Effects of Temperature on Gonadal Sex Determination in Barfin Flounder <i>Verasper moseri</i>

Abstract

This time corresponds to the end of the temperature-sensitive period.These results suggest that gonadal sex was determined by temperature prior to the onset of gonadal sex differentiation in this fish.Key words: barfin flounder, sex determination, temperatureIt has been documented that sex in fish is controlled ge netically. However, many fish do not have a clear chro mosomal sex determining system,1) and therefore, genetic sex and phenotypic sex should be considered as separate en tities. Studies over the past decade have shown that exter nal factors, such as temperature and pH, influence sexual determination in some fish. Thus, temperature-dependent sex determination has been demonstrated in Menidia menidia,2) Oreochromis niloticus3) and Odontesthes bonariensis,4) while pH-dependent sex determination has been demonstrated in Xiphophorus spp.5) and Apistogram ma spp.6) The hirame flounder Paralichthys olivaceus7) in particular, has been well-studied in terms of genetic sex de termination and manipulation of physiological sex by con trolling temperature. It has been demonstrated in this spe cies, in which the male is the heterogametic sex (XX female-XY male), that genetic females reverse their sex to male if the water temperature is manipulated. Not only high but also low temperatures produced male-biased sex ratios, while intermediate temperatures yielded a 1: 1 sex ra tio.Thermolability of sex determination in another flatfish species, the barfin flounder Verasper moseri, a teleost that belongs to the Pleuronectidae family and inhabits cold sea water basins, has not been addressed. It is a commercially important species in Hokkaido and requires establishment of breeding techniques for aquaculture, like the hirame flounder. As in many other teleosts,8-10) gonadal sex in the barfin flounder can be manipulated by treatment with ex ogenous steroids during the period of gonadal sex differen tiation. l'1 However, it became evident that the population frequently had a biased sex ratio under artificial breeding conditions. A supply of seedlings with stable sex ratio for stock enhancement has not yet been realized for this spe cies. As in the hirame flounder, the biased sex ratio in barfin flounder could be caused by temperature influencing sex determination. To test this hypothesis, we evaluated sex ratios following manipulation of the temperature dur ing the larval and juvenile stage. Manipulations were done periodically in order to identify temperature sensitive periods and to relate these periods to gonadal morpholo gy. Materials and Methods AnimalsFertilized eggs of barfin flounder were acquired from a few captive-bred males and females held together in a 1 m3 tank at the Hokkaido Institute of Mariculture, Shikabe, Japan. After egg incubation and hatching at 8•Ž, tempera tures were gradually increased up to 14•Ž in 34 days. Animals were reared in flow-through seawater, larvae being fed Brachionus and Artemia nauplii, while juveniles were fed artificial diets. Experimental DesignThe experimental water temperatures ...

Histological Studies on Early Oogenesis in Barfin Flounder (Verasper moseri)

“…Among fish with a TL of 30mm to 100mm, the degree of gonadal development and gametogenesis depended on the TL. Since these results corresponded with the report of Goto et al (1999), the gonadal development in the barfin flounders used in the present study was normal and the degree of development of the germ cells was associated with the TL.…”

“…In the barfin flounder, morphological differentiation of the gonad into either an ovary or testis becomes distinguishable at a TL of 35 mm (Goto et al , 1999), and the oocytes develop into peri-nucleolus stage oocytes at a TL of 133 mm. In the present study, we first investigated the morphological changes in early oogenesis in barfin flounders with a TL of 30mm (before sex differentiation) to 100mm (perinucleolus stage oocytes exist) by light and electron microscopy.…”

Temperature-Dependent Sex Determination in Vertebrates

“…These patterns and other data summarized by Yamamoto (1999) suggest that an XX-XY system operates at intermediate temperatures, but that the sex of XX individuals is unstable and can be induced to produce male offspring, especially under conditions of extreme high temperature. Similar patterns have been reported in other flatfishes including P. lethosigma (Luckenbach et al 2003), Verasper moseri (Goto et al 1999), and Limanda yokohamae (Goto et al 2000). Kitano et al (1999) demonstrated that the creation of XX males in P. olivaceus at high temperature is associated with suppression of cytochrome P450 aromatase gene expression in the gonad, which at lower temperatures catalyzes the conversion of androgens to estrogens.…”

Comparative study on ovarian structures in scorpaenids: possible evolutional process of reproductive mode

“…In the observation of ovarian development in Sebastes schlegelii, it was shown that the ovarian cavity had been formed on the lateral side of the gonad on the coelomic wall side at fi rst, and then the gonad was differentiated to an ovary. The formation of the ovarian cavity has been enumerated as an initial index of ovarian differentiation in many teleost species having an ovary of type II-1 [Cichlidae: Oreochromis niloticus (see Nakamura and Takahashi, 1973); Gasterosteidae: Gaterosteus aculeatus (see Shimizu and Takahashi, 1980); Tetraodontidae: Takifugu rubripes (see Matsuura et al, 1994); Pleuronectidae: Verasper moseri (see Goto et al, 1999) . Thus, it was shown that the ovary of Sebastes schlegelii differentiated directly into the ovarian structure of type II-1 during the developmental process and did not pass through type II-3 during the ovarian developmental process.…”