Abstract
We found that the sex-ratio of an amphigenic strain of Sciara ocellaris varied widely from progenies with few males to progenies containing a larger proportion of males, with single-sex progenies being rare. The sex-ratio distributions were dependent on the temperature at which the stocks of flies were raised, with the sex-ratio distributions being symmetrical (i.e. about 50% males) at 18 °C and 20 °C while at the higher temperatures of 24 °C and 28 °C the distributions were skewed toward a high proportion of females with the mean proportion of males decreasing to about 30-37% per progeny. Temperature-shift experiments showed that high temperatures were effective only during the last stages of female pupal development plus a period after adult emergence, stages corresponding to oocyte maturation. When imagine females were exposed to temperatures as low as 12 °C the sex-ratio distributions of their progeny were skewed toward a high proportion of males per progeny. No differential fecundity was involved in these progeny sex-ratio modifications. Egg-to-adult survival was lower at 18 °C and 28 °C but no correlations with skewing in the sex ratio distributions were observed, indicating that modifications in progeny sex-ratio did not involve the differential survival of a particular sex.
Highlights
It is well demonstrated that a large number of sex determination mechanisms exist among insects (White, 1973; Bull, 1983; Nothiger and Steinmann-Zwicky, 1985)
The genes involved in this hierarchical set of sex determining genes and their functioning have been determined in Drosophila but differences do exist in other insects, among which are the sciarids Sciara ocellaris (Ruiz et al, 2003), Sciara coprophila, Rhynchosciara americana and Trichosia pubescens (Serna et al, 2004)
There are insect species in which the sex is not fixed at fertilization but is determined by the parental female by the production of oocytes containing factors that dictate the pathway for the ensuing activity of the cascade of sex determination genes
Summary
It is well demonstrated that a large number of sex determination mechanisms exist among insects (White, 1973; Bull, 1983; Nothiger and Steinmann-Zwicky, 1985). The genes involved in this hierarchical set of sex determining genes and their functioning have been determined in Drosophila (see reviews by Sanchez et al, 1994, 1998) but differences do exist in other insects (see Müller- Holtkamp, 1995; Sievert et al, 1997; Meise et al, 1998; Saccone et al, 1998), among which are the sciarids (fungus gnat flies) Sciara ocellaris (Ruiz et al, 2003), Sciara coprophila, Rhynchosciara americana and Trichosia pubescens (Serna et al, 2004) In these species, the Sex-lethal gene does not show differential expression in males and females. In these insects it has been demonstrated that a variety of biotic and abiotic agents like aging of the parental females, temperature, humidity and presence of symbionts seem to interfere with predetermination, leading to progenies with altered sexratios (Brown and Chandra, 1977)
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