Spermiogenesis and sperm ultrastructure in the freshwater musselDiplodon chilensis chilensis(Mollusca: Bivalvia)

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Abstract
Spermiogenesis and sperm ultrastructure of Diplodon chilensis chilensis have been studied. Early spermatids show some peculiar characteristics which include scanty cytoplasm mitochondria restricted to a juxtanuclear region and a poorly developed Golgi complex, a feature present throughout spermiogenesis. Mid and late spermatids show an elongated nucleus and a granular pattern of chromatin condensation. The Golgi complex does not show changes associated to the formation of structures of the acrosomal complex. The mature spermatozoon agrees in all main features with those of bivalves with external fertilization. It is thus of the primitive type with bullet-shaped nucleus and five mitochondrial spheres in the middle piece. The head of the mature sperm lacks acrosomal structures. Previously reported modifications in the reproductive biology of D. ch. chilensis with respect to aspects considered representative of bivalves, have not brought about major modifications in sperm morphology.