| 1. |
- Ekendahl, Anette, et al.
(författare)
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Shell colour variation in Littorina saxatilis Olivi (Prosobranchia : Littorinidae): a multi-factor approach
- 1997
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Ingår i: Biological Journal of the Linnean Society. - 0024-4066. ; 62:3, s. 401-419
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Tidskriftsartikel (refereegranskat)abstract
- The marine snail Littorina saxatilis is highly polymorphic for shell colour. It lives in the heterogeneous intertidal zone, where there are sharp transitions in a number of abiotic factors that may influence the relative fitness of morphs. We investigated the hypothesis of selected variation by relating the colour distribution to five factors (wave exposure, substratum, shore level, sex, snail age), and to interactions between them. We compared patterns from geographical areas in Sweden, Iceland and Russia. Cryptic morphs (tessellated and different dark colours) generally dominated (80-98%) while conspicuous morphs (white, yellow, red and banded) were less common (2-20%). The colour frequencies were often related to wave exposure, substratum and shore level. Frequencies rarely varied with age and never with sex. In order to test the assumption that the different colours are genetically determined we cross-bred snails from Iceland in the laboratory. Both the presence of bands and the ground colours of the shell were inherited, and we have tentative support for a one-locus two-allele model for banding. Our results support a model of selected inherited colour variation, involving a number of different selective agents, the importance of which may vary between populations on local and geographical scales. (C) 1997 The Linnean Society of London.
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| 2. |
- Erlandsson, Johan, et al.
(författare)
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Migratory differences between ecotypes of the snail Littorina saxatilis on Galician rocky shores
- 1998
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Ingår i: Evolutionary Ecology. - 0269-7653. ; 12:8, s. 913-924
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Tidskriftsartikel (refereegranskat)abstract
- Local migration patterns may be crucial to gene flow in species of marine gastropods which do not broadcast pelagic larvae. In some species, dispersal over distances of a few metres may influence micro-scale population structures. We investigated the migration pattern in Galician populations of the snail Littorina saxatilis in which populations of contrasting morphologies occupy different tidal levels of the same rocky shore. Two distinct morphs, one at the upper and one at the lower shore, overlap in distribution in a small mid-shore region where hybrids are produced. We documented the distances and directions of migration of both parental morphs and hybrids 1 month after they had been marked and released at different shore levels. When placed at their native shore level, snails migrated less than about 2 m and usually in independent directions. This supports the suggestion of a low local gene flow. At an alien shore level, however, the morphs often moved further and more directionally compared with native morphs. These differences may help to keep the two morphs separated at different shore levels. As fitness of an individual is highest in its native habitat, this seems to be an adaptive strategy.
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| 3. |
- Grudemo, Johan, et al.
(författare)
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Size of mudsnails, Hydrobia ulvae (Pennant) and H-ventrosa (Montagu), in allopatry and sympatry: conclusions from field distributions and laboratory growth experiments
- 1999
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Ingår i: Journal of Experimental Marine Biology and Ecology. - 0022-0981. ; 239:2, s. 167-181
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Tidskriftsartikel (refereegranskat)abstract
- In most areas, sizes of the two mudsnails Hydrobia ulvae and H. ventrosa diverge when their distributions overlap, while in allopatry they are about equally large. This has been suggested as an example of character displacement, although later studies suggest alternative interpretations. In this study, distribution and snail size of both species was assessed in 30 localities on the Swedish West coast, both allopatric sites of each species and sympatric sites. Overall, mean sizes varied considerably among populations; in H. ulvae mean size range was 2.0-4.5 mm and in H. ventrosa 2.2-3.6 mm. Mean sizes in allopatry were roughly similar, while in sympatry H. ulvae was always larger than H. ventrosa. In the field, snail size of both species increased with increasing silt content of the sediment. However, silt content alone seemed unable to explain the character displacement-like pattern as growth in the laboratory was not differently affected by sandy or silty sediments. In the laboratory, however, snails of both species grew at various rates over sediments from different bays, suggesting environmental effects other than grain size. Furthermore, H. ulvae constantly grew at about 150% the rate of H, ventrosa when grown under the same conditions. This suggests that the divergence in size in sympatric sites is due to intrinsic growth rate differences, which persist over a range of environments. The similar sizes in allopatric sites may be explained by phenotypic adjustments to environmental factors. Our results reject inherited differences in growth rate among populations as an explanation of the character displacement-like pattern of size variation. We also argue that differences in recruitment time seems an unlikely explanation. (C) 1999 Elsevier Science B;V. All rights reserved.
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| 4. |
- Johannesson, Kerstin, 1955, et al.
(författare)
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Allozyme variation in a snail (Littorina saxatilis) - Deconfounding the effects of microhabitat and gene flow
- 1997
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Ingår i: Evolution. - 0014-3820. ; 51:2, s. 402-409
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Tidskriftsartikel (refereegranskat)abstract
- It is commonly observed that a restricted gene flow among populations of a species generates genetic differentiation in, for example, allozyme markers. However, recent studies suggest that microhabital-specific variation may contribute to the total differentiation. To appreciate the relative contributions of geographic variation and habitat-specific variation, we sampled 42 subpopulations of the intertidal snail Littorina saxatilis from three different microhabitats (boulders, low and high rocky intertidal) on five small islands within a distance of 15 km. We used a modified orthogonal version of Nei's gene diversity analysis with a modified analysis of variance (ANOVA) that estimated the significance of habitat and geographic separation and the interaction between them. Between subpopulation differentiation (GST) was usually in the range of 5% to 10% but was exceptionally high in one locus (Aat; 53%). Genetic differentiation attributable to different habitats accounted for 10% to 81% (mean 35%) of the between subpopulation variation and was significant (P < 0.05) in six loci. Differentiation doe to geographic separation accounted for 11% to 61% (mean 36%) and was significant (P < 0.05) in seven loci. Furthermore, three loci showed interactions between habitat and island, suggesting varying effects of habitat in different islands. microhabitat-specific variation, probably through spatially varying fitness, seems particularly likely in Aat and Pgm-2. Moderate levels of habitat associated variation added to the observed differentiation due to gene flow in Pgi, Pnp, and Pgm-1, whereas in the remaining three loci either the habitat effect was confused by strong habitat-island interaction (Ark) or was virtually absent (Pep and Mpi).
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| 5. |
- Johannesson, Kerstin, 1955, et al.
(författare)
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Growth rate differences between upper and lower shore ecotypes of the marine snail Littorina saxatilis (Olivi) (Gastropoda)
- 1997
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Ingår i: Biological Journal of the Linnean Society. - 0024-4066. ; 61:2, s. 267-279
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Tidskriftsartikel (refereegranskat)abstract
- Shell polymorphisms are widespread among those intertidal gastropods that lack a pelagic spreading stage. These polymorphisms may indicate diversifying selection in a heterogeneous habitat, but to do this the variation must be at least partly inherited. Galician populations of Littorina saxatilis (Olivi) living in exposed rocky shores are highly polymorphic in several shell traits, e.g. ornamentation, banding and size. Mature snails of the upper-shore ridged and banded (RB) morph is, for example, often twice as large as mature individuals of the lower-shore smooth and unbanded (SU) morph of the same shore. We investigated the hypothesis that lower-shore snails grow more slowly and that differences in growth rate were at least partly inherited and could be explained by diversifying selection. We released snails of different origin (upper, mid- and lower shore) and morph (RB, SU and hybrids) at different shore levels and compared their shell increment after one month of growth. We found that despite considerable variation among individuals and among replicate samples (together about 53% of the total variation), average rates of growth differed between morphs. RB snails both from the upper and mid-shores grew at a high rate at all shore levels, SU snails grew considerably less, and hybrids grew at intermediate rates, at all levels. Inherited difference among morphs explained about 34% of the total variation while effects of shore levels and the interaction morph x shore level explained only 5 and 7%, respectively. Thus a large part of the difference in growth rate leading to different adult sizes of the two morphs has probably evolved due to spatially varying selection favouring large sizes in upper-shore and small sizes in lower-shore environments. (C) 1997 The Linnean Society of London.
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| 6. |
- Kostylev, Vladimir E., et al.
(författare)
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Microdistribution of the polymorphic snail Littorina saxatilis (Olivi) in a patchy rocky shore habitat
- 1997
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Ingår i: Ophelia. - 0078-5326. ; 47:1, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- Galician populations of the marine snail Littorina saxatilis have two main morphs on exposed rocky shores. The ridged and banded morph inhabits the barnacle (Chthamalus stellatus) dominated upper shore, while the smooth and unbanded morph is mainly found in the lower-shore zone of blue mussels (Mytilus galloprovincialis). The distribution of the two morphs overlaps in the mid shore where hybrids are also present. The mid-shore habitat is a mixture of barnacle and mussel patches. We hypothesised that the different architectural complexity of barnacles and mussels would affect the distribution of the two morphs. Fractal dimensions were used to describe the substratum complexity of 96 small patches from three sites with different proportions of barnacles and mussels. Increased proportions of mussels in a patch increased the fractal dimension, and thus surface complexity. Snail abundance decreased with increased substratum complexity in the smooth and unbanded morph and in the hybrids but not in the ridged and banded morph. Furthermore, snail size increased with surface complexity Barnacles seemed a more suitable habitat than mussels as the interstitial volumes between barnacles available as refuges fbr snails were of a similar size range to that of the snails, while the mussel refuges were considerably larger.
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| 7. |
- Mikhailova, N., et al.
(författare)
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A comparison of different protocols for RAPD analysis of Littorina
- 1998
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Ingår i: Hydrobiologia. - 0018-8158. ; 378, s. 33-42
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Tidskriftsartikel (refereegranskat)abstract
- Randomly amplified polymorphic DNA (RAPD) is a fast and useful method of genome marking that is useful for studies of, for example, parentage, mating patterns, taxonomy of sibling species and intra-specific population genetic structures. Here we compare three different procedures for extracting high molecular weight genomic DNA; phenol-chloroform hexadecyltrimethyl ammonium-bromide (CTAB) and Chelex 100. Double phenol-chloroform and CTAB extractions both generated high amounts of high quality DNA while Chelex 100 failed to do so. We also compared PCR-amplification with different concentrations of template DNA and found that 1-2 ng per 25 mu l of amplification cocktail gave the best results. Amplifying DNA prepared by the three extraction methods revealed that DNA extracted with double phenol-chloroform gave the dearest bands. The double phenol-chloroform extraction seems thus the most suitable extraction method for RAPD in Littorina, however Chelex may be the only method useful for extracting DNA from very small individuals, for example, pre-hatching stages.
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| 8. |
- Rolan-Alvarez, E., et al.
(författare)
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Mechanisms of incomplete prezygotic reproductive isolation in an intertidal snail: testing behavioural models in wild populations
- 1999
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Ingår i: J. Evol. Biol.. ; 12, s. 879-890
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Tidskriftsartikel (refereegranskat)abstract
- Two morphs (ecotypes) of the marine snail Littorina saxatilis coexist along Galician exposed rocky shores. They hybridize, but gene flow is impeded by a partial prezygotic reproductive barrier, and we have earlier suggested that this is a case of incipient sympatric speciation. To assess the mechanisms of prezygotic reproductive isolation, we estimated deviations from random mating (sexual selection and sexual isolation) of sympatric snails in 13 localities on the shore, and performed mate choice experiments in the laboratory. We also investigated the microdistribution of both morphs over patches of barnacles and blue mussels in the hybridization zone. We used computer simulations to separate the mechanisms contributing to reproductive isolation. On the shores sampled, male-female pairs were strongly assortative both with respect to morphs (mean Yule's V = 0.77) and size (mean Pearson's r = 0.47). In the laboratory, males of both morphs mounted other snails and mated other males and juveniles at random. However, mature females of equal sizes mated assortatively with respect to morph. The two morphs were non-randomly distributed over barnacle and mussel patches in the hybridization zone. Monte Carlo simulations showed that this microdistribution could explain about half the morph and size relationships in male-female pairs, while a simple rejection mechanism, rejecting the first 1-3 mates if they were of contrasting morphs, accounted for the remaining part of the reproductive isolation, and for parts of the size relationships found between mates. A size discriminant mate choice mechanism may also, to a lesser extent, contribute to the sexual isolation. Sexual selection was observed for female size (larger ones being favoured) and among certain morphs, but distinct biological mechanisms may cause these processes.
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| 9. |
- Rolan-Alvarez, E., et al.
(författare)
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The maintenance of a cline in the marine snail Littorina saxatilis: The role of home site advantage and hybrid fitness
- 1997
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Ingår i: Evolution. - 0014-3820. ; 51:6, s. 1838-1847
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Tidskriftsartikel (refereegranskat)abstract
- Steep clinal transitions in one or several inherited characters between genetically distinct populations are usually referred to as hybrid zones. Essentially two different mechanisms may maintain strap genetic dines. Either selection acts against hybrids that are unfit over the entire zone due to their mixed genetic origin (endogenous selection), or hybrids and parental types attain different fitness values in different parts of the dine (exogenous selection). Survival rate estimates of hybrids and parental forms in different regions of the dine may be used to distinguish between these models to assess how the dine is maintained. We used reciprocal transplants to test the relative survival rates of two parental ecotypes and their hybrids over microscale hybrid zones in the direct-developing marine snail Littorina saxatilis (Olivi) on the rocky shores of Galicia, Spain. One of the parental forms occupies upper and the other lower shores, and the hybrids are found at various proportions (1-38%) along with both parental forms in a midshore zone a few meters wide. The survival rate over one month was 39-52% of the native ecotype on upper shores, but only 2-8% for the lower-shore ecotype. In contrast, only 4-8% of the upper-shore ecotype but 53% of large (> 6 mm) and 8% of small (3-6 mm) native lower-shore ecotype survived in the lower shores. In the midshores, both the two parental ecotypes and the hybrids survived about equally well. Thus there is a considerable advantage for the native ecotypes in the upper and lower shores, while in the hybrid zone none of the morphs. hybrids included, are favored. This indicates that the dimorphism of L. saxatilis is maintained by steep cross-shore selection gradients, thus supporting the selection-gradient model of hybrid zones. We performed field and laboratory experiments that suggest physical factors and predation as important selective agents. Earlier studies indicate assortative mating between the two ecotypes in the midshore. This is unexpected in a hybrid zone maintained by selection gradients, and it seems as if the reproductive barrier compresses the hybrid zone considerably.
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| 10. |
- Tatarenkov, Andrey, et al.
(författare)
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Evidence of a reproductive barrier between two forms of the marine periwinkle Littorina fabalis (Gastropoda)
- 1998
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Ingår i: Biological Journal of the Linnean Society. - 0024-4066. ; 63:3, s. 349-365
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Tidskriftsartikel (refereegranskat)abstract
- Studies of allozyme variation may reveal unexpected patterns of genetic variation which challenge earlier conclusions of species delimitations based on morphological data. However, allozyme variation alone may not be sufficient to resolve this kind of problem. For example, populations of the marine intertidal snail Littorina fabalis (= Littorina mariae) from wave exposed parts and from protected parts of the same shores are distinguished by different alleles of arginine kinase (Ark) while indifferent, or very nearly so, in another 29 loci. Intermediate populations have large deficiencies of exposed/sheltered heterozygote classes of Ark and we have earlier suggested habitat-related selection in this locus as the explanation. In this study we estimated growth rate of individual snails of different Ark-genotypes in three different habitats (exposed, sheltered and intermediate). In all habitats the snails homozygous for alleles of 'exposed' type grew faster and matured at a larger size than did snails homozygous for alleles of 'sheltered' types. This relationship was indirectly confirmed in three additional sites of intermediate exposure where exposed Ark-genotypes dominated among large (>8 mm) snails while the sheltered genotypes dominated among small (<5 mm) snails of truly sympatric samples. We furthermore found small differences in allele frequencies of two other loci (Pgi and Pgm-2) and in shell colour frequencies, comparing sympatric snails of exposed and sheltered Ark-homozygotes. Although we found no signs of habitat-related selection among snails of different Ark-genotype, or selection against heterozygotes, we cannot reject selection in Ark, as our experiments only covered one island, one season and grown-up snails. The coupling between allozyme and phenotypic characters in strictly sympatric samples of snails suggests the presence of two gene pools. Perhaps the large and small forms of L. fabalis represent very closely related cryptic taxa. However, introgression between them seems a possible explanation for the striking similarities in the vast majority of morphological and allozyme characters. (C) 1998 The Linnean Society of London.
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