L'origine et l'évolution de la reproduction animale
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Sexually reproducing organisms have alternating haploid and diploid phases. Each phase is connected to the other through meiosis and fertilization or conjugation. This article provides a tentative explanation for several features of sexual reproduction, and in particular for anisogamy, i.e. ability to produce gametes of unequal sizes. Chemotropism may be regarded as the driving force which led to acquisition of anisogamy. A chemotropic system functions optimally if the gametes belong to two distinct categories. One category of cells is non-motile and secretes an attractive substance, whilst the other category responds to the attractive signal by moving towards the emitting cells. Anisogamy is thought to result from selection of two types of mutations: those increasing the size of the non-motile gametes, and those reducing the size of the motile gametes. Such modifications have clear advantages. Larger and slower cells increase their attractive power and give rise to zygotes with higher survival fitness. Smaller cells gain swimming speed and can be produced in larger numbers. This gives them a better chance of finding a partner to mate with.
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Denis, H ; Collenot, A, L'origine et l'évolution de la reproduction animale, Med Sci (Paris), 1993, Vol. 9, N° 12; p.1392-403