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The sex of many fish or reptile species is determined by the temperature at which the organism is exposed at the beginning of life, which can allow adaptation of populations to environmental conditions, when they provide a reproductive advantage to one or the other sex. This temperature effect is produced during a well determined period (during incubation in reptiles and during young stages, after hatching, in fish). In natural sea bass populations, the sex-ratio is on the average balanced (as many males as females), with however, variations associated with annual climatic conditions. In breeding, the sex-ratio is very often biased with more males (75 to 95% ) whereas females are more interesting since they have a faster growth and a later puberty than males. Fish farmers and scientists have therefore questionned the origins of these differences.

Sex: a complex mixture of genetic control and temperature effects

Previous studies have shown that the sex of sea bass is determined by genetic factors (no X or Y chromosomes but a male or female "sexual tendancy" associated with a combined effect of many genes) and by the water temperature during the first months of life. A controversy remains, however, on the effect of temperature: whereas most studies showed that a feminine effect is observed at low temperatures, other show the opposite effect (a masculinizing effect at low temperatures). In addition, it seems counter-intuitive that females, which grow faster than males, are favoured by low temperatures, which in fact slow down the growth of cold-blooded animals such as fish.

An evolutionary hypothesis to explain the effect of temperature

"It is this contradictory effect that has allowed us to emit an original hypothesis", explains Marc Vandeputte, from GABI at INRAE. "The idea was that a cold temperature at the beginning of development (at the end of winter) could be an indication of early egg-laying, and therefore a longer growing season and as a consequence, a signal for a more important growth potential during the first year of life". The logical counterpoint was therefore to imagine that at low temperatures on late phases (but nevertheless before the sex was completely determined), by sustainably reducing the possible growth over the coming season, would favor male fish. To test this hypothesis, INRAE and Ifremer's teams raised sea bass at low temperature (16°C) during periods extending from 11 and 240 days after fertilization, before transferring them to a more warmer temperature (21°C). The scientists first confirmed the general hypothesis admitted that a low temperature (<17°C) in the first 56 days of life tends to produce female sea bass. However, beyond 56 days, this same temperature had the opposite effect, favoring male animals, reaching 90% male animals after 230 days at 16°C.

A unique example in the animal kingdom

This is the first observation of opposite effects of temperature on sex determinism at different stages of development, showing the complexity of the effects of temperature on this determinism. These results lead to new perspectives in understanding the evolution of sex determinism systems, effects of climate change on natural populations and the improvement of aquaculture breeding practices. "In this 'cold=male' vs 'cold=female' controversy, both sides were right" chuckled Marc Vandeputte, "it was just a question of timing".

Key words: sex determinism, aquaculture, temperature