Abstract
Monogeny, the production of unisexual broods by individual females, is widely recorded in gall midges (Diptera: Cecidomyiidae). Theoretical models propose that the adjustment of offspring sex ratio by females may pre-dispose the evolution of monogeny in gall midges however empirical studies in this field are superficial. Expressed more simply, monogeny may enable individual female gall midges to decrease or increase the number of male and female progeny they produce in response to changes in environmental conditions. Host quality/size is repeatedly reported to influence sexual investment in insects in terms of sex ratio adjustment. In this paper, we examined the sex ratio of the offspring of the monogenous predatory gall midge Aphidoletes aphidimyza attacking low and high abundances of the cotton aphid, Aphis gossypii. Two consecutive generations of female gall midges were presented with either a low or high abundance of aphids in each generation and the sex ratio of their progenies determined. There was no difference between the sex ratio of the progenies that developed on the high or low abundance of A. gossypii in the two generations. Apparently, the females did not regulate the number of female and male progenies, or adjust the numbers of male or female's eggs they produced in response to the changes in the abundance of prey. Results of this study do not support the theory of sex ratio regulation proposed for monogenous gall midges. It is likely that the skewed sex ratio in most species of monogenous gall midges is a consequence of differential mortality of male and female progenies under harsh conditions. The finding that male and female larvae did not differ in the number of aphids they require to complete their development supports this claim.
Highlights
Gall midges (Diptera: Cecidomyiidae) belong to one of the largest groups of flies with several species of economic importance in agricultural and forest ecosystems (Barnes, 1946; Gagne, 1994)
The occurrence of female biased sex ratios are repeatedly reported for various species of gall midges (Baxendale & Teetes, 1981; Matuszewski, 1982; McClay, 1996; Dorchin & Freidberg, 2004; Rajamani et al, 2004; Shahu et al, 2004; Smith et al, 2004; Ogah et al, 2010), studies on A. aphidimyza indicate that in this species they are slightly male biased in both laboratory and commercial populations (Gilkeson & Hill, 1986; Heimpel & Lundgren, 2000; current study, see Fig. 1)
The sex ratio of the wheat midge Sitodiplosis mosellana is biased toward females by 6–11% as a result of differential mortality during diapause (Smith et al, 2004)
Summary
Gall midges (Diptera: Cecidomyiidae) belong to one of the largest groups of flies with several species of economic importance in agricultural and forest ecosystems (Barnes, 1946; Gagne, 1994). Monogeny in gall midges is a consequence of their unique sex determination system (Stuart & Hatchett, 1991), in which a single maternal-effect autosomal gene (chromosome maintenance gene, Cm) prevents the elimination of the paternally derived X chromosome during embryogenesis, so that all of the offspring of Cm-bearing mothers have a female-determining karyotype. A large number of empirical studies, mainly on haplodiploid Hymenoptera, have shown that parents are able to adjust the sex ratio of their progenies in response to environmental conditions (Charnov, 1982; Werren, 1984; King, 1993; Morrill et al, 2000; Ode & Heinz, 2002; Kuijper & Pen, 2010; Kishani Farahani et al, 2012). In which both males and females develop from fertilized eggs, adjustment by parents of the sex ratio of their offspring seems unlikely (Bull, 1983; Dorchin & Freidberg, 2004)
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