Quantitative Trait in Drosophila melanogaster

The P-M system of transposition in Drosophila melanogaster is a powerful mutator for many visible and lethal loci. Experiments using crosses between unrelated P and M stocks to assess the importance of transposition-mediated mutations affecting quantitative loci and reponse to selection have yielded unrepeatable or ambiguous results. In a different approach, we have used a P stock produced by microinjection of the ryro6 M stock. Selection responses were compared between transposition lines that were initiated by crossing M strain females with males from the “co-isogenic” P strain, and ryso6 M control lines. Unlike previous attempts to quantify the effects of P element transposition, there is no possibility of P transposition in the controls. During 10 generations f election for the quantitative trait abdominal bristle number, none of the four control lines showed any response to selection, indicative of isogenicity for those loci affecting abdominal bristle number. In contrast, three of the four transposition lines showed substantial response, with regression of cumulative response on cumulative selection differential ranging from 15% to 25%. Transposition of P elements has produced new additive genetic variance at a rate which is more than 30 times greater than the rate expected from spontaneous mutation. E VIDENCE for transposable elements in Drosophila initially came from studies of unstable mutations (GREEN 1967, 1969). Male recombination and unusual rates of mutations were discovered to be associated with certain wild second chromosomes (MR chromosomes) (HIRAIZUMI 1971; GREEN 1977). These chromosomes were later found to carry a subset of a family of transposable elements called P elements (KIDWELL, KIDWELL and SVED 1977; ENCELS 1989). The P-M system of transposition in Drosophila melanogaster has demonstrated powerful mutagenic effects for many visible and lethal loci (SIMMONS et al. 1980; ZUSMAN, COULTER and GERCEN 1985; YUKUHIRO, HARADA and MUKAI 1985; INCHAM et al. 1985; TsuBOTA and SCHEDL 1986; SUH and MUKAI 1987). Initial investigations of the influence of P transposition on quantitative genetic variation compared lines derived from crosses of unrelated P and M strains. Selection lines derived from “nondysgenic” (P9 X MS) crosses were used as negative controls for lines from “dysgenic” (M9 x PS) crosses, in which much more transposition was expected (MORTON and HALL 1985; MACKAY 1984, 1985; TORKAMANZEHI, MORAN and NICHOLAS 1988). This type of experiment has two disadvantages. First, it is now realized that transposition actually occurs in lines derived from both dysgenic and nondysgenic crosses, which means that these experiments have lacked a valid control (MACKAY ’ Present address: Faculty of Agriculture, University of Sistan and Baluchistdn, P.O. Box 538, Zabol, Iran. Genetics 131: 73-78 (May, 1992) 1988; PICNATELLI and MACKAY 1990; L ~ r ~ S I a n d MACKAY 1990; TORKAMANZEHI 1990). Second, since the P and M strains are unrelated, both types of cross produce lines that are heterozygous at many loci affecting the trait being selected. Even in the absence of transposition, substantial genetic variation is introduced into control and experimental lines. “Chromosome contamination” experiments, initially used to assess effects of transposition on fitness traits (YUKUHIRO, HARADA and MUKAI 1985; FITZPATRICK and SVED 1986; MACKAY 1986; EANES et al. 1988), are an alternative to selection experiments for analysing the effect of transposition on quantitative variation. For example, LAI and MACKAY (1 990) used compound X chromosomes to construct isogenic X chromosome lines in which the X chromosomes had been exposed to dysgenic, nondysgenic or control crosses. By partitioning of variance components rather than analysis of selection responses, they revealed that even one generation of transposition on one chromosome causes remarkably large increases in genetic variation for abdominal and sternopleural bristle number. A similar effect of transposition in relation to an autosome was reported by MACKAY (1987). In providing clear evidence for the effect of transposition on individual chromosomes, these studies complement selection experiments, which provide evidence in relation to the whole genome. In this paper, we report the results of a selection experiment conducted in such a way as to overcome 74 A. Torkamanzehi, C. Moran and F. W. Nicholas r ~ 10 P x m z Go Uplines ( U l , U 2 ) 6 0 1 2 10 M females Downlinea-(Dl,DP) 6 0 1 2 Uplines ( U l , U 2 ) e 010 < 10/50 I \ 1 Tran.polltlon Ll"..