Developmental expression and biochemical characterization Nassarius obsoleta.

Electrophoretic analysis of catalase isozyme patterns of Nassarius obsoleta indicates that these isozymes are products of two separate loci. Five aminopeptidase isozymes have also been detected in N. obsoleta and the data suggest that at least two loci encode these aminopetidase isozymes. Experiments designed to test for the interconvertibility of the isozymes indicated that the two catalase isozymes and that two of the five aminopeptidase isozymes tested were not conformational artifacts but distinct genetic products. No change in catalase isozyme expression, but considerable quantitative variation in catalase specific activity was noted during early developmental stages of N. obsoleta. Both qualitative and quantitative changes were noted in aminopeptidase expression during early developmental stages. This paper also describes several physicochemical parameters for each of the two enzymes under study.     

Selective grazing by the mud snail Ilyanassa obsoleta

Summary Mud snails (Ilyanassa obsoleta) starved for 48 h were allowed to feed on sediments in laboratory microcosms. Sediment cores sliced at 2 mm intervals were compared to snail stomach contents for per cent carbon and nitrogen, plant pigment contents and species composition of benthic diatoms. Concentrations of carbon, nitrogen, phaeopigments, phycocyanin and chlorophyll were enriched in the top 2 mm of the sediments compared to 7–10 mm depth by a factor of 2–10. In turn, these materials were 20–40 times more concentrated in snail guts than in the surface sediments. Snail feces were enriched for carbon and nitrogen by 5–7 times over the surface sediments. Bacterial chlorophyll peaked at about 3–4 mm in the sediments and was not detectable in the snail stomach contents. The C/N ratio of the snail stomach contents was only 6 compared to a ratio of 8.5 for their feces and 12 for the surface sediments.The percentage of migratory diatoms (e.g. Nitzschia and Navicula) decreased with depth where non-migratory species, such as Fragilaria pinnata, dominated. These migratory species were more common in the snails than in the sediments on which they were feeding.A comparison of daily ingestion rates to the animal's energy budget shows that this selective ingestion is sufficient to meet Ilyanassa's energy needs.     

An unusual cytoplasmic organelle in oocytes of Ilyanassa obsoleta

An unusual organelle in the oocytes of the marine mud snail Ilyanassa obsoleta is described and called a polymerosome, because of its several components. Polymerosomes appear at the beginning of vitellogenesis and persist, in part, through postvitellogenesis. The persistent part is similar to an organelle that has been described by other workers in the vegetal region and the polar lobe of the Ilyanassa egg.     

Trematode accumulation by the estuarine gastropod Ilyanassa obsoleta.

The accumulation of larval trematodes by Ilyanassa obsoleta (Gastropoda) at 2 estuarine sites in Delaware was studied. Initial infection status of snails was assayed by looking for shed cercariae. Native snails (most already infected) were deployed at sites A and B, and sentinel snails (putatively uninfected) were deployed at site B. All were individually marked and, if found, reassessed for infection after being free 1-3 summers. Himasthla quissetensis, Lepocreadium setiferoides, Zoogonus rubellus, Austrobilharzia variglandis, and Gynaecotyla adunca infections were observed in recovered snails. At site A, in 1993, 62 natives were recovered. Among the 26 initially testing uninfected, 15 had infections at recovery. Of 36 that released cercariae, 26 had the infection initially indicated. Of sentinels released at site B in 1996, 3 of 16 had infections when recollected. One probably was infected before transplant; at least 2 were infected at site B during 1996-1997. Among site B natives, 26 were later examined by dissection in 1996; 22 had the infection status initially revealed. Some site B natives (n = 35) escaped recapture in 1996 but were found in 1997, or 1998, or both, and were reexamined, most by cercarial release. The same cercariae were produced by 30. Among natives (both sites, n = 123), 27.6% exhibited some difference in infection status compared to the initial assay. This probably overestimates changes. Some differences were real but most can be discounted as cases where initial infection status was misrepresented.     

Regulation of Protein Synthesis in Embryos of Ilyanassa obsoleta

Much of the protein synthesis during early development in Ilyanassaresults from the translation of oogenetic mRNA. We show thatmicrotubule proteins are products of this translation, and thattheir synthesis is subject to translation level regulation.We also show that translation level regulation is involved inthe function of the polar lobe by making comparisons of theelectrophoretic patterns of synthesis of ¹⁴C labeled proteinsof normal embryos with the patterns of synthesis of ³H labeledproteins of embryos from which the polar lobes had been removedat the trefoil stage. Controls utilizing biochemical and morphologicalmarkers were performed to assure that normal and delobed embryoswere developing at equivalent rates. The expression of significantdifferences in the patterns of protein synthesis were foundbetween normal and delobed embryos, and these differences werenot dependent upon concomitant RNA synthesis. These differenceswere observable as early as after only 24 hours of development,although organogenesis does not begin until much later in development.Therefore, the observed differences probably reflect determinativeevents. The results support the hypothesis that the developmentaldeterminants of the polar lobe may include specific, preformedmRNA sequences, or specific regulators of translation.     

Analysis of ciliary band formation in the mollusc Ilyanassa obsoleta

Two primary ciliary bands, the prototroch and metatroch, are required for locomotion and in the feeding larvae of many spiralians. The metatroch has been reported to have different cellular origins in the molluscs Crepidula fornicata and Ilyanassa obsoleta, as well as in the annelid Polygordius lacteus, consistent with multiple independent origins of the spiralian metatroch. Here, we describe in further detail the cell lineage of the ciliary bands in the gastropod mollusc I. obsoleta using intracellular lineage tracing and the expression of an acetylated tubulin antigen that serves as a marker for ciliated cells. We find that the I. obsoleta metatroch is formed primarily by third quartet derivatives as well as a small number of second quartet derivatives. These results differ from the described metatrochal lineage in the mollusc C. fornicata that derives solely from the second quartet or the metatrochal lineage in the annelid P. lacteus that derives solely from the third quartet. The present study adds to a growing body of literature concerning the evolution of the metatroch and the plasticity of cell fates in homologous micromeres in spiralian embryos.     

Infrapopulation sizes of co-occurring trematodes in the snail Ilyanassa obsoleta

This study addresses the infrapopulation sizes of 2 larval trematode species Himasthla quissetensis and Zoogonus rubellus as they co-occur within their estuarine snail host Ilyanassa obsoleta. Rediae of H. quissetensis and sporocysts of Z rubellus were counted in snails singly infected with each parasite and in snails infected with both. Comparisons of the counts indicate that infrapopulations of H. quissetensis were unaffected by co-occurrence with Z rubellus. However, Z. rubellus infrapopulations were reduced when co-occurring with H. quissetensis. It is proposed that this situation does not result from an interspecific interaction between parasite species. Although this double infection is relatively frequent in certain snail populations, it is contended that these trematode species do not co-occur often enough to evolve responses to one another. However, the host environment must be encountered in each life cycle, and both trematode species must be adapted to use it. On this basis, whatever happens when these 2 species occupy the same host is based on adaptations of the parasites to the host. It is proposed that these parasites are adapted to self-limit their infrapopulations in the snail host. They can, thus, preserve and use the host for many years and thereby enhance total cercarial transmission (fitness). Infrapopulation sizes would be determined by host resource levels, which, among other factors, would be influenced by the presence of multiple parasite species. In single infections, by far the most common situation, host resource levels would be set by the nutritional status or age (size) of the host (or both). The reduced infrapopulation sizes of Z rubellus on co-occurrence suggest that this trematode is more sensitive to host resource levels than is H. quissetensis.     

The cranial development of Elaphe obsoleta (Ophidia,colubridae)

The kinetic skull of snakes is a highly specialized structure that has allowed these limbless organisms to exploit a wide variety of habitats. Here we analyze the development of the cranium in the colubrid snake Elaphe obsoleta quadrivittata based on two sets of embryos raised under controlled conditions and preserved at regular intervals during embryogenesis. Emphasis is on the interactions between dermal and endochondral ossification in the basicranium and in the posterior orbitotemporal region. In Elaphe, the laterosphenoid initially develops as an ascending process of the basal plate and is a combination of membrane and cartilage bone. The maxilla shows a peculiar pattern of differentiation from several centers of ossification.     

Polyadenylation of nascent RNA during the embryogenesis of Ilyanassa obsoleta.

It has been demonstrated that specific changes in carbohydrate-containing cell surface lectin receptor sites occur with differentiation and maturation of sea urchin embryo cells. In this study, evidence is presented, using a quantitative electronic particle counter assay to measure agglutination, which indicates that concanavalin A (Con A) mediated agglutination of dissociated $\text{32}\text{64}$ cell sea urchin embryos differs dramatically with respect to specific cell populations. The migratory cell type, the micromere, is significantly more agglutinable with Con A than the other cell types and colchicine treatment markedly increases sea urchin embryo cell agglutinability. The results indicate that like many malignant cells which display extensive migratory behavior, specific migratory populations of embryonic cells are agglutinable with Con A. The results are discussed with respect to the possible nature of lectin receptor sites on specific populations of embryonic cells and the possible role of colchicine-sensitive structures in controlling the display patterns of these sites.     

Serotonin and Nitric Oxide Regulate Metamorphosis in the Marine Snail Ilyanassa obsoleta

Several neuroactive compounds have been implicated as playingroles in the circuitry that controls larval metamorphosis inmarine molluscs. For the caenogastropod Ilyanassa obsoleta,results of neuroanatomical studies suggest that the productionof nitric oxide (NO) increases throughout the planktonic stageand that NO production is necessary for the maintenance of thelarval state, especially as it becomes metamorphically competent.Bath application or injection of exogenous serotonin (5HT) caninitiate metamorphosis in competent larvae, and exogenous NOcan inhibit such serotonergically-induced metamorphosis. Inhibitionof endogenous nitric oxide synthase (NOS) can also trigger larvalmetamorphosis. The production of endogenous NO appears to decreaseconcurrently with the initiation of metamorphosis, but the specificinteractions between serotonergic and nitrergic neurons areunknown. Evidence in support of NO acting to up-regulate theenzyme guanylyl cyclase (GC) is still equivocal. Thus, we donot yet know if NO exerts its effects through the actions ofcyclic 3',5'-guanosine monophosphate (cGMP) or by a cGMP-independentmechanism. The ubiquity of nitrergic signalling and its significancefor developing molluscan embryos and larvae are still the subjectof speculation and require further investigation.