Você acha que os animais sempre podem ser separados em machos e fêmeas? Você acha que relações entre indivíduos do mesmo sexo e troca de gênero não são “naturais”? São invenções do humano moderno? Nessa série de três matérias, vou apresentar algumas situações muito mais complexas e intrigantes que podemos encontrar na natureza!
Começo falando sobre um tipo de reprodução conhecido como partenogênese (do grego: “nascimento virgem”), que nada mais é do que a formação de um embrião sem fertilização. Isso mesmo, o embrião se forma a partir do óvulo da mãe, sem um espermatozoide. Essa é uma das alternativas encontradas para o tipo de reprodução mais comum: macho + fêmea = filhote.
Você pode estar se perguntando: “mas como nunca ouvi falar disso?”. Se você já ouviu “ai ai ai, carrapato não tem pai”, você já ouviu falar de partenogênese! Existem carrapatos machos e fêmeas que podem cruzar e gerar carrapatinhos machos e fêmeas. Entretanto, na ausência ou na escassez de machos, as fêmeas podem se reproduzir sem eles e, nesse caso, nascem apenas fêmeas [1].
Existem também vertebrados capazes de realizar reprodução unissexual (em que só fêmeas efetivamente participam): lagartos, peixes, salamandras, sapos e aves. Em algumas dessas espécies, as fêmeas precisam de um estímulo do macho para que o embrião se forme, em outras, não é necessária a presença de um macho, sendo que em ambos os casos, os filhotes recebem os genes apenas da mãe.
A descoberta de partenogênese em tubarões aconteceu em uma situação muito curiosa. No aquário do zoológico Henry Doorly, nos Estados Unidos, haviam três tubarões martelo fêmeas e nenhum macho. As três fêmeas foram capturadas na natueza ainda muito jovens, sexualmente imaturas, e viveram no tanque do aquário por três anos sem nunca ter vivido com um macho. Então, um dia surgiu um filhote de tubarão martelo no tanque. Cientistas testaram várias hipóteses e, após um teste de paternidade (no caso, um teste de maternidade), chegaram à conclusão de que uma das fêmeas era a mãe e o pai do filhote! [2]
Tubarão martelo (Foto: Wikipedia Commons)
Como surgem as espécies capazes de realizar partenogênese?
A maioria das espécies unissexuais se originaram a partir do cruzamento de duas espécies diferentes. Se você se lembra das aulas de biologia na escola, quando o professor ensinou que se espécies diferentes se cruzam geram descendentes que não se desenvolvem ou que são estéreis (como o burro e a mula), não se sinta enganado! Essa é a regra geral e a partenogênese é uma das exceções possíveis.
Então, um lagarto macho da espécie A cruza com uma fêmea da espécie B que gera filhotes híbridos. Eventualmente, algumas fêmeas híbridas crescem saudáveis e capazes de se reproduzir, mas nesse caso, sem um macho! Para entender melhor os mecanismos desse tipo de reprodução, leia o artigo aqui [3]. Existe também a possibilidade de ocorrer um conjunto de mutações que levam as fêmeas de uma espécie a desenvolver a capacidade de se reproduzirem sem
um macho [4].
A partenogênese já foi encontrada em aproximadamente 80 espécies de vertebrados, em diversos invertebrados e em plantas, e novas espécies continuam sendo descobertas. Na próxima matéria, vou falar sobre outra situação intrigante e muito comum na natureza, em que um indivíduo pode ser o pai e/ou a mãe, o hermafroditismo. Acompanhe!
por Patricia Sanae Sujii
sujiips@gmail.com
[1] Saito, Y., & Hoogstraal, H. (1973). Haemaphysalis (Kaiseriana) mageshimaensis sp. n.(Ixodoidea: Ixodidae), a Japanese deer parasite with bisexual and parthenogenetic reproduction. The Journal of Parasitology, 569-578.
[2] Chapman, D. D., Shivji, M. S., Louis, E., Sommer, J., Fletcher, H., & Prodöhl, P. A. (2007). Virgin birth in a hammerhead shark. Biology letters, 3(4), 425-427.
[3] Neaves, W. B., & Baumann, P. (2011). Unisexual reproduction among vertebrates. Trends in Genetics, 27(3), 81-88.
[4] Sinclair, E. A., Pramuk, J. B., Bezy, R. L., Crandall, K. A., & Sites Jr, J. W. (2010). DNA evidence for nonhybrid origins of parthenogenesis in natural populations of vertebrates. Evolution, 64(5), 1346-1357.
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