Julia K. Zograf

Ultrastructure of spermatogenesis in the free-living marine nematode Halichoanolaimus sonorus (Chromadorida: Selachinematidae)

Spermatogenesis of the free-living marine nematode Halichoanolaimus sonorus was studied with electron microscopy. The spermatocyte cytoplasm is filled with ribosomes, mitochondria, cisternae of the RER and Golgi bodies. The spermatids are subdivided into the residual body, which includes the entire synthetic apparatus of the cell, and the main cell body with a centrally located nucleus lacking a nuclear envelope. The mitochondria and the precursors of the fibrous bodies form a layer at the periphery of the main cell body. The main cell body surface bears numerous filopodia. The immature spermatozoa from the testes are unpolarised cells with a centrally located nucleus surrounded by spherical fibrous bodies, mitochondria and membranous stacks; the cell surface is covered by numerous short filopodia. Spermatozoa from the uterus do not show the dramatic changes common for activated spermatozoa of nematodes. Their nuclei, mitochondria, fibrous bodies and surface filopodia remain intact. Bundles of filaments appear between the fibrous bodies. Some of the ultrastructural characters of the spermatogenesis of H. sonorus (late appearance of fibrous bodies and their filamentous structure, absence of membranous organelles at all stages of spermatogenesis, occurrence of numerous surface filopodia in the immature and mature spermatozoa) may be utilised as distinctive cytological characters for phylogenetic analysis of the order Chromadorida.

Ultrastructure of spermatozoa in the free-living marine nematode Paracanthonchus macrodon (Nematoda, Chromadorida)

Summary The spermatozoa from testes and uteri of the free-living marine nematode Paracanthonchus macrodon (Cyatholaimidae, Chromadorida) were studied with an electron microscope. The spermatozoa from the testes are unpolarized cells covered by numerous filopodia. The sperm nuclei have oval- or bean-shaped outlines and are surrounded by mitochondria. The peripheral cytoplasm is devoid of organelles and contains evenly distributed filamentous material. The filopodium content resembles that of the sperm cytoplasm. A submembrane layer of 13–16 nm thick, longitudinally oriented, microtubule-like fibres (MLF) strengthens the filopodia. The spermatozoa from the uterus are amoeboid cells with the discrete nucleus surrounded by mitochondria. The bulk of the sperm cytoplasm lacks organelles and is filled with MLF, which also underlie the sperm plasmalemma. The spermatozoa develop large pseudopodia which form hemidesmosome-like junctions with the uterus wall. A layer of a subsurface osmiophilic material concentrated at the junction point is associated with MLF, which run out into the cytoplasm as parallel arrays. The unique cytoplasmic components of the nematode spermatozoa, the membranous organelles and fibrous bodies, were not found in the spermatozoa of P. macrodon; this pattern of the spermatozoon structure is interpreted as a highly reduced condition.

New and known species of Enoplolaimus (Enoplida: Thoracostomopsidae) from the sea of Japan.

Two new thoracostomopsid species, Enoplolaimus pectinatus sp. n., E. patrokli sp. n., and one known species, E. medius , from the Sea of Japan, are described and illustrated with the aid of LM and LSM pictures. Enoplolaimus pectinatus sp. n. belongs to a group of species lacking a preanal supplementary organ and differs from the other species in this group by the shape of the spicules and the gubernaculum, which has a plate-like triangular crura provided with a comb of 14 denticles. Enoplolaimus patrokli sp. n. is characterised by the straight spicules and also differs from other species by the shape of the gubernaculum which has a plate-like cuneus with a dorsal, V-like, thickening and a small proximal dorsal process. Additional information is provided for E. medius on the basis of confocal microscopy. The structure of the head and spicules of E. medius is described. A pictorial key for the genus Enoplolaimus is proposed.

Ultrastructure of the male reproductive system of the free-living marine nematode Paracyatholaimus pugettensis Wieser & Hopper, 1967 (Chromadorida: Cyatholaimidae).

Although nematodes are a well studied group of multicellular organisms, until now the only information on the cellular structure of the male reproductive system of marine nematodes is that on the histology of free-living marine nematode from the order Enoplida. The fine structure of the male reproductive system of the free-living marine nematode Paracyatholaimus pugettensis (Chromadorida: Cyatholaimidae) from the Sea of Japan has been studied using TEM. The testis epithelium has a large distal tip cell similar to that described for representatives of the subclass Rhabditia. The epithelial wall of the testis is differentiated along its length. The proximal part of the epithelial tube consists of relatively large cells bearing numerous surface outgrowths that permeate between the developing spermatocytes. The epithelium in the middle region of the testis is formed from extremely flattened cells. The distal part of the testis – the seminal vesicle – is filled with immature spermatozoa and consists of absorptive cells. The seminal vesicle is followed by the vas deferens . The gonoduct is also differentiated along its length, the first third being formed from synthetically active epithelial cells, the two layers of which form a tiled structure. There is no lumen in the gonoduct and it is probable that, due to the tiled structure, the epithelial cells move apart to create space for the spermatozoa during ejaculation. The posterior two-thirds of the duct is surrounded by muscle cells that create the necessary pressure during ejaculation. The enlarged epithelial cells of the vas deferens show vigorous synthetic activity, which is probably involved in the transformation of immature spermatozoa into mature gametes.

Genital primordium of the free-living marine nematode Halichoanolaimus sonorus (Chromadoria, Chromadorida)

Summary The genital primordium of the first stage juvenile (J1) of the free-living marine nematode Halichoanolaimus sonorus (Chromadorida: Selachinematidae) was studied using transmission electron microscopy. The primordium consists of four undifferentiated cells: two primordial germ cells (PGC) 5–6 μm in diameter and two somatic cells. The PGC have a large nucleus with nucleolus. The centriole was detected in close vicinity of the PGC nucleus. Most of the cell mitochondria are in close contact with the nuclear envelope. The mitochondria are interspersed by 0.2–0.3 μm particles of an electron-dense diffuse substance devoid of surrounding membrane. Both PGC are closely attached to each other and to the neighboring somatic cells of the genital primordium. The elongated somatic cells contain nuclei devoid of nucleoli; the cytoplasm is filled with free ribosomes and contains occasional cisternae of rough endoplasmatic reticulum (RER), Golgi bodies, mitochondria, and transparent vesicles. The genital primordium is separated by a narrow space from of the intestine (dorsally) and the somatic muscles (ventrally). The PGC of H. sonorous are devoid of typical P granules known for previously studied nematodes as distinct markers of germ line cell lineage. Perinuclear particles of dense diffuse substance found in PGC of H. sonorous could be considered as germ determinants analogous to P granules.