TRANSPOSABLE ELEMENT-INDUCED RESPONSE TO ARTIFICIAL SELECTION IN DROSOPHILA MELANOGASTER

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Abstract
The P family of transposable elements in Drosophila melanogaster transpose with exceptionally high frequency when males from P strains carrying multiple copies of these elements are crossed to females from M strains that lack P elements, but with substantially lower frequency in the reciprocal cross. Transposition is associated with enhanced mutation rates, caused by insertion and deletion of P elements, and chromosome rearrangements. If P element mutagenesis creates additional variation for quantitative traits, accelerated response to artificial selection of progeny of M♀♀ × P♂♂ strain crosses is expected, compared with that from progeny of P♀♀ × M♂♂ strain crosses.—Divergent artificial selection for number of bristles on the last abdominal tergite was carried out for 16 generations among the progeny of P–strain males (Harwich) and M–strain females (Canton-S) and also of M–strain males (Canton-S) and P-strain females (Harwich). Each cross was replicated four times. Average realized heritability of abdominal bristle score for the crosses in which P transposition was expected was 0.244 ± 0.017, 1.5 times greater than average heritability estimated from crosses in which transposition was expected to be rare (0.163 ± 0.010). Phenotypic variance of abdominal bristle score increased by a factor of four in lines selected from M♀♀ × P♂♂ crosses when compared with those selected from P♀♀ × M♂♂ hybrids. Not all quantitative genetic variation induced by P elements is additive. A substantial fraction of nonadditive genetic variation is implicated by chromosomal analysis, which demonstrates deleterious fitness effects of the mutations when homozygous.—Several putative "quantitative" mutations were identified from chromosomes extracted from the selected lines; these will form the basis for further investigation at the molecular level of the genes controlling quantitative inheritance.