Sperm dimorphism in the free-living marine nematode Terschellingia glabricutis (Nematoda: Monhysterida: Linhomoeidae)

Abstract

AbstractThe structure and development of the dimorphic spermatozoa of the free-living marine nematode Terschellingia glabricutis (Monhysterida, Linhomoeina, Linhomoeidae) were studied with transmission electron microscope (TEM). The linhomoeid pattern of spermatozoa includes absence of fibrous bodies (FB) in spermatocytes and their appearance in spermatids as spindle-shaped bodies never associating with membranous organelles (MO); presence of centrioles near the sperm nucleus; occurrence of filopodia in the spermatids and immature spermatozoa; unpolarised concentric structure of the mature spermatozoa; microtubule-like elements in spermatids and spermatozoa. The linhomoeins differ from their closest relatives, monhysterins (Xyalidae, Sphaerolaimidae), by total absence of MO at all the stages of spermatogenesis. This peculiarity is also found in the sperm development of some chromadorids, desmodorids and tylenchomorphs. In T. glabricutis the anterior testis produces large (20 μm) 'macrospermatozoa' of the linhomoeid pattern; the posterior testis produces much smaller (10 μm) 'microspermatozoa' of simplified structure. The female genital tubes also contain two types of amoeboid spermatozoa of different size and internal structure. Thus, T. glabricutis demonstrates a pattern of sperm dimorphism in which each testis of the diorchic male produces its own type of spermatozoa. Both types are found in the genital tube of fertilised females. Such sperm dimorphism is known from light microscope observations for several distantly related marine species although the biological significance of this dimorphism is obscure.