Evolution of the alternation of haploid and diploid phases in life cycles. II. Maintenance of the haplo-diplontic cycle

Authors

  • Sophie Richerd,

    1. Laboratoire de Génétique et Evolution des Populations Végétales, URA CNRS 1185, Université Lille I, F-59655 Villeneuve d'Ascq Cedex, France
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  • Denis Couvet,

    1. Laboratoire d'Evolution des Systèmes Génétiques, CEPE/CNRS, Route de Mende, BP 5051, F-34033 Montpellier Cedex, France
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  • Myriam Valéro

    1. Laboratoire de Génétique et Evolution des Populations Végétales, URA CNRS 1185, Université Lille I, F-59655 Villeneuve d'Ascq Cedex, France
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Abstract

The relative duration of the haploid and the diploid phases during the reproductive cycle varies greatly between organisms. This paper addresses the question of the evolution of haploid, diploid, and haplo-diplontic life cycles. When the life span of haploid and diploid individuals is constant whatever their cycle, we show that the haplo-diplontic cycle has an advantage, which depends on the sex-ratio in anisogamous species and on the probability of fertilization in isogamous species. This is because meiosis and fertilization occur half as often in the haplo-diplontic cycle as in haploid or diploid cycles, for the same number of generations of individuals. This argument is demonstrated using a model which considers a genetic determination of the cycle, and fixed haploid and diploid fitnesses. The relevance of measures of fitness of haploid and diploid individuals in predicting the evolution of life cycles is discussed. Measures obtained in algae are compared with theoretical predictions.

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