Exploring the tempo of species diversification in Astragalus section Incani DC. based on nuclear and plastid DNA sequences

Abstract

This study uses phylogenetic relationships of the species-rich section Astragalus (Incani), to follow up on recent evidence pointing to rapid and recent plant diversification patterns in the west of Iran. Section Incani is introduced for its taxonomic complication resulting from overlapping morphological characters, but few studies have been done on this section; hence, we also lack a robust time-calibrated chronogram to address hypotheses (e.g., biogeography and diversification rates) that have implicit time assumptions. Two loci (rpl32-trnL(UAG) and nrDNA ITS) were amplified and sequenced for 87 taxa across Incani for phylogenetic reconstruction and a chronogram in BEAST. Incani is identified as the sister clade to all remaining sections with high support, and within the clade Incani, two strongly supported groups are seen: (1) Clade I includes nine species restricted to eastern Iran and Central Asia, and (2) clade II includes a bulk of the species from west and northwestern Iran, Turkey and southern Europe. Divergence time estimates suggest Incani diverged from remaining sections 3 Mya during the late Pliocene. The crown date for Incani is estimated at 1.5 Mya (Pleistocene). Biogeography showed significant improvement in the likelihood score when the ??jump dispersal? parameter was added. An eastern origin (Central Asia) is implicated as important ancestral area in all deeper nodes. BAMM analyses indicate that the best configuration included one significant shift in diversification rates within Incani: near the crown of Incani (1.5-2 Mya) including clade II. Issues with conducting diversification analyses more generally are examined in the context of scale, taxon sampling, and larger sets of phylogenetic trees.