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T sexual differentiation of P. melanogaster embryos was examined among fish kept in glass aquaria (80x40x35 cm) at 23, 26and29°CwithapH=6.5. Analysis of variance of the data revealed a significant variation (at the 5 % level) of sex composition relative to increase of water temperature .Two mathematical representations were predicted for males and females.Accordingly, the degrees at which all males, or all females, or 1:1 sex ratio had been determined was 18.95, 32.01 and25.4°C respectively. The mechanism of this temperature effect on the sex ratio was discussed where growth offish was suspected to have played a role in this process. INTRODUCTION Sex Ratio The near equal production of males and females by most organisms has fascinated biologists since the days of Darwin. In 1929, Fisher pointed out that, since each individual arising from sexual reproduction, males equal that of parental females. Therefore, if the population sex ratio deviates from equal numbers of males and females, pairs producing the rarer sex will have a selective advantage until the population sex ratio is equalized. This statistical mechanism maintaining equal sex ratios has been later demonstrated by several authors (Shaw &Mohler 1953; Kolman 1960; MacArthur 1965; Vemerl965;Myers,1978;Conover,1984 and Olivier & Kaiser, 1997). However, environmental factors such as pH may affect sex ratio in 30 Elsaved A. Khallaf et al. some Cichlids and Poeciliids (Rubin, 1985). According to Romer and Beisenherz (1995) a correlation between sex ratio of offspring of 33different species of Apistogramma and temperature at certain pH-Ievel could be demonstrated with high significance. At low temperature (23 °C) the sex ratio of offspring of 32 Apistogramma species was skewed towards females, whereas at high temeprature (29°C) towards males and at a temperaure of 26 °C the number of males and females was approximately balanced. However, Apistogramma cacti showed a reversed response to temperature. The previous authors declared that temperature could influence sex ratio of offspring during development within the first weeks of life.On the other hand, in Apistogramma, sex ratio was influenced by pH-level, too. Numerous experimental studies have recently reported, however, that environmental sex determination (ESD) is prevalent among poikilothermic vertebrates. In these vertebrates, incubation temperature determines the course of primary sex differentiation (and thereby the sex ratio of progeny) during a specific period of embryonic or larval development. In some species, lower temperatures produce males and higher temperatures produce females (e.g.turtles) while in others, the pattern is reversed (Lizards, alligators, fishes). The live-bearing fishes of the family Poeciliidae are widely known to exhibit unusual sex ratiosExamples of some reports include Poecilia reticulata (Haskins et ah 1961), Gambusia mani (Krumholz, 1963), Heierandriaformosa {Colsou, 1969), Poeciliopsis monacha (Moore, 1971) and Gambusia holbrooki (Vargas & Sostoa, 1996). Three other examples of fishes with temperature sex dtermination have been recently studied in the laboratory: (l)the Atlantic silver-side, Menidia menidia (Conover & Kynard, 1981; Conver & Heins, 1987 a,b and Conover et all 992), in which some genotypes produced excess of males at high temperatures and excess of females at low temperatures, while others in the same population were strictly genetic sex determination (GSD); (2) the hermaphrodite, Rivuhts marmoratus (Harrington, 1967, 1971), a fish which at its normally warm tropical or subtropical temperatures fertilizer itself and produces hermaphroditic offspring, but when reared at 20°C, produces 75% males; and (3) Poeciliopsis lucida, which is a polymorphic fish with genotypes that have either genetic sex determination (GSD) or temperature sex determination (TSD). As THE EFFECT OF TEMPERATURE VARIATION ON THE SEX COMPOSITION AND GROWTH OF PEOCIUA MELAKOGASTER 3 5 OFFSPRING documented by Sullivan and Schuiz (1986) and Schuiz (1993). In this paper, xhe effect of temperature on the sex ratio of P. rnelanogoster is assessed. M A T E R I A L AND METHODS Adult males and females of Poecilia melanogastcr were collected from large fish tanks containing various fish species at the Department of Zoology II,University of Dtisseldorf (Prof. Dr. Greven, H.sGermany). Fishes were separated and kept in glass aquaria (80 x 40 x35cm)with constantly aerated water. Fresh water plants e.g. Hydrilla and Pistia} were provided to match the natural habitat of the fish. Fishes were maintained under 12 hrs. light/ 12 hrs. dark schedule. They were fed daily with commercially available fish food (Tetramin3Tetra-WerkeJMelIe, Germany). The experimental aquaria were kept al23±l°C,26 ±1°C, or 29±l°Cand pH=6,5.The temperature was checked four times daily, and heating was carried out by heaters, while cooling was reached by use of ice, if such need arised during the experiment. Statistics: The influence of the applied temperatures (23 °C, 26°C and 29°C) on the sex ratios of the offspring were examined statistically, by a two-way ANOVA analysis (Neter and Wasserman,1974).In order to describe the interaction between gender and temperature, a correlation analysis is applied (Pearson Product Moment Correlation).

THE EFFECT OF TEMPERATURE VARIATION ON THE SEX COMPOSITION AND GROWTH OF PEOCILIA MELANOGASTER OFFSPRING