Abstract
Global biodiversity changes are occurring at alarming rates, with the onset of the current mass extinction accelerating during recent years. Protected areas (PA) have been advocated to curb biodiversity loss, as these areas effectively shield ecosystems from anthropogenic stressors. It is well documented that marine systems experience change at pronounced rates, as local impacts spread because of the interconnected nature of ocean basins. The geographic scope of this work is sub-Antarctic Marion Island, which along with Prince Edward Island, forms the Prince Edward Islands archipelago, positioned within a marine protected area (proclaimed in 2004).
Island systems, due to their isolated nature, provide arenas and natural laboratories where research is undertaken to evaluate the micro-evolutionary processes (e.g., colonisation efforts, adaptations, natural selection, genetic drift, population dynamics, spatial processes, and gene flow) within populations. The heterogeneous landscape of Marion Island, driven by past geological events, complex climatic conditions, and the mosaic of intricate vegetation units, is especially suited to study these evolutionary processes. In addition, its geographic location, midway between Antarctica to the south and Africa to the north, makes it ideal to understand how these isolated sub-Antarctic islands serve as conduits for change.
A taxonomic group that dominates terrestrial fauna on all sub-Antarctic islands is springtails (Collembola). Despite their diminutive size, these animals play pertinent roles in soil ecosystems by influencing the microbial properties within the soil, contributing to the recycling of nutrients, and serving as prey for larger predatory arthropods. Springtails can be used as proxies to assess soil ecotoxicology and offer great value to studying evolutionary biology and their adaptive potential in a biochemical and physiological framework.
V
This study aimed to characterise the genomic architecture of the mitochondrial and whole genomic sequence of a springtail species endemic to Marion Island, Cryptopygus antarcticus travei. The knowledge generated through this work will provide novel information on the metabolic pathways that may be under selection, the biological processes, molecular functions, and cellular components that may be enriched within the genome, and the spatial genetic structure of individuals across a fine-scale (typically tens of metres) landscape matrix. Although generated for a taxon endemic to the Prince Edward Islands, the information could be extrapolated to all sub-Antarctic Islands, and could provide the baseline for similar studies on other taxa, and on other sub-Antarctic islands.
The first complete mitochondrial sequence of C. a. travei was assembled and annotated, and added to the current database comprising publicly available mitogenomes for Collembola. Briefly, the mitogenome was assembled to a length of 15,743 bp and annotated. The mitogenomic characteristics conform to what has been previously documented for invertebrate mitogenomes. Additionally, the phylogenetic placement of this species alongside a few others revealed the high divergence and distinctness of C. a. travei compared to its sister species, C. antarcticus, highlighting that C. a. travei, given its long-term isolation to Marion Island, may be a candidate new species and its current taxonomical status should be re-evaluated (this work is published).
A striking characteristic among Collembola is their unique habitat stratification preference (ecological life form). Given that mitochondrial DNA plays a pivotal role in cellular respiration and aerobic metabolism, selection pressures on the mitogenome can provide insight into the functional and physiological requirements of species in different habitats. Therefore, using the mitogenome of C. a. travei, in combination with 34 mitogenomes from online repositories (available at the time of performing the work), an assessment of positive selection linked to ecological life forms was correlated. Interestingly, no evidence for selection was linked to the evolution of life forms but instead hints at the importance of nuclear-mitochondrial interactions (this work is published).
VI
The first draft whole genome sequence of C. a. travei was assembled to a sequence length of ~314 Mb and annotated to identify ~82,000 putative genes with 196 enriched gene ontologies. A single-nucleotide polymorphisms (SNP) dataset was identified and used to successfully decipher population structure at very fine spatial scales. Moreover, variant annotation revealed that most SNPs have a modifying impact and a notable effect on the downstream and upstream gene features.
A fine-scale landscape genetic approach was deployed using species-specific microsatellite markers and novel landscape data, which revealed high levels of genetic diversity to be structured by the local habitat matrix. Additionally, generalised linear models revealed that landscape resistance in combination with the geological history of the sampled area is responsible for shaping structure.
The results of this thesis highlight valuable and novel insight into the complex genomic architecture and genetic inferences of C. a. travei. Moreover, this work points to a paradigm shift regarding the heterogeneity and non-interchangeability of individuals on a fine spatial scale. It highlights the critical importance of considering fine-scale evolutionary processes when mitigating conservation efforts, as variation and population inferences on fine scales are frequently unexplored. Although successful in that all aims were achieved, this work provides fertile ground for future collaborative genomic research for Collembola.