Studies on sex determination and sex differentiation in the cichlid fish Cichlasoma dimerus (Teleostei, Perciformes)

F. MEIJIDE; M. PANDOLFI; G. REY VÁZQUEZ; Y. PIAZZA; M. C. MAGGESE; G. GUERRERO

Kona, USA

Simposio; 5th International Symposium on Vertebrate Sex Determination; 2009

Society for Developmental Biology

Teleost fishes are extremely useful as basic vertebrate models in which to study the processes of sex determination and sex differentiation. Although often used interchangeably, a clear distinction between these two terms must be done. Sex determination refers to the mechanisms that direct sex (gonadal) differentiation, while sex differentiation refers to the development of ovaries or testes from the undifferentiated bipotential gonads. Since most teleost species have external fertilization, it is simple to obtain hundreds of synchronized embryos and larvae that develop externally, allowing direct examinations. Also, large numbers of individuals can be treated easily in experimental manipulations, including larval exposure to chemicals added to the water. In particular, the lability of sex determination in fish makes some species sensitive to environmental pollutants capable of mimicking or disrupting the action of sex hormones. The freshwater cichlid fish Cichlasoma dimerus is a native species that has been extensively used in our laboratory as a model for developmental studies. In particular, several aspects of sex determination and gonadal sex differentiation have been addressed. The analysis of gonadal development by means of light and transmission electron microscopy revealed that C. dimerus is a differentiated gonochorist, in which ovaries and testes develop directly from undifferentiated gonads. Germ cells within the genital ridge (primordial undifferentiated gonad) are located inside an inpocketing of the coelomic epithelium. Ovarian differentiation precedes testicular differentiation, as usual in teleosts. Beginning towards day 42 postfertilization (PF), ovarian differentiation is marked by the appearance of germ cell meiotic figures, and the subsequent formation of the ovarian cavity. In contrast, signs of histological differentiation in the presumptive testis are not observed until day 72 PF. In order to analyze the involvement of the central nervous system in the process of sex differentiation, the activity of the hypothalamus-pituitary axis, via gonadotropin releasing hormone (GnRH) and gonadotropins, was assessed in relation to gonadal differentiation. In C. dimerus, FSH producing cell in the adenohypophysis appear towards day 23 PF, i.e. previous to the onset of sex differentiation, while LH cells are not detected until day 60 PF. A temporal correlation between the innervation of pituitary cells by GnRH fibers and the onset of gonadal sex differentiation was evidenced in both sexes. In addition, when gonadal explants of 30 day old larvae were left alone in culture media for 15 days, the onset of gonadal differentiation did not proceed as observed in vivo, suggesting a role of the brain-pituitary axis in sex differentiation. In contrast, when treated with FSH, 80% of the gonads showed signs of differentiation, including the onset of germ cell meiotic activity. Francis and Barlow’s hypothesis regarding social control of primary sex differentiation, with maleness resulting from large relative size at the juvenile stage, was tested in C. dimerus. Our findings indicate that the relative size has no influence on sex determination in this species. In another set of experiments, the effect of xenoestrogens on gonadal sex differentiation was analyzed. Exposure of larvae to waterborne 17â-estradiol had a pronounced estrogenic effect, as revealed by sex ratios being completely skewed towards the females. Exposure to environmentally relevant concentrations of octylphenol (biodegradation product of a group of non-ionic surfactants used in domestic and industrial activities) had a moderate estrogenic effect, as evidenced by the appearance of a number of individuals exhibiting an intersex condition (testis-ova). Finally, cytogenetic studies established that, as in most teleosts, heteromorphic sex chromosomes are absent in C. dimerus. However, based on sex ratios obtained in the laboratory, we assume that sex is probably under genetic control, as demonstrated in other cichlids. Studies involving back-crossing experimentally sex reversed individuals to normal ones could be performed in order to infer whether this species presents male or female heterogamety.