Fine structure of the utricles and gametogenesis of Codium decorticatum (Caulerpales, Chlorophyta)

Abstract

Codium decorticatum utricles exhibited a large central vacuole surrounded by a thin parietal layer of cytoplasm. Nuclei and other organelles were placed in the outermost portion of the cytoplasm whereas chloroplasts protruded into the vacuole. The utricle cell wall was formed by a single stratum covered by a rugose cuticle. Only gametangia producing a single type of gametes were observed. Progametangia consisted of an apical vacuolate portion and a basal portion with partitioned cytoplasm. The clear apical portion of immature gametangia revealed abundant endoplasmic reticulum, dictyosomes and electron translucent vacuoles. Most of the gametangium volume was occupied by spherical nuclei and small chloroplasts. Gametes were delimitated by the activity of electron translucent vesicles. Mature gametes were pear-shaped with a prominent papilla occupied by the basal apparatus. An anterior nucleus was anteriorly flanked by a large mitochondrion. Spherical non-flagellate cells with a discrete cover were also observed in the gametangia. The gametes of Codium decorticatum resemble the male gametes of other siphonous green algae, in the flagellar apparatus morphology, in the appearance of the capping plate and the structure and location of the terminal caps, and in the presence of one large mitochondrion. We can assert that the Atlantic Argentinian populations produce only male gametes. Therefore, agamic germination of male gametes would be the only asexual reproduction mechanism of the Argentinian populations. Further studies are necessary to confirm the hypothesis that these populations reproduce asexually by germination of only one type of cell, male gametes that in this case may be functionally considered zoospores.