Phenotypic variation along a cline in allozyme and karyotype frequencies, and its relationship with habitat, in the dog-whelk Nucella lapillus, L.

The dog-whelk Nucella lapillus exhibits a number of phenotypic variations and genetic polymorphisms which correlate with habitat-specific environmental pressures, especially those associated with wave action and temperature. This study investigates the relationship between genetic composition (karyotypic and electrophoretic variation) and phenotypic differentiation in N. lapillus sampled at 15 points along an 8 km stretch of coastline. Coincident clinal variation in gene frequencies and shell shape is described; they covary with differences in karyotype and also with growth. Laboratory-reared young show that differences in phenotype (shell shape and growth) are inherited. Experimental evidence is presented that specific shell shapes are adaptive under conditions of thermal stress. Together with the well-established correlations between shell shape and shore exposure, this association provides an argument for a relationship between genetic composition, phenotype and habitat in this species.     

Two different patterns of shell-shape variation in the dog-whelk Nucella lapillus (L.)

Populations of the common dog-whelk, Nucella lapillus (L.), usually show a gradation in mean shell shape related to the exposure of the shore on which they live. Enclaves from exposed headlands have shorter, squatter shells than do their compatriots in shelter. This paper describes the pattern of shell-shape variation shown by populations in the Isles of Scilly and on the north coast of Wales. In both places a gradation is present, but it is not numerically similar, the Welsh population being more elongated. It is concluded that the selective agents are the same in both places. Wave action always eliminates the most elongate-shelled whelks first, and crabs always find it easier to take the animals with the widest apertures to their shells. The reason for the observed differences between the patterns described here lies in the genetic differences between the populations: Scillonian whelks do not have the option of being as elongated as the Welsh ones, which, in turn, cannot show very short squat shells in exposure.     

Shell-shape variation in Faroese dog-whelks (Nucella lapillus (L.))

Nucella lapillus is a common and widely distributed carnivorous gastropod of North Atlantic rocky sea shores. Populations of this animal usually show variation in shell shape according to the exposure of their habitat to wave action, with individuals from exposed sites having a short squat shell and a wide aperture as compared with a more elongated form seen in shelter. The same pattern of variation is seen over most of the species European range, but there are some exceptions. One of these has been described in Sullom Voe (Shetland) and this prompted an investigation of Faroese populations to ascertain whether they behaved in a similar manner. It is shown in this paper that Faroese populations vary in shell shape with exposure in the usual way (and not like those in Shetland) and moreover show such a fine and precise reflection of minor alterations in environmental conditions that measurement of dog-whelk shells may prove to be the best way of 'measuring' exposure in Faroe.     

Shell shape variation in dog-whelks (Nucella lapillus (L.)) from the West Coast of Scotland

European populations of the common dog-whelk, Nucella lapillus (L.), usually show a neat and precise pattern of shell shape variation with the exposure of their habitat to wave action. Whilst this is the case along much of the west coast of Scotland, there are a number of instances where unusually elongated shells occur, somewhat reminiscent of the form described for the Severn Estuary. There are no obvious environmental or geographical correlates to explain the occurrence of these forms and a genetical interpretation is sought.     

Microgeographic variation in allozyme frequencies in relation to the degree of exposure to wave action in the dogwhelk Nucella lapillus (L.) (Prosobranchia: Muricacea)

Nucella lapillus from 15 sites 50 m to 21 km apart in S. Devon, S. W. England were analysed for allozyme variation at eight soluble enzyme loci. Sites were classified as either Exposed or Sheltered and a hierarchical analysis of allozyme variation carried out using Wright's Fixation Index, F_ST. Whelks from Sheltered sites segregated for alleles at the Est-3, Lap-2, Pep-2 and Mdh-1 loci which were virtually absent from exposed sites resulting in high F_ST values (0.289–0.506) when all samples were included. Consequently mean heterozygosity of the Sheltered samples was roughly twice that of the Exposed. Between Sheltered sites the frequencies of these 'additional' alleles varied substantially, even on a microgeographic scale and were found to be highly correlated with exposure. Within individuals the presence of the 'sheltered' alleles was correlated with the possession of the 'sheltered' shell shape as indicated by length divided by aperture height. This must necessarily be the case, however, if both are related independently to exposure. Although these correlations imply the action of selection there is evidence within the data for stochastic factors affecting the pattern observed. Complicating the picture further is the correspondence observed in the distribution of the 'sheltered' alleles with the known distribution of chromosomal translocations. Phenotypic associations within individuals also support the hypothesis that variation at the Est-3, Lap-2, Mdh-1 and Pet-2 loci is related to variation in chromosome number. However, the nature of the relations of karyotype and allozyme variation with shell shape and exposure remain speculative.     

Some observations on shell-shape variation in North American populations of Nucella lapillus (L.)

This paper considers shell-shape variation in samples of the dog-whelk/dog-winkle, ucella lapillus , from the southern half of its North American range, between 41 and 46N, and compares it with the pattern seen in Europe. At the extreme southern limit of its American range, to the south of Cape Cod, the species exhibits features to be expected in an animal close to an environmental limit. It is generally rare and has a patchy distribution. There is little variation in the shell: almost all adult individuals have rather large, thick, white, elongated shells. This contrasts sharply with the situation in Portugal, at the southern limit in Europe, where the animals have small thin, coloured shells of intermediate shape.
Away from the marginal situation, north of Nahant (42^CN) in America, the species shows much the same range of variability in shell size, shape and colour on both sides of the Atlantic. Most populations appear to show the same pattern of shape variation in asociation with the exposure of their habitat to wave action. Animals from exposed sites tend to have shorter, squatter shells than their compatriots in shelter. However, in America as in some parts of Europe, there are occasional enclaves which do not fit the usual pattern. It is interesting to note that the form normally associated with extremely exposed shores from Brittany to Faroe in Europe is found in Atlantic Canada but is apparently absent from southern populations in both the U.S.A and the Iberian Peninsula.     

Allozyme differentiation of populations of the dogwhelk Nucella lapillus, (L.): the relative effects of geographic distance and variation in chromosome number

Geographic patterns of genie differentiation were compared with differentiation between karyotypes in the intertidal snail Nucella lapillus. Samples from 24 sites covering the species range in Europe and North America were analysed for allozyme variation at 16 soluble enzyme loci. Two homokaryotypes have been identified with diploid numbers 2n = 26 and 2n= 36 (variation is Robertsonian and hybrids have intermediate chromosome numbers) and samples were classified (on the basis of published data) according to karyotype. Group 1 consisted of samples from three English Channel populations of higher chromosome number (on average 2n > 32) and Group 2 consisted of the remaining 21 samples (presumed to be 2n= 26). Karyotype variation accounts for roughly the same amount of the absolute allozyme variance as geographic variation (46.3 °, and 53.7°, respectively). Yet the patterns of differentiation seen between karyotypes and with geographic separation are very different. In samples classified as 2n= 26 (Group 2), while there is a significant amount of heterogeneity (F_ST per locus averaged 0.128 for 10 polymorphic loci), allozyme variation occurs independently at different loci so mean genetic identity (Nei) is high: 0.972. There is only a slight decline in genetic identity with distance (genetic identity averaged 0.965 for amphi-atlantic comparisons) indicating that passive transport of juveniles or adults may contribute significantly to gene flow. Conversely, allozyme variation between karyotypes was concordant. High chromosome number populations possessed a suite of alleles at four allozyme loci (Esl-3, Lap-2, Mdh-1 and Pep-2) which were absent or rare in Group 2 samples resulting in high F_ST values for these loci (from 0.294 to 0.472) when karyotypic classes were combined. Consequently the mean genetic identity between these Robertsonian races is low, 0.856, and falls within the range more usually associated with congeneric comparisons than with con-specific comparisons. The mechanisms maintaining this genie difference are unclear. However the distribution of the karyotypes and physiological and morphological differences (in shell shape) between them strongly suggest that karyotypic variation in Nucella is adaptive.     

The evolutionary mechanism maintaining shell shape and molecular differentiation between two ecotypes of the dogwhelk Nucella lapillus

Two ecotypes of Nucella lapillus can occur allopatrically or sympatrically, according to different degrees of wave exposure and microhabitats, on rocky shores of NW Spain. We studied differences in shell size and shape in adults and families of shelled embryos captured in the wild, and molecular variation at 9 microsatellite loci and 4 mtDNA regions. Both adults and shelled embryos of the two ecotypes showed significant differences in size and shell-shape components (the most important being size of the shell aperture). Strong among-family variation suggests that shell-shape variation has an important genetic component, and the pattern of genetic differentiation was in agreement with traits being affected by divergent natural selection. No significant overall molecular differentiation was observed between the two ecotypes for mtDNA, although we found significant genetic structure at two microsatellite loci. Microsatellite differentiation between ecotypes at these two loci was larger in sympatry than in allopatry. These results and the comparison of tenacity suggest that the two forms are distinctly adapted to differences in shore level and degree of exposure to wave action.     

Effect of crab effluent and scent of damaged conspecifics on feeding,growth, and shell morphology of the Atlantic dogwhelk Nucella lapillus (L.)

Juvenile Nucella lapillus of two different shell phenotypes, exposed shore and protected shore, were maintained in running seawater under each of three experimental conditions for 94 d: a) laboratory control, b) exposed to the effluent of crabs (Cancer pagurus) fed frozen fish (fish-crab), and c) exposed to the effluent of crabs fed live conspecific snails (snail-crab). Rates of barnacle consumption and rates of body weight change varied significantly between phenotypes and among experimental conditions. Individuals from the protected-shore consumed consistently fewer barnacles and grew consistently less than those from the exposed shore. Body weight increases in the fish-crab treatments were from 25 to 50% less than those in the controls and body weights in the snail-crab treatment either did not change or actually decreased. The perceived risk of predation thus appears to have a dramatic effect on the rates of feeding and growth of N. lapillus.At the end of the experiment, size-adjusted final shell weights for both phenotypes were consistently higher than controls (no crab) in both the fish-crab and snail-crab treatments. In addition, apertural tooth height, thickness of the lip, and retractability (i.e. the extent to which a snail could withdraw into its shell), with few exceptions all varied in an adaptive manner in response to the various risk treatments. Similar changes in the shell form of starved snails exposed to the same stimuli suggest very strongly that the morphological responses of both phenotypes were not just due to differences in rates of growth. These differences, at least in part, represented a direct cueing of the shell form of Nucella lapillus to differences in the perceived risk of predation. Somewhat surprisingly, the extent of phenotypic plasticity appeared to differ between the populations examined. Both field and laboratory evidence suggest that the exposed-shore population was much more labile morphologically than the protected-shore population.In many instances, particularly among starved snails, the development of antipredatory shell traits was greater in the fish-crab treatment than in the snail-crab treatment. Because the scent of crabs was present in both treatments, these results suggest a) that, at the frequency/concentration used in the experiments, the scent of damaged conspecifics may have been a supernormal stimulus and b) that the morphological response in these treatments might have been greater if the stimulus had been provided at a lower level.     

Biochemical variation and shell shape in populations of the fresh-water snail Lymnaea peregra (Mollusca, Gastropoda, Pulmonata) from south-west Ireland

Lymnaea peregra was collected at sites, predominantly from the Caha plateau, in south-west Ireland. These included sites investigated by previous workers and found to contain populations with a distinctive pattern of variation in shell-shape. Normally spired populations and non-spired ( involuta, sub-involuta ) could occur in adjacent loughs in the same watershed. The biochemical variation in some of these populations was investigated using horizontal starch-gel zone electrophoresis. Fourteen biochemical loci were investigated. Although circumscribed by difficulties in collecting and low sample numbers, the investigation showed that the pattern of variation in shell shape was still present and was not correlated or associated with the biochemical variation. While this did not support hypotheses of geographic race formation or incipient speciation, it was in agreement with the current view of variation and speciation in fresh-water pulmonates.     

Adaptation and speciation of Dolichopoda cave crickets (Orthoptera, Rhaphidophoridae): geographic variation of morphometric indices and allozyme frequencies

Morphometric analyses were carried out for 14 cave dwelling populations of Dolichopoda (Orthoptera, Rhaphidophoridae) from peninsular Italy belonging to the laetitiae-geniculata complex. The selected set of variables includes linear measures of body parts and appendages whose variation is expected to be adaptive and to reflect the degree of specialization to the cave environment.
Inter-population morphometric variation was compared with the patterns of allozyme variation, investigated electrophoretically in a previous paper, by means of ordination and classification multivariate analyses. Results from this comparison reject the hypothesis of parallel geographic variation of morphometric indices and allele frequencies, revealing independent trends for the two sets of characters.     

Epistasis, phenotypic disequilibrium and contrasting associations with climate in the land snail Theba pisana

Hotter conditions favour effectively unbanded (EUB) shells in the snail Theba pisana. T. pisana is also polymorphic for colour of the shell's apex, determined by a pair of alleles at a locus linked to the banding locus. Apex colour is epistatic to shell banding, such that banded snails with a dark apex have darker bands. Annual censuses over 22 years across an ecotone between a sheltered Acacia thicket and open dune vegetation showed a persistent association of both EUB shells and pale apex with the Open habitat. The parallel variation was due partly to strong phenotypic disequilibrium, as the combination of EUB with dark apex was rare. Nevertheless, in fully banded shells the frequency of pale apex was also higher in the Open habitat, confirming independent, parallel associations of the two contributors to paleness. Within the Acacia habitat, temporal variation of the frequencies of banding morphs was much greater than for apex colour, and EUB shells were associated with hotter summers. Consistent with its primary effect only on the very small snails, apex colour did not vary with summer conditions, but instead, higher frequencies of pale apices were associated with sunnier winters. The intensity of selection was lower on apex colour than shell banding, due partly to the constraint of phenotypic disequilibrium. The shell traits in T. pisana are an example of complex responses to climatic variation, in which phenotypic disequilibrium constrains evolution of apex colour, but separate mechanisms of selection are evident.     

Effects of temperature,salinity and aerial exposure on predation and lysosomal stability of the dogwhelk Thais (Nucella) lapillus (L.)

The ingestion and absorption rate of standard length Thais lapillus (L.) stepwise-acclimated to constant temperature-salinity conditions and preying on Mytilus edulis (L.) varied directly with environmental salinity at 10, 15 and 20°C. Dogwhelk ingestion and absorption rates indicate that cold torpor existed at 5°C and heat stress was evident at 20°C. The feeding cycle duration was significantly longer for dogwhelks acclimated to 20%. S than in those acclimated to 30%. S at 10°C even though no significant difference existed between the two groups of snails in the drilling and ingestion or postfeeding phases of the cycle. Ingestive conditioning of dogwhelks to mussels occurred; the duration of the drilling and ingestion and total feeding cycle declined as a function of the number of mussels consumed by a snail. Dogwhelks of all sizes prey on a wide length range of mussels and there is also a high degree of variability in the ingestion rate of snails as a function of their size. A prominent feature of the lack of a relationship between dogwhelk ingestion rate and snail size was that the percentage of nonfeeding snails increased at low salinity and temperature extremes. Digestive-tubule cell lysosomal stability was tested as an index of digestive capability and animal condition; in stepwise-acclimated dogwhelks, it correlated well with their ingestion and absorption rates. The ingestion rate of dogwhelks acclimated to 30%. S and subjected to a 30?17.5?30%. S semidiurnal pattern of fluctuating salinity for 21 days was significantly lower than for snails maintained at 30%. S; however, snails acclimated to 17.5%. S and exposed to the same pattern of fluctuating salinity fed at a higher rate than snails maintained at 17.5%. S. Aerial exposure of snails maintained at 30%. S and 10°C water temperature resulted in an ingestion rate 2.1 times faster than for snails constantly submerged suggesting that tidal emersion is not always stressful to intertidal carnivores. The postfeeding phase of the feeding cycle was shortened in dogwhelks subjected to aerial exposure. Although significant variation occurred in digestive-tubule cell lysosomal stability during the first cycle of fluctuating salinity, the variability had declined significantly by Day 21. This observation suggests that digestive tubule lysosomal stability becomes adapted to a fluctuating osmotic environment, although the initial changes in lysosomal stability are probably related to intralysosomal protein catabolism and production of amino acids for intracellular osmoregulation. Variations in the osmotic environment of T. lapillus have resulted in unexpected outcomes with respect to their ingestion rate under conditions of fluctuating salinity and aerial exposure.     

Salinity Adaptations of the Gastropods Littorina mandshuricaand L. squalidafrom a Marine Bay and an Estuary

The authors examine the effect of decreased salinity on adult and larval periwinkles Littorina mandshuricaand L. squalidafrom two different habitats, a marine bay and an estuary in Vostok Bay, Sea of Japan. Considerable differences were found in salinity tolerance of adult periwinkles, larval growth and survival, and morphology of the egg capsules. Using the stepwise acclimation method, no genotypic differences were found in salinity tolerance limits of gastropod mollusks from different populations. It is assumed that despite marked physiological differences these populations appear not to be different physiological races.     

Temporal and spatial trends in allozyme frequencies in house fly populations,Musca domestica L.

Summary Allelic and genotypic frequencies were sampled from a single age class of the common house fly, Musca domestica L., at five farms on six dates from July 6 to October 12, 1982. Allozymes at six loci were resolved with vertical polyacrylamide gel electrophoresis. No consistent departures from random mating were detected. No consistent linkage disequilibrium was observed. Allele frequencies at the farms changed in independent and unpredictable ways. Gene frequencies at the five farms were initially divergent, converged in midsummer, and then progressively diverged. The divergence occured in mid-August when fly populations were large. Variation in gene frequencies at adjacent farms accounted for a large proportion of the variance in allele frequencies among all farms. These observations are consistent with the hypothesis that allele frequencies in young adult flies reflected the habitat in which they matured as larvae.Journal Paper No. 11718 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, Project No. 2411     

Altitudinal and seasonal variation in microsatellite allele frequencies of Drosophila buzzatii

Variation in climate, particularly temperature, is known to affect the genetic composition of populations. Although there have been many studies of latitudinal variation, comparisons of populations across altitudes or seasons, particularly for animal species, are less common. Here, we study genetic variation (microsatellite markers) in populations of Drosophila buzzatii collected along altitudinal gradients and in different seasons. We found no differences in genetic variation between 2 years or between seasons within years. However, there were numerous cases of significant associations between allele frequencies or expected heterozygosities and altitude, with more than half showing nonlinear relationships. While these associations indicate possible selection and local altitudinal adaptation, direct tests gave strong evidence for selection affecting two loci and weaker evidence for five other loci. Two loci that are located within an inversion (including the one with strongest evidence for selection) show a linear increase in genetic diversity with altitude, likely due to thermal selection. Parallel associations with altitude here and with latitude in Australian populations indicate that selection is operating on chromosomal regions marked by some of the loci.     

Stage-related variation in rapid cold hardening as a test of the environmental predictability hypothesis

The environmental predictability (EP) hypothesis proposes that rapid cold hardening (RCH) might be common in temperate species incapable of surviving freezing events and which also dwell in unpredictable environments. The kelp fly Paractora dreuxi serves as a useful model organism to test this prediction at an intra-specific level because larvae and adults show different responses to low temperature despite occupying a similar unpredictable thermal environment. Here, using acclimation temperatures, which simulated seasonal temperature variation, we find little evidence for RCH in the freeze-intolerant adults but a limited RCH response in freeze-tolerant larvae. In the relatively short-lived adults, survival of -11 degrees C generally did not improve after 2h pre-treatments at -4, -2, 0, 10, 20 or 25 degrees C either in summer- (10 degrees C) or winter (0 degrees C)-acclimated individuals. By contrast, survival of summer-acclimated larvae to -7.6 degrees C was significantly improved by approximately 37% and 30% with -2 and 0 degrees C pre-treatments, respectively. The finding that summer-acclimated larvae showed RCH whereas this was not the case in the winter-acclimated larvae partially supports the predictions of the EP hypothesis. However, the EP hypothesis also predicts that the adults should have demonstrated an RCH response, yet they did not do so. Rather, it seems likely that they avoid stressful environments by behavioural thermoregulation. Differences in responses among the adults and larvae are therefore to some extent predictable from differences in their feeding requirements and behaviour. These results show that further studies of RCH should take into account the way in which differences among life stages influence the interaction between phenotypic plasticity and environmental variability and predictability.     

Seasonal shifts in genotype frequencies in the invasive cladoceran Daphnia lumholtzi in Lake Texoma,U.S.A.

1. We examined the ecological genetics of the invasive cladoceran Daphnia lumholtzi in a reservoir (Lake Texoma) in the southern USA. This species originates from the Old World subtropics and has spread across North America since the late 1980s after its inadvertent introduction to a reservoir in northeastern Texas. 2. The population genetic structure of D. lumholtzi was examined seasonally on 22 dates over a 3‐year period along a natural temperature and salinity gradient. 3. A two‐allele polymorphism at the phosphoglucose isomerase (PGI) locus was detected, while other loci tested showed no variation. Significant temporal heterogeneity existed in genotype frequencies with a major shift between summer and autumn. 4. Results of a multiple linear regression analysis revealed that a significant amount of variation was explained by day length, temperature and conductivity. Additionally, significant spatial heterogeneity of genotype frequencies between lake stations was observed, but was restricted to the summer. 5. Clonal isolates were used in controlled laboratory temperature and salinity tolerance experiments. Results suggest that salinity and temperature tolerance differed between PGI genotypes. 6. Genotype × environment interactions may play a significant role in the micro‐evolutionary dynamics of this invasive species and may have facilitated its rapid expansion across the North American continent.     

Relationship between population size, allozyme variation, and plant performance in the narrow endemic Cochlearia bavarica

We studied population viability in relation topopulation size and allelic variation in thenarrowly-endemic, monocarpic perennial plantCochlearia bavarica in Bavaria. In 1996,we analysed allelic variation by allozymeelectrophoresis in 24 populations ranging from8–2000 flowering individuals. Fitness-relatedcharacters were investigated in 22 of the 24populations in the field in 1996 (reproductiveand vegetative traits) and 1998 (reproductivetraits only). Differences in allozyme patternwere large between a south-eastern and awestern population group. Genetic diversity,assessed by the Shannon-Wiener diversity index,was low within but high among populations.Small populations had fewer alleles per locus,fewer polymorphic loci, lower observedheterozygosity, and lower genetic diversitythan large populations. Environmentalvariables were not significantly correlatedwith population size or fitness with theexception of light availability, indicatingthat habitat quality was similar for large andsmall populations. Population size showedpositive correlations with number of flowers,fruit set per plant, number of seeds per fruit,and total seed output per plant. Fruit set andnumber of seeds per fruit were positivelycorrelated with the observed heterozygosity andthe proportion of polymorphic loci. We usedpath analyses to study the possible causalrelationships among population size, allelicvariation, and reproductive characters. Thesemodels showed that allelic variation had nodirect influence on reproductive characters,whereas population size did. We conclude thatat present population size reduces viabilityand also reduces allelic variation; but thereduced allelic variation may in the longerterm have negative feed-backs on bothpopulation size and viability.