Gene flow between two thick-billed grasswren subspecies with low dispersal creates a genomic pattern of isolation-by-distance

Abstract

Context In the era of the Anthropocene, habitat loss and environmental change threaten the persistence of many species. Genotyping-By-Sequencing (GBS) is a useful molecular tool for understanding how patterns of gene flow are associated with contemporary habitat distributions that may be affected by environmental change. Two parapatric subspecies of the threatened thick-billed grasswren (TBGW; Amytornis modestus) more frequently occur in different plant communities. As such, a preference for plant community type could reduce subspecific introgression and increase genetic diversity at the parapatric boundary. Aims We aimed to measure gene flow within and among two TBGW subspecies and tested whether divergent genomic markers were associated with plant community type. Methods We sequenced 118 individuals from either of the two TBGW subspecies or in the region of parapatry and identified 7583 SNPs through ddRADseq. Key results We found evidence of asymmetric gene flow and a genomic pattern of isolation-by-distance. There were sixteen genomic outliers correlated with plant community type (regardless of location). Conclusions These findings show that plant community type does not prevent introgression in one subspecies (A. m. raglessi), but low dispersal and habitat heterogeneity could contribute to the maintenance of distinct subspecific morphotypes. Local adaptation in different plant community types could also provide a mechanism for future divergence. Implications We suggest subspecific introgression could increase genetic variation and the adaptive potential of the species, facilitating species persistence under conditions of climate change.

A comparative study of two grasswren species reveals strong genetic divergence between a peninsula and mainland population

“…We found strong genetic structuring within both grasswren species. Our finding of genetic divergence between the TBGW subspecies is consistent with a previous SNP study focussing on this species only (Slender et al . 2021).…”

“…The genetic distinctiveness between the Peron and Hamelin WGW populations aligns with the genetic divergences found between Peron Peninsula and the Shark Bay mainland in the Whitewinged Fairywren (Walsh et al 2021), supporting the hypothesis that peninsula populations are genetically isolated from adjacent mainland populations. Knowing that the two grasswren species have different evolutionary histories (Austin et al 2013;Slender et al 2021) further supports the hypothesis as the WGW subspecies is considered to be one ESU, and yet its populations are just as divergent, if not more, than that of what is considered to be two ESUs in the TBGW. This comparison highlights the impact of peninsulas on gene flow, and the potential for Peron and Hamelin to benefit from conservation management as independent ESUs.…”

“…Knowing that the two grasswren species have different evolutionary histories (Austin et al . 2013; Slender et al . 2021) further supports the hypothesis as the WGW subspecies is considered to be one ESU, and yet its populations are just as divergent, if not more, than that of what is considered to be two ESUs in the TBGW.…”

“…It should be noted that the TBGW has a different evolutionary history to the WGW, with the TBGW being a parapatric species having made secondary geographical contact in evolutionary recent times (Slender 2018). The two subspecies have also been argued to belong to two separate evolutionary significant units (ESUs) (Austin et al 2013), however its current lack of genetic differentiation and high level of gene flow has led to the suggestion that they could be regarded as one ESU for conservation purposes (Slender et al 2021). In contrast, the WGW populations remain as one ESU and have likely been in geographical contact throughout much of their evolutionary history (Payne et al 1987;Austin et al 2013).…”

“…The two subspecies have also been argued to belong to two separate evolutionary significant units (ESUs) (Austin et al . 2013), however its current lack of genetic differentiation and high level of gene flow has led to the suggestion that they could be regarded as one ESU for conservation purposes (Slender et al . 2021).…”