Abstract

We examined the role of genetics of rabbit populations in the development of resistance to myxomatosis. We compared the responses of progeny from rabbit populations of Australian arid, mediterranean and subalpine regions when challenged at two ambient temperatures by plaque-purified virus of two myxoma strains of contrasting virulence, Uriarra and Lausanne. Wild rabbit populations have diverged in genetic resistance to myxomatosis since the rabbit was introduced to Australia 125 years previously. Founder effects and natural selection are possible causes. Rabbits from the warm dry regions died of Lausanne strain infection more slowly than rabbits from the cool moist region. Genetic resistance to myxomatosis probably diverged regionally as epizootics selected rabbit populations during the 34 years since the myxoma virus was introduced. Adaptation of rabbit populations to different environments might also have contributed to diverging resistance. Mortality was similar among regional groups and did not correlate with survival time, suggesting either polymorphism within populations, a prerequisite of natural selection, or variation in non-genetic immunity. Warmer conditions did not affect mortality but extended survival times. Therefore myxoma strains predominating in warm regions may be more virulent than those in cooler regions. Mean temperature may cause levels of genetic resistance to vary among regions. Mortality rates were less than those observed previously. Uriarra strain killed < 5% of infected rabbits compared to 35% previously. Lausanne strain killed 88%, formerly 98%. Selection during storage or plaque-purification may have decreased viral virulence, and recent natural selection may have increased genetic resistance of rabbits to myxomatosis.