Sexual reproduction in the marine centric diatom Ditylum brightwellii (Bacillariophyta)

摘要

Ditylum brightwellii (T. West) Grunow in Van Heurck is now a model organism for population genetic studies of marine phytoplankton, but observations of sexual reproduction in this species are sparse and there are controversial aspects to the identification of its gametes. Culture studies and field observations of D. brightwellii from Wadsworth Cove (Castine), Maine (USA) showed that it was homothallic, producing and releasing two naked, spherical eggs from each oogonium and 64 uniflagellate sperm per spermatogonangium. Eggs and sperm were produced by larger and smaller cells, respectively, and were formed in nutrient-replete medium, without special procedures for induction. Size regeneration was achieved from a putatively fertilized egg via a true auxospore. Vegetative cell enlargement occurred in senescent cultures and resulted in intermediate-sized cells. The size structure of natural populations of D. brightwellii was monitored weekly in autumn 2004 and was unimodal throughout, with slight shifts in size accompanying low levels of gametogenesis and auxosporulation. Gametogenesis and size reduction ceased with the onset of winter. Previous reports concerning gametogenesis and auxosporulation in Ditylum are reinterpreted on the basis of our studies.