Spermatogenesis and spermatozoon ultrastructure in Dugesia sicula Lepori, 1948 (Platyhelminthes, Tricladida, Paludicola)

We examine for the first time spermatogenesis, spermiogenesis and spermatozoon ultrastructure in Dugesia sicula Lepori, 1948 a sexual and diploid planarian living in Tunisian springs. This TEM-study shows that early spermatids joined by cytophores have rounded nuclei. During spermiogenesis, a row of microtubules appears in the differentiation zone beneath the plasma membrane and close to the intercentriolar body, which consists of several dense bands connected by filaments. Two free flagella (9+1 configuration) grow outside the spermatid. An apical layer of dense nucleoplasm develops and the flagellum appear, facing in opposite directions before rotating to lie parallel to each other after the intercentriolar body splits into two halves. Mitochondria are closely packed around the spermatocyte nucleus before fusing during spermiogenesis, to form a long mitochondrion, which lies parallel to the elongated nucleus along the ripe spermatozoon. The latter is thread-shaped and consists of two regions: the proximal process and a distal part. The former contains the nucleus and a part of the mitochondrion. The latter contains the rest of the mitochondrion and a tapering tail of the nucleus. Separation between these two regions is marked externally by the insertion zone of the two free flagella. The flagella extend posteriorly along the distal part of the spermatozoon. The spermatozoon nucleus consists of a lucent and a dense component coiled in a screw-like pattern around each other. The single row of peripheral microtubules consists of a maximum 40 microtubules in the middle part with an internal layer of three supplementary microtubules.