Unusual spermatozoa and reproductive modalities ofXenodasys eknomios(Gastrotricha: Xenodasyidae)

Abstract

The rare macrodasyidan Xenodasys eknomios is the first member of the genus to be found in the Mediterranean Sea and the fourth species known worldwide. As Xenodasys has proved to be at the base of the Macrodasyida clade, we provide new data on the reproductive system and spermatozoa to try and shed light on the ground pattern of gastrotrich reproduction. The hermaphroditic system of X. eknomios consists of two testes with ventrolateral pores and two caudal ovaries. A sac-like frontal organ, generally containing a spermatophore, is enveloped by a basal lamina and attached to the body wall by muscular fibres, appearing as a permanent structure. The spermatophore contains mature, filiform, spermatozoa, each composed of acrosome, spiralized nucleus, connecting piece and flagellum. The complex acrosome is the predominant element and forms the axis of the sperm. Most of the acrosome, is surrounded by two helixes, the external one is the nucleus and the internal one is a crystalline-like ribbon structure. The peculiar acrosome-nucleus complex, and the long connecting piece appear as autapomorphies. The structural plans of the reproductive system and the spermatozoa support the current systematization of Xenodasyidae and provide evidence for a possible sperm transfer modality in these species.

Reproductive system of the genus Crasiella (Gastrotricha, Macrodasyida)

“…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is similar to that observed in Xenodasys eknomios (Guidi et al 2009) and Thaumastoderma moebjergi (Ferraguti et al, unpublished data).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 germinal epithelium of the Crasiella species forms the testes wall, like in many species of Macrodasyida (Ruppert 1991). The fusion of the two sperm ducts in a single median, ventral pore is a feature that Crasiella shares with Turbanellidae, Cephalodasys and Megadasys (Remane 1926;Schmidt 1974), whereas most Macrodasyida species have two separate, ventral male pores (Ruppert and Shaw 1977;Ruppert 1978a, b;Guidi et al 2009;Hummon and Todaro 2010). Only a few species have a direct connection between the sperm duct/s and the caudal organ (Mesodasys, Thaumastodermatidae: Ruppert 1991;Ferraguti and Balsamo 1994;Fregni et al 1999).…”

“…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). It is, however, absent in Xenodasys where, surprisingly, the nuclear spring surrounds part of the acrosome (Guidi et al 2009). In Crasiella, the flagellum is devoid of a striated cylinder, which is a characteristic present in Thaumastodermatidae, Cephalodasyidae (Hummon and Todaro 2010) and Macrodasyidae, with the exception of Urodasys anorektoxys (Balsamo et al 2007).…”

“…and different species of Thaumastodermatidae. In addition, hypotheses about these processes have been advanced by Teuchert (1968), Balsamo et al (2002), Kieneke et al (2008) and Guidi et al (2009) based upon microscopical and ultrastructural data on species of other families.…”

The evolution of the reproductive system of Urodasys (Gastrotricha: Macrodasyida)

“…), and Xenodasys (Guidi et al. ), and presence of spermatophore formation in such potentially basal genera (Hochberg & Litvaitis ; Todaro et al. ) suggests it is a plesiomorphic characteristic of gastrotrichs (but see Kieneke et al.…”