The ecology of gene flow between two subspecies of thick-billed grasswren (Amytornis modestus)

Author: Amy L. Slender

Slender, Amy L., 2018 The ecology of gene flow between two subspecies of thick-billed grasswren (Amytornis modestus), Flinders University, School of Biological Sciences

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For decades, population divergence associated with speciation was thought to require the absence of gene flow. Recent empirical studies challenge this view by showing that divergence in the presence of gene flow is common. This study investigates interactions between gene flow and divergence at a landscape scale in two thick-billed grasswren subspecies (Amytornis modestus indulkanna and A. m. raglessi) in arid South Australia. This thesis tests how gene flow between subspecies affects or is affected by the distribution of different markers of divergence. These included (1) a mitochondrial gene, (2) morphology, (3) ecology, (4) nuclear genes, and (5) behaviour. For the first time, I detected thick-billed grasswrens within an area between that previously thought to be the subspecies’ core distributions. Although there were divergent morphotypes and mitochondrial haplotypes associated with each subspecies core area, the intervening area and the core area for A. m. raglessi contained both mitochondrial haplotypes and the intervening area had intermediate morphotypes. I conclude that the subspecies have a parapatric distribution and are also interbreeding. The distribution of different vegetation types correlated with areas of high and low gene flow. Each subspecies’ core area (low gene flow) had different vegetation types and the region of parapatry (high gene flow) had a mixture of both subspecies-specific vegetation types suggesting that natural selection may prevent high gene flow in the core distribution areas. Thick-billed grasswrens were absent from the vegetation type found on sand dunes, which may be a landmark that demarcates where allopatric divergence between the subspecies occurred in the past. Patterns of gene flow across the landscape revealed that gene flow was asymmetric from A. m. indulkanna to A. m. raglessi. Using recorded song from both subspecies, I measured the territorial response of each subspecies to hetero- and con- subspecific song. A. m. indulkanna responded more frequently to hetero-subspecific song compared to A. m. raglessi suggesting that stronger territorial behaviour in A. m. indulkanna may be a mechanism for asymmetric gene flow. This thesis answers questions about the pattern of gene flow at a landscape scale in a contemporary threatened songbird species. Ongoing gene flow between subspecies indicates that subspecies are an appropriate classification for these populations. Given the finding that vegetation type creates stepping zones for high gene flow, the thesis reveals the role of habitat connectivity for increasing adaptive potential and persistence in a region of parapatry. The discovery that asymmetric territorial response to intruder subspecies correlates with the direction of gene flow shows that behaviour is an important factor for introgression in an extant bird species. Finally, these patterns of gene flow and divergence at a landscape scale show how evolutionary studies can be used to manage Australia’s endemic biodiversity.

Keywords: Ecology, Evolution, Passerine, Maluridae, Grasswren, Parapatry, Gene flow, Subspecies, Aridity, Secondary contact, Habitat patches, Sand dunes, Playback, Behaviour, Song, Morphology, Mitochondrial gene.

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: School of Biological Sciences
Supervisor: Sonia Kleindorder