Eric Saillant

Environmental Effects on Fish Sex Determination and Differentiation

Environmental factors affect the sex ratio of many gonochoristic fish species. They can either determine sex or influence sex differentiation. Temperature is the most common environmental cue affecting sex but density, pH and hypoxia have also been shown to influence the sex ratio of fish species from very divergent orders. Differential growth or developmental rate is suggested to influence sex differentiation in sea bass. Studies in most fish species used domestic strains reared under controlled conditions. In tilapia and sea bass, domestic stocks and field-collected populations showed similar patterns of thermosensitivity under controlled conditions. Genetic variability of thermosensitivity is seen between populations but also between families within the same population. Furthermore, in the Nile tilapia progeny testing of wild male breeders has strongly suggested the existence of XX males in 2 different natural populations. Tilapia and Atlantic silverside studies have shown that temperature sensitivity is a heritable trait which can respond to directional (tilapia) or frequency dependent selection. In tilapia, transitional forms within a genetic sex determination (GSD) and environmental sex determination (ESD) continuum seem to exist. Temperature regulates the expression of the ovarian-aromatase cyp19a1 which is consistently inhibited in temperature masculinized gonads. Foxl2 issuppressed before cyp19a1. Recent in vitro studies have shown that foxl2 activates cyp19a1, suggesting that temperature acts directly on foxl2 or further upstream. Dmrt1 up-regulation is correlated with temperature-induced male phenotypes. Temperature through apoptosis or germ cell proliferation could be a critical threshold for male or female sex differentiation.

Sexual growth dimorphism in sea bass Dicentrarchus labrax

Abstract The growth of 100 sea bass feminized by estradiol treatment and 200 individually tagged controls was followed from the age of 10 months [27±1.7 g, mean±confidence interval (95%)] to 45 months (1160±46 g) in order to analyze the evolution of sexual dimorphism as a function of the age and sexual maturity. One hundred percent females were found in the treated group ( n =94) versus 55% in the control nontreated group ( n =171). Males remained significantly smaller than both females and feminized fish from the beginning of the experiment. The resulting weight advantage [(female mean weight−male mean weight)/male mean weight, expressed as a percentage] for females was much more pronounced during the early stages of development (67% at 10 months of age), decreased in the second year of life and then became stabilized around 25%. Females and feminized fish retained the same growth, suggesting that growth is related to phenotypic sex in the sea bass. The growth rate was influenced by sex in the fourth year of life, period during which the females grew faster in terms of both weight and length. Compared to the males, they had a relatively higher ratio of digestive tract to body weight (+26%), but lower ratios of visceral fat to body weight (−49%) and muscle lipid content (−16%) at slaughtering, at which time they had finished their second vitellogenic cycle (45 months).

Effects of rearing density, size grading and parental factors on sex ratios of the sea bass (Dicentrarchus labrax L.) in intensive aquaculture

Abstract The effects of rearing density and size grading on sex ratios in 30 families of sea bass reared in the same tank from the fertilization stage onwards were investigated. Two extreme density treatments (high versus low) were applied in quadruplicates throughout the labile period for sex determination. An excess of males (88%) was found in all groups. Growth was faster under low density from 49 to 191 days post fertilization (p.f.) (27–10% relative difference in length) and was then equivalent between the two groups until the end of the treatments (414 days p.f.). Density had no effect on sex ratio, suggesting that the high densities usually applied in aquaculture are not involved in the systematic excess of males reported in farmed populations. The fish were genotyped at three to six microsatellite loci. This allowed them to be assigned to the individual breeders used in the mating design. The percentage of females was significantly influenced by both the dam and the sire, highlighting the existence of genetic variation of sex ratio. Repeated size grading performed from 84 to 199 days p.f. had no effect on the sex ratio of the overall population studied showing that sex determination had not been affected by the treatment. However, the highest percentages of females were found in the lead classes from 84 days p.f. Most of the fish in this lead group were descended from one dam and the percentage of females found in the individual families were higher in this class than in the non-graded group. These results show that females start growing faster than males long before morphological sexual differentiation occurs.

Production of meiotic gynogenetic and triploidsea bass, Dicentrarchus labrax L. 1. Performances, maturation and carcass quality

Abstract Meiotic gynogenetic and triploid sea bass were produced by pressure shocks according to a previously published protocol. Pressure-treated groups did not survive as well as controls during early development and larval rearing. Performances, sexual maturation and carcass quality were examined over a period of 34–45 months. At the age of 34 months, growth of the gynogenetic fish was comparable to that of the control but inferior in the triploid fish. A predominance of male fish was found within the triploid groups, while diploid and meiotic gynogenetic fish showed equal proportions of the sexes. Gonadal maturation in triploid fish was significantly impaired, particularly in the females that showed rudimentary ovaries. Triploid males exhibited primary maturation but proved to be gametically sterile. Pressure-induced triploids did not grow as well as diploids, but these results might be ascribed to specific on-growing conditions (communal rearing). The performance of gynogenetic sea bass was comparable to that of control. The superiority of diploid fish over their triploid counterparts was confirmed during the final growing period and more clearly so in females. Performances of triploids varied according to their maternal origin. Overall, striking qualitative differences between diploid and triploid fish were found at the age of 34 and 45 months, although the results varied in a gender-specific manner. A strong maternal effect was also observed. The potential advantages of triploid sea bass for aquaculture purposes are discussed.

Sexual differentiation and juvenile intersexuality in the European sea bass (Dicentrarchus labrax)

Sexual differentiation was studied at the histological level using a mixture of 30 families of sea bass Dicentrarchus labrax. Most of the fish (93%) differentiated into males as usually observed in farmed populations. All testes were differentiated when the males reached 12 cm and no more undifferentiated fish were found from 419 days post-fertilization (p.f.). In 28% of the males, among the biggest, sexual differentiation had already begun at 168 days p.f. (8.3–9.5 cm) and these fish started spermatogenesis in their first year of life. The other males differentiated later and remained immature at the end of their first year of life. Ovaries could be identified at the histological level from the age of 168 days p.f. (7.9–9.0 cm) and the females became significantly longer than the males from the age of 191 days p.f., i.e. during the process of ovarian differentiation. In the studied group, 62% of the males developed intratesticular oocytes. Such intersexuality had no consequence on growth rate. Intratesticular oocytes were also recorded in testes of wild males originating from Atlantic (Britain and Gulf of Gascogne) and West Mediterranean showing that juvenile intersexuality is not restricted to farmed populations but is a widespread phenomenon in sea bass.

Saline preferendum for the European sea bass, Dicentrarchus labrax, larvae and juveniles: effect of salinity on early development and sex determination

Abstract The sea bass, Dicentrarchus labrax , is an eurhyaline marine fish. Juveniles of this species are known to frequent estuaries and lagoons where salinity is lower than in the open sea. Sex determination occurs during this phase of fish life and has been shown labile and sensitive to environmental factors. In this work, the effect of rearing salinity on sex determination and early development of the sea bass was investigated. An excess of males (87%) was found in all groups and the salinity level [(natural sea water salinity, mean: 37‰) vs. (15‰)], when maintained constant, had no effect on the sex-ratio. The transfer from low to high salinity at 93 days post-fertilization (p.f.) increased the percentage of males (93%) suggesting that sexual differentiation in this species may be influenced by such an osmotic stress. Growth was improved by a 15‰ salinity at the beginning of larval rearing (14 days p.f.) and at the end of pre-growing (234–458 days p.f.), periods during which low temperatures were applied. Survival during larval rearing and nursery were also improved in the groups reared at low salinity and so was swimbladder inflation. These results show that sea bass juveniles have a low saline preferendum, a finding that corresponds to the conditions they actually encounter in the wild during their juvenile ecophase; sex determination is not directly modulated by the salinity level but seems to be subjected to complex environmental regulations.

Conservation genetics and demographic history of the endangered Cape Fear shiner ( Notropis mekistocholas )

We examined allelic variation at 22 nuclear‐encoded markers (21 microsatellites and one anonymous locus) and mitochondrial (mt)DNA in two geographical samples of the endangered cyprinid fish Notropis mekistocholas (Cape Fear shiner). Genetic diversity was relatively high in comparison to other endangered vertebrates, and there was no evidence of small population effects despite the low abundance reported for the species. Significant heterogeneity (following Bonferroni correction) in allele distribution at three microsatellites and in haplotype distribution in mtDNA was detected between the two localities. This heterogeneity may be due to reduced gene flow caused by a dam built in the early 1900s. Bayesian coalescent analysis of microsatellite variation indicated that effective population size of Cape Fear shiners has declined in recent times (11–25 435 years ago, with highest posterior probabilities between 126 and 2007 years ago) by one–two orders of magnitude, consistent with the observed decline in abundance of the species. A decline in effective size was not indicated by analysis of mtDNA, where sequence polymorphism appeared to carry the signature of an older expansion phase that dated to the Pleistocene (∼12 700 > 1 million years ago). Cape Fear shiners thus appear to have undergone an expansion phase following a glacial cycle but to have declined significantly in more recent times. These results suggest that rapidly evolving markers such as microsatellites may constitute a suitable tool when inferring recent demographic dynamics of populations.

Genetic Identification of Hatchery-Released Red Drum in Texas Bays and Estuaries

Abstract The stock enhancement program for red drum Sciaenops ocellatus in Texas annually releases from 25 to 30 million fingerlings into Texas bays and estuaries and represents one of the largest such programs for marine fishes worldwide. We used 16 nuclear-encoded microsatellites and a 370-base-pair fragment of the mitochondrial DNA (mtDNA) D-loop to assign red drum sampled from two bays along the Texas coast to either hatchery or wild origin. A total of 30 hatchery-released fish were identified among 321 red drum belonging to three year-classes sampled from Galveston Bay, while a total of 11 hatchery-released fish were identified among 970 red drum belonging to four year-classes sampled from Aransas Bay. Allelic richness (microsatellites) was significantly lower among hatchery-released fish than among hatchery broodfish and wild fish. Similarly, the expected number of mtDNA haplotypes in hatchery-released fish (based on simulation analysis) was significantly lower than that expected in a random sample ...

Genetic Effective Size in Populations of Hatchery-Raised Red Drum Released for Stock Enhancement

Abstract Genetic analysis of progeny from 13 spawning events occurring over a 2-week period in a Texas Parks and Wildlife Department (TPWD) hatchery for red drum Sciaenops ocellatus during the spring of 2002 and hatchery spawning and release records over the 2003 spawning season were used to estimate the average genetic effective size of an average spawn and an average hatchery-released population. The purpose of this study was to assess the potential for a Ryman-Laikre effect in the TPWD red drum stock enhancement program. Genetic analysis revealed that 16 of 27 dams (59.2%) and 16 of 18 sires (88.9%) spawned at least once. The average effective size (Ne ) for a single spawn was 2.59, approximately 43% less than the maximum Ne (4.55) predicted if all possible mating (dam × sire) combinations had occurred and family size per mating combination had been equivalent. The reduction in Ne stemming from the actual number of mating combinations was approximately 34% and appeared to be due primarily to nonspawnin...

Production of monosex male populations of European seabass, Dicentrarchus labrax L. by use of the synthetic androgen 17α-methyldehydrotestosterone

An investigation was undertaken to study the effect of the synthetic androgen 17α-methyldehydrotestosterone on the sex reversal and gonadal development of the European sea bass Dicentrarchus labrax L. A combination of three different hormonal doses (0.5, 3, 5 mg/kg food) and three treatment durations (30, 60, 90 days) was tested starting at 84 days post-hatching. Complete and permanent masculinisation occurred in 100% of individuals at all times. The range of treatments did not significantly compromise the gonadal development. However, testicular deformities were observed in low proportions (less than 10%) of both treated and control groups.