Auxosporulation in Biddulphia tridens (Ehrenberg) Ehrenberg (Mediophyceae, Bacillariophyta)

We documented male and female gametogenesis, auxospore structure and development of the biddulphioid diatom species Biddulphia tridens (Ehrenberg) Ehrenberg, a close relative of the generitype. Our mating experiments showed that the B. tridens clones were homothallic, self-fertile and produced two eggs per oogonium. During spermatogenesis, spermatogonia developed by depauperating mitosis from spermatogonangium cells and four hologenous uniflagellate male gametes were produced per spermatocyte. In the early stage of auxospore development, cells were globular to sub-globular in shape and covered with a siliceous envelope. The incunabular scales on these auxospores showed a great diversity in size and shape. In the later stages of development, auxospores grew anisodiametrically, facilitated by transverse perizonial bands. Initial epivalve formation was preceded by the retraction of the protoplast from the auxospore wall. The structural features of sex cells such as incunabular scales found in B. tridens are distinct from other broadly defined biddulphioid diatoms thus far examined. This is consistent with the distant relationship between true Biddulphia species and the Biddulphia-like diatoms indicated by molecular phylogenies. Morphological and molecular examination of B. tridens clones were performed here only to confirm species identity. This complete reproductive dataset from a true Biddulphia species will provide valuable insights for future studies of the relationships between Biddulphia-like diatoms and true Biddulphia species.