Altérations de l'embryogenèse de Marsilea vestita provoquées par un faible abaissement de température et phénomènes de récupération après retour aux conditions initiales

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Abstract
Under controlled conditions, all the fertilized eggs of Marsilea vestita give rise to well-organized embryos in a few hours. The first cleavage divisions of the zygote are fast and synchronous at 24 °C, whereas they are very modified, slowed down, and unstable at 16 °C. Cold treatment induces morphological and structural alterations, the number and the degree of which are correlated with the treatment duration. The most important alteration concerns the mitotic spindle orientation, which is often aberrant. The ensuing cell wall formation is disrupted, branched, and incomplete; consequently the daughter nuclei exhibit abnormal locations. Furthermore they no longer divide synchronously. The identification of the first segments then becomes impossible. In these cells, some chromatic elements having the appearance of mitotic chromosomes are often located near the spindle; these misplaced chromosomes, therefore, are not integrated to the spindle apparatus. A cold treatment applied at fertilization is effective in preventing the polyspermy block. The supernumerary spermatozoids remain in the cytoplasm of the zygote but never fuse with its nucleus. Despite their altered ontogeny, most treated specimens display, when returned to 24 °C, a surprising ability to recover a normal topographic organization.