Genetic variation in a modifier system affecting the expression of Bare mutant of Drosophila subobscura

Abstract

Genetic variation affecting the expression of Bare (Ba), a morphological mutant of Drosophila subobscura that reduces the number of bristles and is located on the 0 chromosome of this species, is reported. Our results show that 0 chromosomes extracted from a natural population (El Pedroso, Santiago de Compostela, Spain) show considerable genetic variation in modifier effect upon Ba expression. The amount of modifier variability is dependent on the chromosomal arrangement (OsT and 03÷4+7), since modifier variation is higher in °ST than in 03÷4+7 chromosomes. Investigations of the genetic architecture of this modifier system on the 0 chromosome carried out using biometrical methods indicate that a relatively small number of genetic "factors" can explain the differences in modifier effect between a wild 0 chromosome of high modifier effect and a marker chromosome of low score. In addition, the modifier effects show a non-uniform distribution along the 0 chromosome and some indication of clustering of modifier "factors" around the major locus (Ba) is also observed.

Measuring selective effects of modifier gene polymorphisms on the Bare locus of Drosophila subobscura

“…The modifier effect of these wild 0 chromosomes on the phenotypic expression of the Bare mutant caused by modifier polygenes located on the 0 chromosome was also evaluated by measuring the difference in mean number of bristles between Ba! + and Va/Ba individuals (see Alvarez et al, 1990). Twelve bristles per individual (four scutellars, four dorsocentrals, two supra-alars and two postalars) of 30 BaI+ and 30…”

“…Bare is a dominant mutant located on the 0 chromosome (map position 54.7 cM) of this species that reduces variably the number of bristles of the fly and is lethal in homozygous condition (Koske & Maynard Smith, 1954;Sperlich et a!., 1977;Loukas et a!., 1979). It has been demonstrated that 0 chromosomes derived from 404 natural populations show considerable genetic variation in modifier effect upon Ba expression (Alvarez et al, 1980(Alvarez et al, , 1990. The analysis of the genetic architecture of this modifier variability carried out using biometrical techniques for locating and mapping polygenes showed that the differences in modifier effect between a wild 0 chromosome of high modifier effect and a marker chromosome of low score can be explained by a relatively small number of polygenic modifier factors (Alvarez et al, 1981(Alvarez et al, , 1990.…”

“…The analysis of the genetic architecture of this modifier variability carried out using biometrical techniques for locating and mapping polygenes showed that the differences in modifier effect between a wild 0 chromosome of high modifier effect and a marker chromosome of low score can be explained by a relatively small number of polygenic modifier factors (Alvarez et al, 1981(Alvarez et al, , 1990. In addition, these modifier factors show a nonuniform distribution along the 0 chromosome and some indication of clustering around the major locus (Ba) (Alvarez et a!., 1981(Alvarez et a!., , 1990). This particular genetic architecture suggests that the Ba locus together with the modifiers probably constitute a coadapted gene complex in such a way that this multilocus complex could be surely considered as a unit of selection.…”

“…In fact, experimental evidence shows that high levels of modifier genetic variability frequently occur in natural populations of species such as Drosophila. Thus, polygenic modifiers affecting the phenotypic expression of morphological mutants (major loci) have been detected in many natural populations (Milkman, 1970;Thoday & Thompson, 1984;Thompson & Spivey, 1984;Alvarez et a!., 1990). At the biochemical level, naturally occurring genetic variation of specific *Correspondence enzyme activities caused by activity modifiers has been reported for a large number of enzymes coded by structural loci (see Laurie-Ahlberg, 1985, for a review).…”

“…It has been demonstrated that 0 chromosomes derived from 404 natural populations show considerable genetic variation in modifier effect upon Ba expression (Alvarez et al, 1980(Alvarez et al, , 1990. The analysis of the genetic architecture of this modifier variability carried out using biometrical techniques for locating and mapping polygenes showed that the differences in modifier effect between a wild 0 chromosome of high modifier effect and a marker chromosome of low score can be explained by a relatively small number of polygenic modifier factors (Alvarez et al, 1981(Alvarez et al, , 1990. In addition, these modifier factors show a nonuniform distribution along the 0 chromosome and some indication of clustering around the major locus (Ba) (Alvarez et a!., 1981(Alvarez et a!., , 1990).…”