The Ultrastructure of Tetrasporogenesis in the Marine Red Alga Chondria tenuissima (Good, et Woodw.) (Ceramiales, Rhodomelaceae)

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Abstract
Tetraspore development has been studied in Chondria tenuissima using light and electron microscopy. The transformation of tetrasporangial mother cells into mature tetrasporangia involves a series of structural changes, especially of dictyosomes and of the nucleus. The youngest stage of tetrasporogenesis consists of a uninucleate tetraspore mother cell with synaptonemal complexes present during early prophase of meiosis I. Mitochondria are aggregated around the nucleus, dictyosome activity is low, and proplastids occur in the peripheral cytoplasm. The cleavage furrows are initiated almost concomitantly with commencement of meiosis. When the cleavage furrows are initiated, spherical bodies bounded by two membranes are found within the cytoplasm; they develop into vacuoles with fibrillar contents (fv1), which increase in size during tetraspore development by fusing with each other and with Golgi vesicles. The Golgi vesicles and the vacuoles with fibrillar contents (fv1) contribute material to the developing tetraspore wall. During the middle stage of tetraspore formation the vacuoles with fibrillar contents (fv1) are dominant, dictyosome activity increases, as well as the number of plastids and mitochondria; starch formation also increases. Stacked cisternae of the endoplasmic reticulum are found within the peripheral part of the nucleus. The same nuclear structures are also observed in tetrasporangia of the marine red alga Gastroclonium clavalum. The final stage is characterized by the disappearance of vacuoles with fibrillar contents (fv1) and of the stacked ER within the nucleus, presence of straight, large dictyosomes which produce cored vesicles, an abundance of starch grains and by the formation of fully developed chlorqplasts. The cored vesicles contain Thiéry-positive material and contribute to the formation of vacuoles with fibrous contents (fv2) as they are dominant in the tetraspores before their liberation.