Ecology and distribution of Alfalfa Mosaic Virus (AMV) and Trifolium repens for the ecological risk assessment of genetically modified AMV-resistant T. repens

Abstract

This thesis presents a study of the ecology, distribution and pathology of the pasture speciesTrifolium repens and its pathogen Alfalfa mosaic virus (AMV) in south-eastern (SE) Australia, to inform an ecological risk assessment of transgenic AMV-resistant T. repens. There are concerns worldwide regarding the environmental release of pathogen-resistant (P-R) pasture plants as pasture species have a history of invasiveness. The key concern is that following release from pathogen pressure, P-R plants may become weedy. The results of this work indicate that T. repens is naturalised in many habitats in SE Australia (at 59% of 213 sites visited in a 300,000 km{u00B2} study region). AMV was detected at 15% of sites, was not restricted by region or habitat type, and was significantly more likely to occur in naturalised T. repens populations that were abundant, close to agriculture, and/or disturbed. Coat protein (CP) analysis of AMV (83 isolates) from naturalised T. repens in SE Australia suggests that the population has little structure by geographic origin, host or community type, suggesting that AMV dispersal is largely human mediated in this region. None of the isolates tested had a CP nucleotide sequence identical to the transgene used for GM T. repens, but most (71%) possessed the same amino acid sequence. The remaining isolates had CP sequences that differed by up to four amino acids to the transgene. Transgenic T. repens, if grown in SE Australia, is likely to be exposed to the full suite of AMV variants observed. Therefore, prior to environmental release, resistance of transgenic T. repens to isolates representing the genetic diversity present in SE Australia should be evaluated. Naturalised and cultivated T. repens genotypes were susceptible to AMV infection and AMV generally reduced T. repens growth (mean reductions up to 35% ). Variability in the infectivity of AMV isolates and host-isolate specificity were observed; growth impacts varied depending on individual clover x virus combinations, indicating that AMV may be more important in reducing host population size for some clover genotypes than others. This thesis illustrates the complex nature of ecological risk assessments of widespread invasive pasture species and demonstrates the need for targeted habitat-and pathosystem-specific assessments. Completion of initial stages of this risk assessment suggest that AMV resistance is likely to increase the fitness of naturalised T. repens populations, and so AMV-resistant T. repens may pose a risk to some native habitats in SE Australia. Show more