The nuanced nature of mesic refugia in arid landscapes: a tale of two peas.

Abstract

Understanding how genetic diversity is distributed and maintained within species is a central tenet of evolutionary and conservation biology, yet is understudied in arid regions of the globe. In temperate, glaciated environments, high genetic diversity in plant species is frequently found in refugial areas, which are often associated with southern non-glaciated landscapes. In arid, unglaciated environments, landscape features providing mesic conditions are likely to be refugia, although our understanding needs more refinement in these biomes. We test whether refugia and nuclear diversity hotspots occur in high-elevation, topographically complex areas for co-distributed shrubs (Petalostylis labicheoides and Indigofera monophylla; Fabaceae) in the ancient, arid Pilbara bioregion of north-western Australia. We conducted extensive sampling of the Pilbara (>1400 individuals from 62 widespread populations) to detect patterns in nuclear diversity and structure based on 13-16 microsatellite loci. Evidence of historical refugia was investigated based on patterns of diversity in three non-coding chloroplast (cp) sequence regions for approx. 240 individuals per species. Haplotype relationships were defined with median-joining networks and maximum likelihood phylogenetic trees. We found cpDNA evidence for a high-elevation refugium in P. labicheoides but not for I. monophylla that instead exhibited extraordinary haplotype diversity and evidence for persistence across a widespread area. Nuclear diversity hotspots occurred in, but were not exclusive to, high-elevation locations and extended to adjacent, low-elevation riparian areas in both species. Phylogeographic refugia in arid environments may occur in high-elevation areas for some species but not all, and may be influenced by species-specific traits: a mesic montane refugium in P. labicheoides could be related to its preference for growth in water-gaining areas, while a lack of such evidence in I. monophylla could be related to maintenance of cpDNA diversity in a large soil seed bank and dynamic evolutionary history. Mesic environments created by the intersection of topographically complex landscapes with riparian zones can be contemporary reservoirs of genetic diversity in arid landscapes.