THE EFFECT OF TEMPERATURE ON SEX-RATIO IN THE ISOPOD PORCELLIONIDES PRUINOSUS - ENVIRONMENTAL SEX DETERMINATION OR A BY-PRODUCT OF CYTOPLASMIC SEX DETERMINATION

The sex ratios of the progenies of woodlice Porcellionides pruinosus ( Crustacea, Isopoda) raised at different temperatures were studied. Fem ales from three French populations sampled in the wild produced highly female-biased broods at 20 degrees C and male-biased broods above 30 degrees C. The effect of high temperature was not due to selective mor tality of females. Sex determination was thus sensitive to temperature in P. pruinosus. We also found an interpopulation variability of sex ratio thermosensitivity and a weak inheritance of male-biased sex rati os at high temperatures. Samples taken from a wild population througho ut the year showed that while the thermal conditions required for chan ges in the sex ratio occurred, there was no significant variation in t he sex ratio. On the other hand, almost all the females and many males in the four populations studied harboured intracytoplasmic bacteria. These maternally inherited symbionts belong to the genus Wolbachia and are known to possess a feminizing effect. While in other arthropods W olbachia are destroyed at high temperatures, the symbionts of P. pruin osus were detected by a PCR procedure whatever the rearing temperature s. In light of these results, we propose that the thermosensitivity of sex determination in P. pruinosus could reflect the removal of the cy toplasmic effect on sex determination rather than environmental sex de termination sensu stricto. The reduction in the amount of bacteria (bu t not their entire elimination), or the inhibition of bacterial metabo lism, may be responsible for sex ratio variations relating to temperat ure. The incomplete inheritance of male-biased sex ratios at high temp eratures might reflect a selection of thermo-tolerant bacterial strain s.