The Comparative Ultrastructure of Sperm Flagella Central Sheath in Clitellata Reveals a New Autapomorphy of the Group

A comparative ultrastructural analysis of sperm tails of the hirudinean Hirudo medicinalis the branchiobdellid Branchiobdella pentodonta , and various microdrile oligochaetes ( Pananais frici, Paranais lioralis, Monopylephorus rubroniveus, Enchytraeus sp. and Bythonomus lemani has revealed a common modification of the primitive axonemal structure. A prominent central sheath embeds the two central tubules of the flagellum for a tract of variable length in the different species. The structure of radial links and link heads appears also modified: link heads, which are three for each radial link, are disposed in rows. Adjacent link heads arc also transversely connected to each other. The absence of these peculiar modifications in megadriles is interpreted as a secondary phenomenon. Since no homologous modification of the sperm tails is present in other spermatozoa, the prominent central sheath and radial links here described in three of the four taxa belonging to the Clitellata is considered a new autapomorphy for the whole group.