Evidence for incipient speciation in the agile antechinus (Antechinus agilis)

2017-02-14T02:50:42Z (GMT) by Draper, Tara Louise
Antechinus are small Australian dasyurid marsupials that show remarkable geographic variation in teat number, thought to be associated with local climate and resource availability. For most antechinus species, this geographic pattern of teat number is generally consistent within a regional metapopulation, where one teat number is common and individuals with one or two additional or fewer teats are rare. However, within the Otway Ranges in southwest Victoria, there are populations of both 6- and 10-teat phenotypes of agile antechinus (Antechinus agilis), which can co-exist at zones of contact between forms, along with a small number of individuals with intermediate teat numbers. As teat number limits the number of young a female antechinus can wean, it is of great interest to investigate evolutionary drivers of divergence in a trait closely linked with reproductive success. Previous research revealed substantial nuclear DNA and morphological differentiation, suggesting limited gene flow between 6- and 10-teat forms in the Otway Ranges. However, it remains unclear what processes are driving or at least maintaining reproductive isolation between the two forms. Reproductive isolation is a key mechanism in the process of speciation and often evolves as a by-product of ecologically driven phenotypic and/or genotypic divergence. Here I examine several ecological, behavioural and temporal isolating barriers that may act as drivers of divergence and reproductive isolation between 6- and 10-teat forms. I selected multiple contact zone sites where 6- and 10-teat forms co-exist and have the opportunity to interbreed (syntopic sites), as well as sites where they occur separately (allopatric sites). I used a combination of morphological and genetic data to characterise 6- and 10-teat forms, and to assess the degree of genetic mixing between forms and its phenotypic consequences. Two distinct morphological and genetic clusters associated with teat number were revealed. At syntopic sites, distinct 6- or 10-teat forms persist, despite the presence of intermediate (hybrid) teat-number forms (~40%) (Chapter 2). Although interbreeding between pure teat-number forms clearly occurs, the resulting hybrids have a degree of reduced fitness, and gene flow between forms is insufficient to overcome the persistence of distinct 6- and 10-teat forms (Chapters 2 and 4). Allopatric sites had significantly different habitats and climate conditions, despite the small geographic scale of the study (<10 km linear map distance) and syntopic sites had intermediate habitat and climate conditions, supporting the suggestion of ecologically driven trait divergence (Chapter 3). My results also suggest reinforcement of observed genetic and morphological divergence via assortative mating (i.e. mating with like teat-number) (Chapter 4) and possibly via later onset of activity in 10-teat animals (temporal niche partitioning) (Chapter 5). Analysis of nuclear DNA and morphological differentiation between 6- and 10-teat forms suggests functional and genetic differentiation through ecological specialisation, consistent with the ‘ecological gradient/ divergence-with-gene-flow’ model of speciation. Overall, the multiple lines of evidence presented here suggest the divergence of 6- and 10-teat forms of agile antechinus in the Otway Ranges likely represents a mid to late stage in the development of complete reproductive isolation, and a putative small mammal example of incipient speciation.