Temperature and the progeny sex-ratio in Sciara ocellaris (Diptera, Sciaridae)

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.

Sex-determining mechanisms in insects

“…It has been shown that this variation depends on temperature: at 18-20°C, the sex ratio distribution, although variable, shows a median at approximately 50%, but at 24-29°C the sex ratio moves towards the production of more females. This change in sex ratio is not caused by a higher mortality among males, but by a transformation of male into females (Nigro, 1995;Campos et al, 1996;Nigro et al, 2007). This increase in female production is due to an increase in the number of embryos that eliminate one instead of the two paternally-derived chromosomes.…”

“…This increase in female production is due to an increase in the number of embryos that eliminate one instead of the two paternally-derived chromosomes. Temperature-shift experiments have shown that the temperature-sensitive period for the determination of the final sex ratio is from the mid-pupa stage to the emergence of the adult females (Nigro, 1995;Nigro et al, 2007), the period during which oogenesis takes place (Berry, 1941). It should be recalled that in Sciara females the number of oocytes is fixed during the early larval stages and that no further mitosis occurs nor are new oocytes produced in the pupal/adult stages (Berry, 1941).…”

Sex-Determining Mechanisms in Insects Based on Imprinting and Elimination of Chromosomes

“…This change in sex ratio is not caused by a higher mortality among males, but by a transformation of males into females; this fact results in an increase in the number of embryos that eliminate 1 instead of the 2 paternally derived X chromosomes [Nigro et al, 2007]. Temperature-shift experiments have shown that the temperature-sensitive period for the determination of the final sex ratio is from the mid-pupa stage to the emergence of adult females [Nigro et al, 2007], the period during which oogenesis takes place [Berry, 1941]. Hence, S. ocellaris females produce at distinct temperatures different ratios of oocytes predetermined to eliminate either 1 or 2 X chromosomes.…”

Sciara as an experimental model for studies on the evolutionary relationships between the zygotic, maternal and environmental primary signals for sexual development

“…This change in sex ratio is not caused by a higher mortality among males, but by a transformation of male into females; i.e., an increase in the number of embryos that eliminate one instead of two paternal-derived X chromosomes (Nigro et al 2007). Temperature-shift experiments have shown that the temperature-sensitive period for the determination of the final sex ratio is from the mid-pupa stage to the emergence of adult females (Nigro et al 2007), the period during which oogenesis takes place (Berry 1941). Hence, the S. ocellaris females produce at distinct temperatures different ratios of oocytes that either contain or do not contain the maternal factor for X chromosome elimination.…”