Green Algae and the Origins of Multicellularity in the Plant Kingdom

The green lineage of chlorophyte algae and streptophytes form a large and diverse cladewith multiple independent transitions to produce multicellular and/or macroscopically complex organization. In this review, I focus on two of the best-studied multicellular groups of green algae: charophytes and volvocines. Charophyte algae are the closest relatives of land plants and encompass the transition from unicellularity to simple multicellularity. Many of the innovations present in land plants have their roots in the cell and developmental biology of charophyte algae. Volvocine algae evolved an independent route tomulticellularity that is captured by a graded series of increasing cell-type specialization and developmental com-plexity. The study of volvocine algae has provided unprecedented insights into the innova-tions required to achieve multicellularity. The transition from unicellular to multicellu-lar organization is considered one of the ma-jor innovations in eukaryotic evolution (Szath-máry and Maynard-Smith 1995). Multicellular organization can be advantageous for several reasons. Foremost among these is the potentia