Auxospore fine structure and development in the surirelloid diatom <i>Campylodiscus</i> cf. <i>neofastuosus</i> (Bacillariophyta)

The development of the auxospore is reported in the surirelloid diatom Campylodiscus cf. neofastuosus. As in most pennate diatoms, growth of the auxospore is accompanied and constrained by the formation of a transverse perizonium composed of finely structured bands, which are added one after another as the auxospore expands. However, unlike in most pennate diatoms, in which the transverse perizonium develops bidirectionally from a hoop-like or shortly cylindrical primary band formed around the equator of the zygote, in C. cf. neofastuosus, development is unidirectional, outward from a heart-shaped cap at one end of the auxospore. Limited evidence from other surirelloid diatoms suggests that unidirectional perizonium development may be typical of the group. This developmental pattern correlates with the profound symmetry shifts that have occurred during the evolution of the circumferential raphe system of Surirellaceae and may have a common origin with these, despite the difference in life-cycle stage. The longitudinal perizonium of Campylodiscus also exhibits differences from the arrangement typical of most raphid diatoms, since it comprises three concentrically organized bands, including a unique, heavily silicified secondary band, which runs around the whole circumference of the wide primary band. In this and some other Surirellaceae, the longitudinal perizonium seems to play an important role in the morphogenesis of the initial cells by controlling the shape of the protoplast during a contraction before the formation of the initial epivalve. Small extra perizonial elements, unlike any reported previously in raphid diatoms, were detected beneath the suture formed by the ends of the transverse perizonial bands.