Atogenesis and Euspermatozoa in Cerithidea Obtusa (Lamarck 1822) (Caenogastropoda: Potamididae) Jintamas Suwanjarat*1 & Waltraud Klepal2
Total Page：16
File Type：pdf, Size：1020Kb
Ultrastructural Investigations of Eusperm- atogenesis and Euspermatozoa in Cerithidea obtusa (Lamarck 1822) (Caenogastropoda: Potamididae) Jintamas Suwanjarat*1 & Waltraud Klepal2 1 Department of Biology, Prince of Songkla University, Hat-Yai, Thailand 90110. 2 Institute of Zoology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria. *Author to whom correspondence should be addressed. E-mail: [email protected] Abstract Abstract. Species of the Potamididae occupy the full range of aquatic habitats and differ not only in the morphology and size of their shells but also in sperm morphology. In the past, several species were classified as Cerithiidae. Characters of the developing and the mature spermatozoa have been used to gain better insight into their taxonomy. Cerithidea obtusa (Lamarck 1822) is the most dominant brackish water gastropod of the mangrove forests in Southern Thailand. Spermatological data are scarce in this species. In order to confirm its taxonomic position within the Potamididae, euspermatogenesis and euspermatozoa are examined by transmission electron microscopy. The morphological changes during spermiogenesis such as nucleus condensation, acrosome formation and development of the midpiece are described. Problem The Potamididae is a large and important family of Cerithioidea which varies greatly in shell characters and shell size. Houbrick (1991) commented that because of the unawareness of the significant anatomical differences among cerithioidean gastropods many Potamididae were erroneously classified as Cerithiidae. In addition, many genera of each potamidid subfamily were mistaken for each other and for Cerithiidae. The modern classifications of prosobranchs, to which the Caenogastropoda belong, are generally achieved by light- and electron microscopy (Haszprunar, 1988). In particular, studies of mature spermatozoa and spermiogenesis have provided insight into characters which have become incorporated into prosobranch systematics (Franzén, 1955, 1970; Healy, 1988a). The morphological diversity of spermatozoa in prosobranchs has been considered as a guide to judge the phylogenetic and taxonomic relationships within molluscs. Many authors have confirmed the taxonomic position of prosobranch gastropods by considering sperm-data (Morton & Young, 1964; Healy, 1983; Robertson, 1985; Healy, 1988b, c; Minniti, 1993). Although a number of recent investigations have dealt with sperm ultrastructure of the superfamily Cerithioidea, information on sperm- atogenesis of euspermatozoa of potamidid gastropods is scarce (Healy, 1982b, 1986a, b; Attiga & Al-Hajj, 1996), and a detailed description of the early stages of spermatogenesis at the ultrastructural level is missing. The present investigation therefore aims to fill this gap by providing a detailed description of spermatogenesis and the mature euspermatozoon of the potamidid Cerithidea obtusa (Lamarck 1822). Similarities and differences in the spermatozoa of C. obtusa will be correlated with those of closely - related species as well as with other caenogastropods. Apart from providing further evidence for the phylogenetic relationship of this species, this study also has ecological implications. Cerithidea obtusa is one of the most dominant and conspicuous brackish water gastropods found on the mudflats in the mangrove forests of Thailand and is therefore used as an index of fertility. Despite their abundance, accessibility and economical importance as food (Amornjaruchit, 1988), their biology, and especially their reproductive biology, is hardly known. Attempts have been made to examine their ecological adaptation, population dynamics, feeding activity (Kasinathan & Natarajan, 1981; Kitting, 1989) and even radular morphology (Suwanjarat, 1994; Suwanjarat & Suwaluk, 1997 observing was carried out at the Institute of Zoology, University of Vienna, Austria. In Cerithidea obtusa, as in many other caenogastropods, spermatogenesis occurs in the testes, in which the different developmental stages are found. 1. Spermatogonia Spermatogonia are close to the wall of each germinal follicle. The large, round nucleus of the spermatogonium has one or two nucleoli. Small clumps of electron-dense chromatin are loosely distributed in the nucleoplasm (Fig. 1). The narrow rim of cytoplasm around the nucleus contains a few rounded mitochondria, a small amount of endoplasmic reticulum and a Golgi body. 2. Spermatocytes The primary spermatocytes of Cerithidea obtusa are roughly spherical to irregular in shape. Their round or ovoid nuclei contain patchy chromatin with one large and highly electron-dense nucleolus. There are many pores in the nuclear membrane. The amount of cytoplasm around the nucleus is greater than in the previous stage; several round mitochondria and electron-dense inclusions are scattered within it. Rough endoplasmic reticulum, arranged concentrically, surrounds one large electron-dense granule (Fig. 2). The secondary spermatocytes are smaller than the primary spermatocytes. Their nuclei are round with some electron-dense chromatin dispersed throughout the nucleoplasm and there are numerous pores in the nuclear membrane. The cytoplasm is granular and contains a well-developed Golgi body, round mitochondria, rough endoplasmic reticulum and some moderately sized electron-dense granules. The developing spermatocytes are connected by intercellular bridges (Fig. 3). 3. Spermatids The spermatid may be distinguished from the spermatocyte by its general denser appearance and by the numerous organelles in the cytoplasm. During spermiogenesis the spermatids undergo differentiation to become mature spermatozoa. The following major steps are taken: nucleus condensation, acrosome formation, development of midpiece and axonemal complex. a. Nucleus condensation The early spermatid has a spherical, eccentric nucleus. There are still many pores in the membrane. The nucleoplasm is granular with some aggregation of heterochromatin and thus it appears more electron- dense than in the previous stages (Fig. 4). The nucleoplasm progressively accumulates electron-dense chromatin, concomitantly with the change of the nuclear shape to irregular (Figs. 5, 6). As spermiogenesis proceeds, the granular chromatin thickens at the periphery and begins to accumulate at the posterior nuclear pole. The nucleus is compressed posteriorly and laterally expanded; it has its greatest diameter perpendicular to the flagellar axis. The nucleus is now in its fibrous phase with parallel fibrillar appearance of the chromatin (Fig. 7). The granular cytoplasm of the early spermatid shows a Golgi body surrounded by vesicles, several large mitochondria and rough endoplasmic reticulum. Cytoplasmic bridges connecting spermatids are often observed (Fig. 8 electron-dense fibrils which are elongated and oriented longitudinally along the nuclear vertical axis. At the basal pole of the nucleus is a deep invagination, the indendation fossa, with the axoneme insertion (Figs. 9, 13). As the next step of nuclear condensation the chromatin fibrils thicken and form lamellae (Figs. 10, 14). During further development the nucleus lengthens and the chromatin lamellae thicken, leaving only a few spaces between them (Fig. 11). The condensation is clearly correlated with nuclear elongation. At the late spermatid stage, the nucleus is columnar with a length to breadth ratio of no more than 4 : 1. b. Acrosome formation Acrosome formation begins with the proacrosomal vesicle arising from the Golgi apparatus concomitantly with the nuclear condensation (Fig. 15). The proacrosomal vesicle elongates and differentiates to the pre-attachment acrosome. It is surrounded by microtubules oriented longitudinally; parallel to its wall it has an inner supporting structure and at one end of the vesicle there is an electron-dense granule (Fig. 16). The vesicle develops into the hollow acrosomal cone with the electron-dense granule fixed to its base (Fig. 17). This pre- attachment acrosome moves anteriorly towards a slight apical depression of the condensing nucleus (fibrous phase), then it tilts and moves into a vertical position (Fig. 18). As it begins to tilt, the large dense granule separates from the base of the acrosomal cone and attaches to the depression in the nuclear apex (Fig. 19). Here the dense granule flattens to form a basal plate with an acrosomal rod arising in its center. The internal supporting structure of the acrosomal cone disappears, while an external supporting structure appears and remains during the process of acrosomal development until it is gradually obscured in the mature acrosome. The acrosomal cone, which is invaginated at the base, is then placed upon the basal plate with its prominent acrosomal rod extending into the subacrosomal space (Fig. 20). The acrosome then undergoes pronounced elongation whereby the inner lining of the acrosomal cone thickens (Fig. 21). At the apex the tapering inner lining fuses, thus causing a constriction in the subacrosomal space. The external supporting structure still exists at this phase (Figs. 22, 23, 24). As development proceeds towards maturation the axial rod becomes increasingly obscured and scarcely appears in the mature acrosome. c. Development of midpiece and axonemal complex In the early spermatid the mitochondria distributed throughout the cytoplasm begin to aggregate at the posterior pole of the nucleus. As the nuclear chromatin condenses they fuse to form four large, spherical mitochondria surrounding the axoneme (Fig. 8). In the course of the midpiece-development