We tested the hypothesis that aridification of Australia during the Pleistocene promoted the isolation and divergence of three lineages of a migratory fish. We found support for this using an integrative framework of environmental and genomic modelling.
Above: Golden perch, Macquaria ambigua. Photo credit: Peter Unmack.
The Australian landscape has not always been so arid. In fact, if you travelled back to the early Cenozoic, in place of the central deserts you would find rainforests and lavish wetlands supporting very different ecosystems than we see today. But over millions of years, the plants and animals of Australia have had to adapt to a gradually drier climate, with heightened aridification during the glacial phases of the Pleistocene (~2.6 million to 11.7 thousand years ago) having caused major changes to the distribution and connectivity of populations. Despite having a good understanding of how this aridification has impacted terrestrial species, less is known about how it has affected the diversification of freshwater taxa.
Cover article (free-to-read for two years):
Booth, E. J., Sandoval-Castillo, J., Attard, C. R., Gilligan, D. M., Unmack, P. J. & Beheregaray, L. B. (2022). Aridification-driven evolution of a migratory fish revealed by niche modelling and coalescence simulations. Journal of Biogeography, 49, 1726– 1738. https://doi.org/10.1111/jbi.14337
In this study, we were curious to understand how historical aridification of Australia has influenced the evolution of an iconic freshwater fish, the golden perch (Macquaria ambigua). Specifically, we wanted to find out whether the divergence of three lineages of golden perch from three different river basins has been driven (or at least reinforced) by aridification. Previous research has found strong genetic differentiation between these lineages, and it has been proposed that they are actually three different ‘cryptic’ species. Clarifying this taxonomic ambiguity is important for the management and conservation of golden perch, especially since the species is regularly stocked from hatcheries into rivers and impoundments to support its recreational fishery.
We already had a ton of genome-wide data from previous work, so we thought we’d repurpose it to run some complex ‘coalescent’ models to better understand when these lineages diverged and experienced demographic changes. But what we needed first were some specific hypotheses to test. Rather than making hypotheses up out of thin air (or should I say water?), we used an environmental data set to develop contemporary and historical species distribution models. These models allowed us to predict how the amount of suitable habitat for golden perch has changed over time and theorize about how population sizes and connectivity of the lineages might have fluctuated between glacial and interglacial times.
Species distribution models for three golden perch lineages that are endemic to three major Australian drainage basins: Fitzroy (FIT), Lake Eyre (LEB), Murray–Darling (MDB). We found support for reduced habitat availability during the Last Glacial Maximum (~21 ka) compared to the present day.
What’s really cool is that our two independent data sets (environmental and genomic) found support for the same story. We discovered that during the Last Glacial Maximum (~21 thousand years ago), all three lineages had much smaller population sizes compared to the current day. Furthermore, the connectivity of suitable habitat between drainage basins was reduced at that time. This supports the idea that aridification caused the isolation (and facilitated the divergence) of the three golden perch lineages. We also found phylogenetic support for the delimitation of these lineages as separate species.
Our paper provides an exciting new insight into the diversification of freshwater taxa in Australia, and this integrative analytical framework could be applied to other study systems in the future. This research also has relevance for understanding how anthropogenic climate change might affect the connectivity of freshwater lineages, but that’s a story to explore elsewhere …
PhD candidate, Molecular Ecology Laboratory, College of Science and Engineering, Flinders University, Adelaide, South Australia, Australia
Twitter: @molecolflinders; @EmilyJBooth