Ultrastructural study of sporogenesis in Ophioglossum vulgatum L.

Sporogenesis is a key process during asexual reproduction in pteridophytes. However, the formation of sporocyte and spore coats during sporogenesis and the origin of the lamellar layer and inner exospore are less understood. In the present investigation, we examined the process of sporogenesis in Ophioglossum vulgatum L. using transmission electron microscopy. Results showed that the sporocyte coat enclosed the spore mother cells, which underwent meiosis, with the coat then digested after the second meiotic division. A new spore coat was then formed to enclose each tetrad spore. Within the spore coat, the lamellar layer and inner exospore were formed, indicating that both were derived from spore cytoplasmic material. The spore coat was finally digested, after which the plasmodium directly enclosed each spore and the orbicules in the plasmodium were deposited on the lamellar layer to form the outer exospore. Simultaneously, the endospore was formed under the inner exospore. Lastly, the outermost perispore, which was composed of deeply stained material and membranous structures, was formed to cover the exospore. Our results indicated that the so-called surface coat actually consists of two coats, i.e., the sporocyte coat and the tetrad spore coat, which form individually and show no inheritance relationship. The lamellar layer and inner exospore are formed by spore cytoplasm material. The current study elucidated the formation of the sporocyte and spore coats during meiosis and the origin of exospore material in O. vulgatum, and thus should help improve our understanding of sporogenesis in pteridophytes.