Life in the extreme

: when did tardigrades colonise Antarctica?

Abstract

Antarctica has long been isolated from its nearest landmasses and has been at least partially glaciated for the past ~35 million years. This long-term isolation combined with the fragmentation of habitats creates a unique environment for the examination of biogeographic patterns. Previously it was believed that the repeated full glaciation of Antarctica wiped out the pre-existing terrestrial fauna and flora and that the continent was, therefore, subsequently re-colonised after periods of glacial retreat, such as from the particularly extensive Last Glacial Maximum. However, recent molecular studies of a variety of taxonomic groups have shown that much of the current terrestrial life found on the continent is generally ancient and must have survived these glaciation events. The main body of Antarctica (East or continental Antarctica) and West Antarctica and the Antarctic Peninsula show differing geological histories and distinct biogeographic patterns in a variety of invertebrates including mites, springtails and nematodes, indicating distinct evolutionary histories. However, despite tardigrades being the most common and widespread invertebrate within Antarctica there has been limited study of their Antarctic biogeography. This thesis set out to investigate the evolutionary and biogeographic patterns of two eutardigrade species (Acutuncus antarcticus and Mesobiotus furciger). At the initiation of this study, A. antarcticus was believed to be a pan-Antarctic species, with a Gondwanan distribution, and M. furciger a globally common taxon and a Southern hemisphere species with a widespread Antarctic distribution. Individuals were sampled that originated from multiple locations across Antarctica. These individuals were then used to document a range of morphometric characters and underwent molecular extraction targeting the three gene regions of 18S, 28S and COX1. Further sequence data were added from GenBank. Phylogenetic analyses using both Maximum Likelihood and Bayesian methods showed that both species are likely to be species groups, with highly distinct lineages in the maritime and continental Antarctic regions. The levels of divergence detected strongly support cryptic speciation. This conclusion was further supported by morphological analyses, which identified significant differences between material sampled from each of these regions. Ancestral state and molecular clock analyses showed that both of these species groups had ancient Antarctic origins, requiring long-term regional survival within Antarctica, consistent with patterns identified in other invertebrate taxa. Morphological comparisons between individuals within single genetic clades obtained from different habitats also identified significant differences, supporting phenotypic plasticity in response to environmental conditions. This finding, not noted previously in tardigrades, has important taxonomic implications, as species identification relying on morphological characters alone may now result in miss-identification.

Date of Award	21 Jan 2021
Original language	English
Awarding Institution	The University of Bristol
Sponsors	British Antarctic Survey, NERC
Supervisor	Davide Pisani (Supervisor), Daniela N Schmidt (Supervisor), Peter Convey (Supervisor) & Sandra McInnes (Supervisor)