Volvox is a green alga that belongs to a single clade that includes colonial forms as well as the single-celled Chlamydomonas. Figure 18.104.22.168 shows a cluster of Volvox that average apporimxately 350 µm in diameter and is visible to the naked eye. Each organism consists of over 2,000 doubly-flagellated cells (each looking like a Chlamydomonas cell) embedded in the surface of a gelatinous sphere of extracellular matrix (ECM). Within the sphere are some 16 gonidia (the darker objects).
Figure 22.214.171.124: Volvox is a genus of multicellular green algae. Image used with permission by http://www.mikro-foto.de (CC-SA-BY Frank Fox)
Volvox can reproduce both asexually and sexually. In asexual reproduction, the gonidia develop into new organisms that break out of the parent (which then dies). In sexual reproduction, the presence of an inducing chemical causes
- The gonidia of the males to develop into clusters of sperm.
- The gonidia of the females to develop into new spheres each of whose own gonidia develops into a pair of eggs.
- The sperm break out of the male parent and swim to the female where they fertilize her eggs.
- The zygotes form a resting stage that enables Volvox to survive harsh conditions.
The genome of Volvox carteri consists of 14,560 protein-encoding genes - only 4 more genes than in the single-celled Chlamydomonas reinhardtii! Most of its genes are also found in Chlamydomonas. The few that are not encode the proteins needed to form the massive extracellular matrix of Volvox.