MORPHOLOGICAL AND MOLECULAR VARIATION ACROSS A MIGRATORY DIVIDE IN WILLOW WARBLERS,PHYLLOSCOPUS TROCHILUS

A migratory divide is a narrow region in which two populations showing different migratory directions meet arid presumably also mate and hybridize. Banding of willow warblers, Phylloscopus trochilus, in Europe has demonstrated a migratory divide latitudinally across central Scandinavia. In autumn, southern birds migrate southwest to tropical West Africa, whereas northern birds migrate southeast to East and South Africa. The migratory divide is associated with concordant differences in size and plumage coloration. Based on morphology, we estimate the width of the transition zone between northern and southern willow warblers to be less than 350 km. We found indication of linkage disequilibria around the migratory divide, in that measures of body size were correlated with plumage coloration within the contact zone, but uncorrelated within the populations south or north of the contact zone. The presence of linkage disequilibria and the fact that several morphological clines occur together suggest that the hybrid zone is a result of secondary contact between populations that have differentiated in allopatry. This interpretation is in accord with the knowledge of the recolonization pattern of the Scandinavian peninsula after the last glaciation; animals and plants appeared to have colonized either from the south or from the north around the northern bay of the Baltic Sea. If northern and southern willow warblers resided in allopatric populations during late Pleistocene glaciations and the hybrid zone is a result of postglacial range expansions, we would expect some degree of genetic differentiation accumulated during the period in isolation. In contrast, northern and southern willow warblers are near panmictic in the frequencies of alleles of mitochondrial DNA and at two microsatellite loci. The observed pattern, clear morphological and behavioral differentiation without genetic differentiation at neutral loci, suggests either that the differences are maintained by strong selection on the expressed genes in combination with high levels of current gene flow or, in the case of weak gene flow, that the divergence in morphology and behavior is very recent.     

Clinal variation for amino acid polymorphisms at the Pgm locus in Drosophila melanogaster

Clinal variation is common for enzymes in the glycolytic pathway for Drosophila melanogaster and is generally accepted as an adaptive response to different climates. Although the enzyme phosphoglucomutase (PGM) possesses several allozyme polymorphisms, it is unique in that it had been reported to show no clinal variation. Our recent DNA sequence investigation of Pgm found extensive cryptic amino acid polymorphism segregating with the allozyme alleles. In this study, we characterize the geographic variation of Pgm amino acid polymorphisms at the nucleotide level along a latitudinal cline in the eastern United States. A survey of 15 SNPs across the Pgm gene finds significant clinal differentiation for the allozyme polymorphisms as well as for many of the cryptic amino acid polymorphisms. A test of independence shows that pervasive linkage disequilibrium across this gene region can explain many of the amino acid clines. A single Pgm haplotype defined by two amino acid polymorphisms shows the strongest correlation with latitude and the steepest change in allele frequency across the cline. We propose that clinal selection at Pgm may in part explain the extensive amino acid polymorphism at this locus and is consistent with a multilocus response to selection in the glycolytic pathway.     

Conflicting patterns of mitochondrial and nuclear DNA diversity in Phylloscopus warblers

Molecular variation is often used to infer the demographic history of species, but sometimes the complexity of species history can make such inference difficult. The willow warbler, Phylloscopus trochilus, shows substantially less geographical variation than the chiffchaff, Phylloscopus collybita, both in morphology and in mitochondrial DNA (mtDNA) divergence. We therefore predicted that the willow warbler should harbour less nuclear DNA diversity than the chiffchaff. We analysed sequence data obtained from multiple samples of willow warblers and chiffchaffs for the mtDNA cytochrome b gene and four nuclear genes. We confirmed that the mtDNA diversity among willow warblers is low (pi = 0.0021). Sequence data from three nuclear genes (CHD-Z, AFLP-WW1 and MC1R) not linked to the mitochondria demonstrated unexpectedly high nucleotide diversity (pi values of 0.0172, 0.0141 and 0.0038) in the willow warbler, on average higher than the nucleotide diversity for the chiffchaff (pi values of 0.0025, 0.0017 and 0.0139). In willow warblers, Tajima's D analyses showed that the mtDNA diversity, but not the nuclear DNA diversity, has been reduced relative to the neutral expectation of molecular evolution, suggesting the action of a selective sweep affecting the maternally inherited genes. The large nuclear diversity seen within willow warblers is not compatible with processes of neutral evolution occurring in a population with a constant population size, unless the long-term effective population size has been very large (N(e) > 10(6)). We suggest that the contrasting patterns of genetic diversity in the willow warbler may reflect a more complex evolutionary history, possibly including historical demographic fluctuations or historical male-biased introgression of nuclear genes from a differentiated population of Phylloscopus warblers.     

Weak geographical structure in sperm morphology across the range of two willow warbler Phylloscopus trochilus subspecies in Scandinavia

Sperm morphology is highly diversified among species and at higher taxonomic levels. In birds, there is also increasing evidence of geographical differentiation in sperm traits within species, especially in those with strong sperm competition. Geographical divergences in sperm traits might imply the formation of a reproductive barrier in a speciation process. Here we study sperm morphology variation of willow warblers Phylloscopus trochilus in a geographical context in Scandinavia, across the range of two subspecies that are differentiated in certain genetic markers, morphology and migratory routes. We describe geographical patterns in genotypes (two previously described single‐nucleotide polymorphism (SNP) markers and four polymorphic microsatellites); in wing length, tarsus length and body mass; and in sperm traits of 330 male willow warblers sampled at 33 localities across Norway (58°N–69°N). Birds were on average larger and longer‐winged in the north (spp. acredula) than in the south (spp. trochilus), and showed a sigmoid change in the SNP allele frequencies and body morphology around 65°N. We found no evidence of genetic structuring in the microsatellites. There was no geographical variation in sperm traits across Norway, except that sperm heads were on average longer in the south. Sperm head length was also associated with the two SNP markers, with longer sperm heads for the southern alleles, and midpiece length was weakly associated with one of the SNP markers. Similar among‐male variances in total sperm length among the 33 sampling sites indicate uniform levels of sperm competition across Norway. We conclude that sperm morphology remains a rather undifferentiated trait between the two willow warbler subspecies in Scandinavia, which is consistent with a pattern of a shallow genetic divergence. This indicates that sperm morphology is not a reproductive barrier maintaining the narrow hybrid zone.     

Allelic Variation in a Willow Warbler Genomic Region Is Associated with Climate Clines

Local adaptation is an important process contributing to population differentiation which can occur in continuous or isolated populations connected by various amounts of gene flow. The willow warbler (Phylloscopus trochilus) is one of the most common songbirds in Fennoscandia. It has a continuous breeding distribution where it is found in all forested habitats from sea level to the tree line and therefore constitutes an ideal species for the study of locally adapted genes associated with environmental gradients. Previous studies in this species identified a genetic marker (AFLP-WW1) that showed a steep north-south cline in central Sweden with one allele associated with coastal lowland habitats and the other with mountainous habitats. It was further demonstrated that this marker is embedded in a highly differentiated chromosome region that spans several megabases. In the present study, we sampled 2,355 individuals at 128 sites across all of Fennoscandia to study the geographic and climatic variables associated with the allele frequency distributions of WW1. Our results demonstrate that 1) allele frequency patterns significantly differ between mountain and lowland populations, 2) these allele differences coincide with extreme temperature conditions and the short growing season in the mountains, and milder conditions in coastal areas, and 3) the northern-allele or “altitude variant” of WW1 occurs in willow warblers that occupy mountainous habitat regardless of subspecies. Finally these results suggest that climate may exert selection on the genomic region associated with these alleles and would allow us to develop testable predictions for the distribution of the genetic marker based on climate change scenarios.     

Molecular Analysis of an Allozyme Cline: Alcohol Dehydrogenase in Drosophila Melanogaster on the East Coast of North America

Clines may either be selectively maintained or be the by-product of nonadaptive processes related to population structure and history. Drosophila melanogaster populations on the east coast of North America show a latitudinal cline in the frequencies of two common electrophoretically distinguishable alleles at the alcohol dehydrogenase locus (Adh), designated Adh-S and Adh-F. This cline may either be adaptive or an artifact of a possible recent dual founding of North American D. melanogaster populations in which frequencies of Adh alleles differed between founder populations. By means of a high resolution restriction-mapping technique, we studied the distribution of 113 haplotypes derived from 44 polymorphic DNA markers within the Adh region in 1533 individuals from 25 populations throughout the cline. We found significant clinal differentiation at the polymorphisms determining the mobility-difference causing amino acid replacement between Adh-F and Adh-S alleles. Hitchhiking was limited, despite extensive linkage disequilibrium, and other sites did not vary clinally. Such a pattern of differentiation implies that selection is responsible for the cline. To investigate whether selection acts only on the Adh-F/S site, we performed a ``selective equivalence'''' test under the assumption that all variability within the specified allelic class is selectively neutral. This revealed selective equivalence among Adh-S-bearing haplotypes, whose frequencies showed no differentiation throughout the cline, implying high levels of frequency-homogenizing gene flow. Geographical heterogeneity among Adh-F-bearing haplotypes implied the action of selection on one or more additional variants in linkage disequilibrium with Adh-F. In a further study of a subset of the data (n = 1076 from 18 populations), we found a combined insertion/deletion polymorphism, designated &1, located in the 5'' adult intron and in linkage disequilibrium with Adh-F, to show more marked clinal variation than Adh-F/S. Although the unequivocal identification of the precise target(s) of selection requires further study, we suggest that clinal selection may be acting epistatically on the Adh-F/S and &1 polymorphisms.     

Resistance of flight feathers to mechanical fatigue covaries with moult strategy in two warbler species

Flight feather moult in small passerines is realized in several ways. Some species moult once after breeding or once on their wintering grounds; others even moult twice. The adaptive significance of this diversity is still largely unknown. We compared the resistance to mechanical fatigue of flight feathers from the chiffchaff Phylloscopus collybita, a migratory species moulting once on its breeding grounds, with feathers from the willow warbler Phylloscopus trochilus, a migratory species moulting in both its breeding and wintering grounds. We found that flight feathers of willow warblers, which have a shaft with a comparatively large diameter, become fatigued much faster than feathers of chiffchaffs under an artificial cyclic bending regime. We propose that willow warblers may strengthen their flight feathers by increasing the diameter of the shaft, which may lead to a more rapid accumulation of damage in willow warblers than in chiffchaffs.     

Testing potential selective agents acting on leaf shape in Ipomoea hederacea: predictions based on an adaptive leaf shape cline

Leaf shape is a highly variable phenotype, and is likely influenced by many sources of selection. Ipomoea hederacea exhibits an adaptive latitudinal cline in leaf shape, which is controlled by a single Mendelian locus: lobed individuals dominate the north with entire‐shaped individuals mostly in the south. We test if the following candidate selective agents, suggested by the literature, are responsible for the cline: differential insect herbivory, genetic correlations with other clinal traits like flowering time and growth rate, and thermoregulatory differences. We planted 1680 F₃ individuals, segregating for leaf shape, in the north of I. hederacea's range, where we expected lobed genotypes to have higher fitness. Individuals were assigned to insect removal or control treatments, and we scored herbivory, flowering time, growth rate, leaf temperature, and fitness (seed number). Herbivory, flowering, and growth rate had significant fitness effects, but none differed between leaf shapes. Lobed leaves were consistently warmer at night, but no performance advantage was detected. Finally, we detected no overall fitness differences between leaf shape genotypes, whether we controlled for other traits under selection or not. Our data suggest these candidate selective agents may not be important contributors to the cline, and alternative approaches to understanding the mechanisms maintaining the leaf shape cline in I. hederacea may be necessary.     

Molecular population genetics of the melanic plumage polymorphism in Arctic skuas (Stercorarius parasiticus): evidence for divergent selection on plumage colour

The Arctic skua (Stercorarius parasiticus) is a classic example of an avian plumage polymorphism, with variation in melanin‐based ventral plumage coloration defining pale, intermediate and dark morphs in adults of both sexes. However, despite several decades of field research, there is an incomplete understanding of how the polymorphism in ventral plumage colour is maintained and the selective forces involved. Here, we investigate selection on a locus (MC1R) that is strongly associated with plumage colour variation in Arctic skuas using patterns of nucleotide variation and comparison to neutral loci (nuclear introns and mtDNA). We find that three linked nonsynonymous mutations in MC1R, including the single mutation described previously, are associated with plumage colour in the Arctic skua. The position of nonsynonymous mutations on a MC1R haplotype network implies that divergent selection drove the initial evolution of the colour morphs. Comparisons of F_STs of MC1R vs. nuclear introns among five skua populations differing in proportion of dark morphs along an approximate north–south cline reveal a signature of divergent selection on MC1R. In contrast, we find limited evidence for balancing selection on MC1R within populations, although the power is low. Our results provide strong evidence for both past and ongoing selection on MC1R, and, by implication, plumage colour in Arctic skuas. The results suggest that a fruitful avenue for future ecological studies will be analysis of selection on morphs in colonies at the extremes along the morph ratio cline.     

Evidence for a link between local and seasonal cycles in gene frequencies and latitudinal gene clines in a cyclic parthenogen

In an earlier study (Rhombergh et al., Can. J. Genet. Cytol. 27: 224-232, 1985) of natural populations of the cyclic parthenogenetic Rose aphids, Macrosiphum rosae, 6 out of 31 loci were found to be polymorphic and one locus (Esterase-4) showed cyclic seasonal changes in gene and genotypic frequencies. Assuming that the Est-4 polymorphism was balanced and due to some climatic factor that varies seasonally, and realizing that most environmental factors that vary seasonally also vary latitudinally, we predicted existence of a latitudinal gene cline at this locus. In the present study we surveyed four polymorphic loci (chosen to be used as markers) in six geographic populations spanning over 1200 km between the United States and Canada and found all four loci to have latitudinal clines. We think that the gene clines are due to a latitudinal cline in the degree of advancement of local populations through the seasonal cycle, and have called such a pattern a 'seasonal phase cline'. The results are discussed in relation to the temporal instability of local patterns and persistence of genetic variability on the large scale in aphids. It is argued that population structure of aphids makes retention of selectively neutral or weakly selected polymorphisms difficult.     

An inversion supergene in Drosophila underpins latitudinal clines in survival traits

Chromosomal inversions often contribute to local adaptation across latitudinal clines, but the underlying selective mechanisms remain poorly understood. We and others have previously shown that a clinal inversion polymorphism in Drosophila melanogaster, In(3R)Payne, underpins body size clines along the North American and Australian east coasts. Here, we ask whether this polymorphism also contributes to clinal variation in other fitness‐related traits, namely survival traits (lifespan, survival upon starvation and survival upon cold shock). We generated homokaryon lines, either carrying the inverted or standard chromosomal arrangement, isolated from populations approximating the endpoints of the North American cline (Florida, Maine) and phenotyped the flies at two growth temperatures (18 °C, 25 °C). Across both temperatures, high‐latitude flies from Maine lived longer and were more stress resistant than low‐latitude flies from Florida, as previously observed. Interestingly, we find that this latitudinal pattern is partly explained by the clinal distribution of the In(3R)P polymorphism, which is at ~ 50% frequency in Florida but absent in Maine: inverted karyotypes tended to be shorter‐lived and less stress resistant than uninverted karyotypes. We also detected an interaction between karyotype and temperature on survival traits. As In(3R)P influences body size and multiple survival traits, it can be viewed as a ‘supergene’, a cluster of tightly linked loci affecting multiple complex phenotypes. We conjecture that the inversion cline is maintained by fitness trade‐offs and balancing selection across geography; elucidating the mechanisms whereby this inversion affects alternative, locally adapted phenotypes across the cline is an important task for future work.     

DNA sequence variation and latitudinal associations in hsp23, hsp26 and hsp27 from natural populations of Drosophila melanogaster

Heat shock genes are considered to be likely candidate genes for environmental stress resistance. Nucleotide variation in the coding sequence of the small heat shock genes (hsps) hsp26 and hsp27 from Drosophila melanogaster was studied in flies originating from the Netherlands and eastern Australia. The hsp26 gene was polymorphic for an insertion/deletion of three extra amino acids and two nonsynonymous changes in all populations. The hsp27 gene exhibited two nonsynonymous changes and three synonymous mutations. The hsp26 polymorphism showed a latitudinal cline along the east coast of Australia. This pattern was not confounded by the fact that the shsps are located in the inversion In(3 L)P which also shows a latitudinal cline in eastern Australia. A similar latitudinal cline was found for the previously described variation in hsp23, while frequencies of hsp27 alleles did not change with latitude. These findings suggest that variation at two of the shsps or closely linked loci are under selection in natural populations of D. melanogaster.     

Rapid evolution of an adaptive cyanogenesis cline in introduced North American white clover (Trifolium repens L.)

White clover is polymorphic for cyanogenesis (HCN production after tissue damage), and this herbivore defence polymorphism has served as a classic model for studying adaptive variation. The cyanogenic phenotype requires two interacting biochemical components; the presence/absence of each component is controlled by a simple Mendelian gene (Ac/ac and Li/li). Climate-associated cyanogenesis clines occur in both native (Eurasian) and introduced populations worldwide, with cyanogenic plants predominating in warmer locations. Moreover, previous studies have suggested that epistatic selection may act within populations to maintain cyanogenic (AcLi) plants and acyanogenic plants that lack both components (acli plants) at the expense of plants possessing a single component (Acli and acLi plants). Here, we examine the roles of selection, gene flow and demography in the evolution of a latitudinal cyanogenesis cline in introduced North American populations. Using 1145 plants sampled across a 1650 km transect, we determine the distribution of cyanogenesis variation across the central United States and investigate whether clinal variation is adaptive or an artefact of population introduction history. We also test for the evidence of epistatic selection. We detect a clear latitudinal cline, with cyanogenesis frequencies increasing from 11% to 86% across the transect. Population structure analysis using nine microsatellite loci indicates that the cline is adaptive and not a by-product of demographic history. However, we find no evidence for epistatic selection within populations. Our results provide strong evidence for rapid adaptive evolution in these introduced populations, and they further suggest that the mechanisms maintaining adaptive variation may vary among populations of a species.     

Polymorphism in the couch potato gene clines in eastern Australia but is not associated with ovarian dormancy in Drosophila melanogaster

Natural selection can generate parallel latitudinal clines in traits and gene frequencies across continents, but these have rarely been linked. An amino acid (isoleucine to lysine, or I462K) polymorphism of the couch potato (cpo) gene in Drosophila melanogaster is thought to control female reproductive diapause cline in North America (Schmidt et al. 2008, Proc Natl Acad Sci USA, 105, 16207-16211). Here, we show that under standard diapause-inducing conditions (12 °C and short photoperiod) (Saunders et al. 1989, Proc Natl Acad Sci USA, 86, 3748-3752), egg maturation in Australian flies is delayed, but not arrested at previtellogenic stages. At 12 °C, the phenotypic distribution in egg development was bimodal at stages 8 and 14 and showed a strong nonlinear pattern on the east coast of Australia, with incidence of egg maturation delay (ovarian dormancy) increasing both toward tropical and temperate climates. Furthermore, we found no evidence for an association between the cpo I462K polymorphism and ovarian dormancy at either 12 or 10 °C (when egg maturation was often delayed at stage 7). Owing to strong linkage disequilibrium, the latitudinal cline in cpo allele frequencies was no longer evident once variation in the In(3R)P inversion polymorphism was taken into account. Our results suggest that the standard diapause-inducing conditions (12 °C and short photoperiod) were not sufficient to cause the typical previtellogenic developmental arrest in Australian flies and that the cpo I462K polymorphism does not explain the observed delay in egg development. In conclusion, ovarian dormancy does not show a simple latitudinal cline, and the lack of cpo-dormancy association suggests a different genetic basis to reproductive dormancy in North America and Australia.     

LATITUDINAL VARIATION OF WING:THORAX SIZE RATIO AND WING-ASPECT RATIO IN DROSOPHILA MELANOGASTER

In dipterans, the wing-beat frequency, and, hence, the lift generated, increases linearly with ambient temperature. If flight performance is an important target of natural selection, higher wing:thorax size ratio and wing-aspect ratio should be favored at low temperatures because they increase the lift for a given body weight. We investigated this hypothesis by examining wing: thorax size ratio and wing-aspect ratio in Drosophila melanogaster collected from wild populations along a latitudinal gradient and in their descendants reared under standard laboratory conditions. In a subset of lines, we also studied the phenotypic plasticity of these traits in response to temperature. To examine whether the latitudinal trends in wing:thorax size ratio and wing-aspect ratio could have resulted from a correlated response to latitudinal selection on wing area, we investigated the correlated responses of these characters in lines artificially selected for wing area. In both the geographic and the artificially selected lines, wing:thorax size ratio and wing-aspect ratio decreased in response to increasing temperature during development. Phenotypic plasticity for either trait did not vary among latitudinal lines or selective regimes. Wing:thorax size ratio and wing-aspect ratio increased significantly with latitude in field-collected flies. The cline in wing:thorax size ratio had a genetic component, but the cline in wing-aspect ratio did not. Artificial selection for increased wing area led to a statistically insignificant correlated increase in wing:thorax size ratio and a decrease in wing-aspect ratio. Our observations are consistent with the hypotheses that high wing-thorax size ratio and wing aspect ratio are per se selectively advantageous at low temperatures.     

Genetic, morphological, and feather isotope variation of migratory willow warblers show gradual divergence in a ring

The circular distribution of the willow warbler Phylloscopus trochilus around the Baltic Sea shares many features with the classic examples of ring species; however, the system is much younger. It has previously been shown that a secondary contact zone is located in central Scandinavia, where there are narrow clines for several morphological traits coincident with a migratory divide. Here we analyse multiple traits and genes from > 1700 males captured on breeding territories at 77 sites spread around the Baltic Sea to test the following hypothesis. If the secondary contact zone in Scandinavia is a result of divergence in two allopatric refuge populations during the last glaciation, we expect to find a similar secondary contact zone somewhere else around the circular distribution. Our results show that the trait clines were wider and displaced from each other along the eastern side of the Baltic Sea. Analyses of 12 microsatellite loci confirmed that the genome is very similar between the terminal forms ( F _ST = 0). Two AFLP-derived markers filtered out from a genomic scan instead appear to be maintained by selection. These markers exhibited steep clines at the secondary contact zone in Scandinavia, but as for the phenotypic traits, had vastly different cline centres east of the Baltic Sea. The trait clines along the ring distribution outside the Scandinavian secondary contact zone thus seem to have been shaped by independent action of selection or drift during the process of postglacial colonization.     

Autumn migratory orientation and displacement responses of two willow warbler subspecies (Phylloscopus trochilus trochilus and P. t. acredula) in South Sweden

Topography and historical range expansion has formed a so-called migratory divide between two subspecies of willow warbler (Phylloscopus trochilus) in central Scandinavia. The autumn migratory directions of individuals assigned molecularly to both subspecies and possible hybrids were recorded using orientation cage experiments in southwest and southeast Sweden. We found pronounced differences in willow warblers’ orientation in respect to genotype. The mean directions registered in the control experiments were in accordance with the ringing recoveries and analyses of stable isotopes for Scandinavian willow warblers. With the same individuals we performed displacement experiments between both sites. They resulted in non-significant orientation, which could be explained by the intermediate distance of the displacement or reactions to housing, transportation and location. On a separate set of birds we tested whether stress following transportation could explain the disorientation and found that orientation before and after transport was unchanged. Experimental studies of effects of intermediate displacements across longitudes and studies of orientation of hybrid individuals in the zones of migratory divides are crucial for understanding the mechanisms underlying orientation behaviour. Our work further stresses the importance of knowing the migration genotype of a particular bird under study, in order to correctly evaluate expected migration routes.     

A comparison of nuclear and mitochondrial cline shapes in a hybrid zone in the Sceloporus grammicus complex (Squamata; Phrynosomatidae)

The F5 and FM2 chromosome races of the Sceloporus grammicus complex form a hybrid zone in the Mexican state of Hidalgo. Previous studies of this zone have assessed genetic structure by averaging estimates of shape and width across three diagnostic chromosome markers. This approach is likely to mask subtle differences in cline shape among loci (e.g. selected vs. neutral), and obscure any displacement of cline centres (if present). Here we use maximum likelihood methods to construct the best fitting individual clines for three chromosomal markers, and also add two new markers; the mitochondrial DNA (mtDNA) locus, and the nuclear ribosomal DNA (rDNA) repeat. For each locus, hybrid zone models were fitted by cline shape and width, and the position and number of segments describing the centre of the zone. Pairwise comparisons between all clines revealed concordance between chromosomes 2 and 6, but significant discordance in cline structure among all other paired combinations. The concordance of chromosomes 2 and 6 suggests that these clines are maintained by genome-wide forces. The discordance of the chromosome 1 cline suggests an influence of asymmetric introgression, while the mtDNA cline is probably influenced by selection and drift. The rDNA locus reveals a pattern best explained by either extreme asymmetric introgression or gene conversion. The structure of zone indicates that genome-wide processes and locus specific selective forces as well as drift, are operating to different degrees on different loci. The locus-by-locus approach used here permits a finer discrimination among possible mechanisms responsible for the maintenance of the individual clines.     

Latitudinal clines in body size,but not in thermal tolerance or heat‐shock cognate 70 (HSC70), in the highly‐dispersing intertidal gastropod Littorina keenae (Gastropoda: Littorinidae)

Natural populations of widely‐distributed animals often exhibit clinal variation in phenotypic traits or in allele frequencies of a particular gene over their geographical range. A planktotrophic intertidal snail, Littorina keenae is broadly distributed along the north‐eastern Pacific coast through a large latitudinal range (24°50′N–43°18′N). We tested for latitudinal clines in two complex phenotypic traits – thermal tolerance and body size – and one single locus trait – heat shock cognate 70 (HSC70) – in L. keenae along almost its entire geographical range. We found only weak evidence for a latitudinal cline in the thermal tolerance and no evidence for a cline in allele frequencies at HSC70. However, as predicted by Bergmann's rule, we detected a strong latitudinal cline that accounted for 60% of the variance in body size (R² = 0.598; P < 0.001). In contrast, body size did not significantly affect thermal tolerance. HSC70 showed no genetic differentiation among the populations, supporting our previous mitochondrial gene‐based estimate of high gene flow during this snail's free‐swimming larval stage. Given that L. keenae experiences panmixia along its species range, the observed size cline may be partially or entirely caused by a phenotypically plastic response to local thermal environments rather than by genetic divergence in body size among populations in response to locally optimizing natural selection. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 494–505.     

Characterization of a divergent chromosome region in the willow warbler Phylloscopus trochilus using avian genomic resources

Genome scans have made it possible to find outlier markers thought to have been influenced by divergent selection in almost any wild population. However, the lack of genomic information in nonmodel species often makes it difficult to associate these markers with certain genes or chromosome regions. Furthermore, the extent of linkage disequilibrium (LD) in the genome will determine the density of markers required to identify the genes under selection. In this study, we investigated a chromosome region in the willow warbler Phylloscopus trochilus surrounding a single marker previously identified in a genome scan. We first located the marker in the assembled genome of another species, the zebra finch Taeniopygia guttata, and amplified surrounding sequences in Fennoscandian willow warblers. Within an investigated chromosome region of 7.3 Mb as mapped to the zebra finch genome, we observed elevated genetic differentiation between a southern and a northern population across a 2.5-Mb interval comprising numerous coding genes. Within the southern and northern populations, higher values of LD were mostly found between SNPs within the same locus, but extended across distantly situated loci when the analyses were restricted to sampling sites showing intermediate allele frequencies of southern and northern alleles. Our study shows that cross-species genome information is a useful resource to obtain candidate sequences adjacent to outlier markers in nonmodel species.