Geographic variation in hybridization and ecological differentiation between three syntopic, morphologically similar species of montane lizards

Abstract

To understand factors shaping species boundaries in closely related taxa, a powerful approach is to compare levels of genetic admixture at multiple points of contact and determine how this relates to intrinsic and extrinsic factors, such as genetic, morphological and ecological differentiation. In the Australian Alps, the threatened alpine bog skink Pseudemoia cryodroma co-occurs with two morphologically and ecologically similar congeners, P. entrecasteauxii and P. pagenstecheri, and all three species are suspected to hybridize. We predicted that the frequency of hybridization should be negatively correlated with genetic divergence, morphological differentiation and microhabitat separation. We tested this hypothesis using a mitochondrial locus, 13 microsatellite loci, morphological and microhabitat data and compared results across three geographically isolated sites. Despite strong genetic structure between species, we detected hybridization between all species pairs, including evidence of backcrossed individuals at the two sites where all three species are syntopic. Hybridization frequencies were not consistently associated with genetic, morphological or ecological differentiation. Furthermore, P. entrecasteauxii and P. pagenstecheri only hybridized at the two sites where they are syntopic with P. cryodroma, but not at the largest site where P. cryodroma was not recorded, suggesting that P. cryodroma may serve as a bridging species. This study reveals the complex dynamics within a three species hybrid zone and provides a baseline for assessing the impact of climate change and anthropogenic habitat modification on future hybridization frequencies.

Triad hybridization via a conduit species

“…We used neutral markers, microsatellites, to assign individuals to species or hybrids, using an arbitrary but often used and biologically justified criterion (36,39,43,46) to separate them (SI Appendix, section 2). Assignments are subject to error (SI Appendix, section 2), and it is possible that the small number of loci results in an inflated number of erroneous assignments to hybrid classes.…”

“…Triad hybridization has been reported in fish (32)(33)(34), frogs (35), lizards (36), birds (17,37,38), mammals (39)(40)(41), snails (42), butterflies (43), mosquitoes (44), as well as several plant taxa (45,46). One species acts as a genetic bridge (33,47,48) or conduit (41,49) for the passage of genes (alleles) between two others that, in some cases, rarely if ever hybridize (33,39,41).…”

“…One species acts as a genetic bridge (33,47,48) or conduit (41,49) for the passage of genes (alleles) between two others that, in some cases, rarely if ever hybridize (33,39,41). Exchange of genes is generally not studied directly with breeding pairs but is inferred from the presence of individuals of mixed (trispecies) ancestry (33,39,41) or from the fact that hybridization occurs, or is enhanced, only in the presence of a third species (36,50).…”

Impacts of human‐induced environmental disturbances on hybridization between two ecologically differentiated Californian oak species

“…Thus, as done in previous studies, we summed q-values over all hybrid genotype frequency classes (e.g. Cullingham et al, 2011;Hasselman et al, 2014;Haines et al, 2016). Given that NEWHYBRIDS can only accommodate two species/clusters and our analyses support genetic substructure within Q. berberidifolia (see the Results section), we conservatively based subsequent analyses on estimates of admixed ancestry obtained from STRUCTURE (e.g.…”

“…Given that NEWHYBRIDS can only accommodate two species/clusters and our analyses support genetic substructure within Q. berberidifolia (see the Results section), we conservatively based subsequent analyses on estimates of admixed ancestry obtained from STRUCTURE (e.g. Haines et al, 2016). Details of STRUCTURE and NEWHYBRIDS settings are presented in Notes S1.…”