Atherinidae is a family of fishes known to include at least three species with documented thermolabile sex determination. In Odontesthes bonariensis, fish exposed to about 17°C from hatching to juvenile stage become all-female, whereas groups exposed to about 25°C become male-biased. In contrast, sex in another atherinid, Patagonina hatcheri, appears to be under stronger genotypic control because the sex ratios are always balanced (1:1) within the same range of temperatures. The main purpose of this study was to determine the effect of temperature on the sex ratios of both species over their full range of viable temperatures. In addition, we determined the critical time of thermolabile sex determination in O. bonariensis at female-and male-forming temperatures. O. bonariensis larvae survived at temperatures between 13 and 29°C, but growth was negligible at 13 and 15°C. The proportions of females were 100% at 13-19°C, 95% at 21°C, 81.2% at 23°C, 29.4% at 25°C, 10% at 27°C, and 0% at 29°C. These results indicate a gradual transition from female-to male-forming temperatures evenly spread over the range of viable temperatures and hence the absence of any marked thermal threshold for thermolabile/ genotypic control of sex; they also suggest weak or no genotypic control of sex in this species. In addition, prolonged exposure to 29°C caused disappearance of germ cells. The critical time of thermolabile sex determination in O. bonariensis was estimated to be 3-5 wk, 2-4 wk, and 1-4 wk after hatching at 17, 19, and 27°C, respectively. Survival of P. hatcheri was limited to temperatures between 13 and 27°C (only 8.7% at the latter temperature) with slow but steady growth at 13 and 15°C. The proportions of females were 88.9 and 89.5% at 13 and 15°C, respectively, about 50% at temperatures between 17 and 23°C, and 30.8% at 25°C, hence, showing one clear threshold for thermolabile/genotypic control of sex at 15-17°C and possibly another at or around 25°C. These results confirmed that gonadal sex in this species is under genotypic control at intermediate temperatures but also showed that this control can be overridden at low (and possibly also at high) temperatures. J. Exp. Zool. 278:167-177, 1997. © 1997 Recent studies have shown that the phenotypic sex can be altered by the thermal conditions prevailing during the period of primary sex differentiation (a phenomenon termed thermolabile sex determination [TSD]) in two South American atherinids, Odontesthes bonariensis and O. argentinensis (Strüssmann et al., '96a,b). In O. bonariensis, for instance, thermal treatments of larvae and juveniles yielded 98-100% females at around 17°C, 85-100% females at around 21°C, and 14-83% females at around 25°C (Strüssmann et al., '96a). These results parallel those for another atherinid, the Atlantic silverside Menidia menidia (Conover and Kynard, '81), in that low temperatures favor the formation of females, whereas high temperatures yield more males. For this species, Conover and Heins ('87a) determined that the shape of the resp...

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The concept of reducing nonaxial loading of dental implants has been widely regarded as the standard procedure. The aim of this study was to reveal the biomechanical stress distribution in supporting bone around an implant and a natural tooth under chewing function. Three-dimensional finite element models of the mandibular first molar and the titanium implant both with the mandible in the molar region were constructed. The directions of displacement constraints were determined according to the angles of the closing pathways of chopping type and grinding type chewing patterns. The tooth model showed smooth stress distribution in the supporting bone with low stress concentration around the neck of the tooth. The implant model showed stress concentration in the supporting bone around the neck of the implant, especially in the buccal area. The grinding type model of the implant showed higher tensile stress concentration than the chopping type model at the lingual neck of the implant. The results of this study suggested the importance of considering occlusion under chewing function for understanding the biomechanics of oral implants.

This study evaluated the effects of different temperatures on the histological process of sex differentiation in the pejerrey Odontesthes bonariensis, a fish with marked temperature-dependent sex determination (TSD), at feminizing, neutral, and masculinizing temperatures. Fish reared at three temperatures (17 degrees C, 24 degrees C, and 29 degrees C) from hatching were sampled weekly until 11 weeks and their gonads were examined by histology. The percentages of females at 17 degrees C, 24 degrees C, and 29 degrees C were 100%, 73%, and 0%, respectively. Sex differentiation occurred earlier and at a smaller body size at higher temperatures in both sexes. The first signs of ovarian differentiation were observed at 4 and 7 weeks at 24 degrees C and 17 degrees C, respectively, and those of testicular differentiation at 4 and 7 weeks at 29 degrees C and 24 degrees C, respectively. Body or gonadal growth rates before sex differentiation were not proportional to temperature and showed no sexual dimorphism at 24 degrees C, where both sexes were present. Thus, differential growth rate is probably not a factor in TSD or histological sex differentiation in pejerrey. Blood vessels were formed before sex differentiation in both sexes and at all temperatures, and may be important for sex differentiation. No signs of intersexuality were found in any of the groups, and this characterizes pejerrey as the differentiated type of gonochorist even at feminizing and masculinizing temperatures. Ovaries were formed by the same histological processes at feminizing (17 degrees C) and neutral (24 degrees C) temperatures and without any pathological features such as germ cell degeneration. The process of testicular formation was generally similar at 24 degrees C and 29 degrees C, but some fish at 29 degrees C had widespread germ cell degeneration before sex differentiation. This suggests that pathological processes leading to germ cell death, such as heat-induced dysfunction of the supporting somatic cells, could be involved in masculinization of the genetic females at high temperatures.

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SynopsisThe present study investigated the effects of water temperature (18,21, and 25 "C) on the histological process of gonadal sex differentiation of two commercially important atherinid fishes from South America, Odontesthes argentinensis (sea pejerrey) and Patagonina hatcheri (Patagonian freshwater pejerrey). In both species, female gonadal sex differentiation began with the formation of lateral stromal cell outgrowths and the appearance of meiotic oocytes. The male gonads remained quiescent for about twice as long as the female gonads, with differentiation becoming evident by the formation of the main sperm duct and of cysts of germ cells at the periphery of the gonads. Meiosis in males occurred relatively long after somatic differentiation of the testis. The ovaries of 0. argentinensis differentiated at 28 days (20.3 mm) at 25 "C, 42 days (24.0 mm) at 21 "C, and 56 days (23.8 mm) at 18 OC. In the males, differentiation was observed at 98 days at 25 and 21 "C (39.4 mm and 40.4 mm, respectively), but at 112 days under 18 OC (40.7 mm). In l? hatcheri, differentiation of females occurred at 21 days (17.8 mm) at 25 OC, 28 days (20.8 mm) at 21 OC, and 35 days (23.2 mm) at 18 "C. Male differentiation became evident at 56 days under 25 and 21 "C (30.8 and 32.7 mm, respectively), and at 70 days (37.7 mm) at 18 "C. The sex-ratios of 0. argentinensis reared at 18 or 21 "C were female-biased whereas those at 25 OC were not; groups reared at 18 "C had significantly more females than groups from the same progeny reared at 25 "C. In contrast, the sex-ratios in all groups of l? hatcheri did not differ significantly from 1:l and no significant differences were found between groups of the same progeny reared at different temperatures. These results suggest the occurrence of thermolabile sex determination (TSD) in 0. argentinensis whereas in I? hatcheri gonadal sex appears to be strongly genetically determined.

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