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Polar eveolution: molecular genetic and physiological parameters of Antarctic arthropod populations : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Molecular Biosciences at the Allan Wilson Centre of Molecular Ecology and Evolution, Institute of Molecular Biosciences, Massey University, Palmerston North, New Zealand
This thesis is presented as a collection of research papers synthesising knowledge
gained during the period of candidacy. Its underlying focus is the examination of
evolution from a variety of perspectives for terrestrial arthropods (springtails) in an
Antarctic setting. These perspectives include investigation of the ways in which
springtail populations respond both physiologically and genetically to environmental
variability over historical and contemporary time-scales. While the physiological and
genetic may seem two worlds apart, this thesis recognises that, in reality the two are
inextricably linked. Thus, when genetic differentiation between populations of the same
species can be demonstrated, physiological differentiation of these populations may also
be predicted (and vice versa). Therefore, across several locations and springtail species,
physiological and genetic parameters of individuals and populations are examined both
separately and, where possible, in concert.
The physiological aspect of this thesis focuses on the springtail Gomphiocephalus
hodgsoni from continental Antarctica. In addition to providing the first metabolic rate
data for a continental Antarctic springtail, seasonal variation in metabolic rates is
examined across multiple temporal and spatial scales to evaluate the ways in which
individuals and populations respond to environmental variability. Metabolic activity in
this species is intricately linked to a variety of factors, both intrinsic and extrinsic.
These include biological function, temperature profiles in the local microclimate, and
body mass and genetic differences among populations.
In the genetically-focused aspect of this thesis, population genetic patterns of G.
hodgsoni from several continental locations and Cryptopygus antarcticus antarcticus
from locations across the Antarctica Peninsula are compared. Here, the importance of
differing evolutionary histories in influencing patterns of contemporary genetic
population structure is highlighted. While both species have been similarly affected
genetically by Pleistocene (2 Ma – present) glacial cycling, it is clear that differences in
timing of colonisation events and subsequent population expansions have left distinct
genetic signatures in each species. In a separate molecular study, phylogenetic analyses
are employed to study members of the circum-Antarctic springtail family Isotomidae.
The genetic ancestry among these closely related species is shown to reflect a diverse
evolutionary origin in the Miocene (23 – 5 Ma), subsequent to which both vicariant and
dispersal processes have been important. Phylogenetic re-constructions tease out the
relationships among sister species, and the identification of several genetically distant
lineages suggests that a revision of current species designations is required.
Finally, two studies that integrate the physiological and molecular genetic are
presented. First, metabolic rate variation across several locations on sub-Antarctic
Marion Island in the springtail Cryptopygus antarcticus travei is examined. This
variation is related to the genetic structure of populations to show that historical and
contemporary environmental characteristics have left their trace in the expression of
both genetic and physiological variability of these populations. Second, the perceived
association between metabolic rate and genetic (mutation) rate is investigated more
closely - a sophisticated Bayesian correlation analysis detects that there is an indirect
relationship between metabolic rate and underlying species phylogeny in C. a. travei.
Thus, the physiological and molecular genetic elements of this thesis test or
advance important hypotheses within their own fields, and the integrated approach
applied is a new step in interpreting evidence of physiological adaptation in Antarctic
species. In its multi-faceted approach to evolutionary studies, this thesis enhances
understanding of the current picture of springtail evolution in polar environments.