Laboratory-induced apogamy and apospory in ceratopteris richardii

Abstract

All land plants progress through a life cycle that alternates between two multicellular generations, the haploid gametophyte and the diploid sporophyte. The lifespan and autotrophy of the two generations have evolved from bryophytes to seed plants in opposite directions: the sporophyte generation became long-lived and autotrophic whereas the gametophyte generation became ephemeral and -heterotrophic during the land plant evolution. In ferns, seedless vascular plants of the Monilophyte clade, both generations are free living. In addition to the normal life cycle using meiosis to generate spores and sexual union to form zygotes, in nature many fern species can switch from one generation to another asexually. In the asexual pathways, termed apospory and apogamy, a gametophyte is generated from sporophytic cells without meiosis and a sporophyte is generated from gametophytic cells without fertilization, respectively. The model fern Ceratopteris richardii does not reproduce asexually through apospory and apogamy in nature but both pathways can be induced in the laboratory using specific culture conditions. The independence of the two generations in ferns and the ease of switching from one generation to the other through the asexual pathways offer a system suitable for studying how each generation is initiated. This developmental plasticity of crossing- generation barriers, i.e., meiosis and fertilization, is not unique to ferns and is manifested in the complex pathways leading to apomixis in some seed plants. This review summarizes the establishment of apogamy and apospory induction systems in C. richardii and their possible application in studying the initiation of each generation. The asexual pathways in ferns are compared and contrasted with the current understanding of apomixis in seed plants.