Connecting social system dynamics, population genetics and symbiotic interactions : insights from the mountain brushtail possum

Abstract

The mating system and dispersal patterns of a species can profoundly affect fine-scale spatial genetic structure. Conversely, fine-scale spatial genetic patterns can have important ramifications for kin cooperation and mate choice. In my PhD, I explored these connections between social system dynamics and spatial genetic patterns using a combination of empirical research and modelling techniques. My empirical research centred on a field study of the mountain brushtail possum in the central highlands of Victoria, Australia. This semi-social, arboreal marsupial shelters in tree hollows, alone or shared, during the day and interacts at night. I used proximity logger collars to document the social interactions of this elusive species. By combining these data with microsatellite markers and mitochondrial sequences, I found that maternal lineages are a better predictor of interactions between individuals than bi-parental relatedness. This is likely to represent, in part, mother-offspring associations and suggests that kin recognition may occur through familial cues. Furthermore, the duration of nocturnal interactions were longer between females than males, which may be attributed to an interaction between kin selection and male-biased dispersal. Sex-biased natal dispersal can facilitate inbreeding avoidance by spatially separating kin. However, I showed through spatially explicit simulation modelling that the spatial clustering of kin cannot be prevented by sex-biased dispersal when dispersal distances are limited or a species has overlapping generations. Consistent with these simulations, spatial genetic autocorrelation analysis revealed that in mountain brushtail possums opposite-sexed kin remain in proximity, despite short range male-biased dispersal. To investigate how mountain brushtail possums respond to this inbreeding risk, I explored two inbreeding avoidance mechanisms; mate choice and mate-fidelity. Genetic parentage assignment analyses suggested that this species actively avoids kin during mate choice. Additionally, using probability modelling, I determined that individuals are more genetically monogamous than expected by chance. This high level of genetic monogamy may be due to social pair-bonding, which can be characterised by frequent interactions and overlapping home ranges. In addition to species-level processes, community-level interactions can influence social system dynamics. For instance, pathogen transmission can potentially constrain social interactions. To understand transmission dynamics associated with mountain brushtail possum behaviour, I used commensal Escherichia coli strains as a model system. I found high levels of temporal variability in E. coli community structure within host individuals, suggesting that strain sharing between individuals likely represents contemporary transmission. Additionally, within and among host E. coli strain abundance were correlated and predicted by the same aspects of the strains' functional genotype. Finally, I showed that strain sharing was better explained by social interactions than spatial proximity, suggesting that host-to-host contact maybe an important transmission route in this study system. Furthermore, I revealed that nocturnal interactions were more strongly associated with strain sharing than den sharing. Together, my results highlight how a multidisciplinary approach can provide detailed insights into the factors shaping a species social system dynamics. Further, they show how recent advances in animal tracking and surveillance technology combined with new genetic techniques can allow us to understand the socio-biology of elusive species.