Sperm ultrastructure and spermatogenesis in the interstitial polychaete Sphaerosyllis hermaphrodita (Syllidae: Exogoninae)
|ANNELIDA; FERTILIZATION; HESIONIDAE; MALDANIDAE; MICROMALDANE; Polychaeta; Reproductive Biology; SIZE; sperm types; SPERMATOZOA; spermatozoon; ultrastructure; Zoology
|TAYLOR & FRANCIS LTD
|INVERTEBRATE REPRODUCTION & DEVELOPMENT
The mature spermatozoa of Sphaerosyllis hermaphrodita (Polychaeta: Syllidae) were reconstructed from electron micrographs. Their filiform shape (length 60 mu m), the elongated, helically coiled acrosome and the similarly shaped nucleus identify them as belonging to the modified sperm type often found in interstitial polychaetes. No discrete middle piece is present; the single mitochondrion, a long oval structure, is situated within the proximal urns of the nucleus. The axoneme of the long tail section contains microtubules in the conventional 9x2+2 arrangement, except that in the distal part the central microtubules are absent. A notable feature of the tail section is a cortex comprising a flat cuff, which coils around the flagellum in closely packed helicoid turns. The mature sperm are grouped in very orderly bundles of 20 or more, presumably held together by a matrix that surrounds the tail sections and is characterized by tubular structures oriented parallel to the long axis of the sperm cells. The primary spermatocytes are joined to one another by cytoplasmic bridges; a striking characteristic of these stages is the presence of two pairs of centrioles. The functional significance of this sperm morphology and their;grouping in bundles regarding the reproductive biology of this species are discussed, as is the recent debate about the direction in which sperm structure has evolved within the Bilateria.
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