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Environmental sex determination implies that sex is defined by environmental factors influencing the maternal organism and/or oocytes. It provides population sex ratios that will maximise the sustainability of the population under incipient environmental conditions. Environmental factors responsible for sex determination include temperature, photoperiod, food availability, and population density. These cues stimulate the release of chemical signalling molecules (i.e. hormones) that determine the sex of newborns.We have analysed the responses of three Daphnia pulex clones (I, K, and S), isolated from different natural populations, to juvenile hormone methyl farnesoate (MF) in terms of survivorship, fecundity, production of males and ephippia, and expression of genes involved in sex determination (dsx1) and hormonal regulation (JHAMT, Met, RXR). Clonal individuals were exposed to: (1) high MF concentration (0.8 µM) combined with stressful conditions; (2) low MF concentration (0.15 µM) combined with optimal conditions of population density and food availability in two subsequent generations.The genetically different clones of D. pulex differed in the propensity for male and ephippia production and in the sensitivity to the stimuli that lead to the switch from parthenogenetic to sexual reproduction.A significant induction of male production was observed in all three clones when exposed to MF. In conditions mimicking crowding, only clone S produced male offspring even in absence of MF and showed higher expression levels of JHAMT and dsx1 genes. This response suggests that clone S has a high propensity to produce males probably due to a high sensitivity of receptors to environmental stimuli activating the endogenous biosynthesis of MF. However, clone S exposed to high MF concentration produced fewer males than clones I and K, which generally have a low natural propensity to produce males. High MF concentration coupled with conditions mimicking crowding, probably activated a feedback mechanism in clone S via the differential modulation of genes involved in the synthesis as well in the degradation of MF.Exposure to low MF concentration for two subsequent generations resulted in a reduction of male progeny production from the first (G1) to the second (G2) generation. All analysed genes were up‐regulated in both generations treated with MF, but with lower expression in G2. The reduced up‐regulation of the dsx1 gene matched with the lower male production. Based on our results and literature data, we hypothesised that the maternal dsx1 mRNA probably plays some role early in oocyte development, and causes the eggs that receive the dsx1 signal to develop into males.Exposure to MF for two generations significantly increased ephippia production. These results suggest that the induction of male and ephippia might have a common root in the MF pathway. The effect of MF on life‐history traits (delay in the age at maturity and fecundity reduction), sex ratio, and ephippia production suggests a direct impact of the juvenile hormone on the level of population growth. Thus, as a consequence of the switch from parthenogenetic to sexual reproduction, MF influences Daphnia population metrics and in this way probably maximises the sustainability of the population under variable environmental conditions. [ABSTRACT FROM AUTHOR]

Abstract: In a laboratory-selected Cr-tolerant strain of the unicellular green alga Scenedesmus acutus, the capacity to synthesize higher amounts of cysteine (Cys) and reduced glutathione (GSH) than the wild-type was demonstrated to underlie tolerance to Cd and Cr(VI). In photosynthetic organisms sulfate constitutes the main sulfur source for the biosynthesis of GSH and its precursor Cys, hence it was hypothesized that the sensitivity of the two strains to Cr(VI) could be modified after culturing in sulfate-deprived medium. Both strains were grown in the presence of different concentrations or in the absence of sulfate (sulfur-starved) and then assayed for Cr(VI) tolerance in standard medium. Unstarved, sulfur-starved and sulfur-replete cells (cells maintained in standard medium after S-starvation) were analysed for Cys, GSH and sulfur content. Sulfur-starved cells showed a greater tolerance to Cr(VI) than unstarved ones. The increased tolerance was ascribable to a transient physiological change and can be considered as specifically due to sulfur deprivation, since it was lost after a 3-day culture in standard medium and was not exhibited by nitrogen-starved cells. The comparison between Cys, GSH and sulfur content in sulfur-starved and sulfur-replete cells of the two strains suggests that the higher tolerance to Cr(VI) after S-starvation could depend on the up-regulation of sulfate uptake mechanisms, and that the primary reason for the higher tolerance to chromium in the selected strain could be due to greater sensitivity to the decrease in negative intracellular end-products (free Cys and GSH) leading to an earlier up-regulation of sulfate assimilation processes. [Copyright &y& Elsevier]

Abstract: The gene expression of the wild type (S2-N) and a Cr-tolerant strain (S2-T) of the unicellular green alga Scenedesmus acutus has been compared in order to get more insight on their different chromium sensitivity. The RNA of the two strains was extracted after 4 days of culture in standard medium without chromium and analyzed by means of RNA differential display. The two strains showed differential gene transcription even in the absence of the heavy metal and six putatively differential amplicons were evidenced in the Cr-tolerant strain. Among the isolated amplicons, S 2 -T A63 was much more pronouncedly transcribed in the tolerant than in the wild type strain and was further characterized. S 2 -T A63 corresponding gene is present with the same copy number in the wild type and tolerant genomes and corresponds to an mRNA of about 2000nt. The corresponding transcript is overexpressed in the Cr-tolerant strain after a 4-day culture and is not up-regulated by chromium exposure. The S 2 -T A63 sequence, obtained up to now, does not show significant homologies with any known gene. However, the analysis of the putative translation product reveals the presence of an interrupted fasciclin domain. This extracellular domain has been found in proteins from mammals, insects, echinoderms, plants, yeast and bacteria and is usually involved in cell adhesion. This finding suggests that the product for the S 2 -T A63 translation has an extracellular collocation, maybe as surface or secreted protein involved in external chromium detoxification. [Copyright &y& Elsevier]

The gene expression of the wild type (S2-N) and a Cr-tolerant strain (S2-T) of the unicellular green alga Scenedesmus acutus has been compared in order to get more insight on their different chromium sensitivity. The RNA of the two strains was extracted after 4 days of culture in standard medium without chromium and analyzed by means of RNA differential display. The two strains showed differential gene transcription even in the absence of the heavy metal and six putatively differential amplicons were evidenced in the Cr-tolerant strain. Among the isolated amplicons, S2-T A63 was much more pronouncedly transcribed in the tolerant than in the wild type strain and was further characterized. S2-T A63 corresponding gene is present with the same copy number in the wild type and tolerant genomes and corresponds to an mRNA of about 2000 nt. The corresponding transcript is overexpressed in the Cr-tolerant strain after a 4-day culture and is not up-regulated by chromium exposure. The S2-T A63 sequence, obtained up to now, does not show significant homologies with any known gene. However, the analysis of the putative translation product reveals the presence of an interrupted fasciclin domain. This extracellular domain has been found in proteins from mammals, insects, echinoderms, plants, yeast and bacteria and is usually involved in cell adhesion. This finding suggests that the product for the S2-T A63 translation has an extracellular collocation, maybe as surface or secreted protein involved in external chromium detoxification.