Adaptive phenotypic plasticity in a clonal invader

Organisms featuring wide trait variability and occurring in a wide range of habitats, such as the ovoviviparous New Zealand freshwater snail Potamopyrgus antipodarum, are ideal models to study adaptation. Since the mid‐19th century, P. antipodarum, characterized by extremely variable shell morphology, has successfully invaded aquatic areas on four continents. Because these obligately and wholly asexual invasive populations harbor low genetic diversity compared to mixed sexual/asexual populations in the native range, we hypothesized that (1) this phenotypic variation in the invasive range might be adaptive with respect to colonization of novel habitats, and (2) that at least some of the variation might be caused by phenotypic plasticity. We surveyed 425 snails from 21 localities across northwest Europe to attempt to disentangle genetic and environmental effects on shell morphology. We analyzed brood size as proxy for fitness and shell geometric morphometrics, while controlling for genetic background. Our survey revealed 10 SNP genotypes nested into two mtDNA haplotypes and indicated that mainly lineage drove variation in shell shape but not size. Physicochemical parameters affected both shell shape and size and the interaction of these traits with brood size. In particular, stronger stream flow rates were associated with larger shells. Our measurements of brood size suggested that relatively larger slender snails with relatively large apertures were better adapted to strong flow than counterparts with broader shells and relatively small apertures. In conclusion, the apparent potential to modify shell morphology plays likely a key role in the invasive success of P. antipodarum; the two main components of shell morphology, namely shape and size, being differentially controlled, the former mainly genetically and the latter predominantly by phenotypic plasticity.     

Microgeographical variation in shell size of the land snail Chondrina clienta

Microgeographical variation in shell morphology of the rock-dwelling land snail Chondrina clienta , collected from 30 sites within an area of 0.5 km² on the island of Öland, Sweden, was examined in relation to its own population density and that of a potential competitor (the land snail Balea perversa ) and to environmental factors. Dispersal of marked individuals averaged 83 cm per year within a stone pile and 291 cm in an area of exposed bedrock. Local population density of active C. clienta ranged from 5 to 794 individuals per m². Shell characters were highly intercorrelated, both within and between populations. Principal component analysis revealed that most of the interpopulational variation could be expressed by the single character of shell height, which ranged from 5.54 to 6.94 mm. In all populations, snails of a given size had the same whorl number. Shell size was not influenced by habitat type (exposed rock surface, stone pile or stone wall) or proportion of calcareous stones within habitat. It was, however, negatively correlated with local population density, indicating intraspecific competition, and positively correlated with the degree of plant cover within the habitat. Analysis of variance revealed additionally a density effect of B. perversa on shell size in C. clienta , probably as a result of interspecific competition. Breeding experiments using C. clienta from different sites and carried out under unifrom conditions caused most of the phenotypic variation to disappear, demonstrating the high phenotypic plasticity of the species.     

GEOGRAPHICAL VARIATION OF SHELL MORPHOLOGY IN CYPRAEA ANNULUS (GASTROPODA: CYPRAEIDAE)

Macro- and microgeographic variation of shell morphology inCypraea annulus was studied. Surveys of phenotypic variationin 19 populations from the Indian and West Pacific Oceans revealeda significant positive relationship between average seawatertemperature and callus thickness. In contrast, juvenile shellsize after maturation had no significant relationship with seawatertemperature. Research on within-population variation in OkinawaIsland indicated that shell morphology is correlated with someecological factors such as temperature and algal abundance.Both juvenile shell size and callus thickness decreased withincreasing density of individuals. These microgeographical patternssuggest that ecophenotypic variation, rather than genetic differentiation,causes the latitudinal clines in shell morphology that havebeen reported in several cypraeid species. (Received 18 November 2004; accepted 8 April 2005)     

Phenotypic clines, plasticity, and morphological trade-offs in an intertidal snail

Understanding the genetic and environmental bases of phenotypic variation and how they covary on local and broad geographic scales is an important goal of evolutionary ecology. Such information can shed light on how organisms adapt to different and changing environments and how life-history trade-offs arise. Surveys of phenotypic variation in 25 Littorina obtusata populations across an approximately 400-km latitudinal gradient in the Gulf of Maine revealed pronounced clines. The shells of snails from northern habitats weighed less and were thinner and weaker in compression than those of conspecifics from southern habitats. In contrast, body size (as measured by soft tissue mass) followed an opposite pattern; northern snails weighed more than southern snails. A reciprocal transplant between a northern and southern habitat revealed substantial plasticity in shell form and body mass and their respective measures of growth. Southern snails transplanted to the northern habitat produced lighter, thinner shells and more body mass than controls raised in their native habitat. In contrast, northern snails transplanted to the southern site produced heavier, thicker shells and less body mass than controls raised in their native habitat. Patterns of final phenotypic variation for all traits were consistent with cogradient variation (i.e., a positive covariance between genetic and environmental influences). However, growth in shell traits followed a countergradient pattern (i.e., a negative covariance between genetic and environmental influences). Interestingly, body growth followed a cogradient pattern, which may reflect constraints imposed by cogradient variation in final shell size and thickness. This result suggests the existence of potential life-history trade-offs associated with increased shell production. Differences in L. obtusata shell form, body mass, and their respective measures of growth are likely induced by geographic differences in both water temperature and the abundance of an invading crab predator (Carcinus maenas). Water temperatures averaged 6.8 degrees C warmer during the transplant experiment and C. maenas abundance is greater in the southern Gulf of Maine. Because both increased water temperature and crab effluent affect shell form in the same way, future experiments are needed to determine the relative importance of each. Nevertheless, it is clear that phenotypic plasticity has an important role in producing geographic variation in L. obtusata shell form. Moreover, the evolution of phenotypic plasticity in L. obtusata and other marine gastropods may be driven by architectural constraints imposed by shell form on body mass and growth.     

GENETIC AND CONCHOLOGICAL COMPARISON OF SNAILS (HELIX ASPERSA) DIFFERING IN SHELL DEPOSITION OF LEAD

Populations of snails inhabiting areas with different historiesof Pb contamination differed in their deposition of Pb in shellrelative to soft tissues. Genetic variation, measured usingisozymes, was not related to Pb history nor geographic distancebetween populations. Shell characteristics were significantlydifferent among sites; shell dry weight was strongly relatedto soil calcium levels. Shells of snails from areas with longhistories of Pb contamination were significantly more robust(greater shell width/shell height ratio) than snails from otherlocations. H. asprsa adaptation to Pb contamination may involvesignificant changes in shell characteristics but these do notcorrelate with genetic traits assessed with allozymes (Received 29 December 1994; accepted 15 October 1995)     

Habitat-specific variation in life-history traits,clonal population structure and parasitism in a freshwater snail (Potamopyrgus antipodarum)

High risk of infection by parasites may select for early reproduction in natural host populations. In a previous study of a freshwater snail (Potamopyrgus antipodarum) we found (1) that different clones of the snail are associated with different depth-structured vegetation zones and (2) that snails in shallow water, where the age-specific risk of infection is highest, mature at a smaller size than snails in deeper habitats. This result suggests that there has been selection for early reproduction in these snails, and that different clonal genotypes have different life-history strategies. Alternatively, the observed life-history variation in the snails might be due to ecological factors that are independent of parasites, but correlated with depth. In the present study, we decoupled parasitism and depth by examining life histories and clonal population structure in a second lake (Lake Tennyson) where the mean prevalence of trematode parasites was low and unrelated to depth. Consistent with the previous results, clones were structured according to vegetation zones in Lake Tennyson. However, we found no relationship between depth and life-history traits, which is inconsistent with the idea that depth-associated factors other than parasites affect snail life histories. Taken together, these results suggest that life-history variation is more likely to result from a depth-specific risk of infection than from depth per se, and that partitioning of habitat zones by different groups of clones may be a general phenomenon in P. antipodarum populations.     

Genetic variability and drift load in populations of an aquatic snail

Abstract Population genetic theory predicts that in small populations, random genetic drift will fix and accumulate slightly deleterious mutations, resulting in reduced reproductive output. This genetic load due to random drift (i.e., drift load) can increase the extinction risk of small populations. We studied the relationship between genetic variability (indicator of past population size) and reproductive output in eight isolated, natural populations of the hermaphroditic snail Lymnaea stagnalis . In a common laboratory environment, snails from populations with the lowest genetic variability mature slower and have lower fecundity than snails from genetically more variable populations. This result suggests that past small population size has resulted in increased drift load, as predicted. The relationship between genetic variability and reproductive output is independent of the amount of nonrandom mating within populations. However, reproductive output and the current density of snails in the populations were not correlated. Instead, data from the natural populations suggest that trematode parasites may determine, at least in part, population densities of the snails.     

GROWTH, SURVIVORSHIP, AND POPULATION SIZE IN THE LAND SNAIL RHAGADA CONVICTA COX, 1870 (PULMONATA: CAMAENIDAE) FROM A SEMIARID ENVIRONMENT IN WESTERN AUSTRALIA

Basic characteristics of a population of the camaenid land snailRhagada convicta were studied in a semiarid environment nearNorth West Cape, in Western Australia. Consistent evidence frommark-recapture studies and analysis of size-frequency distributionindicates an average annual increase in shell diameter of 3.6mm, with an average period of 5 years from hatching to reachadult size. These rates are low, compared with most other landsnails studied, including cama-enids from wetter environments.Estimates of mortality rate and the rate of recruitment intothe adult population indicate that the snails live an averageof approximately 5 years after reaching adult size, which meansthat the period of turnover for this population is approximately10 years. The population density was estimated to be 0.8 adultsper m², with a total population size (± s.e.) of 875± 164. The population is very localised, with an estimateddiameter of the neighbourhood area of only 38 m. (Received 28 August 1990; accepted 12 February 1991)     

Resource allocation,demography and the radiation of life histories in rough periwinkles (Gastopoda)

Applicability of life-history theory to higher levels of comparison (from populations, through ecotypes to sibling species) was investigated in rough periwinkles, whose life histories have diversified since colonization of the North Atlantic by an oviparous ancestor in the upper Pliocene. Comparisons were made among populations of the ovoviviparous Littorina saxatilis, between L. saxatilis and its ecotype, L. neglecta (with an annual life history) and between the sibling species L. saxatilis and L. arcana, the latter of which retains the ancestral oviparity. Resource-allocation priority, reproductive effort and related trade offs were compared between the ecotypes and the sibling species by measuring changes in flesh mass and reproductive output in snails subjected to different degrees of food deprivation, and by measuring mortality rate of snails stressed by desiccation, high temperature and low salinity. Body size had a marked effect on all parameters, but after statistically removing this effect there remained no significant differences in allocation among ecotypes or species. Published demographical data were reviewed for correlations between habitat, mortality regime and life-history characteristics. Populations of L. saxatilis varied principally in size at birth and in adult size. Theoretical premises based on density-dependent versus density-independent mortality regimes could not explain these trends. Instead, size at birth may have reflected the mechanical, physiological or biological nature of mortality risk rather than its density dependence or independence. Adult size reflected the available sizes of crevices used for shelter and perhaps also the quality of feeding conditions. Radiation of life histories within the rough periwinkles is interpreted as a series of adaptations to a progressively wider range of habitats. The transition from oviparity to ovoviviparity allows colonization of estuaries, saltmarshes and pebble beaches too hazardous for naked egg masses. The transition from a perennial to an annual life history in barnacle ecotypes follows from allometric re-scaling of morphological and physiological parameters, enabling reproduction and brooding to occur at the small body size necessary for life within empty barnacle tests. This suite of adaptations allows exploitation of a relatively benign microhabitat that occurs almost ubiquitously on exposed rocky shores of the temperate North Atlantic. The persistence of oviparous forms, presumably in the face of competition from sympatric ovoviviparous forms, remains unexplained.     

Reproductive strategies and body shape in the European pond turtle (Emys orbicularis) from contrasting habitats in Italy

Comparisons within and among populations offer important insights into variation in life-history traits and possible adaptive patterns to environmental conditions. We present the results of observed differences in body size, body shape and patterns of reproduction in four separate populations of the European pond turtle Emys orbicularis in central and southern Italy – coastal ( n =3) and mountainous ( n =1) sites and pond ( n =2) and canal ( n= 2) habitats – to determine whether phenotypic plasticity affects reproductive output. Although we did not find any significant latitudinal variation in body size, we observed significant differences in body shape between canal (rounded body shape) and pond (elongated body shape) systems and smaller size with rounded shape in the mountainous population. Reproductive output is similar among populations (median=5 eggs per clutch), whereas reproductive investment (relative clutch mass to maternal body mass) is higher in the mountain population (one clutch per year) than in coastal populations (two clutches per year), suggesting differential trade-offs between geographic locality, elevation and habitat type. Turtle shell shape and geographic location together affect reproductive output in E. orbicularis in Italy.     

Assessment of the status of wild populations of land snail (escargot) Helix pomatia L. in Moldova: the effect of exploitation

Helix pomatia L., the Roman snail, is a species faced with growing commercial interest in Moldova. Its life history characteristics (slow maturation and recruitment, high mortality among juveniles and low fecundity) along with its strong spatial aggregation, makes it especially vulnerable to exploitation. In this study, differences in density, shell size and age distribution were assessed in 7 unexploited and 10 exploited sites in the northern and central parts of Moldova. A significant impact of exploitation on snail population densities, shell size of adult snails and age distribution was revealed. Exploited sites had much lower densities than unexploited ones and in two places no live snails were found. This may suggest that exploitation is currently carried out at an unsustainable level, but additional information on the demography of populations and intensity of exploitation is required in order to make inferences regarding sustainability and long-term population management. There was a higher proportion of adult snails in exploited sites than in non-exploited, because of the collection strategies: not only adults, but also all other age groups are gathered. Bigger adult shell size in exploited sites may be related to lower population density, but further study is required to confirm this. Establishing of well-organized population monitoring systems and development of snail breeding enterprises are proposed in order to conserve the species in Moldova.     

Covariation of larval gene expression and adult body size in natural populations of Drosophila melanogaster

Understanding adaptive phenotypic variation is one of the most fundamental problems in evolutionary biology. Genes involved in adaptation are most likely those that affect traits most intimately connected to fitness: life-history traits. The genetics of quantitative trait variation (including life histories) is still poorly understood, but several studies suggest that (1) quantitative variation might be the result of variation in gene expression, rather than protein evolution, and (2) natural variation in gene expression underlies adaptation. The next step in studying the genetics of adaptive phenotypic variation is therefore an analysis of naturally occuring covariation of global gene expression and a life-history trait. Here, we report a microarray study addressing the covariation in larval gene expression and adult body weight, a life-history trait involved in adaptation. Natural populations of Drosophila melanogaster show adaptive geographic variation in adult body size, with larger animals at higher latitudes. Conditions during larval development also affect adult size with larger flies emerging at lower temperatures. We found statistically significant differences in normalized larval gene expression between geographic populations at one temperature (genetic variation) and within geographic populations between temperatures (developmental plasticity). Moreover, larval gene expression correlated highly with adult weight, explaining 81% of its natural variation. Of the genes that show a correlation of gene expression with adult weight, most are involved in cell growth or cell maintenance or are associated with growth pathways.     

CONTRIBUTION OF GENETIC AND ENVIRONMENTAL FACTORS TO SHELL SHAPE VARIATION IN THE LOTIC SNAIL SEMISULCOSPIRA REINIANA (PROSOBRANCHIA: PLEUROCERIDAE)

The genetic and environmental factors affecting shell shapein the freshwater snail Semisulcospira reiniana at Takahashiin Kyoto, central Japan, were studied by means of a rearingexperiment and field observations. Shell shape was characterizedby three parameters; W (whorl expansion rate), T (whorl translationrate), and S (roundness of generating curve). Estimated heritabilitieswere low in all three parameters and the largest component ofthe great shell variation in the Takahashi population was environmentalvariance, suggesting that the main source of shell variationwas phenotypic modulation, in response to the external environment.In the field, snails that were active in fast currents had largerW and smaller T, that is, a larger body whorl and a lower spire,than snails in slow currents. Substratum conditions relatedto T in resting periods although its cause was unclear. Mechanismsare suggested which in the absence of selection of genetic variationcould cause and maintain shell variation in S. reiniana in differentmicrohabitats. (Received 8 March 1996; accepted 3 November 1997)     

Crayfish induce a defensive shell shape in a freshwater snail

Abstract. This study was undertaken to determine whether intraspecific variation in shell morphology of the freshwater snail Elimia (= Goniobasis ) livescens is caused by predator-induced morphological changes. Juvenile snails from 3 populations were grown in the presence of effluent from predatory crayfish feeding on conspecific snails or in effluent from conspecific snails only. Snails from one population, Clear Creek, exhibited a predator-induced morphology; they grew a narrow body whorl when exposed to the effluent from crayfish. Experimental feeding trials with crayfish and snails from Clear Creek were conducted to determine whether a narrow body whorl reduced predation in the presence of the crayfish Orconectes propinquus . In the feeding trials, snails with a narrow body whorl were eaten less frequently than snails with wider shells. However, there was no difference in overall size (length) between snails that were eaten and those that were not. Thus, juvenile snails from Clear Creek exposed to the presence of crayfish were induced to develop relatively narrow shells, which reduce the risk of successful attack by these crayfish. Hence the induced shell morphology is probably a defense against predation by crayfish.     

Population regulation in the land snail Arianta arbustorum: density effects on adult size,clutch size and incidence of egg cannibalism

Summary The relationships between local population density and adult size, clutch size and spatial distribution of egg batches were investigated in 11 natural populations of the land snail Arianta arbustorum in a forest near Uppsala, Sweden. Shell size of adults decreased with increasing population density as did clutch size. Within populations, clutch size scaled allometrically with shell size indicating size-specific fecundity. It is hypothesized that food unpalatability caused by mucus deposition slows down juvenile growth rate in high density populations, resulting in small adults and thus reducing their fecundity in subsequent years. The influence of the distance between batches on the incidence of egg cannibalism by hatchlings was examined in a laboratory experiment. In this experiment the number of eggs cannibalized increased with decreasing distance to the batch of hatching snails. Thus, in the field, eggs of highly aggregated batches suffer a high risk of cannibalism. In the 3 populations with the highest snail density, 21–39% of all batches were deposited close to each other (nearest neighbour distance 5 cm, i.e. less than hatchlings more within 1 day). These findings indicate that egg cannibalism can act as a population regulating factor.     

Living in isolation – population structure,reproduction, and genetic variation of the endangered plant species Dianthus gratianopolitanus (Cheddar pink)

The endangered plant species Dianthus gratianopolitanus exhibits a highly fragmented distribution range comprising many isolated populations. Based upon this pattern of distribution, we selected a study region in Switzerland with a lower magnitude of isolation (Swiss Jura) and another study region in Germany with a higher degree of isolation (Franconian Jura). In each region, we chose ten populations to analyze population structure, reproduction, and genetic variation in a comparative approach. Therefore, we determined population density, cushion size, and cushion density to analyze population structure, investigated reproductive traits, including number of flowers, capsules, and germination rate, and analyzed amplified fragment length polymorphisms to study genetic variation. Population and cushion density were credibly higher in German than in Swiss populations, whereas reproductive traits and genetic variation within populations were similar in both study regions. However, genetic variation among populations and isolation by distance were stronger in Germany than in Switzerland. Generally, cushion size and density as well as flower and capsule production increased with population size and density, whereas genetic variation decreased with population density. In contrast to our assumptions, we observed denser populations and cushions in the region with the higher magnitude of isolation, whereas reproductive traits and genetic variation within populations were comparable in both regions. This corroborates the assumption that stronger isolation must not necessarily result in the loss of fitness and genetic variation. Furthermore, it supports our conclusion that the protection of strongly isolated populations contributes essentially to the conservation of a species' full evolutionary potential.     

Temperature-related diversity of shell colour in the intertidal gastropod Batillaria

The intertidal snail Batillaria exhibits remarkable variationin the shell colour within and among populations. Field studywas conducted to determine the factors in maintaining observedshell colour polymorphism. Geographical variations in shellcolour polymorphisms in B. attramentaria were significantlycorrelated with the temperature of the locality of the population.Darker morphs were predominant in colder regions, whereas lightermorphs increase their proportion in warmer regions. A consistentassociation was also found in B. multiformis that co-existedwith B. attramentaria. Strong predatory pressure imposed bydigenean trematode parasites was observed in B. attramentaria.However, it is unlikely to affect the colour variations, becauseno correlation exists between colour morphs and trematode parasitism.Although visual selection may also contribute to colour variationin Batillaria, no evidence is found for the existence of visualpredators that affect colour patterns of these snails. The deficitof variation in cold regions is possibly due to selection againstbrighter morphs, because bright colours reflect heat. Althoughdark shells absorb sunlight and may therefore be exposed tothe risks of overheating and drying up in a hot habitat, thedarkest morph was frequently observed in the warmer regions,suggesting that physical selection on the colour morphs canbe relaxed in the warmer environment. Our results suggest thatclimatic selection is one of the significant factors maintainingshell colour polymorphism in these intertidal snails. (Received 13 December 2006; accepted 14 April 2007)     

GENETIC AND MORPHOLOGICAL VARIATION IN THE LAND WINKLE POMATIAS ELEGANS (MULLER) (CAENOGASTROPODA: POMATIASIDAE)

Population genetics and shell morphology have been studiedin 11 populations of the poorly colonizing land caenogastropodPomatias elegans. The total area of suitable habitats in northwestEurope is shrinking and many isolated populations are becomingvulnerable to extinction. In this study we tested whether theconcept of management units (i.e. groups of population withsignificantly different allele frequencies due to demographicindependence) is applicable to the conservation of P. elegans.F_st values indicated strong genetic differentiation and thuslittle genetic exchange between populations. Allozyme differentiationcould be explained with an isolation by distance model, whereasmorphological differentiation could not. A morphological differenceexists between sexes but not sufficient to discriminate malesand females. A Mantel test showed no significant relationshipbetween morphological distance (size corrected or not) and geneticdistance. Since allele frequencies differed even among populationsin areas where P. elegans is not threathened, we conclude thatin the case of P. elegans, defining management units with allozymesmay not be an appropriate way to select the most suitable populationsfor conservation. (Received 3 April 2000; accepted 25 August 2000)     

Life-history variation in selfing multilocus genotypes of the land slug Deroceras laeve (Pulmonata: Agriolimacidae)

The land slug Deroceras laeve shows an enormous variation inlife-history traits. Here, we compared several life-historycharacters (i.e. length of the prereproductive period, growthduring the egg-laying period, number of eggs, hatching success)within and among five selfing homozygous multilocus genotypes(MLG) and investigated a possible trade-off between the numberof eggs produced during the egg-laying period and growth duringthe egg-laying period. Life-history traits were highly variable.There was no significant correlation between slug size and fecundity.Slugs that matured early produced more eggs than slugs thatmatured later. Life-history traits were significantly differentamong MLGs, suggesting that genetic factors may affect and explainat least part of the variation in life-history traits in D.laeve. Finally, we found no evidence of a trade-off betweenthe number of eggs produced during the egg-laying period andgrowth during the egg-laying period. (Received 1 June 2005; accepted 28 October 2005)     

昆虫体型及性体型二型性的地理变异

体型是昆虫基本的形态特性,它会影响到昆虫几乎所有的生理和生活史特性。同种昆虫不同地理种群在体型上常表现出明显的渐变,导致这些渐变的环境因素包括温度、湿度、光照、寄主植物、种群密度等,并且多种环境因素也会对昆虫种群内个体体型产生影响。雌雄个体的体型存在差异,称性体型二型性。性体型二型性也显示了地理差异。这些差异形成的途径已经得到详细的分析,其形成机制导致多个假说的提出,这些假说又在多种昆虫中得到验证。本文从同一种昆虫不同种群间、同一种群内、雌雄虫个体间3个水平,对种内昆虫体型变异的方式,影响昆虫种群间体型变异和种群内昆虫体型的变异的环境因素,以及昆虫性体型二型性及其地理变异的现象等方面的研究进行了综述,并对未来的相关研究提供了建议。