OOGENESIS AND STALK MEDIATED FERTILIZATION IN PHYLLOSPORA-COMOSA, (LABILLARDIERE) AGARDH,C. (SEIROCOCCACEAE, PHAEOPHYTA)

Species of the family Seirococcaceae have been described as producing an oogonial collar and egg stalk. Oogonial collar, stalk and egg structure were investigated in Phyllospora comosa (Labillardiere) C. Agardh using light and electron microscopy techniques. Fertilization was observed using videomicroscopy. Oogonia develop a layered wall and an 'oogonial collar' of the inner wall layer around the oogonium apex. A single egg is extruded from each oogonium and held in close proximity to the receptacle surface by an elongate, mucilaginous stalk. The upper component of the stalk is derived from the inner oogonial wall layer and collar, with a persistent attachment to the outer oogonial wall layer or stalk base in the region of previous collar development. Upon release supernumerary nuclei are segregated as vestigial cells. Eggs are fertilised within the stalk matrix. Spermatozoid contact with stalks appears to be random, and spermatozoid shape appears to facilitate penetration of the stalk. It is suggested that stalk evolution in the Seirococcaceae may be related to the subtidal habitat of species in the family. The oogonial collar provides a release mechanism in the absence of periodic drying of the frond as occurs in intertidal habitats. Retention of eggs on stalks may retain eggs in an optimal position for fertilisation through successive sperm releases.