Sex differentiation in the endangered Neotropical species Steindachneridion parahybae (Siluriformes: Pimelodidae): a cytogenetic and morphological study

HONJI, R. M.; MEDRADO, A. T.; MAZZONI, T. S.; CANEPPELE, D.; MOREIRA, R. G.

Olhão

Simposio; 10th International Symposium on Reproductive Physiology of Fish; 2014

Introduction: Fish are extremely versatile and with various patterns of sex determination/differentiation, including gonocorists and hermaphrodite animals. Several fish species with temperature sex determination (TSD) have been proposed, and may be regarded as biomarkers of biological impacts on global warming, since changes in temperature induced sex ratio shifts, which are a direct response to the environmental thermal fluctuations. Steindachneridion parahybae is a freshwater catfish endemic to the Paraíba do Sul River Basin, Brazil, classified as an endangered Neotropical species. We investigated the sex differentiation in S. parahybae during the first hundred days after hatching (DAH), reared in different temperatures (19º, 24º and 29ºC). Methods: Artificial breeding of S. parahybae broodstock was performed at the CESP fish farm and the fertilized eggs transferred to an aquarium with water recirculation in the IB-USP biotherium, with controlled temperature (19º, 24º and 29ºC) and photoperiod (14:10, L:D). During the first ten DAH, five animals were sampled; thereafter, five animals were sampled every three DAH, until the one hundreth DAH. The animals sampled were fixed in Bouin solution; serial sections (8μm) were obtained and then stained with haematoxylin/eosin, periodic-acid-Schiff (PAS)/haematoxylin/metanil yellow. Additionally, karyotypic data were obtained from 2 females and 2 males of S. parahybae. Results and Discussion: This species showed a conserved diploid number of 2n = 56 chromosomes, with no morphological evidence of differentiated sex chromosomes or supernumerary chromosomes. The best temperatures for the development of this catfish were 24º and 29ºC and the lower temperature (19ºC) was limited by stage of the development of this species, since abnormal development observed. Canibalism rates increased at higher temperatures and decreased at lower temperatures. Undifferentiated gonads were observed until the sixtieth to the sixty fifth DAH (29ºC); until the seventieth to the eightieth DAH (24ºC); and until the sixty seventh DAH (19ºC). The beginning of the ovarian differentiation was identified from the seventieth to the ninetieth DAH (29º) and from the seventieth to the eightieth (24ºC), with initial formation of the ovarian cavity (extention of ovary wall). Furthermore, no extention of gonad wall was observed in testes differentiation (eightieth to ninetieth DAH (29ºC); and seventieth to seventy eighth DAH (24ºC)). Additionally, when rearing in a different temperature, S. parahybae showed a different sex ratio. On the other hand, probably no ?thermo sensitive period? was observed. Conclusion: It can be considered that this catfish evolved genotypic sex determination but with influence of temperature in this process. Furthermore, this data provide important tools for further investigations of reproduction of this threatened catfish.