Volvocaceae

The simplest of the Volvocaeans are ordered assemblies of cells, each similar to the related unicellular protist Chlamydomonas and embedded in a gelatinous matrix.

In all Volvocaceans, the fertilization reaction results in the production of a dormant diploid zygote (zygospore) capable of surviving in harsh environments.

Once conditions have improved the zygospore germinates and undergoes meiosis to produce haploid offspring of both mating types.

Immediately after, the cell layer is inside out compared with the adult configuration—the apical ends of the embryo protoplasts from which flagella are formed, are oriented toward the interior of the plakea.

The mechanism of inversion has been investigated extensively at the cellular and molecular levels using a model species, Volvox carteri.

Schematic representation of the phylogenetic relationships of the volvocine algae and the parallel evolution of the spheroidal colony. Volvocine algae range from the unicellular Chlamydomonas to the multicellular Volvox through various intermediate forms and are used as a model for research into the evolution of multicellularity. The spheroidal colony is thought to have evolved twice independently within this group: once in the Volvocaceae, from Pandorina to Volvox, and the other in the genus Astrephomene. The phylogeny is based on previous reports. All drawings and photographs represent side views of individuals with anterior ends orienting toward the top of this figure. ^{[

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