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Gonium
''Gonium'' is a genus of colonial algae, a member of the order Volvocales. Typical colonies have 4 to 16 cells, all the same size, arranged in a flat plate, with no anterior-posterior differentiation. In a colony of 16 cells, four are in the center, and the other 12 are on the four sides, three each. A description by G.M. Smith (1920, p. 94):〔Smith, GM. Phytoplankton of Inland Lakes of Wisconsin, Part I, Wisconsin Geological and Natural History Survey, Madison, WI. (1920).〕
Gonium Mueller 1773: Colonies of 4-8-16 cells arranged in a flat quadrangular plate and embedded in a common gelatinous matrix or connected by broad gelatinous strands. Cells ovoid to pyriform, with a single cup-shaped chloroplast containing one pyrenoid. Each cell with two cilia of equal length, contractile vacuoles at the base of the cilia, and an eyespot. Four- and eight-celled colonies with the cilia on the same side ; sixteencelled colonies with the four central cells having their cilia on the same side and the twelve marginal cells with radially arranged cilia.
Asexual reproduction by simultaneous division of all cells in the colony to form autocolonies, or by a formation of 2-4 zoospores in each cell.
Sexual reproduction isogamous, by a fusion of biciliate zoogametes.
==Evolution==
The genus Gonium represents species closely related to single celled Chamydomonas and multicellular differentiated Volvox. The order Volvocales has long been a well recognized model system for the study of multicellular evolution. Gonium and the genus Tetrabaenecae contain species representative of colony formation among unicells. Gonium's morphology of colonies of alike cells suggest it is more genetically similar to Chlamydomonas than Volvox, a fact confirmed by phylogenetic analysis.
The Volvocales have been hypothesized to have evolved in twelve discrete steps. Gonium represents the first six evolutionary steps of multicellularity;〔 (1) incomplete cytokinesis, (2) partial inversion, (3) rotation of the basal bodies, (4) organismal polarity, (5) transformation of the cell wall into extra-cellular matrix (ECM), (6) genetic control of cell number. Although the exact order and progression through David Kirk's twelve steps of multicellular evolution 〔 are probably not necessarily linear and each occurs more dynamically than originally thought.〔
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