Spermiogenesis and spermatozoa in Acanthodasys aculeatus (Gastrotricha, Macrodasyida): an ultrastructural study

Abstract

The spermatogenesis, the spermiogenetic process and the structure of the mature spermatozoon of Acanthodasys aculeatus (Gastrotricha, Macrodasyida) are described from an ultrastructural point of view. Several spermatogonia in mitotic divisions were seen, proving that euthely of gastrotrichs does not concern gonads. Spermiogenesis is characterized by the early formation of both the acrosome and the axoneme, by the subsequent appearances of a perinuclear helix and of a complex axial tubular structure in the acrosome and by the late development of the peraxonemal striated cylinder. The mature spermatozoon is filiform, and composed of a spiralized acrosome, a helical nuclear–mitochondrial complex and a long flagellum. The acrosome contains an axial tubular structure and the spring‐shaped nucleus delimits a single, long mitochondrion. A perinuclear helix formed by the pro‐acrosome surrounds the nuclear–mitochondrial complex extending for its whole length. A monolayered, obliquely striated cylinder encloses the 9 × 2 + 2 axoneme; its terminal part is empty because of the shortness of the axoneme.

Reproductive system of the genus Crasiella (Gastrotricha, Macrodasyida)

“…The presence in Crasiella of two club-shaped and hollow testes, which extend into sperm ducts confirms the basic plan of the male system of Macrodasyida (Ruppert 1991;Fischer 1994;Guidi et al 2003). However, it has been stated that Lepidodasys sp.…”

“…Region one is, in fact, also found in Urodasys anorektoxys (Balsamo et al 2007), Acanthodasys aculeatus (Guidi et al 2003), and in the Thaumastodermatinae examined so far; region two is present in Acanthodasys aculeatus, while region four is similar to that observed in Xenodasys eknomios (Guidi et al 2009) and Thaumastoderma moebjergi (Ferraguti et al, unpublished data). A nucleo-mitochondrial region, formed by a spring-shaped nucleus surrounding the mitochondrion, is also present in all of the Thaumastodermatidae, Lepidodasyidae and Turbanellidae studied so far, as well as in Mesodasys (Ferraguti and Balsamo 1994).…”

“…The fusion of mitochondria into a single element is followed by the penetration in the nucleus by this mitochondrial element. A comparable two-step process has also been described in Acanthodasys aculeatus (Thaumastodermatidae), but here the mitochondria are incorporated into the nucleus one by one, and it is only afterwards that they fuse into a single element (Guidi et al 2003). In Urodasys anorektoxys (Macrodasyidae), spermatogenesis is completely different, because the single mitochondrion gets lost at the end of the process, thus forming a highly specialized sperm that is lacking any mitochondria (Balsamo et al 2007).…”

“…The major steps in the process have been documented in five species belonging to five families: Turbanella cornuta, Lepidodasys sp., Cephalodasys maximus, Acanthodasys aculeatus and Urodasys anorektoxys (Teuchert 1976;Fischer 1994;Guidi et al 2003Guidi et al , 2004Balsamo et al 2007). Like in Turbanella cornuta (Turbanellidae), Lepidodasys sp.…”

Ultrastructural observations on the spermatozoa of a species of Lepidodasys (Gastrotricha, Macrodasyida)

“…Several recent studies have shown that gastrotrich spermatozoa are species-speciWc, and are useful phylogenetic characters (Balsamo et al 1998;Balsamo and Todaro 2002;Guidi et al 2003aGuidi et al , b, 2004Marotta et al 2005 previously examined has prompted us to observe its spermatozoa for integrating the available data on the genus.…”