Monterey Bay Aquarium Research Institute
Marine Botany

Plocamium

Evidence for the Plocamiales

Recently the taxonomy of the genera Plocamium has been revised. Plocamium used to be in the Gigartinales, but has been placed a new order, the Plocamiales. Saunders and Kraft (1993) describe the similarities and differences of Plocamiales with the Gigartinales and Ceramiales, and cite these and their PCR analysis of small-subunit rRNA gene sequences as evidence for making Plocamiales a separate division.

Similarities with Gigartinales (and differences with Ceramiales) include disc-type germination of sporlings; three-celled carpogonial branches; pseudoparenchymatous pericarp formed by cortical cell divisions; and zonate cleavage of tetrasporangia.

Similarities with Ceramiales (and differences with Gigartinales) include rami-sympodial growth; production of four pericentral cells; fusion of the fertilized carpogonium with the supporting cell (or auxiliary cell); a single, outward oriented gonimoblast initial, on which successive gonimoblasts arise to form clusters in which most cells mature into carpospores; and tetraspores produced in stichidia. Saunders and Kraft found differences of 87 to 105 nucleotides between the small sub-unit (SSU) rRNA of Plocamiaceae and representatives of the Sphaerococcaceae, Phacelocarpaceae, and Nizymeniaceae, with which the Plocamiaceae have been historically allied, while the Phacelocarpaceae and Nizymeniaceae differed by only nine base pairs, indicating a close evolutionary relationship.

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