Crowding stress during the period of sex determination causes masculinization in pejerrey Odontesthes bonariensis, a fish with temperature-dependent sex determination

Abstract

The pejerrey is an atherinopsid species from South America that presents a combination of genotypic and environmental (temperature-dependent) sex determination whereby low and high temperatures induce feminization and masculinization, respectively. Masculinization involves a heat-induced stress response leading to increased circulating cortisol and androgens. We tested whether crowding would elicit a similar response as high temperature and affect the sex ratios of pejerrey. Larvae with XX and XY genotypes were reared at 15, 62 and 250 larvae/L in 0.4, 1.6, and 6.4 L containers during a period considered critical for sex determination at 25 °C, a mixed-sex promoting temperature. Fish were analysed at 3–7 weeks for whole-body cortisol and 11-ketotestosterone (11-KT) titer and hydroxy-steroid dehydrogenase (hsd11b2) mRNA transcript abundance, and after completion of gonadal sex differentiation (10–14 weeks) for determination of phenotypic and genotypic sex mismatches. Crowding was associated with depressed growth, higher cortisol and 11-KT titers, increased hsd11b2 transcription, and increased frequency of masculinization compared to intermediate and/or low rearing densities. Perceived crowding (by rearing in containers with mirror-finish, reflecting walls) also caused masculinization. These results suggest the possibility that other environmental factors besides temperature can also affect sex determination in pejerrey and that a stress response leading to increased cortisol and androgen levels, which is potentially perceived by the brain, may be a common feature among different forms of environmental sex determination in this species.