Evolution, systematics & geographic parthenogenesis of Ilyodromus (Crustacea, Ostracoda)

Author

Rylan James Shearn, Edith Cowan UniversityFollow

Date of Award

2015

Document Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Sciences

Faculty

Faculty of Health, Engineering and Science

First Supervisor

Dr Annette Koenders

Second Supervisor

Dr Stuart Halse

Third Supervisor

Professor Isa Schön

Fourth Supervisor

Professor Koen Martens

Files

Abstract

Most multicellular organisms reproduce sexually at some point in their life cycle. This is paradoxical because being asexual is theoretically far more advantageous. Asexual organisms do not need to find and court new mates, they reproduce at a faster rate, and with no males, all members of the population contribute toward population growth rate. With over 20, often mutually exclusive hypotheses, this paradox resists a synthesised explanation, and continues to represent one of the largest gaps in our understanding of fundamental evolutionary theory. Clearly, more real world studies are required that document the selective mechanisms underlying differences in evolutionary fitness between sexual and asexual organisms.

Some species can use either sexual or asexual reproduction, and remarkably, populations that use sexual reproduction can have distinctly different geographic distributions from those that use asexual reproduction. This phenomenon, whereby different distributions of reproductive mode can be observed within the same or closely related species, is called geographic parthenogenesis. These patterns hold promise in providing real world evidence for mechanisms leading to differences in evolutionary fitness between sexual and asexual lineages, because the two regions that sexual and asexual lineages occupy can be characterised by environmental tolerance ranges that in turn may be associated with selection pressure.

This thesis addresses the lacking real world evidence for selective mechanisms behind differences in fitness between sexual and asexual organisms, by seeking to develop and use a model system from which the importance of environmental parameters in explaining reproductive mode can be quantified. A freshwater crustacean (Ilyodromus, Ostracoda) occurring in rock outcrop pools along an ecological gradient in south western Australia was investigated. This crustacean employs both reproductive modes to varying degrees, such that the proportion of males in a population is indicative of the extent to which sexual reproduction is used. Unfortunately, since the systematics of this genus were poorly understood, one could not be sure of whether variation in reproductive mode observed between populations could also be due to lineage divergence, and an extensive systematic revision was also necessary for the development of thismodel system.

The key findings of this thesis are:

1. An improved understanding of the systematics of Ilyodromus, specifically:

a) Revised characters that are considered typical of the genus.

b) The boundaries between Ilyodromus and other similar genera.

c) Detailed species descriptions for ten nominal species, and three as yet unnamed species.

d) An improved ability to identify female-only populations

2. The model system constructed and used in this thesis enables the importance of environmental parameters in explaining reproductive mode to be quantified

3. Parthenogenesis tends to occur more in less stable (or more arid) habitats, while sexual reproduction tends to occur more in more stable (higher rainfall) habitats.

4. This, alongside patterns of species distribution, suggests that variation in reproductive mode is closely linked with processes of speciation and adaptive response to emptied niches.

Related Publications

Shearn, R., Halse, S., Koenders, A., Schön, I., & Martens, K. (2014). Redescriptions of six species of Ilyodromus Sars, 1894 (Crustacea, Ostracoda, Cyprididae) from New Zealand and Eastern Australia. Zootaxa, 3878(2), 101-145. doi:10.11646/zootaxa.3878.2.1

Comments

Shearn, R. J. (2015). Thesis template (based Edith Cowan University format requirements) written in LaTeX. Available from https://github.com/rylanshearn/rylan-thesis-template.

Recommended Citation

Shearn, Rylan James, "Evolution, systematics & geographic parthenogenesis of Ilyodromus (Crustacea, Ostracoda)" (2015). Theses. 1.
https://ro.ecu.edu.au/theses_ebooks/1