Volvocaceae are a family of unicellular or colonial biflagellate algae, including the typical genus Volvox, and are collectively known as the volvocine algae. The family was named by Ehrenberg in 1834, and it is known in older classifications as the Volvocidae. All species are colonial and typically inhabit freshwater environments. They are particularly useful as model organisms for studying the evolution of multicellularity, the evolution of sex, and cellular motion and mechanics.

Description

Volvocine algae consist of multiple, biflagellate cells. Each cell has a cell membrane, a central nucleus, multiple mitochondria, and one large chloroplast with an associated stigma (also known an eyespot). The stigma is reddish due to the presence of rhodopsin. The apical end of the cell has two equal-length flagella; the flagella beat in a manner similar to a breaststroke.<!--- ref for entire para --->

thumb|left|Volvox sp.

Other genera of Volvocaceans represent another principle of biological development as each organism develops differented cell types. In Pleodorina and Volvox, most cells are somatic and only a few are reproductive. In Pleodorina californica a colony normally has either 128 or 64 cells, of which those in the anterior region have only a somatic function, while those in the posterior region can reproduce; the ratio being 3:5. In Volvox only very few cells are able to reproduce new individuals, and in some species of Volvox the reproductive cells are derived from cells looking and behaving like somatic cells. In V. carteri, on the other hand, the division of labor is complete with reproductive cells being set aside during cell division, and they never assume somatic functions or develop functional flagella. Thus, the simplest Volvocaceans are colonial organisms but others are truly multicellular organisms. Colony inversion during development is a special characteristic of this order that results in new colonies having their flagella facing outwards. During this process reproductive cells first undergo successive cell divisions to form a concave-to-cup-shaped embryo or plakea (curved plate) composed of a single cell layer. 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. Then the embryo undergoes inversion, during which the cell layer inverts to form a spheroidal daughter colony with the apical ends and flagella of daughter protoplasts positioned outside. This process enables appropriate locomotion of spheroidal colonies of the Volvocaceae. The mechanism of inversion has been investigated extensively at the cellular and molecular levels using a model species, Volvox carteri. Another species Volvox globator has a similar mode of colony inversion, but begins at the posterior instead of the anterior. Volvocine algae, including the taxa Tetrabaenaceae, Goniaceae and Chlamydomonas, are well-studied for their transitions between multicellularity. Unlike most multicellular lineages (such as animals and land plants) whose transition from unicellularity to multicellularity was long ago, Volvocaceae and their multicellular relatives diverged relatively recently from the unicellular Chlamydomonas reinhardtii. Genera within Volvocaceae have been defined based on morphological characters such as the number and shape of cells, the spacing between the cells, number of pyrenoids, and mode of sexual reproduction. However, it is known that many of these genera are polyphyletic or paraphyletic.

Phylogeny

Phylogenomic studies suggest the following relationships (not all genera and species are included):

Notes

References

  • (Research on Volvox carteri)