Histochemical and electron microscope observations on the reproductive tract of Biomphalaria glabrata (Australorbis glabratus), intermediate host of Schistosoma mansoni

LOSP: A putative marker of parasperm lineage in male reproductive system of the prosobranch mollusk Littorina obtusata

Reproductive isolation is the key attribute of biological species and establishment of the reproductive barriers is an essential event for speciation. Among the mechanisms of reproductive isolation, gamete incompatibility due to the variability of gamete interaction proteins may drive fast divergence even in sympatry. However, the number of available models to study this phenomenon is limited. In case of internally fertilized invertebrates, models to study gamete incompatibility and sperm competition mechanisms are restricted to a single taxon: insects. Here, we propose a group of closely related Littorina species as a new model for such studies. Particularly since periwinkles are already thoroughly studied in terms of morphology, physiology, ecology, phylogeny, and ecological speciation. Earlier, we have identified the first species-specific Littorina sperm protein (LOSP) with no known conservative domains or homologies. LOSP is relatively abundant component of sperm extracts and might be involved in gamete incompatibility. Here, we characterize its definitive localization and mRNA expression pattern in the male reproductive system by immunocytochemistry and RNA in situ hybridization. We demonstrate that LOSP distribution is limited to the parasperm cells. Losp gene expression occurs only at the early stages of parasperm development. The protein is stored within granules of mature parasperm and, most likely, is released after ejaculation inside female reproductive system. Thus, LOSP is the only described molluscan paraspermal protein to date, and there is a possibility for LOSP to be involved in gamete incompatibility since heterospermy is a common phenomenon among Littorina.

Neuro-endocrine control of reproduction in hermaphroditic freshwater snails: mechanisms and evolution

Invertebrates are used extensively as model species to investigate neuro-endocrine processes regulating behaviors, and many of these processes may be extrapolated to vertebrates. However, when it comes to reproductive processes, many of these model species differ notably in their mode of reproduction. A point in case are simultaneously hermaphroditic molluscs. In this review I aim to achieve two things. On the one hand, I provide a comprehensive overview of the neuro-endocrine control of male and female reproductive processes in freshwater snails. Even though the focus will necessarily be on Lymnaea stagnalis, since this is the best-studied species in this respect, extensions to other species are made wherever possible. On the other hand, I will place these findings in the actual context of the whole animal, after all these are simultaneous hermaphrodites. By considering the hermaphroditic situation, I uncover a numbers of possible links between the regulation of the two reproductive systems that are present within this animal, and suggest a few possible mechanisms via which this animal can effectively switch between the two sexual roles in the flexible way that it does. Evidently, this opens up a number of new research questions and areas that explicitly integrate knowledge about behavioral decisions (e.g., mating, insemination, egg laying) and sexual selection processes (e.g., mate choice, sperm allocation) with the actual underlying neuronal and endocrine mechanisms required for these processes to act and function effectively.

Pallial oviduct of Pomacea canaliculata (Gastropoda): ultrastructural studies of the parenchymal cellular types involved in the metabolism of perivitellins

Seasonal variations in the morphology of the parenchymal mass and function of the albumen gland/capsule gland complex have been studied in Pomacea canaliculata, together with the cellular types involved in the synthesis and secretion of perivitellin fluid components. The two major parenchymal cell types, albumen secretory cells (AS) and labyrinthic cells (LC), undergo seasonal variations throughout the annual reproductive cycle, which is divided into three periods. Both cellular types show maximal development and structural complexity during the reproductive period (spring and summer). AS cells have a well-developed Golgi complex and rough endoplasmic reticulum and their secretory granules show electron-dense particles of about 20 nm (probably galactogen). These cells are uniquely involved in ovorubin and PV2 perivitellin synthesis and their secretory granules are the single storage site for these two major perivitellins, as revealed by immunoelectron microscopy. AS also possess calcium deposits that infiltrate the cytoplasmic matrix. The luminal surfaces of LC exhibit long cilia intermingled with sparce short microvilli. Basally, the plasma membrane shows deep irregular folds that extend through the cytoplasm up to the subapical region. Calcium deposits infiltrate the cytoplasm and accumulate in the extracellular space of the basal labyrinth. Nerve terminals seem to be involved in the regulation of parenchymal cell secretion. At the post-reproductive period, AS markedly change their aspect following the release of most of the secretory granules into the acinar lumen. LC decrease in volume, the number of their cilia decreases, their cytoplasmic folds are much thinner and their extracellular spaces lack calcium particles. At the pre-reproductive period (winter), AS and LC recover and prepare for the subsequent period.

The influence of self-fertilization performance and copulation behaviour in reproduction by cross-fertilization in groups of Biomphalaria tenagophila (Mollusca, Planorbidae)

The following hypotheses were tested for groups of simultaneous hermaphrodites Biomphalaria tenagophila: (a) snails that have low reproductive success during the process of self-fertilization do not increase their reproductive success after the end of grouping; (b) the copulation behaviour and the presence of one snail whose eggs have a low viability rate influence the partner's reproductive success by cross-fertilization. Groups were constituted by a homozygous pigmented snail and two albinos: one with a viability rate higher than 70% ("good reproducers") and the other less than 10% ("bad reproducers"). All pigmented snails had viability rates higher than 70%. The "good" and "bad" reproducer albino snails had similar copulation behaviour. However, after the end of grouping, the "bad reproducers" continued to have viability rates less than 10% over 30 days. In 100% of the cases that pigmented snails copulated (performing either a male role or simultaneously male and female roles) exclusively with "good" reproducer albinos, they presented high reproductive success (producing, on average of 8.4 pigmented embryos/egg-mass). However, in 100% of the cases that pigmented snails copulated with both partners, the "good" reproducer albino snails produced none or very few embryos (the highest average was 2.2 pigmented embryos/egg-mass). Therefore, the production of viable embryos by cross-fertilization was more influenced by self-fertilization performance than by copulation behaviour. The presence of a snail whose eggs have a low viability rate could decrease their partners reproductive success.

Egg capsule formation and hatching in the marine snail Littorina sitkana

The process of egg capsule formation and hatching in Littorina sitkana , which includes the secretion of albumen layer, egg covering, egg capsule and jelly layer, has been studied with light and electron microscopy. Fertilization occurs in an expanded portion of the oviduct, or in the lumen of the albumen gland. There the zygotes are coated with a thick layer of albumen, followed by a thin egg covering in the covering gland. Specialized cilia with dense flexible tips manipulate the secretions into place. The characteristic shape of the egg capsule is moulded inside the capsule gland by opposing lips of the gland, which are under muscular control. The two halves of the capsule gland contain different types of gland cells, the secretions of which differ both in content and quantity. The resulting egg capsule is heterogeneous with a thick half composed solely of loose filamentous secretions and a thin half comprising a stratified mixture of loose and dense filamentous secretions. The dense filaments degrade more slowly. Thus during larval development the thick half of the egg capsule weakens first. When the juvenile snail is close to hatching it uses both its shell and radula to wear down and chew through the egg covering. The pre-hatching juvenile crawls around the inside surface of the capsule and periodically exerts pressure on the capsule wall by rapidly expanding the foot and shell. Eventually the thick side of the capsule fractures and the juvenile uses the thin side as a platform for thrust and escape. Rapid dissolution of parts of the egg capsule during hatching indicates that a hatching enzyme may be released.

Morphology of neurosecretory cells in basommatophoran snails homologous with egg-laying and growth hormone-producing cells of Lymnaea stagnalis

In a light and electron microscope study, neurosecretory cells morphologically homologous with the egg-laying hormone-producing caudodorsal cells (CDC) and growth hormone-producing dorsal cells (DC, light green cells) of the freshwater basommatophoran snail Lymnaea stagnalis have been found in five genera (seven species) of Basommatophora, viz. in Lymnaea palustris and Lymnaea ovata, in Planorbis planorbis and Planorbis vortex, in Planorbarius corneus, in Bulinus truncatus, and in Biomphalaria glabrata. It is concluded that the functions of these cells are homologous as well. The homologies of the respective neuron types regard their locations in the cerebral ganglia, their clustering in groups, location of their neurohemal area (CDC: cerebral commissure; DC: median lip nerves), and ultrastructural characteristics (e.g., abundance of rough endoplasmic reticulum, well-developed Golgi apparatus, presence of two types of neuron-specific secretory granules, and release of granule contents by exocytosis into the hemolymph). In addition, CDC show large electron-dense granules and DC reveal infoldings of the plasma membrane at the abaxonal side of the soma as well as synaptic input. On the other hand, each neuron type shows species-specific characteristics, particularly with regard to the number of cells and the structure of the neurohemal area. Furthermore, the CDC show marked differences between genera in the morphology (especially the mean diameter) of type 2 and, particularly, type 1 secretory granules. The morphology of the two types of secretory granules in the DC differs strongly between species. The possible relation between the morphology and the chemical contents of secretory granules has been discussed.

Structure and function of the spermatheca in a snail host of schistosomiasis, Biomphalaria glabrata

The histochemistry and ultrastructure (SEM and TEM) of the spermatheca of Biomphalaria glabrata was investigated to elucidate the function of this organ and to compare its structure and function to similar organs found in other species. The spermatheca has a debris-filled lumen surrounded by a thin wall of tissue. The cells adjacent to the lumen are of three columnar epithelial cell types. Two cell types have abundant microvilli and mammalian cell-like organelle distribution and morphology. The above cell types differ in the electron density of their cytoplasms, nuclear morphologies, and organelle content. The third cell type differs from the other two in its cytoplasmic makeup. However, the most distinctive difference is the presence of large numbers of cilia at the apical surface with no evidence of microvilli. These columnar cells rest on a basal lamina adjacent to a two to three cell thick muscle layer. The entire organ is surrounded by an adventitia of unusual morphology. Histochemical investigation demonstrated that DNAase, RNAase, and protease are present in the lumen, alkaline phosphatase is associated primarily with the microvilli, small amounts of acid phosphatase are concentrated in the midcell area of the columnar epithelium, and ATPase activity is localized in the muscle cells and just below the absorptive surface of the microvillous cells. The luminal contents and adventitial areas are Sudan Black B positive, all areas of the lumen and organ wall are PAS positive, the cell nuclei and amorphous masses in the lumen showed Feulgen staining, and large vesicles in the columnar cells were Oil Red O positive. Apparently, the spermatheca of B. glabrata is both a digestive and absorptive structure. Although this organ shares functional similarities with those found in opisthobranchs and terrestrial pulmonates, the epithelia of the spermatheca differ dramatically in these groups.

Endocrine control of galactogen synthesis in the albumen gland of the slug, Limax maximus

An in vitro method for culturing Limax maximus albumen glands is described in which the biosynthetic activity of the slug albumen gland was monitored by measuring the incorporation of [14C]glucose into galactogen. Homogenates of the central nervous system were shown to cause a 3.5- to 12-fold increase in galactogen synthesis in albumen gland explants as compared to controls. The major sources of the galactogen-synthesis stimulating factor (Gal-SF) were found to be the cerebral ganglia and their surrounding connective tissue. Gal-SF was demonstrated to be peptidase sensitive and heat labile suggesting that it is probably a polypeptide. Autoradiographs of semithin araldite sections supported the incorporation data: in albumen gland explants cultured with cerebral ganglion homogenate considerably more label was found over secretory granules than in control-cultured explants. The possible cellular source of Gal-SF is discussed in relation to its possible origin in other investigated pulmonates.

Development of Trichobilharzia ocellata in Lymnaea stagnalis and the effects of infection on the reproductive system of the host

Juvenile specimens of Lymnaea stagnalis were exposed to 0 or 4 miracidia of Trichobilharzia ocellata. Highly productive infections developed in all infected snails. Qualitative and/or quantitative light microscopic observations were performed at various intervals between day 0 and 83 post exposure (p.e.). Gonadal maturation started between day 4 and 7 p.e. in all snails, but in infected animals gonadal growth was retarded considerably after day 7 and was blocked from day 19 onwards. This resulted in very small gonads (10% of the volumes of those of controls on day 83 p.e.) in infected snails, in which however, gametogenesis was initially normal. Due to the small gonad volume gamete production was highly reduced. From day 33 onwards spermatogenesis appeared to be inhibited and around day 83 oocyte maturation seemed to be impaired. The accessory sex organs (ASO) of infected snails showed a similar reduced growth. The cellular differentiation and production of secretory granules in these small ASO was normal albeit delayed in both the male and female organs. The severe reduction in growth of the gonad and the ASO started in the presence of immature primary sporocysts, the later effects on gametogenesis were simultaneous with the production of massive numbers of cercariae. These results are discussed in view of the available data on the endocrine control or reproduction in L. stagnalis. It is suggested that either the secretion of gonadotrophic hormones or the response of their targets is reduced by humoral effects of the parasites.

Spermatogenesis and the role of Sertoli cells in the freshwater snail Biomphalaria glabrata

The various stages of spermatogenesis and the Sertoli cells of Biomphalaria glabrata were studied with histochemical and electron microscope techniques. During spermatogenesis a manchette of microtubules is formed around the nucleus and the mid-piece of the spermatids. This manchette becomes helically coiled and probably plays an important role in the spiralisation of the nucleus and of the mitochondrial sheath. During spermatogenesis so-called chromatoid bodies (CB) occur, which consist of arginine-rich proteins. These CB disintegrate during the early spermatid stage. The results suggest that the CB are either involved in histone transition or in the formation of microtubules. The remaining cytoplasm of the spermatids is phagocytised by the Sertoli cells. Apparently this process of phagocytosis is an important part of the mechanism of spermiation. Morphological measurements of the Sertoli cells showed that the relative volume of most organelles decrease during spermatogenesis, indicating a general decrease in cell activity. Possible functions of the Sertoli cells, such as transportation and nutrition of spermatogenic cells and hormone production, are discussed. It is concluded on the basis of the histochemical and ultrastructural observations that the Sertoli cells are involved in the nutrition of spermatogenic cells. It seems unlikely that they are hormone producing cells.