The effects of a chromosomal inversion on adult size and male mating success in the seaweed fly, Coelopa frigida

Abstract

SUMMARYAn association is reported between the a//3 inversion polymorphism on chromosome I and adult size as assessed by the length of wings. aa flies are larger than (3/3 flies, with heterokaryotypes intermediate, and the differences are more marked in males than in females. Laboratory mating experiments were performed in which a single female was given a choice of two males. By examining the genotypes of the progeny larvae, it is shown that the larger male is successful in a significantly greater proportion of trials than the smaller one. This mating success is dependent on the size difference between the males and on the female size. Together these observations suggest an indirect influence of the inversion on male mating success. The possible relevance of this effect to the maintenance of the inversion polymorphism in natural populations is discussed.

Female mating preference for large size in Coelopa frigida (seaweed fly)

“…The males were stored at -25°C and subsequently their size measured and their Adh genotype determined using starch gel electrophoresis (Butlin et al, 1982b). Sizes of adults were determined by measuring wing length, which is strongly correlated with other body dimensions (see Butlin et a!., 1982b). After the 2-day egg-laying period the females were treated in the same manner as the males.…”

“…At the end of the trials the females were placed individually onto freshly minced seaweed for 2 days, and allowed to oviposit. The males were stored at -25°C and subsequently their size measured and their Adh genotype determined using starch gel electrophoresis (Butlin et al, 1982b). Sizes of adults were determined by measuring wing length, which is strongly correlated with other body dimensions (see Butlin et a!., 1982b).…”

“…As the chromosome I inversion system is very large, comprising approximately 10 per cent of the entire genome (Aziz, 1975;Day et a!., 1982), it is difficult to distinguish between the effects of size, the Adh locus and the inversion system. There is known to be strong linkage disequilibrium associated with this inversion system (Collins, 1978;Day et a!., 1982Day et a!., , 1983, no recombination in heterokaryotypes or in males (Day et a!., 1982), and strong heterokaryotypic advantage (Butlin et al, 1982b). Under these circumstances there would seem to be the ideal situation for the operation of a viability indicator system.…”

Adult size, longevity and fecundity in the seaweed fly, Coelopa frigida

“…Much of this variation is due to competition amongst larvae (Butlin and Day, 1984), but there is also an inherited component of variation, in males at least, associated with the a/f3 inversion polymorphism on chromosome I (Butlin et a!., 1982). Under laboratory conditions large males have greater mating success than small males (Butlin et al, 1982).…”

The evolution of female choice when the preference and the preferred trait are linked to the same inversion system

“…There is considerable variation in adult size and natural populations exhibit high coefficients of variation in both sexes (8.6-19.4 in males; 5.2-13.4 in females), although in every population males are more variable than females (Gilburn & Day, 1994b). Under controlled laboratory conditions, adult size has been shown to have a heritability between father and son of about 70 per cent (Wilcockson et al, 1995), and much of the genetic variation can be attributed to the cfl inversion system on chromosome I (Butlin et al, 1982;Gilburn & Day, 1994b;Wilcockson et al, 1995). homokaryotypes are the largest, ff3s the smallest, and /3s are intermediate. A similar pattern exists in females, although the differences between the kaiyotypes are considerably smaller.…”