On Alternation of Generations in Embryophytic Plants

摘要

Multiple diverse eukaryote lineages independently evolved a life-cycle that alternates between free-living haploid and diploid generations. The life-cycles of the red, brown and green algae constitute variations of alternation of generations, which can also be found in some embryophyte land plants. The maintenance of two multicellular stages necessary for sexual reproduction indicates that this alternation confers adaptive advantages. The evolution of this type of life-cycle early in the embryo-phytic land plant lineages may have assisted with their colonization of land. This issue has led to the development of ecological and genetic hypotheses and tests in attempts to explain the adaptive advantages of maintaining two generations. However, only one hypothesis, proposed by Svedelius in 1927, has been found to be applicable across the diverse lineages that exhibit alternation of gen-erations. We discuss the evolution of the embryophytic land plants, their transition to land and shift from a haplobiontic life-cycle in their green algal ancestors to a diplobiontic life-cycle in the basal lineages of embryophytes. The evolution of matrotrophy and a multicellular sporophyte through de-layed zygotic meiosis are of particular interest. We then provide a brief summary of the ecological and genetic hypotheses regarding the advantages of maintaining two multicellular generations, and conclude in favor of the hypothesis proposed by Svedelius in 1927 that a lengthened diploid genera-tion and increased complexity can lead to increased genetic diversity.