A revision of the genus Cyclophora and description of Astrosyne gen. nov. (Bacillariophyta), two genera with the pyrenoids contained within pseudosepta

Abstract

Ashworth M.P., Ruck E.C., Lobban C.S., Romanovicz D.K., and Theriot E.C. 2012. A revision of the genus Cyclophora and description of Astrosyne gen. nov. (Bacillariophyta), two genera with the pyrenoids contained within pseudosepta. Phycologia 51: 684–699. DOI: 10.2216/12-004.1The araphid pennate diatom genus Cyclophora is characterized by an elliptical to circular pseudoseptum at the centre of one valve; C. tenuis is the only commonly reported species. New species have included some with pseudosepta on both valves, and we emended the generic description to accommodate these. Three species of Cyclophora were described from light and scanning electron microscopy: C. castracanei sp. nov., C. tabellariformis sp. nov. and C. minor sp. nov. Of these, C. castracanei was isovalvar with a pseudoseptum on both valves; C. tabellariformis was heterovalvar but differed in shape from the type species; and C. minor was very small and had both iso- and heterovalvar frustules. Other differences included stria density and arrangement of slits in the apical fields. Developmentally, the areolae within the boundary of the pseudoseptum filled in as the pseudoseptum grew. Also described was Astrosyne radiata gen. nov., sp. nov., which possessed a pseudoseptum in both valves but was radially symmetrical in valve outline. Transmission electron microscopy (TEM) observations of cytoplasmic ultrastructural organization and three-gene sequencing (nuclear-encoded small subunit rRNA, rbcL and psbC) of cultured cells was carried out on all except C. minor. TEM showed that pyrenoids were localized within the pseudoseptum in both Cyclophora and Astrosyne. Phylogenetic analysis of the DNA sequences also supported the close relationship between these genera. While A. radiata was not the first radially symmetrical diatom suggested to be derived from an araphid pennate lineage, the localization of pyrenoids within a central pseudoseptum in the valve may be the most distinctive synaopomorphy amongst diatoms with such different valve outlines.