Developmental Morphology of Reproductive Structures of Zostera and a Reconsideration of Heterozostera (Zosteraceae)

Abstract

The Zosteraceae, a family of marine sea grasses that are aquatic monocotyledons, is presently composed of three genera: one dioecious genus, Phyllospadix, and two monoecious genera, Zostera and Heterozostera. The reproductive structures of all genera in this family are unique, consisting of a flattened spadix with flowers on one side, all enclosed within a spathe. Using techniques of epi-illumination light microscopy, SEM, and paraffin embedding, we examined the developmental morphology of the complex and unusual reproductive structures in Zostera and Heterozostera Floral development studies showed that retinacules (speculated to be modified perianth parts) arise first, followed by anthers consisting of two bisporangiate thecae joined by a connective, and finally the gynoecium, consisting of a unilocular ovary. The best interpretation of these reproductive structures is that they are male and female parts of a complex inflorescence. Based on reproductive and vegetative comparative studies, the similarities between the genera Zostera and Heterozostera do not warrant placing Heterozostera tasmanica into a distinct genus but do warrant reinstating Zostera tasmanica into the genus Zostera in the subgenus Heterozostera.