflora subsp. occidentalis, which in this figure are treated as one unit. Range maps should be considered tentative, particularly in northern Canada and west-central USA, pending a thorough taxonomic and phytogeographical study.

This highlights the important role that host-switching plays in the evolution and diversification of parasitic plants. Orobanche cooperi is a root parasite of Hymenoclea salsola and several other Astereaceae in the southwestern North American desert. Extensive morphological reduction of these plants masks the extensive diversity of western-hemisphere Orobanche. Most of these plants parasitize unique hosts, suggesting the likely role that host-switching plays in speciation.

Reference List

Adam C. Schneider, Alison E. L. Colwell, Gerald M. Schneeweiss, Bruce G. Baldwin, 2016, 'Cryptic host-specific diversity among western hemisphere broomrapes (Orobanche s.l., Orobanchaceae)', Annals of Botany, vol. 118, no. 6, pp. 1101-1111 http://dx.doi.org/10.1093/aob/mcw158