Heterothallic and homothallic sexual reproduction in Tabularia fasciculata (Bacillariophyta)

Sexual behaviour, pattern of auxosporulation, and breeding system of the araphid diatom Tabularia fasciculata were studied using 64 clones derived from two natural populations from the Canadian Atlantic seashore and the Black Sea. Both inter-and intraclonal reproductions were observed. Heterothallic reproduction was morphologically and behaviourally heterogamous. The heterothallic nature of sexual reproduction in this species allowed the determination of sex ratio in the population, which was 1:1 male:female. A novel type of active movement of gametes designated as male was described and pathways traced and recorded. A few male clones could reproduce intraclonally, but this was not abundant compared to interclonal reproduction. The advantages and inferred significance of each mode of reproduction are discussed. In the semi-natural population of T. fasciculata, two size classes were detected among sexualized individuals, and they gave rise to two size groups of initial cells. Sexual compatibility of all derived clones was independent of cell size class. In cultures, the apical length of initial cells was also strongly correlated to the size of parental gametangia. Cell sizes corresponding to cardinal points in the life cycle were established. Interbreeding of clones derived from the Canadian Atlantic and the Black Sea suggests a pan-Atlantic reproductive compatibility of populations of diatoms conforming to current morphological delineation of T. fasciculata.