THE INFLUENCE OF INTRASPECIFIC VARIATION IN DISPERSAL STRATEGIES ON THE GENETIC STRUCTURE OF PLANTHOPPER POPULATIONS

The hypothesis that levels of gene flow among populations are correlated with dispersal ability has typically been tested by comparing gene flow among species that differ in dispersal abilities, an approach that potentially confounds dispersal ability with other species-specific differences. In this study, we take advantage of geographic variation in the dispersal strategies of two wing-dimorphic planthopper species, Prokelisia marginata and P. dolus, to examine for the first time whether levels of gene flow among populations are correlated with intraspecific variation in dispersal ability. We found that in both of these coastal salt marsh–inhabiting species, population-genetic subdivision, as assessed using allozyme electrophoresis, parallels geographic variation in the proportion of flight-capable adults (macropters) in a population; in regions where levels of macroptery are high, population genetic subdivision is less than in regions where levels of macroptery are low. We found no evidence that geographic variation in dispersal capability influences the degree to which gene flow declines with distance in either species. Thus, both species provided evidence that intraspecific variation in dispersal strategies influences the genetic structure of populations, and that this effect is manifested in population-genetic structure at the scale of large, coastal regions, rather than in genetic isolation by distance within a region. This conclusion was supported by interspecific comparisons revealing that: (1) population-genetic structure (G_ST) of the two Prokelisia species correlated negatively with the mean proportion of flight-capable adults within a region; and (2) there was no evidence that the degree of isolation by distance increased with decreasing dispersal capability. Populations of the relatively sedentary P. dolus clustered by geographic region (using Nei's distances), but this was not the case for the more mobile P. marginata. Furthermore, gene flow among the two major regions we surveyed (Atlantic and Gulf Coasts) has been substantial in P. marginata, but relatively less in P. dolus. The results for P. marginata suggest that differences in the dispersal strategies of Atlantic and Gulf Coast populations occur despite extensive gene flow. We argue that gene flow is biased from Atlantic to Gulf Coast populations, indicating that selection favoring a reduction in flight capability must be intense along the Gulf. Together, the results of this study provide the first rigorous evidence of a negative relationship within a species between dispersal ability and the genetic structure of populations. Furthermore, regional variation in dispersal ability is apparently maintained by selective differences that outweigh high levels of gene flow among regions.     

Folate-mediated one-carbon metabolism and neural tube defects: balancing genome synthesis and gene expression

Neural tube defects (NTDs) refer to a cluster of neurodevelopmental conditions associated with failure of neural tube closure during embryonic development. Worldwide prevalence of NTDs ranges from approximately 0.5 to 60 per 10,000 births, with regional and population-specific variation in prevalence. Numerous environmental and genetic influences contribute to NTD etiology; accumulating evidence from population-based studies has demonstrated that folate status is a significant determinant of NTD risk. Folate-mediated one-carbon metabolism (OCM) is essential for de novo nucleotide biosynthesis, methionine biosynthesis, and cellular methylation reactions. Periconceptional maternal supplementation with folic acid can prevent occurrence of NTDs in the general population by up to 70%; currently several countries fortify their food supply with folic acid for the prevention of NTDs. Despite the unambiguous impact of folate status on NTD risk, the mechanism by which folic acid protects against NTDs remains unknown. Identification of the mechanism by which folate status affects neural tube closure will assist in developing more efficacious and better targeted preventative measures. In this review, we summarize current research on the relationship between folate status and NTDs, with an emphasis on linking genetic variation, folate nutriture, and specific metabolic and/or genomic pathways that intersect to determine NTD outcomes.     

Is there a familial link between Down's syndrome and neural tube defects? Population and familial survey

Objective To verify whether Down''s syndrome and neural tube defects arise more often in the same family than expected by chance.Design Population and familial survey.Setting Network of maternity hospitals in the Latin American collaborative study of congenital malformations (ECLAMC) in Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Paraguay, Peru, Uruguay, and Venezuela between 1982 and 2000.Probands 2421 cases of neural tube defects, 952 of hydrocephalus, and 3095 of Down''s syndrome registered from a total of 1 583 838 live births and stillbirths.Main outcome measures Observed number of cases of Down''s syndrome among siblings of probands with a neural tube defect or hydrocephalus and number expected on the basis of maternal age; observed number of cases of neural tube defects or hydrocephalus among siblings of probands with Down''s syndrome and number expected according to the prevalence in the same population.Results Five cases of Down''s syndrome occurred among 5404 pregnancies previous to a case of neural tube defect or hydrocephalus, compared with 5.13 expected after adjustment by maternal age. Twelve cases of neural tube defect or hydrocephalus occurred among 8066 pregnancies previous to a case of Down''s syndrome, compared with 17.18 expected on the basis of the birth prevalence for neural tube defects plus hydrocephalus in the same population.Conclusion No association occurred between families at risk of neural tube defects and those at risk of Down''s syndrome.     

Levels of variation in stress resistance in drosophila among strains, local populations, and geographic regions: patterns for desiccation, starvation, cold resistance, and associated traits

Stress resistance traits in Drosophila often show clinal variation. Although these patterns suggest selection, there is generally no attempt to test how large differences at the geographical level are relative to levels of variation within and between local populations. Here we compare these levels in D. melanogaster from temperate Tasmania versus tropical northern Queensland by focusing on adult resistance to desiccation, cold and starvation stress, as well as associated traits (size, lipid content). For starvation and desiccation resistance, levels of variation were highest among strains from the same population. whereas there was little differentiation among local populations and a low level of differentiation at the geographic level. For adult cold resistance, there was local differentiation and strain variation but no geographic variation. For size (thorax length), geographic differentiation was higher despite some overlap among strains from the tropical and temperate locations. Finally, for lipid levels there was only evidence for variation among strains. The low level of differentiation among geographic locations for stress resistance was further verified with the characterization of isofemale strains from 18 locations along a coastal transect extending from Tasmania to northern Queensland. Crosses among some of the isofemale strains showed that results were not confounded by inbreeding effects. Strains derived from a cross between a tropical and temperate strain differed for all traits, and variation among strains for body size was higher than strain variation within the geographic regions. Unlike in previous studies, lipid content and starvation resistance were not correlated in any set of strains, but there was a correlation between cold resistance and lipid content. There was also a correlation between desiccation resistance and size but only in the geographic cross strains. These findings suggest a large amount of variation in stress resistance at the population level and inconsistent correlation patterns across experimental approaches.     

Factors Affecting Geographic Variation in Echolocation Calls of the Endemic Myotis davidii in China

The sensory drive hypothesis of speciation predicts that divergence in communication systems will occur when environments differ and that this sensory divergence can ultimately promote speciation. The factors affecting geographic evolution in acoustic signals remain poorly understood, especially in the contexts of high gene flow. This study investigated variation patterns in peak frequency emitted by the Chinese endemic Myotis davidii on a broad geographic scale by evaluating the relative importance of morphological, environmental, geographic, and genetic variables. Significant variation in peak frequency was observed among regions, but peak frequencies among populations within region had some percentage of similarity. Differences in peak frequency were not associated with morphological difference, genetic structure, and geographic distance among regions, which suggested that peak frequency divergences in M. davidii were not the primary driver of regions' isolation in a context of weak gene flow. Within the Middle East Plain (MEP), one of the regions delineated in this study, peak frequency differences of M. davidii were not significantly correlated with genetic distance and geographic distance among populations, suggesting that peak frequency was not be subject to cultural drift within MEP. Our results provide evidence that geographic variation in echolocation call design may evolve as a consequence of local adaptation to climate conditions.     

Body Size and Geographic Range Do Not Explain Long Term Variation in Fish Populations: A Bayesian Phylogenetic Approach to Testing Assembly Processes in Stream Fish Assemblages

We combine evolutionary biology and community ecology to test whether two species traits, body size and geographic range, explain long term variation in local scale freshwater stream fish assemblages. Body size and geographic range are expected to influence several aspects of fish ecology, via relationships with niche breadth, dispersal, and abundance. These traits are expected to scale inversely with niche breadth or current abundance, and to scale directly with dispersal potential. However, their utility to explain long term temporal patterns in local scale abundance is not known. Comparative methods employing an existing molecular phylogeny were used to incorporate evolutionary relatedness in a test for covariation of body size and geographic range with long term (1983 – 2010) local scale population variation of fishes in West Fork White River (Indiana, USA). The Bayesian model incorporating phylogenetic uncertainty and correlated predictors indicated that neither body size nor geographic range explained significant variation in population fluctuations over a 28 year period. Phylogenetic signal data indicated that body size and geographic range were less similar among taxa than expected if trait evolution followed a purely random walk. We interpret this as evidence that local scale population variation may be influenced less by species-level traits such as body size or geographic range, and instead may be influenced more strongly by a taxon’s local scale habitat and biotic assemblages.     

Isozyme Frequency Patterns in DROSOPHILA PAVANI Associated with Geographical and Seasonal Variables

Fourteen population samples of Drosophila pavani were obtained from a number of localities in Chile. The populations sampled were dispersed over 7 degrees of latitude and 1800 meters of elevation, and were drawn at three different times. Sixteen electrophoretic loci were assayed for each population; eight of the loci were analyzed statistically for geographic variation; the other eight were essentially monomorphic. For all eight variable loci, variation in allelic frequencies among populations was highly significant. In all cases, a significant portion of the variation among populations was associated with variation in gross environmental variables (latitude, elevation, month of collection). The implications of the evidence were discussed, and the authors concluded that there was suggestive evidence for selection.     

Local adaptation: Mechanical fit between floral ecotypes of Nerine humilis (Amaryllidaceae) and pollinator communities

Geographic variation in floral morphology is often assumed to reflect geographic variation in pollinator communities and associated divergence in selective pressures. We studied populations of Nerine humilis (Amaryllidaceae) to assess whether geographic variation in floral form is the result of local adaptation to different pollinator communities. We first tested for associations between floral traits and visitor communities, and found that populations with similar floral morphologies were visited by similar insect communities. Mean style length in each population was also closely associated with the mean body length of the local visitor community. A reciprocal translocation experiment demonstrated that native phenotypes set more seed than translocated phenotypes. Single visitation experiments showed that native flowers received more pollen, and set more seed per visit, than introduced phenotypes in both populations. This suggests that the effectiveness of pollinator visits is determined by the degree of mechanical fit between flowers and visitors. We provide strong evidence that the observed among‐population variation in floral traits is an adaptive response to geographic variation in the pollinator community.     

Host plant associations and geography interact to shape diversification in a specialist insect herbivore

Disentangling the processes underlying geographic and environmental patterns of biodiversity challenges biologists as such patterns emerge from eco‐evolutionary processes confounded by spatial autocorrelation among sample units. The herbivorous insect, Belonocnema treatae (Hymenoptera: Cynipidae), exhibits regional specialization on three plant species whose geographic distributions range from sympatry through allopatry across the southern United States. Using range‐wide sampling spanning the geographic ranges of the three host plants and genotyping‐by‐sequencing of 1,217 individuals, we tested whether this insect herbivore exhibited host plant‐associated genomic differentiation while controlling for spatial autocorrelation among the 58 sample sites. Population genomic structure based on 40,699 SNPs was evaluated using the hierarchical Bayesian model entropy to assign individuals to genetic clusters and estimate admixture proportions. To control for spatial autocorrelation, distance‐based Moran's eigenvector mapping was used to construct regression variables summarizing spatial structure inherent among sample sites. Distance‐based redundancy analysis (dbRDA) incorporating the spatial variables was then applied to partition host plant‐associated differentiation (HAD) from spatial autocorrelation. By combining entropy and dbRDA to analyse SNP data, we unveiled a complex mosaic of highly structured differentiation within and among gall‐former populations finding evidence that geography, HAD and spatial autocorrelation all play significant roles in explaining patterns of genomic differentiation in B. treatae. While dbRDA confirmed host association as a significant predictor of patterns of genomic variation, spatial autocorrelation among sites explained the largest proportion of variation. Our results demonstrate the value of combining dbRDA with hierarchical structural analyses to partition spatial/environmental patterns of genomic variation.     

Similar rhythms of seasonal conceptions in neural tube defects and schizophrenia: a hypothesis of oxidant stress and the photoperiod

BACKGROUND: Numerous studies have found that people with schizophrenia tend to be born most often in late winter and least often in late summer. The same rhythm appears in the birth of children with neural tube defects (NTDs). In the northern hemisphere, both disorders thus show a conception peak in May-June and a trough around November-December. The senior author found the same May-June conception peak among left-handed American baseball players and the opposite effect (a November-December peak) among extreme right-handed players. A similar rhythm appeared with respect to characteristics related to artistic as opposed to scientific modes of thought. DISCUSSION: Schizophrenia has been proposed to involve a deficit in the establishment of lateral asymmetry, as does left-handedness. The artist-scientist dichotomy is also believed to involve cerebral dominance. Thus, the similarity of seasonal variation in month of conception between NTDs, schizophrenia, left-handedness, and artistic intuition suggests that these four conditions may share some factor affecting the cellular processes involved in both neural tube closure and asymmetry development during the early-fourth week, neural-fold stages of embryogenesis. We propose that maternal oxidant stress, which can rise with exposure to intense solar radiation, may interfere with both neural tube closure and asymmetry development. The June and December extremes of sunlight would thus explain the peak times of the seasonal fluctuations. Moreover, regardless of mechanisms, the parallel between the two conception rhythms suggests that the same periconceptional folate regimens found effective in preventing NTDs may also lower the risk of schizophrenia. This paper reviews some of the clinical and experimental evidence supporting this hypothesis.     

Patterns and sources of variation in pollen deposition and pollen tube formation in flowers of the endemic monoecious shrub Cnidoscolus souzae (Euphorbiaceae)

Pollen deposition and pollen tube formation are key components of angiosperm reproduction but intraspecific variation in these has rarely been quantified. Documenting and partitioning (populations, plants and flowers) natural variation in these two aspects of plant reproduction can help uncover spatial mosaics of reproductive success and underlying causes. In this study, we assess variation in pollen deposition and pollen tube formation for the endemic monoecious shrub Cnidoscolus souzae throughout its distribution range in Mexico, and determine how this variation is structured among populations, plants and flowers. We also infer the relative importance of pollen quantity and quality in determining pollination success in this species. While we found no evidence suggesting that pollen receipt limits C. souzae reproduction across 19 populations, we did find extensive variation in pollen load size and pollen tube number per flower. Total variation in pollen receipt and pollen tube number was mostly explained by intra‐individual and among‐population variance. Furthermore, pollen load size had a stronger effect on the number of pollen tubes at the base of the style than pollen germination rate, suggesting that pollen quantity may be more important than quality for pollen tube success in C. souzae. Our results suggest that both small within‐plant flower differences and broad‐scale differences in community attributes can play an important role in determining pollination success. We emphasise the need to evaluate patterns and sources of variation in pollen deposition and pollen tube formation as a first step in understanding the causes of variation in pollination success over broad spatial scales.     

Nuclear RFLP variation in Eucalyptus camaldulensis Dehnh. from northern Australia

Eucalyptus camaldulensis Dehnh. is the most widely planted eucalypt in the tropics. Natural populations are riparian and sampling strategies for breeding programmes have assumed that gene flow among drainage basins is limited. RFLP variation, within and among 31 populations from river systems across northern Australia, was analysed to test this hypothesis. To allow comparisons within and between river systems, trees were sampled from up to three populations per river system. Allele frequencies were correlated with longitude for more than half the 33 RFLP loci surveyed. Genetic identity was greatest between populations in closest geographic proximity, irrespective of river system, suggesting that sampling strategies for breeding programmes should be based on geographic distance rather than river system. The level of genetic variation was similar throughout the geographic range examined (mean H(E) = 0.49). However, there was evidence of a barrier to gene flow between populations in the east and west of the species range. The RFLP data support morphological evidence of hybridisation between E. camaldulensis and E. tereticornis Sm. in several populations in northeast Queensland and the genetic divergence of E. camaldulensis subsp. simulata Brooker and Kleinig.     

Ecogeographic analysis of morphological and life-history variation in the Italian treefrog

We describe the spatial pattern of variation in body size, body shape, and adult population age structure of the Italian treefrog (Hyla intermedia). By means of a non-parametric test of matrix association (the Partial Mantel Test), we investigate the role of climate and geography in explaining the patterns of phenotypic variation. Body size is the most important source of morphometric variation, both at intra- and inter-populational levels. Body shape, despite its secondary role in explaining variation, still maintains high discriminative power among populations. Age structure differs significantly among populations, since body size correlates positively to age, these differences partly explain geographic variation of body size. The patterns of variation in both age and body size are congruent with climate variation, suggesting causal relationship. However, we find no evidence for adaptive explanations and suggest that climate-induced demographically differential survivorship might be the proximate cause responsible for the associations observed. Unlike body size, body shape varies congruently with the pattern of between-population geographic distances, which we show to be highly correlated with their genetic distances, thus suggesting isolation by distance as the proximate cause of the association.     

Relationships of Nematodirus species and Nematodirus battus isolates (Nematoda: Trichostrongyloidea) based on nuclear ribosomal DNA sequences

Nuclear ribosomal sequence data from the internal transcribed spacers (ITS-1 and ITS-2), 5.8S subunit, and regions of the 18S and 28S genes were used to investigate sequence diversity among geographic samples of Nematodirus battus, and to infer phylogenetic relationships among Nematodirus species. Phylogenetic analysis of these data yielded strong support for relationships among species, depicting Nematodirus helvetianus and Nematodirus spathiger as sister-taxa and a clade of these 2 species and Nematodirus filicollis. This tree is consistent with caprine bovids as ancestral hosts, with a subsequent host shift to Bovinae in N. helvetianus. Eleven of 14 N. battus sequences were unique, with 19 variable sites among sequences representing 5 geographic samples. The lowest number of variable nucleotide sites was observed in samples representing apparently recent introductions to the United States and Canada, which is consistent with a population bottleneck concomitant with translocation. Comparison of directly sequenced polymerase chain reaction products and clones revealed evidence for intraindividual variation at some of the sequence sites, and this pattern of variation and that within geographic samples indicates incomplete rDNA repeat homogenization within species. This pattern of variation is not conducive for inferring phylogenetic relationships among sequences representing N. battus or addressing the putative history of introduction.     

INTERSPECIFIC CROSSES IN KALMIA

Interspecific crosses among Kalmia species are reported for the first time. All possible crosses between K. angustifolia, K. cuneata, K. latifolia, K. hirsuta, K. polifolia var. rosmarinifolia and K. p. var. microphylla were attempted. Several crosses yielded no hybrids, many produced albino or yellow-green inviable seedlings, and six combinations produced at least a few seedlings with near-normal growth habit. Stylar inhibition of pollen tube growth prevented some species from hybridizing; reciprocal differences in crossability and pollen tube growth were noted. Kalmia species are highly heterozygous, but no evidence was found within a species for geographic variation of combining ability except for the two varieties of K. polifolia which had different crossing patterns with the other species. The anthocyaninless form of K. angustifolia, in contrast to the normal, has unique combining ability with K. latifolia. No genetic evidence was found to support the placement of K. hirsuta in another genus (Kalmiella) or to designate K. angustifolia var. caroliniana as a distinct species. Use of all the species to breed improved ornamentals is limited by genetic barriers and sterility of F₁ hybrids.     

The evolutionary response of predators to dangerous prey: hotspots and coldspots in the geographic mosaic of coevolution between garter snakes and newts

The "geographic mosaic" approach to understanding coevolution is predicated on the existence of variable selection across the landscape of an interaction between species. A range of ecological factors, from differences in resource availability to differences in community composition, can generate such a mosaic of selection among populations, and thereby differences in the strength of coevolution. The result is a mixture of hotspots, where reciprocal selection is strong, and coldspots, where reciprocal selection is weak or absent, throughout the ranges of species. Population subdivision further provides the opportunity for nonadaptive forces, including gene flow, drift, and metapopulation dynamics, to influence the coevolutionary interaction between species. Some predicted results of this geographic mosaic of coevolution include maladapted or mismatched phenotypes, maintenance of high levels of polymorphism, and prevention of stable equilibrium trait combinations. To evaluate the potential for the geographic mosaic to influence predator-prey coevolution, we investigated the geographic pattern of genetically determined TTX resistance in the garter snake Thamnophis sirtalis over much of the range of its ecological interaction with toxic newts of genus Taricha. We assayed TTX resistance in over 2900 garter snakes representing 333 families from 40 populations throughout western North America. Our results provide dramatic evidence that geographic structure is an important component in coevolutionary interactions between predators and prey. Resistance levels vary substantially (over three orders of magnitude) among populations and over short distances. The spatial array of variation is consistent with two areas of intense evolutionary response by predators ("hotspots") surrounded by clines of decreasing resistance. Some general predictions of the geographic mosaic process are supported, including clinal variation in phenotypes, polymorphism in some populations, and divergent outcomes of the interaction between predator and prey. Conversely, our data provide little support for one of the major predictions, mismatched values of interacting traits. Two lines of evidence suggest selection is paramount in determining population variation in resistance. First, phylogenetic information indicates that two hotspots of TTX resistance have evolved independently. Second, in the one region that TTX levels in prey have been quantified, resistance and toxicity levels match almost perfectly over a wide phenotypic and geographic range. However, these results do not preclude the role the nonadaptive forces in generating the overall geographic mosaic of TTX resistance. Much work remains to fill in the geographic pattern of variation among prey populations and, just as importantly, to explore the variation in the ecology of the interaction that occurs within populations.     

二型花柱植物海仙花报春花部性状随地理梯度的变异

植物的花部性状在异质环境中表现出不均一的适应性进化, 其自然变异可能在时空格局上呈现一定的规律性。选择同一物种的不同地理居群进行花部表型变异分析, 能揭示花部性状随地理梯度的变异模式。海仙花报春(Primula poissonii)属于典型的二型花柱植物, 依赖昆虫传粉实现严格的型间异交。该物种广布于横断山地区亚高山-高山草甸, 其分布海拔跨度大且花部性状在种内具有较高变异, 但这些变异在不同地理梯度(海拔梯度和经纬度梯度)的特定选择因子作用下的变化规律尚不清楚。本研究选择海仙花报春16个居群, 对8个花部关键性状和二型花柱繁殖器官的互补度与海拔和经纬度的关系进行研究, 探究花部性状随地理梯度变异的模式及其潜在的选择因素。研究表明, 海仙花报春两种花型的花冠管开口大小、花药高度以及短柱花柱头高度与海拔均呈正相关, 但两种花型的花冠大小, 长柱花的花瓣长度、柱头到花冠管开口的距离, 以及短柱花的花冠管长度与海拔高度间均呈负相关, 其余性状与海拔无显著相关性。除短柱花中柱头高度以外的性状均随着纬度升高而逐渐减小。长柱花中除花药和柱头间的距离以及柱头到开口的距离外, 其余性状均随着经度的增加而减小; 短柱花的花瓣长度、花药高度以及花药和柱头间的距离随着经度的增加而变大, 其余性状均随着经度的增加而减小。繁殖器官间的互补度并不随地理环境的变化而变化。花部性状的地理变异可能受访花昆虫组成的地理变化驱动。繁殖器官间互补程度的高度保守表明非选型交配在居群二态性的维持和稳定过程中起关键作用。本研究为进一步深入开展报春花属(Primula)花部性状及其选择压力的地理变异研究奠定了基础。     

Character displacement in sailfin mollies, <Emphasis Type="Italic">Poecilia latipinna</Emphasis>: allozymes and behavior

Synopsis We analyzed variation in allozymes and mating preferences in 12 populations across much of the range of the sailfin molly, Poecilia latipinna. Sailfin mollies can be sympatric with its sexual parasite Amazon mollies, P. formosa. Amazon mollies must co-exist and mate with bisexual males of closely related species (including sailfin mollies) to induce embryogenesis but inheritance is strictly maternal. Where sailfin and Amazon mollies are sympatric there is evidence of reproductive character displacement as males show a significantly stronger mating preference for sailfin molly females over Amazon mollies compared to preferences of males from allopatric populations. From the allozyme data we found a moderate amount of genetic variation across all populations but this variation did not reveal significant partitioning between sympatric and allopatric populations. Additionally, we found no evidence for isolation by distance as genetic distance was not significantly correlated with geographic distance. While allozyme variation also did not significantly correlate with male mating preferences, there was a significant correlation between male mating preferences and geographic distance. This correlation between mating preferences and geographic distance may have arisen from coevolution with Amazon mollies resulting in reproductive character displacement. Taken together, the distribution of genetic and behavioral variation among sympatric and allopatric populations suggests that behavioral evolution has outpaced evolution at the allozyme loci we examined in P. latipinna.     

Neural tube defects without neural crest defects in splotch mice.

Homozygous Splotch mutant mice (Sp/Sp) die on day 14 of gestation with neural tube defects, curly tail, and malformations of neural crest derivatives. Sp1H mice, which have a radiation-induced allele of Splotch with a similar phenotype, were used for this study. The neural tube defects are always located in the lumbosacral region and in 50% of the cases also in the region of the hindbrain. In this report, rare cases of neural tube defects and tail defects among the offspring of crosses between Splotch (Sp1H) heterozygotes are presented, which are not associated with a neural crest defect. This suggests that the development of the neural tube and neural crest defects in this mutant is caused by independent mechanisms or is dependent on the dosage of the mutant gene, with different thresholds being pathogenetic in the neural tube and neural crest, respectively.     

Metabolic rate, genetic and microclimate variation among springtail populations from sub-Antarctic Marion Island

Measurement of metabolic rates (made at 10°C) of individuals of the springtail Cryptopygus antarcticus travei from six geographically distinct populations on sub-Antarctic Marion Island were combined with mitochondrial DNA (COI) haplotype analysis to examine in parallel both physiological and genetic variation of distinct populations. We found evidence of genetic differentiation among populations and a general indication of long-term isolation with limited gene flow. While we found support for an overall pattern of metabolic rate structure among populations from different geographic locations on the island (mean rate = 0.0009–0.0029 μl O₂ μg⁻¹ h⁻¹ for populations of a mean individual mass of 8–26 μg), we were unable to demonstrate a coherent common pattern between this and genetic variation. However, spatial structure in metabolic rate variation was strongly related to the extent of variability in microclimate among sites, and also showed some indication of a phylogeographic signal. Thus, over the relatively short timescale of Marion Island’s history (<1 million years), the periodic geographic barriers that have driven population differentiation from a molecular perspective may also have resulted in some physiological differentiation of populations.