quarta-feira, 2 de Maio de 2007

Determinação do sexo...

A determinação do sexo na espécie humana é bastante… aborrecida. O sistema é simples: espermatozóides podem ter um cromossoma X ou um cromosoma Y, enquanto que os óvulos apenas cromosomas X. Se o bebé for XX, será uma rapariga, se for XY um rapaz… sem dúvidas e sem possibilidade de troca (fora alguns conhecidos, mas raros, síndromes). A determinação do sexo pelos cromossomas não é específica dos humanos. Muitos usam o mesmo sistema. As galinhas e outros pássaros tem um sistema um pouco diferente – os machos são ZZ e as fêmeas ZW.

O mais interessante é que em alguns animais não são os cromossomas que definem o sexo dos filhos… mas o ambiente. O exemplo mais conhecido é o de muitos répteis, em que a temperatura dos ovos tem um papel determinante. Por exemplo, em algumas tartarugas, se a temperatura dos ovos for acima dos 29º, nascem fêmeas, se for abaixo são machos. Noutros casos, como em alguns crocodilos, uma temperatura intermédia leva a nascerem machos, e temperaturas mais baixas ou mais altas levam a fêmeas. Este sistema é muito prático, pois ajuda a regular a proporção de machos e fêmeas numa população. Claro que aqui vemos já um dos problemas do aquecimento global. A subida de um grau pode ser o suficiente para produzir excesso de um certo sexo…

Mas existem situações ainda mais interessantes… As lagartas marinhas Bonellia viridis, antes de se tornarem adultas, apresentam um fase intermédia, sob a forma de larva. As larvas não têm sexo definido. Na altura de se tornarem adultos, no entanto, tudo depende de quem se encontrar por perto. Na presença de muitas fêmeas diferenciam-se em machos. Se não, transformam-se elas mesmo em fêmeas.

E agora o meu caso favorito- existe uma espécie de peixe dos recifes de coral, Halichoeres melanurus, em que cada macho tende a controlar um harém de fêmeas. Uma experiência realizada em 1994 teve resultados muito curiosos- a um harém de fêmeas foi retirado o macho dominante… qual não foi a surpresa ao verificar que ao fim de 2 a 3 semanas, a maior das fêmeas se havia transformado… num macho. E como se isso não bastasse, quando o macho foi reintroduzido, a fêmea-agora-macho voltou a ser fêmea…

E vai uma pessoa pensar que mudanças de sexo eram uma coisa nova…

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