Phylogeography of Austral soil invertebrates

Abstract

Soil invertebrates are terrestrial animals belonging to ancient phyla that emerged almost half a billion years ago. They have since spread throughout all known landmasses, with contemporary distributions governed by geological and environmental change across spatial and temporal gradients across the globe. However, limited knowledge of southern hemisphere (Austral) species hampers our ability to discern the general patterns of distribution and speciation. The lack of robust taxonomic information has also constrained our understanding of the evolutionary relationships and functional roles of the diverse soil fauna. This thesis capitalises on the development in molecular tools and improved sequence libraries to explore the factors that define the distribution and diversity of common soil invertebrates, specifically oribatid mites (Acari), springtails (Collembola) and nematodes (Nematoda). I investigated communities at continental-scales from maritime Antarctica and Australia to enable greater resolution of the drivers of distribution that might be applicable to southern hemisphere taxa more broadly. In a literature review I introduce the bioinformatic approaches using phylogeography to resolve evolutionary theories concerning soil fauna indigenous to Antarctica. Phylogenetic evidence supports most soil faunal groups as having ancient origins, refugial survival and repeated colonisation, whilst also highlighting the benefits of comparative analyses over larger scales. In addition, I show in a perspectives paper that morphological and functional traits are phylogenetically constrained in nematodes and springtails, allowing function to be partially conferred for 'unknown' species using sequencing approaches. Baseline biodiversity across a transect through maritime Antarctica found contrasting distributions of mites and springtails and the influences of climatic factors at broad scales and soil microhabitat conditions at local scales. Detailed population genetic analysis of genotypes of the oribatid mites Podacarus auberti and Membranoppia loxolineata alongside the springtail Cryptopygus antarcticus revealed the importance of multiple dispersal events in their ancestral past, supporting theories of refugial survival. Comparative analysis of phylogeographic reconstructions with an analogous Australian transect highlighted that the importance of dispersal differs among mites and springtails and supported the influences of climate and edaphic factors on assemblage structure. These different influences of biogeography and climatic variability related to inherent morphological and physiological traits of the study organisms demonstrate potentially contrasting responses to future episodes of environmental change. With such knowledge, conservation strategies of Austral soil fauna can be re-focussed to ensure their continued persistence in terrestrial ecosystems.

Date of Award	2023
Original language	English