Heterothallism in the pennate diatom Haslea ostrearia (Bacillariophyta)

Twelve clonal cultures of the marine pennate diatom Haslea ostrearia (Gaillon) Simonsen were used to study sexual reproduction and auxosporulation. After crossing, compatible gametangia paired girdle to girdle, with no visible surrounding mucilage. Two gametes were formed per gametangium; these were not attached to the thecae of the parental frustule and gametes from one gametangium fused with the gametes from the other gametangium. Sexual reproduction was isogamous, resulting in the production of two zygotes per paired gametangia. The pattern of reproduction in H. ostrearia can be classified as type IB2a according to Geitler's system. The expansion of auxospores, which was bipolar, was accompanied by the formation of a transverse perizonium. Superfluous nuclei resulting from meiosis were not visible. The upper limit of the cell size range suitable for sexualization was c. 68 m, i.e. about half the maximum species-specific size of about 140 m. Sexual reproduction has been shown by previous authors to occur in clonal cultures and our observations confirm that two modes of sexual reproduction, homothallic and heterothallic, coexist in H. ostrearia.