Analysis of sexually dimorphic expression of genes at early gonadogenesis of pejerrey Odontesthes bonariensis using a heterologous microarray

The process of morphological development of a differentiated gonad from an undifferentiated primordium is a very important step of gonadogenesis. Studies on sexually dimorphic gene expression are important to increase our understanding of this process and to investigate how environmental factors such as temperature can regulate gonadal development. The aim of this study was to identify putative genes involved in sex differentiation in pejerrey (Odontesthes bonariensis) reared at male- and female-producing temperatures (MPT and FPT, respectively) using a microarray heterologous from the medaka (Oryzias latipes), a closely phylogenetic species. Genes related to numerous processes presented higher expression at MPT, including those involved in muscular contraction, metabolic pathways, developmental processes, and reproduction. Genes induced by FPT were classified under the gene ontology terms of response to stimulus, transport and proteolysis. From genes selected for validation, at MPT ndrg3 expression was observed in the somatic cells, whereas pen-2 was detected in germ cells in the caudal portion of the gonads, where no apoptotic signals were observed. Finally, hsp90 was highly expressed in somatic cells of the gonads at the FPT. The results suggest that the interplay of pro-apoptotic and anti-apoptotic genes is important during the masculinization process and for the prevention of sterility following exposure to warm temperatures.

Establishment of a strain inheriting a sex-linked SNP marker in Patagonian pejerrey (Odontesthes hatcheri), a species with both genotypic and temperature-dependent sex determination

The Patagonian pejerrey Odontesthes hatcheri is an atherinopsid species presenting genotypic sex determination (GSD) at intermediate temperatures and temperature-dependent sex determination at the low and high ranges of thermal tolerance. A recent study revealed the presence of a sex-linked SNP marker in some males of this species, but a strain which inherits the marker faithfully has not been established. This research was conducted to develop such a strain, for use as a tool to study the molecular mechanisms of gonadal sex differentiation and sexual dimorphism, and to obtain basic information on the GSD mode in this species. For these purposes, we performed backcrosses and full-sibling crosses using males and females whose presumptive genotypic sex was inferred from the presence of the sex-linked SNP marker. Four backcrosses between SNP(-) daughters and their SNP(+) father generated balanced sex ratios with the phenotypic sex matching the genotypic sex in most cases (98.21%) at an intermediate, sexually neutral temperature (21 degrees C). Full-sibling crosses between these four SNP(-) females and their SNP(+) brothers produced three progenies with balanced sex ratios and one with 94.4% males. The results of this study confirm that a strain inheriting the sex-linked SNP marker was successfully developed. Moreover, the inheritance pattern of the marker and the sex ratios of the progenies provide strong evidence that the GSD mode in O. hatcheri is the XX-XY system.

Experimentally induced depletion of germ cells in sub-adult Patagonian pejerrey (Odontesthes hatcheri)

Germ cell (GC) transplantation (GCT) is a novel reproductive technology with application in seed production and conservation of endangered species. This study examined the suitability of treatment with Busulfan, a cytotoxic agent, and warm water, known to cause GC degeneration, for depletion of endogenous GCs in sub-adult Patagonia pejerrey Odontesthes hatcheri intended as hosts in GCT. In two experiments, fish were treated with six combinations of temperature (intermediate and high, 20 and 25 degrees C, respectively) and Busulfan (0, 20, and 40 mg/kg body weight), given intraperitoneally (ip) as a single (0 week) or repeated (0 and 4 week) dose. The effectiveness of the treatments was assessed by gonado-somatic index, histology, and (germ cell-specific) vasa gene expression after 8 weeks. Fish were allowed to recover at 17 degrees C for 4-8 weeks after the treatments to ascertain the permanency of the effects. The high temperature (25 degrees C) alone induced only incipient gonadal degeneration and germ cell loss, but was highly effective in combination with double administration of 40 mg/kg Busulfan. Males tolerated Busulfan better and were more easily depleted of germ cells than females. Animals treated for 8 weeks were severely devoid of germ cells, but were still capable of gametogenesis. Thus, the combination of Busulfan and high water temperature appeared to be efficient for depletion of GCs in adult fish; and the treated gonads retained the ability to support GC proliferation and differentiation. Furthermore, quantitative analysis of vasa transcript levels was found to be an useful to monitor the degree of gonad sterility during treatment.

105. TRANSPLANTED GERM CELLS CAN COLONIZE THE GONADS OF SEXUALLY COMPETENT FISH AND PRODUCE FUNCTIONAL GAMETES

Germ cell (GC) transplantation (GCT) has great potential for seed production and conservation of valuable germlines. Currently available approaches to GCT in fish rely on sophisticated equipment and skills for cell transplantation into the blastodisc of embryos and/or the peritoneal cavity of small, sometimes few millimeters long larvae. Moreover, transplanted individuals may take years to grow to maturity, adding to the cost of producing surrogate gametes. In this context, the use of intragonadal GCT into sexually competent recipients that have been experimentally depleted of endogenous GCs might overcome these constraints. Here we demonstrate the feasibility of xenogeneic GCT in sexually competent fish. Spermatogonial cells isolated from pejerrey (Odontesthes bonariensis, Atherinopsidae) donors were implanted surgically into the testes of congeneric Patagonian pejerrey (O. hatcheri) that were severely depleted of endogenous GCs by treatment with Busulfan (40 mg/kg) and elevated water temperature (25°C) (Fig. 1).Donor cell behavior inside the recipient gonads was tracked using fluorescent cell linkers (CFDA-SE and PKH-26) and showed that transplanted spermatogonial cells were able to migrate towards, settle and multiply at the blind ends (cortical region) of the seminiferous lobules. The presence of donor-derived sperm was confirmed by PCR in 20% of the surrogate Patagonian pejerrey fathers at 6 months and fertilization of pejerrey eggs with surrogate sperm produced 1.2-13.3% pure pejerrey offspring (Fig. 2). These findings indicate that transplantation of spermatogonial cells into sexually competent fish can shorten considerably the production time of surrogate gametes and offspring. Ongoing studies are examining (low-tech) refinements in the proposed approach, such as non-surgical transplantation of GCs through genital papilla, and the suitability of GCT for generation of female gametes, for which cryopreservation techniques have not yet been developed. The results obtained so far have been encouraging and these developments will make GCT invaluable for the timely rescue of fish speciesfacing imminent extinction.

Temperature-dependent sex determination in Hd-rR medaka Oryzias latipes: gender sensitivity, thermal threshold, critical period, and DMRT1 expression profile

The developmental time and thermal threshold for temperature-dependent sex determination (TSD), gender differences in temperature sensitivity, the fertility of thermally sex reversed fish, and the effect of temperature on the expression of two major sex determination/differentiation genes (DMY/DMRT1bY and DMRT1) were examined in the Hd-rR strain of medaka, Oryzias latipes. Fertilized eggs were exposed from either shortly after fertilization (8-16 cells; embryonic stages 5-6) or from middle embryogenesis (heart development stage; stage 36) until hatching to temperatures ranging from 17 degrees C to 34 degrees C. Secondary sexual characteristics, gonadal histology, progeny testing, sex-linked body coloration and gene expression were used to determine phenotypic and genotypic sex. Sex determination was unaffected by low or high temperatures in genotypic (XY) males. In contrast, genotypic (XX) females treated from stages 5-6 showed increasing rates of sex reversal into phenotypic males at temperatures above 27 degrees C up to 100% at 34 degrees C. Thermal manipulation of sex was ineffective after stage 36, indicating that gonadal fate in medaka is determined considerably earlier than histological differentiation (stage 39). High temperature induced DMRT1 expression in genotypic females, which was observed already from stage 36. Sex-reversed males had histologically normal testes, were capable of sexual courtship and, with the exception of fish from 34 degrees C, sired viable progeny when mating with fertile females. These results clarify the pattern of TSD in medaka and provide important clues to understand the mechanism of sex determination in this species. They also suggest that a brief exposure to high temperature early in life could impair the fertility of medaka as adults.