Geropogon hybridus (L.) Sch.Bip. (Asteraceae) exhibits micro-geographic genetic divergence at ecological range limits along a steep precipitation gradient

Christina M Müller,Michael Ristow,Birgit Gemeinholzer,Benjamin Schulz,Volker Wissemann,Daniel Lauterbach

doi:10.1007/s00606-016-1354-y

Christina M Müller, Michael Ristow + Show 4 more

Open Access

https://doi.org/10.1007/s00606-016-1354-y

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Abstract

We analyzed the population genetic pattern of 12 fragmented Geropogon hybridus ecological range edge populations in Israel along a steep precipitation gradient. In the investigation area (45 × 20 km2), the annual mean precipitation changes rapidly from 450 mm in the north (Mediterranean-influenced climate zone) to 300 mm in the south (semiarid climate zone) without significant temperature changes. Our analysis (91 individuals, 12 populations, 123 polymorphic loci) revealed strongly structured populations (AMOVA ΦST = 0.35; P < 0.001); however, differentiation did not change gradually toward range edge. IBD was significant (Mantel test r = 0.81; P = 0.001) and derived from sharply divided groups between the northernmost populations and the others further south, due to dispersal or environmental limitations. This was corroborated by the PCA and STRUCTURE analyses. IBD and IBE were significant despite the micro-geographic scale of the study area, which indicates that reduced precipitation toward range edge leads to population genetic divergence. However, this pattern diminished when the hypothesized gene flow barrier was taken into account. Applying the spatial analysis method revealed 11 outlier loci that were correlated to annual precipitation and, moreover, were indicative for putative precipitation-related adaptation (BAYESCAN, MCHEZA). The results suggest that even on micro-geographic scales, environmental factors play prominent roles in population divergence, genetic drift, and directional selection. The pattern is typical for strong environmental gradients, e.g., at species range edges and ecological limits, and if gene flow barriers and mosaic-like structures of fragmented habitats hamper dispersal.

Highlights

Species’ distribution ranges are commonly defined by ecological limits, which are most often determined by ecological gradients
amplified fragment length polymorphism (AFLP) analyses with three different primer combinations and 91 analyzed individuals resulted in 123 unambiguously scorable polymorphic loci, ranging from 50 to 450 base pairs
We found low Isolation by distance (IBD) and no Isolation by environment (IBE) correlations, the vast amount of unaccounted variation there must largely be due to other non-spatially structured biological or unmeasured environmental variables, and/or random processes triggered by ecological drift and dispersal (Legendre et al 2009; Huang et al 2015)

Summary

Introduction

Species’ distribution ranges are commonly defined by ecological limits, which are most often determined by ecological gradients. As species approach their range limits, their populations typically become smaller and more fragmented (Bridle and Vines 2007). Range edge (margins or verge) populations often feature decreased genetic diversity due to reduced gene flow as result of the more fragmented distribution or random genetic drift effects enhance genetic diversity due to neutral, negative, or positive mutations which can appear and increase in frequency over time (e.g., Ellstrand and Elam 1993). If gene flow patterns align with geographic distance (isolation by distance, IBD; Wright 1943), this affects the.

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