Gotowa bibliografia na temat „Littorinid snails”

Littoraria ardouiniana and Littoraria melanostoma are common snails in Hong Kong, living and feeding on mangrove trees. Gut content and stable isotopic analyses were conducted to investigate the littorinid's diets. Gut content analyses revealed these snails ingested bark, epidermal plant cells, fungi, and microalgae, but that broken plant cells were the most abundant food items in the stomach and faecal contents. The gut contents of the two littorinid species, either from the mangrove trees Kandelia candel or Aegiceras corniculatum, were similar and showed little temporal variation throughout the year. Dual stable isotopic analysis, which investigated the δ13C and δ15N values of the littorinids and their potential food items, indicated that these littorinids might feed on mixed diets composed of parts of the mangrove trees and other items available on the trees such as phylloplane fungi, microalgae and cyanobacteria. These epiphytic mangrove littorinids are generalist grazers which graze on the substratum non-selectively as they are constrained on their host trees, and their diets are, therefore, dependent on food availability on the trees themselves.

Background The introduction of DNA-based molecular markers made a revolution in biological systematics. However, in cases of very recent divergence events, the neutral divergence may be too slow, and the analysis of adaptive part of the genome is more informative to reconstruct the recent evolutionary history of young species. The advantage of proteomics is its ability to reflect the biochemical machinery of life. It may help both to identify rapidly evolving genes and to interpret their functions. Methods Here we applied a comparative gel-based proteomic analysis to several species from the gastropod family Littorinidae. Proteomes were clustered to assess differences related to species, geographic location, sex and body part, using data on presence/absence of proteins in samples and data on protein occurrence frequency in samples of different species. Cluster support was assessed using multiscale bootstrap resampling and the stability of clustering—using cluster-wise index of cluster stability. Taxon-specific protein markers were derived using IndVal method. Proteomic trees were compared to consensus phylogenetic tree (based on neutral genetic markers) using estimates of the Robinson–Foulds distance, the Fowlkes–Mallows index and cophenetic correlation. Results Overall, the DNA-based phylogenetic tree and the proteomic similarity tree had consistent topologies. Further, we observed some interesting deviations of the proteomic littorinid tree from the neutral expectations. (1) There were signs of molecular parallelism in two Littoraria species that phylogenetically are quite distant, but live in similar habitats. (2) Proteome divergence was unexpectedly high between very closely related Littorina fabalis and L. obtusata, possibly reflecting their ecology-driven divergence. (3) Conservative house-keeping proteins were usually identified as markers for cryptic species groups (“saxatilis” and “obtusata” groups in the Littorina genus) and for genera (Littoraria and Echinolittorina species pairs), while metabolic enzymes and stress-related proteins (both potentially adaptively important) were often identified as markers supporting species branches. (4) In all five Littorina species British populations were separated from the European mainland populations, possibly reflecting their recent phylogeographic history. Altogether our study shows that proteomic data, when interpreted in the context of DNA-based phylogeny, can bring additional information on the evolutionary history of species.

Within-shore and among-shore patterns of distribution, abundance and size structure of Littorina unifasciata Gray were identified on a number of shores in New South Wales. There was significant patchiness in distribution, abundance and size of L. unifasciata among patches of shore only a few metres apart, at different heights on the shore and from shore to shore. On a particular shore, the sizes of snails were strongly correlated with densities. In contrast, differences in densities at different heights from one shore to another were not correlated with mean size of snails. Density and size were each strongly correlated with the height on the shore at which snails were found. At any one height, differences in densities and size were also correlated with the distribution of particular microhabitat variables, such as the slope of the rock surface, the presence of pits and shallow pools and the presence of barnacles. Densities were also negatively correlated with densities of the large microalgae-grazing limpet Cellana tramoserica but were independent of other littorinids. A number of alternative models have been proposed to account for these patterns of distribution, abundance and size. Although processes that might account for these patterns were not investigated here, quantification of such patterns at a number of spatial scales is necessary before potential factors that might affect small-scale spatial variation in densities and sizes of L. unifasciata can be identified and investigated.

Interactions between evolutionary forces such as natural selection, genetic drift and gene flow are complex. Natural selection can lead to parallel formation of phenotypes under similar environmental conditions. Phylogenetic relationships can be inferred from the accumulation of genetic variation caused by genetic drift, regardless of phenotypes. Gene flow between populations can sometimes facilitate the formation of species by natural selection. In this thesis, two groups of marine snails in the diverse subfamily Littorininae were studied. The three rocky-shore species Littorina saxatilis, Littorina arcana and Littorina compressa form one such group, whereas the two sister-species Littoraria cingulata and Littoraria filosa form the other group. Previous studies of the L. saxatilis complex have shown high levels of phenotypic and genetic diversity both at local and broader scales. Previous studies of L. cingulata and L. filosa have found some of the typical signatures of reinforcement. Chapters II and III focused on analysing morphological and genetic variation, respectively, within and among species in the Littorina saxatilis complex from the British Isles. Geometric morphometrics analyses revealed a diversity of shell shape among species, but especially so within L. saxatilis. Shell shape was better explained by environment rather than by geography. Molecular data obtained by high-throughput targeted capture showed the opposite pattern, i.e. genetic variation showed a strong phylogeographic pattern. Chapter IV focused on testing whether reinforcement had contributed towards speciation between Littoraria cingulata and Littoraria filosa. Approximate Bayesian computation analysis supported absence of gene flow between the species in sympatry, suggesting that reinforcement did not contribute towards speciation. However, the results need to be validated and more complex models tested. This thesis highlights the relevance of marine snails in the subfamily Littorininae as model species for addressing a wide range of evolutionary questions. It also provides a wealth of data for many potential follow-up studies.

According to Darwin, any inheritable traits that enhance fitness (i.e. survival and reproductive success) would be adaptive in a given environment, and therefore subject to natural selection. Some traits that enhance reproductive success but not necessarily survival may, however, evolve through the process of sexual selection. The importance of sexual selection has been intensively studied in “higher animals” birds, mammals and insects but has largely been neglected in “lower animals” such as gastropods. Using two mangrove snails, Littoraria ardouiniana HEUDE 1885 and L. melanostoma GRAY 1839 (Littorinidae) as model species, this thesis documents their various reproductive traits and the occurrence of sexual selection, in an attempt to understand how these animals optimize their fitness. Being one of a few gastropod families that live closely associated with mangrove habitats, various adaptive reproductive traits are likely to have evolved in the genus Littoraria. The two Littoraria species exhibited several, contrasting, reproductive traits (e.g. reproductive mode; length of reproductive season; seasonal fecundity and egg- or larvae-releasing rate), which may represent species-specific strategies to optimize reproductive success. Both species adopted mucus trail following as a mate-searching strategy; with males being able to discriminate the trails laid by conspecific females and trail orientation during the mating season. Since these two snails mate in the complex habitat of mangrove tree canopies, incorporating some cue into mucus trails to aid mate recognition would increase potential encounter rates and hence reproductive success. 1D proteomic techniques identified two potential pheromones in the trail mucus of female L. ardouiniana, but further investigations are needed to confirm their possible role as sex pheromones. Evidence for sexual selection was found in L. ardouiniana through male mate choice and male-male competition. Large male L. ardouiniana showed a preference for mucus trails laid by large (= more fecund) females, but this preference was not seen in small males, suggesting a size-based male mate preference. Males of this species also mated with larger females for a longer duration and they showed physical aggression (i.e. they pushed each other) when they encountered a female. These sexual selection mechanisms may drive size-assortative mating patterns in littorinid species and in other animals. Conversely, L. melanostoma exhibited no obvious male mate preference or male aggression. Such differences in sexual selection patterns from L. ardouiniana were proposed to be a result of differences in operational sex ratio and variation in female quality. As many gastropods show similar mating behaviour to littorinids (i.e. trail following, shell mounting and copulation), sexual selection may operate in a comparable manner in other gastropods. By revealing the complexity of sex roles of males and females (e.g. the mutual occurrence of male mate choice and male-male competition) and condition-dependent male mate preference in these mangrove snails, this thesis has contributed to a missing link between sexual selection and the “lower animals” and provides new insights into the operation of sexual selection in animals.
HKU 3 Minute Thesis Award, Champion (2012)
published_or_final_version
Biological Sciences
Doctoral
Doctor of Philosophy

With the anticipated effects of climate change due to global warming, there is concern over how animals, especially ectotherms, will respond to or tolerate extreme and fluctuating environmental temperature stress. Littorinid snails are intertidal ectotherms that live high on the shore where they experience both extreme and variable conditions of temperature and desiccation stress, and are believed to live close to their tolerance limits. This study investigated the thermal biology of littorinid snails of the genera Afrolittorina, Echinolittorina and Littoraria from temperate, subtropical and tropical regions in South Africa and Brunei Darussalam using thermal tolerance, heart function, and proteome approaches. The effects of conditions, such as rate of change in temperature, acclimation, heat shock, season and starvation were also tested. In addition, the evolutionary relationships and genetic diversity between and within the South African Afrolittorina spp. were investigated using mitochondrial and nuclear markers. Genetic results confirmed that these are two distinct species, with the brown to black A. knysnaensis predominant in the cool-temperate region of South Africa and the pale blue-grey A. africana in the subtropical region. There was low genetic variation and differentiation within each species, suggesting high gene flow among populations as a result of the effects of ocean currents on the dispersal of their planktotrophic larvae. Tests using exposure to high temperatures revealed differences in the thermal tolerances, heart performance and protein profiles of species from different latitudes, regions and zones on the shore. Thermal tolerance conformed to expectations, with clear, statistically significant trends from high tolerance in subtropical species to lower tolerance in temperate species. However, for Afrolittorina spp., there were no significant differences in the thermal tolerances of conspecifics from different regions, though there was a significant difference in thermal tolerance between juveniles and adults. Overall, adults of all species showed higher thermal tolerances than juveniles. Although lethal temperatures for these species were higher in summer than winter, laboratory acclimation had no effect on heat coma temperatures. All species showed some regulation of heart rate, with a degree of independence of heart rate from temperature across mid-range temperatures. The tropical species showed quick induction and good regulation of heart rate followed by the subtropical and temperate species, which displayed mixed responses including regulation, partial regulation and lack of regulation. Overall, tropical Echinolittorina spp. showed good regulation, while the subtropical E. natalensis and Littoraria glabrata exhibited a mixture of partial regulation and regulation. The subtropical/temperate Afrolittorina spp. showed high individual variability, some animals exhibiting regulation, while others did not. These effects seem to be largely phylogenetically determined as there were no differences in the heart rate responses of Afrolittorina spp. from different regions. The temperatures at which heart rate became independent of temperature (thermoneutral zone) were within the range experienced under natural conditions. In addition, there were differences in Arrhenius breakpoint and endpoint temperatures, showing a trend from higher in tropical animals to lower for temperate animals. Conditions such as acclimation, heat shock and starvation had little or no effect on heart performance. However, a slow increase in temperature induced good regulation of heart rate with noticeable shifts of breakpoints and endpoints for Afrolittorina spp. Lastly, there were differences in the proteome responses between and within Afrolittorina spp. as a function of species, size and treatment. Although both large and small A. knysnaensis had a greater number of protein spots in their proteome than A. africana (though the difference was not significant), the later showed significantly higher differential expression of certain proteins following heat stress. In addition, juveniles of both species displayed greater numbers of protein spots in their proteome than adults. The results indicate a difference in the physiological and biochemical responses (i.e. adaptations) of these snails to temperature, and this seems to relate to differences in biogeography, phylogeny, species identity and ecology. The ability to regulate heart rate is phylogenetically determined, while thresholds and lethal limits correspond to biogeography and species ecology. The proteome seems to correspond to species ecology. The results also indicate that these littorinids can tolerate high temperature stress and in this respect they are well suited to life in the intertidal zones or habitats where temperature and other stresses or conditions are extreme and can change abruptly. However, the limited ability of these snails to acclimate to different temperatures suggests that they are already living close to their tolerance limits with small safety margins or narrow thermal windows and so may be vulnerable to small rises in substratum temperature and/or solar radiation.

Speciation is a key process underlying biodiversity. This process is facilitated by local adaptation, when divergent selection overcomes gene flow, resulting in the accumulation of reproductive barriers. Theory suggests that this accumulation is strongly dependent on the genetic architecture of the traits underlying local adaptation. The aim of this project was to investigate the genetic architecture of locally adaptive traits in the marine snail Littorina saxatilis. This marine snail (Littorina saxatilis) is an excellent model to study speciation and local adaptation. Two diverging ecotypes live a few metres apart in distinct habitats and face divergent selection pressures dominated by crab predation and wave action. The ecotypes have evolved traits to adapt locally that make them behaviourally and structurally distinct. The most observable differences are seen in the shell size, shape, colours and patterns. Despite the differences, the two ecotypes meet in narrow contact zones and hybridize. Intermediates between the two parental ecotypes are observed in a crab-wave environmental gradient across the hybrid zones. This situation provides an excellent opportunity to exploit the power of association mapping in the hybrid zone to elucidate the genetic architectures of the locally adaptive traits. However, a prerequisite for the application of evolutionary genetic approaches is a genomic toolbox. In Chapters 2 and 3, I describe the construction of a transcriptome assembly and high-density linkage map for this species. These genetic resources were utilized in the subsequent analyses and other studies in this system. In Chapter 4, I investigate the genetic architecture of the adaptive shell traits. Theory suggests that the ground colours or banding patterns possess Mendelian inheritance and may respond directly to selection or may be linked with genes that respond to the physical environment and may thus be affected by selection. Shell morphometric characters (size and shape) may have a more complex pattern of inheritance and tend to be responsive to the environmental conditions. Thus, shell characteristics are excellent to study divergent selection pressures and local adaptation while making it imperative to understand their underlying genetic architecture. In the current study, we applied association analysis to a single hybrid zone in Sweden to elucidate the genes underlying six shell phenotypic traits (size, shape, banding pattern, ground colours - beige, black and dark beige). We sampled individuals from the hybrid zone and implemented targeted capture-sequencing to obtain genotypic data. We identified loci associated with the black and beige ground colours and banding pattern of the shell. No significant associations with the shell shape and size were found which may suggest polygenic and complex architecture, consistent with the theoretical expectation. In addition, our analysis suggests a possible role for chromosomal inversion underlying locally adaptive traits. This thesis addressed longstanding questions regarding the genetic architecture of the adaptive shell traits in this organism and provides directions for the future follow-up studies. The genetic resources described in this thesis will assist the future studies that may address a wide-range of evolutionary questions in this species.

I examined variability in the shell form of 25 Littorina obtusata populations from Massachusetts to northern Maine. I chose this scale because the history of one of L. obtusata's principal predators, the crab Carcinus maenas, has changed dramatically in the past 100 years. Before 1900, Carcinus did not occur north of Cape Cod, Massachusetts, but by 1950 it had reached Canadian border. Moreover, in summer water temperatures during average 6-8&\sp\circ&C colder at northern locations. Shell thickness and mass increased and body mass decreased with increasing latitude. to test whether these patterns reflect plasticity in response to predators, snails from two northern and two southern populations were raised in the laboratory with and without Carcinus effluent. Snails raised with Carcinus produced thicker shells than conspecifics raised without Carcinus. This response was accompanied by reduced body size and body growth. Another experiment examined whether geographic differences in water temperature induce changes in shell form. Snails reciprocally transplanted between a northern (Maine) and southern (Massachusetts) exhibited substantial plasticity in shell form. Southern snails transplanted north produced significantly thinner, lighter, shells than snails raised at their native shore, while northern snails transplanted south produced thicker, heavier shells than snails raised at the native shore. In addition, snails producing thicker, heavier shells exhibited reduced body mass and growth. Although patterns in final phenotypes exhibited cogradient variation, growth in both shell thickness and mass exhibited countergradient gradient variation. Most examples of countergradient variation are associated with temperature differences suggesting that differences in water temperature are responsible for this pattern. A third experiment involved reciprocally transplanting snails between the same two sites (temperature effect) and raising them with and without Carcinus effluent. In general, Carcinus effluent and warmer water temperatures induced thicker, heavier shells and reduced body mass and growth. Overall, it appears that predator effluent and water temperature have similar effects on shell form. My work on phenotypic plasticity encourages a more pluralistic view of phenotypic variation. Moreover, my results suggest that phenotypic plasticity is a ubiquitous strategy in adapting to different environments and that its evolution may be driven by life history trade-offs.

Several parasites that require two or more hosts to complete their life cycles are known to manipulate host behaviour, enhancing their transmission to the next host. The intertidal snail, Littorina sitkana, is host to a diverse assemblage of parasites dominated by trematodes. Trematodes often use snails as first intermediate host and vertebrates as definitive host. Trematode infections can affect host behaviours such as dispersal and foraging. I identified four sites in Barkley Sound that varied in trematode prevalence and species richness. I measured dispersal of snails at these sites and in the laboratory to assess effects of trematode infection on behaviour. I measured feeding rate under laboratory conditions. Trematode effects lowered snail grazing activity at three of the four sites studied, suggesting trematode infection lowers feeding rate of L. sitkana, potentially affecting algal composition of the intertidal zone. Infected male snails travelled longer distances in some sites but shorter distances in others. There was an almost significant effect of trematode infection on vertical displacement of L. sitkana in the field. I estimated survival rates on each site through intensive capture-mark-recapture experiments. There was differential survival among sites, but no negative correlation between survival estimates and trematode prevalence.
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