Genetic diversity and structure lag the effects of contemporary environmental changes in a platypus meta-population.

The platypus is an evolutionary unique mammal on the east coast of mainland Australia and throughout Tasmania. The species is dependent on freshwater ecosystems, is declining throughout its range, and is listed as Vulnerable in the state of Victoria, and Near Threatened on the IUCN Red List. This relatively long-lived species is cryptic and nocturnal making it difficult to study in natural populations. Relatively little is known about its demographic history or the forces that shape genetic variation. We use a unique genomic dataset comprising 2715 single-nucleotide polymorphisms from 545 individual platypuses sampled from five catchments across Melbourne, Victoria. This dataset enabled us to describe the genetic variation across the catchments and test hypotheses relating to migration, effective population size, and potential negative effects of anthropogenic barriers. We found relatively consistent levels of genetic diversity in platypuses across Melbourne's catchments, moderate levels of within-catchment migration, and genetic differentiation both between and within catchments. This genetic structure is explained by several factors, including isolation-by-river-distance, isolation-by-environment and within-catchment sex biased dispersal at short distances. These patterns are likely explained by a temporal lag between indirect and direct anthropogenic changes to the environmental and genetic variation, and these contemporary analyses likely reflect historical demographic patterns. In addition, we find that anthropogenic barriers such as dams have not measurably affected migration in these catchments. Our study highlights future evolutionary challenges that exist for platypuses in Melbourne's catchments, which could be representative of their entire range along the east coast of Australia.