Bio21 - Theses
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ItemFitness trade-offs involved in adaptation to host plants, diapause-egg phenotypes and pesticides pressure in the redlegged earth mite, Halotydeus destructor( 2018)The redlegged earth mite, Halotydeus destructor, is an invasive species introduced into Australia from South Africa. In Australia, this species enters summer diapause at the egg stage which can survive desiccation, heat exposure and applications of pesticides over summer, while annually it produces approximately three generations of active mites in the cool and wet period. Hatching of diapause eggs approximately synchronize the germination of annual plants and the post-diapause mites (1st generation), thus causing serious damage on seedlings. Understanding the mechanism and fitness cost of adaptation to host plants, environmental changes and pesticides pressure in this species is crucial for development of an integrated pest management (IPM) strategy. My experiment reveals that host performance of H. destructor is influenced by the interaction between mite populations and host plants. Mite survival, net reproductive output, development and feeding damage on plants are reduced if the previous host plant of a population is different from the host plant of the introduced microcosm, revealing that a fitness cost is involved in host adaptation. A typical type of diapause eggs with a thick chorion is known to survive sprays of pesticides and stressful summer. A program, Timerite®, has been designed in Western Australia to predict the onset of the typical diapause (TD) egg production based largely on daylength, and farmers spray pesticides according to Timerite® predicted date to suppress populations of pre-diapause mites. However, effectiveness of Timerite® is less in eastern areas of Australia than Western Australia. My study reveals a cryptic type of diapause (CD) egg which lacks a thick chorion and thus is morphologically similar to the non-diapause (ND) egg. Production of diapause eggs (including both TD and CD types) was triggered by longer-daylength, as well as hotter and drier conditions in comparison with conditions leading to ND eggs. However, CD egg can be produced under shorter daylength and cooler temperatures in comparison with TD eggs. Coexistence of CD and ND eggs reflects a bet-hedging strategy in that some individuals can enter diapause for unpredictable conditions. Nevertheless, a fitness penalty of this strategy is revealed by lower diapause intensity of CD eggs when parental mites were reared in cooler and moister conditions. Heavy reliance of synthetic pyrethroid chemicals targeting active mite has led to target-site resistance (kdr) evolving. The L1024F substitution in the voltage-gated para sodium channel is incompletely recessive. Heterozygote (RS) individuals which are detectable in genetic screening are therefore undetectable by bioassays. Only the resistant homozygote (RR) survive 100 mgL-1 bifenthrin which is the discriminating dose and also the field rate, while RS heterozygotes do not survive. Refuges untreated by pyrethroids could reduce resistance evolution because susceptible homozygous (SS) mites in refuges mate with RR survivals in sprayed areas to produce RS. Furthermore, the resistant population has decreased net reproductive output in comparison with susceptible populations, revealing a fitness cost of the L1024F substitution. The frequency of resistant alleles in a population is therefore decreased in the absence of pyrethroid chemicals.
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ItemClimate adaptation in Eucalyptus microcarpa (Grey Box) and implications for conservation( 2017)Restoration is an important component of conservation management, especially in highly modified landscapes. In the face of rapid environmental change, the mere presence of vegetation doesn’t necessarily equate to the long term sustainability of populations. Rather, there is a need to consider evolutionary potential in conservation planning, including restoration. This thesis investigates two key components of evolutionary potential pertinent to restoration – namely genetic diversity and climate adaptation – in an important restoration tree species in south-eastern Australia, Eucalyptus microcarpa. This thesis aims to understand how genetic diversity and local adaptation are distributed across the range of E. microcarpa and how this knowledge may help inform seed sourcing and enhance resilience of restoration plantings under climate change. To begin, I use a landscape genomic approach to explore genomic diversity in E. microcarpa and how diversity in small habitat remnants and revegetation (restored) sites compare to large remnants. This work found that small, habitat remnants and revegetation sites largely, but not completely captured patterns of genomic diversity across the landscape. Whilst overall genomic diversity was similar between site types, patterns of diversity across the genome varied between site types. These results suggest important genomic differences between site types that may influence future adaptive potential of revegetation sites and small habitat remnants. I then investigated adaptation to climate in E. microcarpa using multiple approaches. Firstly, using a landscape genomic approach, I found evidence of genomic climate adaptation in E. microcarpa. These results suggest climate adaptation to be a genome-wide phenomenon, involving many genes and genomic regions. Exploration of genomic changes that may be required to match projected climate change suggest adaptation to be via shifts in allele frequency from standing variation. In addition to suggesting a number of climate variables associated with adaptation in E. microcarpa, these results highlight the importance of genetic diversity and standing variation for maintaining adaptive potential. Utilising existing genetic resources for this species, I found evidence of heritable, genetic variation in growth and leaf traits of E. microcarpa growing in a provenance trial. Furthermore, significant trait variation between provenances and associations with climate variables suggest climate as a driver of adaptive differences. Finally, I combined the independent genomic and phenotypic analyses to provide stronger support for climate adaptation in E. microcarpa, including links between genomic variants and adaptive traits. Associations between traits and single nucleotide polymorphisms (SNPs) using putatively adaptive SNPs genotyped in provenance trial trees validated genomic results, suggesting some trait variation could be explained by these SNPs. Furthermore, links between all three sources of variation relevant to local adaptation – genotype, phenotype and climate – corroborated findings of the two independent analyses. This approach therefore provides greater support for adaptation to climate in E. microcarpa. Together these analyses address the current genetic state of restoration in E. microcarpa as well as the structure of genetic diversity and climate adaptation across its distribution. These results suggest adaptive differences within E. microcarpa that could be utilised to enhance evolutionary potential within restoration plantings.
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ItemGeographic range and the mountain niche: ecology, adaptation and environmental change( 2015)The geographic range is one of the most fundamental traits of a species. For this reason, understanding the ecological and evolutionary drivers of the size, position, and structure of the range is a key research challenge. The geographic range also has an overriding influence on the environments to which a species is exposed and their spatial and temporal variation. This study addresses four questions relevant to understanding interactions between the environment and individuals, populations, species and communities, with a focus on mountain regions where environmental variation is particularly pronounced. First, using meta-analysis and a single-genus case study, I explore the relationship between geographic range size and characteristics of the ecological niche. Range size can vary by several orders of magnitude among closely related species, but is strongly and consistently associated with both niche breadth and niche position: the most widespread species tend to be those with a broader niche and/or those that utilise resources that are common across the landscape. Second, I investigate the relationship between niche and range limits by testing variation in physiological tolerance across environmental gradients in two mountain systems: beetles (Carabidae: Nebria Latreille) from the North American Cascade Range and grasshoppers (Acrididae: Kosciuscola Sjösted) from the Australian Alps. Whereas the Nebria, distributed across a 2000 m elevational gradient, showed almost no variation in thermal tolerance, the Kosciuscola showed significant interspecific variation in cold tolerance, consistent with the decrease in average temperature with elevation. I suggest that cold tolerance limits might constrain the upper range edge of at least one species. Third, I explore how past climate cycles and Australia’s dissected mountain landscape have influenced the population structure of an alpine-endemic grasshopper (Kosciuscola tristis) using a combination of genetic methods. Despite continuity of alpine habitat during Pleistocene glacial cycles and, by global standards, small-scale disjunctions in the present distribution of these environments, K. tristis showed deep lineage divergence associated with geographic breaks in alpine conditions. Fine-scale structure in the absence of clear geographic barriers suggests that habitat heterogeneity might structure populations at a regional scale. The last component of this work tests the response of alpine invertebrate species and communities to reduced winter snow cover. This is a likely future scenario in the Australian high country, where the winter snowpack is already marginal. I show that Australia has a diverse subnivean arthropod fauna, characterised by the high relative abundance of springtails (Collembola), mites (Acari), spiders (Araneae) and beetles (Coleoptera). Experimental reduction of the winter snowpack caused shifts in community composition, driven by a small number of abundant arthropod taxa. These effects were apparent at a small spatial and temporal scale, with rapid recovery from experimental perturbation in spring. Mountain ecosystems are threatened by climate change as they are already rare at a landscape scale, are typically fragmented and have limited scope for climate tracking. The work presented here highlights effects of small-scale environmental variation on species traits, genetic structure and communities, which could act to either buffer or exacerbate landscape-scale climatic changes.