THE FOUR‐CELLED SPECIES OF GONIUM

STEIN, JANET R. (U. California, Berkeley.) The four-celled species of Gonium. Amer. Jour. Bot. 46(5): 366-371. Illus. 1959.-A review is given of the 4-celled species of Gonium 0. F. Muller 1773 (Volvocales). From a study of clonal isolates, it has been possible to distinguish 2 distinct species, G. sociale (Dujardin) Warming 1876 and G. sacculiferum Scherffel 1904. The former may be divided further, on the basis of vegetative morphology and sexual reproduction, into 2 varieties: var. sociale and var. sacculum var. nov., and a possible third variety, var. majus Hansgirg 1888. A brief discussion of morphology and sexual reproduction (reported for the first time in G. sociale var. sacculum and G. sacculiferum) is presented as is a key to the species and varieties. IN THE COLONIAL volvocalean genus Gonium 0. F. Muller, the cells are arranged in a square, slightly curved plate. The colonies consist of 4, 8, 16, or 32 cells with the number usually constant for a given species. Since colonies with 8 or more cells sometimes tend to fall apart, at times forming 4celled fragments, it is often difficult to recognize a true 4-celled species without first maintaining the alga in clonal culture. The present study was undertaken with the purpose of establishing whether criteria exist whereby these 4-celled species can be delimited. At present, two 4-celled species of Gonium are recognized: G. sociale (Dujardin) Warming 1876 (fig. 1-4) and G. sacculiferum Scherffel 1904 (fig. 5-8). Gonium sociale has been further sub-divided into several varieties. The cells of all the 4-celled taxa contain a single bowl-shaped chloroplast and generally a single pyrenoid, a small anterior eyespot, 2 anterior contractile vacuoles and 2 flagella that arise on either side of an anterior papilla. HISTORY.-The most familiar 4-celled species is Gonium sociale. As early as 1841 Dujardin described a 4-celled motile colonial alga which he named Cryptomonas (Tetrabaena) socialis. He thought that it might be a species of Goniurn (sensu Ehrenberg, 1838) but the colony appeared to lack a sheath and each of the cells seemingly possessed only I flagellum. Braun in 1847 (according to Cohn, 1877) and in 1875, Fresenius in 1856, and Archer in 1865 also noted a 4-celled Gonium-like alga, but Warming in 1876 was the first to refer it to the genus Gonium. In his notes, Warming referred to the alga as G. quadrijuga, but, after surveying the literature, he concluded that the organism described by Dujardin as Cryptomonas socialis was identical to his manuscript species. He therefore made the new combination, G. sociale (Dujar. din) Warming. In 1877, Cohn published a description of a 4'Received for publication December 4, 1958. 2 This study was started at Indiana University, Bloomington, while the author was the holder of the Fanny Bullock Workman Fellowship awarded by Wellesley College. celled species of Goniurm. He was aware of Warming's work but chose to call the alga G. tetras A. Braun because Braun, as early as 1847, had recognized the 4-celled alga as a species of Goniun. In 1888 Hansgirg placed G. tetras A. Braun ex Cohn 1877 in synonomy with G. sociale (Dujardin) Warming 1876. As recently as 1902, the invalid name G. tetras was still used by some workers (e.g., Snow, 1902). Even though Warming recognized the 4-celled alga as an autonomous species, he thought it not unlikely that it was a stage in the life-history of the 16-celled Gonium pectorale 0. F. Muller 1773. Since Warming's publication, it has been shown that there definitely is a 4-celled species (Hansgirg, 1888; Migula, 1890; Chodat, 1894; Lemmermann, 1897; Pascher, 1927; Smith, 1930 ; Starr, 1955; Pocock, 1955) as well as a 4-celled developmental stage of G. pectorale. This developmental stage is the germ colony produced by meiosis during germination of the zygospore (Schreiber, 1925; Stein, 1958b). In 1896, W. and G. S. West described a 4-celled Gonium-like alga as a new genus Tetragoniun, with the single species T. lacustre; but G. S. West in 1904 placed the species in Gonium. Pascher (1927) did not consider G. lacustre an autonomous species and placed it in the species G. sociale. The Wests believed that their organism was a different species because the 4 cells diverged markedly from one another and the long flagella (2 per cell) vibrated only at their tips owing to the presence of a thick gelatinous matrix. Korshikov (1940) in studying colonies of G. sociale noted that in his material the matrix sometimes was quite thick. He believed this to be the result of environmental conditions, possibly low pH. The divergence of the cells of the colony of G. lacustre probably is not a significant enough character to warrant recognition of this taxon as an independent species, although it may represent a variety of G. sociale. The second 4-celled species, Gonium sacculiferum Scherffel 1904, is less well-known and there has been some discussion as to its validity as a separate entity. Chodat (1894), who originally illustrated it,