The Evolution of Complex Multicellularity in Streptophytes

The streptophyte lineage of green plants, comprised of charophyte algae and land plants, inhabits freshwater ecosystems and dominates nearly every terrestrial environment. Land plants form a clade within a paraphyletic assemblage of charophyte algae comprising a phylogenetic grade implying land plants evolved from an ancestral freshwater alga. Within the streptophyte lineage of green plants, the evolution of complex multicellular bodies in both haploid and diploid generations is unique to the lineage of land plants. The grade of charophyte algal lineages, on the other hand, exhibit a gradient of increasing complexity from unicellular, simple multicellular to relatively complex multicellular gametophyte, the haploid gamete producing generation, but always retain a unicellular sporophyte, the diploid spore producing generation. Here we discuss the origins of multicellularity in streptophytes and examine streptophyte characters that pre-date the origin of land plants, and which may have helped facilitate the evolution of a complex multicellular form in both generations during the colonization of land. We conclude by examining molecular genetic innovations underpinning complex tissue formation in land plants and considering whether the complex multicellularity in sporophyte generation of land plants evolved de novo or by co-option of an ancestral gametophyte program.