Implications for Understanding the Origin of Land Plant Life Cycles

Two evolutionary ideas have been put out regarding the life cycles of putative “green algal” progenitors to explain the genesis of the alternation of generations in terrestrial plants.

1. Antithetic theory
2. Alternative homologous theory

Antithetic theory

According to the Antithetic theory, terrestrial plants descended from green algae with haploid, haplobiontic life cycles and anisomorphic alternation of generations is similar to Coleochaete, a “charophycean algae.” In a haploid, haplobiontic life cycle, the sporophyte (zygote) is unicellular, and the initial sporophytic cell division is meiotic. However, the gametophyte generation is multicellular. According to this theory, the sporophyte generation of terrestrial plants descended from somatic cell divisions crosslinked in the zygote before meiosis.

Alternative homologous theory

According to the Alternative homologous theory, the first green algae had an isomorphic alternation of generations and a diplobiontic life cycle. According to this theory, several characteristics of the sporophyte and gametophyte generations of land plants may have previously existed in the algal ancestor.

Non-flowering plants’ gametophytes and sporophytes are simple to monitor and regulate. As a result, they have frequently been used to study the developmental processes that underlie the alternation of generations. Moss, such as Physcomitrella patens, is amongst them and is an early-diverging land plant, making it a suitable model to determine the origin and evolution of developmental pathways. The recent advances in plant systematics have had a significant impact on current theories regarding the evolution of alternation of generations, which are based on life cycles in existing taxa.