Mature spermatozoa of Brevibucca sp (Nematoda: Rhabditida: Brevibuccidae)

Mature spermatozoa found in the female gonoduct of the nematode Brevibucca sp. (Brevibuccidae, Rhabditida) were studied with transmission electron microscopy. As in other nematodes, the spermatozoa in Brevibucca sp. represent an aberrant type of male gametes characterised by the absence of an axoneme and an acrosome. Mature spermatozoa of Brevibucca sp. are amoeboid bipolar cells Ca 4-4.5 mu m diam.; each cell is subdivided into a main cell body and a pseudopod devoid of organelles. The main cell body includes a condensed nucleus, many mitochondria and so-called 'membranous organelles' (MO). These unique organelles are characteristic of developing, as well as of mature, spermatozoa of many nematodes. The MO in Brevibucca sp. sperm resemble large (ca 0.5 diam.) vesicles with transparent content and a system of internal finger-like projections of the outer membrane. Each MO is joined to the plasmalemma of the main cell body and is open to the intercellular exterior via a pore. Each spermatozoon has a prominent pseudopod filled with the filamentous components of the cytoskeleton. The structure and development of the rhabditid (order Rhabditida apud De Ley & Blaxter, 2002) sperm is somewhat conserved throughout the order. That is, the general pattern of spermatozoon recognized as "rhabditid" in Brevibucca sp. is similarly expressed in Spiruromorpha, Ascaridomorpha, Panagrolaimomorpha, Tylenchomorpha and Rhabditomorpha. Thus, such conservation lacks phylogenetic pointers at the scale of order. The spermatozoa of the rhabditid pattern observed also in Brevibucca sp. may be considered as an ancestral or symplesiomorphic character for several clades of the order Rhabditida.