Genital morphology and taxonomy of the water strider Aquarius remigis (Say) (Insecta, Hemiptera-Heteroptera: Gerridae)

The shape of the dorsal vesical plate was used to determine the extent of morphological differentiation among populations of the water strider Aquarius remigis (Say) (Hemiptera-Heteroptera: Gerridae), and between species assigned to the A. remigis species group. Populations were sampled throughout North America and Mesoamerica, and included paratypes of A. remigoides Gallant & Fairbairn, and material collected from the type localities of A. amplus (Drake & Harris) and A. nyctalis (Drake & Hottes). If taxonomy is to be inferred from the shape of the dorsal vesical plate, as suggested by previous authors, several taxonomic issues need to be reconsidered. First, there was one species distributed widely across North America and deep into Mexico that is very likely to represent A. remigis , meaning that populations from the western USA previously assigned to ' A. nyctalis ' should be synonymized with this species. Second, A. remigoides as currently defined also includes some populations of A. remigis , and is probably much less widely distributed in the south-eastern USA than previously thought. A hybridization zone in Pennsylvania between A. remigis and A. remigoides , as inferred from measurements of body size and allozyme electrophoresis, is not congruent with data from the male genitalia, which indicated a transition zone between the two forms in Virginia much further to the south. Finally, A. amplus was widespread in Mexico, reaching into Guatemala to the south and into Arizona to the north.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 381–398.     

Effects of the East Australian Current on the genetic structure of a direct developing muricid snail (Bedeva hanleyi, Angas): variability within and among local populations

This paper examines the model that the East Australian Current (EAC) is an important determinant of genetic structure for the direct developing intertidal snail Bedeva hanleyi. Above latitude 33^o S on the east coast of Australia the EAC typically flows parallel to the shore in a southerly direction, whilst below 33^o S the direction of flow is predominantly offshore. Based on this flow-pattern it was predicted that B. hanleyi should exhibit greater genetic subdivision in the region south of 33^o S, due to smaller amounts of inter-population gene-flow via rafting. To test this hypothesis, collections of 45 individuals were made from each of 10–32 local populations/˜ 400 km region. Data for six polymorphic and apparently unlinked allozyme loci were consistent with the hypothesized effects of the EAC. Local populations from the northern region exhibited significantly more alleles per locus (mean = 2.2, cf. 1.8 for southern populations) and greater Hardy-Weinberg-expected heterozygosities (mean = 0.299, cf. 0.185 for southern populations) (one-tailed t -test; P <0.05 in each case). Moreover, the weighted jackknife ±mean SE) standardized genetic variance (θ) among southern populations (0.156 ± 0.025) was more than 50% higher than that among northern populations (0.103 ± 0.018).     

THE EXTENT OF INTROGRESSION OUTSIDE THE CONTACT ZONE BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (TELEOSTEI: CYPRINIDAE)

The cyprinid fishes, Notropis cornutus and N. chrysocephalus, hybridize in a long, narrow zone in the midwestern United States. To quantify the extent of introgression of genetic markers outside of this zone, samples were collected along transects starting near the region of contact (as defined by morphological characters), followed by samples progressively more distant. Diagnostic allozymic and mitochondrial DNA (mtDNA) restriction site markers were used to estimate the extent of introgression outside of the zone, while polymorphic allozyme and mtDNA markers were used to evaluate the potential for gene flow among populations within transects. Analysis of populations from the northern transect provided evidence for differentiation of populations for some of the markers; however, on average, enough gene flow has occurred to overcome substantial differentiation. Introgressed mtDNA and allozyme haplotypes were rare and found only in the population closest to the contact zone. The rarity of introgressed alleles in the more northern populations is consistent with the recent origin of these populations after the Wisconsin glaciation (less than 12,000 years bp) and/or selection maintaining the northern boundary of the contact zone. Analysis of populations from the southern transect revealed evidence for population subdivision but no evidence for introgression at the diagnostic allozyme loci; however, nearly all individuals from this transect possessed introgressed mtDNA haplotypes, with samples furthest from the contact zone exhibiting the highest frequencies of introgression. Patterns of variation for one of the polymorphic allozyme markers (Est-A) and introgressed mtDNAs were highly correlated, suggesting that allozymic heterogeneity at this locus is also the result of introgression. The most likely explanation for these data is that these introgressed haplotypes are indicators of a more southern position of the contact zone during the Pleistocene, with the contact zone shifting northward with the recession of the glacial front. Such movement implicates selection in the maintenance of distributional limits of these species, and hence, the width and position of the contact zone.     

Influence of northern limit range on genetic diversity and structure in a widespread North American tree,sugar maple (Acer saccharum Marshall)

Due to climate change, the ranges of many North American tree species are expected to shift northward. Sugar maple (Acer saccharum Marshall) reaches its northern continuous distributional limit in northeastern North America at the transition between boreal mixed‐wood and temperate deciduous forests. We hypothesized that marginal fragmented northern populations from the boreal mixed wood would have a distinct pattern of genetic structure and diversity. We analyzed variation at 18 microsatellite loci from 23 populations distributed along three latitudinal transects (west, central, and east) that encompass the continuous–discontinuous species range. Each transect was divided into two zones, continuous (temperate deciduous) and discontinuous (boreal mixed wood), based on sugar maple stand abundance. Respective positive and negative relationships were found between the distance of each population to the northern limit (D_north), and allelic richness (A_R) and population differentiation (F_ST). These relations were tested for each transect separately; the pattern (discontinuous–continuous) remained significant only for the western transect. structure analysis revealed the presence of four clusters. The most northern populations of each transect were assigned to a distinct group. Asymmetrical gene flow occurred from the southern into the four northernmost populations. Southern populations in Québec may have originated from two different postglacial migration routes. No evidence was found to validate the hypothesis that northern populations were remnants of a larger population that had migrated further north of the species range after the retreat of the ice sheet. The northernmost sugar maple populations possibly originated from long‐distance dispersal.     

Phylogeographic analysis of the green python,Morelia viridis,reveals cryptic diversity

Green pythons, which are regionally variable in colour patterns, are found throughout the lowland rainforest of New Guinea and adjacent far northeastern Australia. The species is popular in commercial trade and management of this trade and its impacts on natural populations could be assisted by molecular identification tools. We used mitochondrial nucleotide sequences and a limited allozyme data to test whether significantly differentiated populations occur within the species range. Phylogenetic analysis of mtDNA sequences revealed hierarchal phylogeographic structure both within New Guinea and between New Guinea and Australia. Strongly supported reciprocally monophyletic mitochondrial lineages, northern and southern, were found either side of the central mountain range that runs nearly the length of New Guinea. Limited allozyme data suggest that population differentiation is reflected in the nuclear as well as the mitochondrial genome. A previous morphological analysis did not find any phenotypic concordance with the pattern of differentiation observed in the molecular data. The southern mitochondrial lineage includes all of the Australian haplotypes, which form a single lineage, nested among the southern New Guinean haplotypes.     

Allozyme Variation and the Genetic Structure of Populations of the Rare Sedge Carex misera (Cyperaceae)

Abstract Carex misera is a rare sedge, endemic to rocky outcrops and mountain summits within the southern Appalachian Mountains from northern Georgia to northern North Carolina. We assessed allozyme diversity for 406 individuals from nine populations over most of the geographic range. Twenty-seven putative loci were assayed and eight (30%) were found to be polymorphic. Nei's gene diversity statistics ( H _T =0.043, H _S =0.019, G _ST =0.551) indicated low levels of variation but relatively highly differentiated populations, suggesting little gene flow. Significant deviations from genotypic expectations under Hardy-Weinberg equilibrium, high positive fixation indices, and the existence of small genetic neighborhoods within populations suggest that at least some inbreeding occurs. Cluster analysis of Nei's genetic identity statistics and principal component analysis of allele frequency data showed high similarity among the six southern populations with the two northern populations more differentiated from them and from each other. These results suggest that preservation of the northern populations is necessary to conserve the already low levels of genetic diversity within the species.     

Postglacial migration and the loss of allozyme variation in northern populations of Asclepias exaltata (Asclepiadaceae)

The recent Wisconsin glaciation has provided opportunities for examining the effects of postglacial recolonization on the population genetics of plant and animal communities. In this study allozyme Variation was examined in 19 populations of the herbaceous perennial Asclepias exaltata occurring in previously glaciated regions of North America. These northern populations of A. exaltata possess significantly fewer polymorphic loci (46.31 ± 2.7; mean ± 1 SD), alleles per polymorphic locus (1.84 ± 0.24), and expected heterozygosity (0.133 ± 0.031) than populations found in the Pleistocene refugium in the southern Appalachians. Population-level allozyme diversity decreased linearly from south to north and from east to west. Nineteen uncommon alleles previously observed in southern Appalachian populations were undetected in the northern region. Seven common alleles exhibited a clinal change in allele frequency. Of these, only Pgd-1a and Mnr-1c were at low-frequency in the southern Appalachians and increased significantly with increasing latitude and longitude, respectively. Despite this loss of allozyme diversity following postglacial migration, northern populations of A. exaltata have higher allozyme diversity and lower population differentiation (G" = 0.1 17) than mean values for other long-lived herbaceous perennials. Increased habitat fragmentation in northern regions and potential habitat loss in the southern Appalachians are likely to reduce the historically rich gene pool that has provided the genetic stock for postglacial recoveries.     

Systematics of dusky salamanders, Desmognathus (Caudata: Plethodontidae), in the mountain and Piedmont regions of Virginia and North Carolina, USA

We analysed mitochondrial (cytochrome  b ) nucleotide sequences, nuclear allozyme markers, and morphometric characters to investigate species boundaries and phylogenetic relationships among dusky salamanders ( Desmognathus ) in the southern Blue Ridge and adjacent Piedmont Physiographic Provinces of Virginia and North Carolina. Our results revealed four distinct mitochondrial DNA clades that are also characterized by distinct allozyme markers. One clade consists of sequences derived from populations distributed from New England to south-western Virginia that are referable to Desmognathus fuscus Rafinesque, 1820, although there is considerable sequence and allozyme divergence within this clade. A second clade consists of sequences derived from populations referable to Desmognathus planiceps Newman, 1955, a form that we resurrect from its long synonymy under D. fuscus . Desmognathus planiceps and D. fuscus also differ in mandibular tooth shape. Two other cytochrome  b sequences recovered from populations along the Blue Ridge escarpment in southern Virginia are quite divergent from those of the previous two clades, and these populations may represent yet another distinct species. Sequences from a population in the Brushy Mountains in the Piedmont of northern North Carolina are similar to those of Desmognathus carolinensis . Population groupings indicated by allozyme data generally correspond to the cytochrome  b clades. Cryptic diversity in Appalachian desmognathan salamanders clearly requires further study. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society , 2008, 152 , 115–130.     

Subtle population structuring within a highly vagile marine invertebrate, the veined squid Loligo forbesi, demonstrated with microsatellite DNA markers

Microsatellite DNA markers were applied for the first time in a population genetic study of a cephalopod and compared with previous estimates of genetic differentiation obtained using allozyme and mitochondrial DNA (mtDNA) markers. Levels of genetic variation detected with microsatellites were much higher than found with previous markers (mean number of alleles per locus=10.6, mean expected heterozygosity ( H _E)=0.79; allozyme H _E=0.08; mtDNA restriction fragment length polymorphism (RFLP) H _E=0.16). In agreement with previous studies, microsatellites demonstrated genetic uniformity across the population occupying the European shelf seas of the North East Atlantic, and extreme genetic differentiation of the Azores population ( R _ST/ F _ST=0.252/0.245; allozyme F _ST=0.536; mtDNA F _ST=0.789). In contrast to other markers, microsatellites detected more subtle, and significant, levels of differentiation between the populations of the North East Atlantic offshore banks (Rockall and Faroes) and the shelf population ( R _ST=0.048 and 0.057). Breakdown of extensive gene flow among these populations is indicated, with hydrographic (water depth) and hydrodynamic (isolating current regimes) factors suggested as possible barriers to migration. The demonstration of genetic subdivision in an abundant, highly mobile marine invertebrate has implications for the interpretation of dispersal and population dynamics, and consequent management, of such a commercially exploited species. Relative levels of differentiation indicated by the three different marker systems, and the use of measures of differentiation (assuming different mutation models), are discussed.     

Hierarchical organization of genetic variation in the Costa Rican livebearing fish Brachyrhaphis rhabdophora (Poeciliidae)

I examined the geographic distribution of genetic variation in the livebearing freshwater fish Brachyrhaphis rhabdophora in northwestern Costa Rica as revealed by allozymes and mitochondrial DNA sequences. Allelic variability at 11 enzyme-coding loci surveyed across 12 localities revealed marked genetic differentiation among populations within drainages (0_P= 0.36) and among drainages within regions (0_D=0.17), but not between northern and southern geographic regions (0_R=– 0.02). Allozyme variation was hierarchically organized such that populations found within stream drainages were more similar to each other than to populations found in adjacent drainages, a result confirmed by cluster analysis. In contrast to the allozyme data, there was extremely little DNA sequence variation among populations in the mitochondrial control region (3 variable nucleotide positions out of 444 bp examined). The difference in genetic divergence between allozyme and mtDNA markers was unexpected and is discussed in terms of biogeographical colonization events and a molecular selective swéep on the mitochondrial genome, both processes that could explain the lack of mitochondrial variability in this highly subdivided species.     

Isolation by distance and post-glacial range expansion in the rough-skinned newt, Taricha granulosa

Allozymes and mitochondrial DNA sequences were used to examine the phylogeographical history of the rough-skinned newt, Taricha granulosa, in western North America. Nineteen populations were analysed for allozyme variation at 45 loci, and 23 populations were analysed for cytochrome b sequence variation. Both data sets agree that populations in the southern part of the range are characterized by isolation by distance, whereas northern populations fit the expectations of a recent range expansion. However, the northern limit of isolation by distance (and the southern limit of range expansion) is located in Oregon State by the mtDNA data, and in Washington State by the allozyme data. Nevertheless, both data sets are consistent with the known Pleistocene history of western North America, with phylogenetically basal populations in central and northern California, and a recent range expansion in the north following the retreat of the Cordilleran ice sheet 10,000 years ago. Additionally, a population in Idaho, previously considered introduced from central California based on morphometric analyses, possesses a distinct mtDNA haplotype, suggesting it could be native. The relevance of these results for Pacific Northwest biogeography is discussed.     

Structure of allozyme variation in Nordic Silene nutans (Caryophyllaceae): population size, geographical position and immigration history

We investigated allozyme variation in 34 populations of the perennial herb Silene nutans from Sweden and northern Finland, areas that were ice-covered during the last (Weichselian) glaciation. The present geographical structure of genetic variation in S. nutans in Sweden and northern Finland appears to have been mainly shaped by ancient historical processes. Patterns of variation in allele frequencies suggest two major postglacial immigration routes into Sweden, with populations entering the area from both the south and the east and forming a contact zone with admixed populations in central Sweden. While estimates of within-population genetic diversity and allelic richness are significantly correlated with present population size and geographical position (latitude), population size is not correlated with latitude. Low genetic diversity in the northern populations is more likely to have resulted from ancient stochastic events during the process of immigration than from recent population fragmentation. F _IS values are high and increase with latitude. Evidence of recent bottlenecks was detected in several southern Swedish populations: these can be interpreted in terms of population fragmentation as a result of anthropogenic disturbance. Soil pH is uncorrelated with population size and position.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 357–371.     

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.     

INTRASPECIFIC CHLOROPLAST DNA VARIATION AND BIOGEOGRAPHY OF NORTH AMERICAN LIRIODENDRON L. (MAGNOLIACEAE)

Restriction site variation in chloroplast DNA (cpDNA) was surveyed to analyze population dynamics in Liriodendron tulipifera L., a woody angiosperm found in eastern North America. Two cpDNA haplotypes, differing by the presence or absence of five restriction site changes (nucleotide sequence divergence estimated as approximately 0.15%) are geographically structured; 61 widespread populations possess the “northern” haplotype and three isolated populations of central Florida possess the “southern” haplotype. This geographic break in cpDNA distribution corresponds to patterns of geographic distribution revealed by a previous survey of allozyme variation, with the exception that analyses of allozyme data further divided the populations containing the northern cpDNA haplotype into two groups, a widespread upland group and a coastal intermediate group. Analyses of these two independent data sets together support the hypothesis that L. tulipifera survived the glacial advances of the Pleistocene in two distinct refugia, possibly as different taxa, and the intermediate coastal group was putatively formed from recent hybridizations between these entities.     

Morphological and genetic differentiation in Isodon umbrosus by altitudinal variation in bumblebee pollinator assemblages

The corolla tube length of the bumblebee-pollinated plant Isodon umbrosus shows conspicuous geographical variation, corresponding with the proboscis length of its bumblebee pollinators across its distributional range. We hypothesized that altitudinal variation in the pollinator assemblage is a principal factor mediating morphological and genetic differentiation among I. umbrosus populations. We examined determinants of the morphological and genetic differentiation of Isodon umbrosus by analyzing floral morphology and allozyme variation across the distributional range. A reanalysis of previous data confirmed that altitude was a good indicator of pollinator assemblages. Corolla tube length was highly variable among the 15 study populations, and genetic differentiation among the populations ( G _ST = 0.360) was also highly significant. The differentiation in corolla tube length was explained by altitudinal difference, a proxy of the difference in pollinator assemblages. Genetic differentiation among populations also tended to be affected by the same factor, but statistical support was weak. To better understand the mechanisms responsible for morphological and genetic differentiation in I. umbrosus , we need to investigate altitudinally different populations over a narrower geographical scale.     

Genetic and morphological variation in the venus clam Cyclina?sinensis along the coast of China

Six geographic samples of Cyclina sinensis were collected from the coast along China and analyzed to reveal morphological and genetic variation by using nine allozyme loci and 11 morphological variables. The discriminant function analysis (DFA) of morphology suggested a clear separation between the southern and northern populations. Polymorphism was detected at nine loci across all six populations. The mean allele number ranged from 2.44 to 2.78, and the mean observed heterozygosity ranged from 0.218 to 0.296. High level of genetic differentiation was found between three northern populations and three southern populations. The marked genetic differentiation can be explained by the upwelling of the Zhejiang province and the freshwater outflow of Yangtze River. The results obtained in this study indicate that the northern and southern populations of C. sinensis should be treated as separate units for conservation management.     

Genetic and morphological differentiation in the clam Coelomactra antiquata (Bivalvia: Veneroida) along the coast of China

Coelomactra antiquata is a commercially exploited bivalve, and its populations have been severely declining in the coast of China during the last two decades. In order to provide guidelines for conservation strategies and management programs, four populations of C. antiquata, representing the most important populations in China, were analysed morphologically and genetically to reveal the genetic structure by using 7 polymorphic allozyme loci and 12 morphological variables. The mean allele richness ranged from 2.79 to 3.53, while the mean heterozygosity ranged from 0.326 to 0.519. All genetic distance metrics revealed high level of genetic differentiation between the three northern populations and one southern population. The marked genetic differentiation can be explained by the freshwater outflow of Yangtze River and upwelling of Zhejiang province. The morphological analyses were in accordance with the allozyme data, which also disclosed a high degree of differentiation between the northern and southern populations. The results obtained in this study indicate that the northern and southern populations in C. antiquata should be treated as separate units for conservation management.     

Micro-evolution in the weevil genus Larinus: the formation of host biotypes and speciation

Data are presented on allozyme variation between 15 populations of the stenophagous capitulum weevil, Larinus cynarae , and three populations of its congener, L. latus , that had been collected throughout the northern mediterranean range of these species. A phenetic analysis of these data revealed no direct relationship between genetic variation and host-plant association within L. cynarae , but there was a strong geographical structuring of allozyme patterns. Most of the genetic variation was due to differences between geographical regions and variation within these was small. Wright's F _ST values showed that Italian and Greek populations of L. cynarae were most distinct from L. latus , with southern Iberian, northern Spanish and French populations increasingly less so. This pattern was associated with a cline in the frequencies of certain alleles along this geographical arc from France to Greece. A phenogram of Nei's genetic distances indicated the close genetic relationship between the two species of Larinus and separated the populations of L. cynarae into three allopatric groups. These groups have different host-plant spectra - dominated by Cynara cardunculus in Italy and Greece, Cynara humilis/Onopordum in southern Iberia and Onopordum spp. in France/Northern Spain - and can be considered to be host biotypes of L. cynarae. L. latus , which occurs in Greece and further east is also an Onopordum specialist. An analysis of the phylogeny of this group of Larinus indicates a primary separation into eastern ( L. latus ) and western ( L. cynarae ) taxa, with further branching of the L. cynarae lineage into the putative host-biotypes. An hypothesis for the evolution of these taxa is given, based on the evolutionary history of host-plant taxa and geographical constraints.     

Genetic diversity of trout (genus Salmo) from its most eastern native range based on mitochondrial DNA and nuclear gene variation

Russia and western Asia harbour trout populations that have been classified as distinct species and subspecies, most often on the basis of morphological and ecological variation. In order to assess their origins and to verify whether traditional taxonomy reflects their evolutionary distinctiveness, we documented their genetic relationships on the basis of mitochondrial DNA (mtDNA) RFLP, mtDNA sequence analysis, and allozyme variation. Both mtDNA and nuclear gene variation defined two ancient phylogenetic assemblages of populations distributed among northern (Baltic, White, Barents), and southern (Black, Caspian, Aral) sea basins, between which gene flow has been possible but limited in postglacial times. These results supported the traditional taxonomic differentiation between populations of these two regions. They provided weak support for the taxonomic distinction of southern brown trout (Salmo trutta) populations based on their basin of origin. They also refuted the hypothesis that L. Sevan trout (Salmo ischchan) diverged from a primitive brown trout ancestor. Nevertheless, all trout populations from southern sea basins possessed private alleles or mtDNA genotypes and were genetically distinct Therefore, they represent unique gene pools that warrant individual recognition for conservation and management.     

ASSOCIATION BETWEEN LATITUDINAL VARIATION FOR EMBRYONIC DEVELOPMENT TIME AND CHROMOSOME STRUCTURE IN THE GRASSHOPPER CALEDIA CAPTIVA (ORTHOPTERA: ACRIDIDAE)

From southeastern Queensland to southern Victoria, over a transect of 11° latitude, the Moreton taxon of the Australian grasshopper Caledia captiva exhibits a cline in chromosome structure that involves change from a metacentric to an acrocentric genome. In this study, we show that embryonic development time covaries with chromosome structure along the transect. Both development time and chromosome short arm length exhibit an overall negative correlation with latitude, but with maxima just south of the northern limit of the taxon's distribution. Selection for such a pattern appears to arise from changes in voltinism along the cline in season length that exists along the transect. Populations with the highest temperature thresholds for avoidance of embryonic diapause also have the slowest development time and probably represent the northern extreme of a primarily univoltine life cycle. North of this region bivoltinism increases in frequency and, as expected from a split of the season length, development time decreases. Maximum chromosome short arm length occurs in the vicinity of the northern univoltine populations, rather than at the limit of distribution where bivoltinism prevails. We conclude that variation in chromosome structure could be contributing to the heritable variation for development time that forms the basis for adaptive change in this trait. These results provide justification for investigating causal relationships between chromosome structure and development time, with an ultimate aim of understanding the adaptive significance of chromosomal variation in C. captiva.