Gonium

Gonium consists of flat, planar colonies of four to 32 cells, all identical with the colony having no anterior-posterior differentiation. In one species Gonium dispersum, single cells may also be found. A description by G.M. Smith (1920, p. 94): ==Life cycle==

Gonium being evolutionarily related to Chlamydomonas has a life cycle that is derivative of that of Chlamydomonas. Gonium cells grow asexually as colonies of either 4, 8 or 16 colonial cells. Cell and colony growth of Gonium is uncoupled from cell division just like Chlamydomonas and each cell within the colony divides by multiple-fission. Thus, each cell within the colony will divide 2, 3 or 4 times, thus producing 2"n" daughter cells, or 4, 8 or 16 cells within the colony. Unlike Chlamydomonas where each of the daughter cells separate from each other, Gonium daughter cells remain attached to each other in their ECM. The sexual cycle of Gonium is also very similar to that of Chlamydomonas. The sexual program of Gonium is induced by nitrogen deprivation where each vegetative cell within the colony differentiates in gametes. Gonium gametes are isogamous, or equal-sized, and unicellular. Thus unicellular Gonium gametes break apart from the multicellular colonies when the sexual program is initiated. Also like Chlamydomonas, there are two "sexes", plus or minus controlled by a genes homologous to those found in Chlamydomonas and Volvox. ==Evolution==

The genus Gonium represents species closely related to single celled Chlamydomonas and multicellular differentiated Volvox. The order Volvocales has long been a well recognized model system for the study of multicellular evolution. Gonium and the family Tetrabaenaceae contain species representative of colony formation among unicells. Gonium 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. ==Taxonomy==

Gonium is the type genus of the family Goniaceae, which includes one other genus, Astrephomene. The two genera are characterized by having similar extracellular matrix ultrastructure. The similar genera Tetrabaena and Basichlamys were previously included in Gonium. Species of Gonium are distinguished based on morphological characters, such as the typical number of cells in a colony, shape of cells and their extracellular matrix, number of pyrenoids, and arrangement of cells within the colony. The following is a phylogeny based on the ITS region of nuclear ribosomal DNA (not all species are included): }} }} }} ==References==