Ultrastructure of spermiogenesis and synthesis of enigmatic cytoplasmic inclusions in the spirally shaped spermatozoon of the polychaete Spio setosa (Annelida: Spionidae)

Abstract

AbstractThe sperm of Spio setosa (Polychaeta, Spionidae) are known to be very unusual in form; here, spermiogenesis and the structure of the spermatozoon in this species are described by transmission electron microscopy. While spermiogenesis is similar to that described for many other polychaetes, two notable exceptions to this process include the synthesis of abundant ring‐shaped and tubular, membrane‐bounded cytoplasmic inclusions in the midpiece, and the differentiation of a spirally shaped sperm head. Spermatids develop as free‐floating tetrads in the male's coelom. A microtubular manchette does not develop during chromatin condensation and nuclear elongation, and the spiral acrosome forms as a single Golgi‐derived vesicle that migrates anteriorly to become housed in a deep anterior nuclear fossa. Early in spermiogenesis, numerous Golgi‐derived, membrane‐bounded cytoplasmic inclusions appear in the cytoplasm; these ultimately occupy the sperm midpiece only. The mature spermatozoon in the male has a 15‐μm‐long head consisting of a nucleus coiled like a spring and a spiral acrosome with differentiated substructure, the posterior two thirds of which sits in an anterior nuclear fossa. The midpiece is wider than the rest of the spermatozoon and contains 9–10 spherical mitochondria surrounding the two centrioles, as well as numerous membrane‐bounded conoid and tubular cytoplasmic inclusions. The axoneme has a 9 + 2 arrangement of microtubules. By contrast, stored sperm in the female's seminal receptacles have lost the midpiece inclusions but contain an abundance of glycogen. The function of the midpiece inclusions remains unresolved, and the significance of their absence in stored sperm within the seminal receptacle and the appearance of midpiece glycogen stores remains unclear and requires additional investigation.