Delimitation, diversification and adaptation in Lachnagrostis.
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Date
2013
Authors
Brown, Austin James
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Advisors
Lowe, Andrew
Cross, Hugh
Cross, Hugh
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Thesis
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Abstract
This thesis explores the potential influence of a range of biological mechanisms (polyploidy, breeding system and edaphic adaptation - salinity tolerance) on diversification of Lachnagrostis Trin.(Poaceae): a largely Australian and New Zealand genus, with a few South African and South American taxa, recently segregated from Agrostis L. The genus occurs across a wide range of temperate environments from coastal to montane conditions, from grassland to forest and from non-saline to saline soils. Morphological differences between some taxa are subtle but nevertheless distinct and have led to the ongoing recognition of new species. The relationship between Lachnagrostis and other members of the subtribe Agrostidinae was examined through the construction of a phylogeny based on the cpDNA regions, rbcL and matK and the nDNA region, ITS, derived from sampling and sequencing herbaria collections in order to answer i) whether or not Lachnagrostis should be regarded as a genus in its own right, ii) whether the non-Australasian taxa are part of a Lachnagrostis clade and if so, with which Australasian species they are most closely related and iii) whether a phylogenetic signal exists within Lachnagrostis that could provide clues to historical or contemporary speciation events. Sequencing was supplemented with an AFLP analysis of the majority of Lachnagrostis taxa to provide an enhanced phylogeny of the genus. Polyploidy was examined through determination of C-values across a range of nursery grown populations and thumb-squash chromosome counts on selected examples. Breeding system was examined through nursery observations of the timing of anther dehiscence in relation to anthesis. Salinity tolerance was examined through the measurement of seed germination and plumule and radicle response. The distribution of each trait was examined across a broad range of taxa and populations and the potential of each interpreted against the phylogenetic relationship amongst taxa. None of the DNA regions proved to be variable enough to provide a fully resolved phylogeny at either the subtribe or genus level, although subtribal relationship inferences were greatly improved when the three regions were combined. Phylogenetic analysis did not support a monophyly for Lachnagrostis and in particular, split the NZ taxa between two clades. The AFLP analysis provided better taxonomic definition within the genus but backbone support for a robust phylogeny was again lacking. These lines of evidence suggest incomplete lineage sorting and recent, and in some cases contemporaneous diversification within the genus: probably linked to increasing landscape fragmentation. High-level polyploidy is the norm within Lachnagrostis but is poorly correlated with C-value. Among taxa with the highest C-values were the perennial L. billardierei and the highly salt-tolerant L: adamsonii and L. robusta while the lowest C-value and chromosome counts were found for L. punicea: a sister taxon of the Australian clade. Inbreeding appears to be common, even in widespread species and suggests inflorescence dispersal as a major mechanism for gene flow between populations. A high degree of salt tolerance, even among taxa normally growing in non-saline environments suggests that adaptation to salinity is as much a response to phenotypic plasticity as to natural selection.
School/Discipline
School of Earth and Environmental Sciences
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2013
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This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals