Chapter Four: Life Cycles in Major Lineages of Photosynthetic Eukaryotes, with a Special Reference to the Origin of Land Plants

Abstract

Meiosis and fertilization represent two fundamental events in the origin, development, and evolution of the eukaryotic life cycle. A life cycle with alternation of generations evolved independently in multiple diverse eukaryote lineages, and variations of this type of life cycle can be found in the photosynthetic red, brown, and green algae and embryophyte land plants. The adaptive advantages of maintaining two multicellular stages necessary for sexual reproduction have been explored on both ecological and genetic levels. However, no hypothesis has been found to be applicable across the diverse lineages. This review summarizes the types of life cycles found in photosynthetic eukaryotes, focusing on those lineages that exhibit an alternation of generations and the adaptive advantages such life cycles may confer. The evolution of the embryophyte lineage and its transition from an aquatic to terrestrial habitat are discussed, as is how the first-diverging embryophyte lineage, the liverworts, display intermediate life cycle characteristics between those of their green algal ancestors and later-evolving embryophytes. In addition, the adaptive advantages of maintaining two multicellular stages are reviewed, particularly the evolution of a multicellular sporophyte through delayed zygotic meiosis and its resulting genetic complexity. The hypothesis developed by Svedelius in 1927 is favored that a lengthened diploid generation with simultaneous increase of the body size allows production of a large number of genetically different gametes via meiosis, which further leads to production of diverse offspring after fertilization.