Evidence of sexual reproduction inSmithora naiadum(Erythropeltidales, Rhodophyta) and its evolutionary significance

Abstract

Smithora naiadum blades, which grow epiphytically on seagrasses, are monoecious gametophytes. Previous reports of male gametes are confirmed using light and electron microscopy. Spermatogenesis is initiated when vegetative cells of the monostromatic thallus divide anticlinally twice to give an associated group of four cells. Each of these cells then divides periclinally producing a distromatic thallus. A subsequent, unequal periclinal division of each of these cells forms a small cell that becomes the spermatangium and a larger cell that becomes the carpogonium. Abundant fibrous vesicles are produced during spermatogenesis. Female gametes and fertilization are reported for the first time. The fate of the zygote remains unresolved, but available evidence suggests germination into a diminutive, as yet unknown, life history stage. Because of its systematic placement, near the root of the red algal phylogenetic tree, the occurrence of sexual reproduction in Smithora has significant evolutionary implications.