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Reduction in fitness of flea beetles which are homozygous for an autosomal gene conferring resistance to defences in Barbarea vulgaris

Major resistance genes are present in Danish flea beetle (Phyllotreta nemorum) populations, enabling the beetles to utilize a defended plant, Barbarea vulgaris ssp. arcuata, as a host plant, whereas this plant is unsuitable for beetles lacking the resistance genes. Two lines of beetles carrying a resistance gene have been established which are near-isogenic with a susceptible line. Larval survival of offspring from crosses between flea beetles carrying resistance genes and susceptible beetles, tested in bioassays on the defended B. vulgaris, and sex ratios of the survivors, were consistent with the presence of a dominant, autosomal resistance gene in each of the lines. An attempt to produce pure-breeding lines for the autosomal genes revealed that beetles that are homozygous for the resistance gene suffer a high mortality. This result was repeatable for both lines, and when both resistant males and females were used in the crosses. The high mortality was also independent of the plant (defended B. vulgaris or suitable radish) on which the beetles were reared. The results suggest that the time of death of homozygous resistant beetles is variable. The spread and maintenance of resistance genes in flea beetle populations are discussed.