Isolation of the sex-determining gene Sex-lethal (Sxl) in Penaeus (Litopenaeus) vannamei (Boone, 1931) and characterization of its embryogenic, gametogenic, and tissue-specific expression.

The Pacific white shrimp Penaeus vannamei is the most cultured shrimp species around the world. Because females grow larger than males, the culture of 'only females' is of great interest, but knowledge on sex determination and differentiation is required for producing only females. In an effort to obtain information associated with reproduction in P. vannamei, transcriptomic data from female gonads was generated, and partial sequences of a transcript were identified as Sex-lethal (Sxl). Its characterization indicated that, differently from other penaeids in which this gene has been isolated, there are six isoforms of the Sxl transcript in P. vannamei (PvanSxl 1-6). These isoforms result from alternative splicing at three splice sites (SS1, SS2, SS3). The first splice-site is unique to P. vannamei, as it has not been reported for other Arthropod species; the second splice-site (SS2) is common among crustaceans, and the third splice-site (SS3) is also unique to P. vannamei and when spliced-out, it is always together with SS2. All isoforms are expressed during embryogenesis as well as gametogenesis of both genders. The two shorter isoforms, PvanSxl-5 and PvanSxl-6, which result from the splicing of SS2 and SS3, were found mostly expressed in adult testis, but PvanSxl-6 was also expressed in oocytes during gametogenesis. During oogenesis, the second largest isoform, PvanSxl-2, which splices-out only SS1, and PvanSxl-4 that splices-out SS1 and SS2 were highly expressed. These two isoforms were also highly expressed during embryonic development. In situ hybridization allowed pinpointing more specifically the cells where the PvanSxl transcripts were expressed. During embryogenesis, hybridization was observed from the one-cell stage embryo to late gastrula. In the female gonad in previtellogenesis, hybridization occurred in the nucleus of oocytes, whereas in secondary vitellogenesis the transcript also hybridized cytoplasmic granules and cortical crypts. Finally, in situ hybridization corroborated the expression of PvanSxl also in the male gonad during spermatogenesis, mostly occurring in the cytoplasm from spermatogonia and spermatocytes.

Assessment of metallothioneins in tissues of the clam Megapitaria squalida as biomarkers for environmental cadmium pollution from areas enriched in phosphorite.

The aim of this study was to evaluate the use of metallothionein (MT) concentrations in tissues of the clam Megapitaria squalida as biomarkers of environmental cadmium (Cd) pollution from phosphorite enrichments in the marine environment, which resulted from mining activities in La Paz Bay, Baja California Sur, Mexico. Cd and MT were quantified in gills, digestive gland, and kidney of clams exposed to 0.2 or 0.5 mg Cd l(-1) for 10, 20, or 30 days. In addition, clams from four strategically selected natural sites of La Paz Bay were collected for analysis. In tissues of bioassayed and untreated clams, the gradient of Cd concentrations was digestive gland>>gills>kidney, whereas that of MT was digestive gland>gills>kidney. Digestive gland of the clams exposed to 0.5 mg Cd l(-1) for 30 days showed the highest concentrations of Cd (16.3+/-3.9 microg Cd g(-1)). The highest statistically significant MT concentrations were found in digestive gland at 10 days of exposure to Cd. In the untreated clams, one of the highest Cd concentrations, but not MT levels, was found in digestive glands of the organisms collected from the area close to phosphorite mining activities. For environmental monitoring, MT levels in digestive gland can be used as a first approximation of the presence of high levels of divalent metals in the environment. However, in this study, MT levels did not correlate with high Cd levels in clams that had been collected from areas associated with phosphorite enrichment.