系统发生学与进化生物学杂志

Nuclear and Plastid DNA Sequence-based Molecular Phylogeography of Salvadora oleoides (Salvadoraceae) in Punjab, India Reveals Allopatric Speciation in Anthropogenic Islands Due to Agricultural Expansion

Abstract

Felix Bast and Navreet Kaur

Salvadora oleiodes is a tropical tree species belonging to the little-known family Salvadoraceae and distributed in the arid regions of Africa and Asia. Aims of our study were to trace the microevolutionary legacy of this tree species with the help of sequence-based multi-local phylogeography and to find the comparative placement of family Salvadoraceae within angiosperm clade malvids. A total 20 geographical isolates were collected from different regions of North India, covering a major part of its species range within the Indian Subcontinent. Sequence data from nuclear-encoded Internal Transcribed Spacer region (ITS1-5.8S-ITS2) and plastid-encoded trnL-F spacer region, were generated for this species for the first time in the world. ITS-based Bayesian phylogeographic analysis revealed the existence of four clades while trnL-F spacer based Bayesian analysis revealed one clade for this species distributed in the Indian subcontinent. Between these two loci, ITS revealed more distinct phylogeographic clades, indicating the phylogeographic utility of this locus for the systematics of Salvadoraceae. Interestingly, the ITS phylogeny indicated the existence of allopatric speciation in this tree species. Factors such as habitat destruction through agricultural expansion of plains might have forced the remaining population of this threatened tree species to isolated patches of thorn forest biotas, the ‘anthropogenic islands’, analogous to an archipelago. Phylogenetic analyzes based on trnL-F spacer suggested a synonymy of this species with Salvadora angustifolia. Maximum Likelihood gene tree based on ITS sequence data revealed that Salvadoraceae belongs to Sapindales rather than Brassicales. However, in the gene tree based on trnL-F spacer sequence, this family clustered within Brassicales. An evolutionary congruence of S. oleoides isolates across its range in North India is revealed in this study. Given the conflicting results on the relative placement of Salvadoraceae in Brassicales and Sapindales, the need for further phylogenetic analyses of malvids using supermatrix approach is highlighted.