Genetic diversity, climatic selection and speciation of Sphincterochila landsnails in Israel

Allozymic variation in proteins encoded by 29 loci was analysed electrophoretically in 364 adult specimens representing 12 populations and five species of the landsnail Sphincterochila in Israel along a north-south general transect of increasing aridity. In addition, geographic variation in three morphological body variables of these snails was also studied. The results indicate that: (i) most loci (86%) are strongly polymorphic; (ii) most loci (65%) display fixation of alternative alleles either within or between species; (iii) most of the variant alleles (51%) are not widespread, and genie differentiation is very high (66%) between populations and species, indicating sharp local and regional geographic differentiation; (iv) clinal patterns are rare or nonexistent; (v) populations of Sphincterochila display average estimates of mean alleles per locus,A=1.53; polymorphism, P (5% criterion) = 0.31; heterozygosity, H=0.07; and genie diversity, He= 0.11; (vi) wide geographic variation within and between species is displayed in A= 1.18-2.07; P=0.11–0.61; H=0.02-0.15, and He = 0.042-0.22. Wright's fixation index, F, ranges from 0.03 to 0.65. (vii) Genie diversity, He, increases southwards with aridity from 0.051 to 0.145. (viii) A differential amount of variation in different functional classes of enzymes follows the Gillespie-Kojima hypothesis, (ix) Coefficients of genetic distance, D, between populations are high, D= 0.34, range 0.09-0.58, and between species, D= 0.27, range 0.12-0.40. D's within species may be higher than between species. Likewise, D's increase clinally southwards, (x) Deviations from Hardy-Weinberg equilibria were found in several loci in some populations and species, (xi) A statistically significant (P< 0.001) amount of morphological variation of body variables exists within and between species. Size between three species increases eastwards and southwards with aridity, (xii) P, H, He, and allozymic variation in several gene loci are significantly correlated with, and predictable by, climatic variables, primarily those related to the moisture index, (xiii) Allozymic and morphological variations are partly correlated, (xiv) Significant microgeographical climatic differentiation was found in three critical tests. The pattern of genetic variation within and between species suggests that: (a) climatic selection plays a conspicuous role in allozymic morphological differentiation into ecologically adaptive patterns; (b) the environmental variation model seems to be a good predictor of genetic variation in Sphincterochila; (c) adaptive radiation of the five species of Sphincterochila in Israel occurred during Pleistocene times in accord with climatic differentiation and apparently involved few changes of structural genes.     

Genetic determinants of phenotypic diversity in humans

New technologies for rapidly assaying DNA sequences have revealed that the degree and nature of human genetic variation is far more complex then previously realized. These same technologies have also resulted in the identification of common genetic variants associated with more than 30 human diseases and traits.     

Genetic diversity of wild emmer wheat in Israel and Turkey

Summary Allozyme variation in the tetraploid wild emmer wheat, Triticum dicoccoides, the progenitor of all cultivated wheats, was studied for the proteins encoded by 42 gene loci in 1815 plants representing 37 populations - 33 from Israel and 4 from Turkey - sampled in 33 localities from 1979 to 1987. The results showed that: (a) 6 loci (14%) were monomorphic in all populations, 15 loci (36%) were locally polymorphic, and 21 loci (50%) were regionally polymorphic. These results are similar to those obtained earlier on 12 Israeli populations. All polymorphic loci (except 4) displayed high local levels of polymorphism (>/ 10%). (b) The mean number of alleles per locus, A, was 1.252 (range: 1.050–1.634); the proportion of polymorphic loci per population averaged 0.220 (range: 0.050–0.415); genic diversity, He, averaged 0.059 (range: 0.002–0.119). (c) Altogether there were 119 alleles at the 42 putative loci tested, 114 of these in Israel, (d) Genetic differentiation was primarily regional and local, not clinal; 70% of the variant alleles were common (>/ 10%) and not widespread, but rather localized or sporadic, displaying an archipelago population genetics and ecology structure. The coefficients of genetic distance between populations were high and averaged D = 0.134; range: 0.018–0.297, an indication of sharp genetic differentiation over short distances, (e) Discriminant analyses differentiated Israeli from Turkish populations, and within Israel, between central and 3 marginal regions, as well as between different soil-type populations, (f) Allozymic variation comprised 40% within and 60% between populations, (g) Gametic phase disequilibria were abundant, their number being positively correlated (r_s = 0.60, P<0.01) with the humidity, (h) Multilocus organization was substantive, also positively correlated with humidity, (i) Allozyme diversity, overall and at single loci, was significantly correlated with, and partly predictable by, climatic and edaphic factors, (j) The distrubition of the significant positive and negative values and the absence of autocorrelations in the correlogram revealed no similar geographic patterns across loci, eliminating migration as a prime factor of population genetic differentiation. These results suggest: (I) during the evolutionary history of wild emmer, diversifying natural selection, through climatic and edaphic factors, was a major agent of genetic structure and differentiation at both the single and multilocus levels; (II) wild emmer harbors large amounts of genetic diversity exploitable as genetic markers in sampling and abundant genetic resources utilizable for wheat improvement.     

Genetic diversity and ecological relationships of marsh frog populations in Israel

Summary Allozymic variation in proteins encoded by 28 loci was analyzed electrophoretically in 340 mostly adult specimens representing 11 populations, 8 central and 3 isolated, of aquatic marsh frogs, Rana ridibunda in Israel, along a north-south transect of generally increasing aridity. In addition, geographic variation in 3 morphological variables of 144 frogs and in vertebral stripe color polymorphism of 262 frogs were also studied. The results indicate that. (a) Of the 28 loci examined, 12 (= 43%) are largely monomorphic in all populations; out of the remaining loci, 6 were locally and weakly polymorphic and 10 regionally and strongly polymorphic. (b) No fixation of alternative alleles was found in any of the 28 loci and 11 populations studied. The commonest allele predominated across all populations, central as well as isolates, (c) Clinal patterns associated with increasing aridity southwards and eastwards occurred in polymorphism, P; heterozygosity, H; and in allele frequencies of Esterase-1, Xanthine dehydrogenase, Aldehyde oxidase and Albumin. (d) In the 3 estimates of genie variation, mean number of alleles per locus, A, mean proportion of polymorphic loci per population, P, and heterozygous loci per individual, H, marsh frogs displayed average estimates of genetic variation. The 3 estimates were: A =1.14 (range, 1.18–1.57); P = 0.33 (range, 0.14–0.54): H = 0.069 (range, 0.032–0.094). (e) Central populations harbored distinctly more genic variation than isolated populations. (f) Genic similarity between populations was high. (g) Significant deviations from Hardy-Weinberg equilibrium were found in 8 out of 11 populations involving 8 loci, (h) P, H, and allozymic variation in several gene loci were significantly correlated and predictable by environmental variables, primarily those related to water and temperature. (i) A significant amount of morphological variation was found between localities for body length, foot length, and weight in both sexes. Body weight in females was negatively correlated with temperature; and all three morphological variables in females were predicted significantly by a combination of temperature and humidity. (j) The three vertebral stripe color phenotypes, gray, green and red occurred in the following frequencies: 0.59, 0.24, 0.17, respectively. The red morph increased clinally southwards and was significantly correlated with most temperature and water variables. The geographic variation in both the green and red morphs was predicted significantly by climatic variables, both colors blending with local substrates.The spatial patterns and environmental correlates of genetic and morphological variation in Rana ridibunda in Israel suggest that (i) protein polymorphisms are at least partly adaptive and that part is moulded by natural selection rather than by stochastic processes or neutrality; (ii) the environmental variation model seems to be a good predictor of genetic variation in marsh frogs; (iii) body size varies adaptively, presumably determined primarily through thermoregulation; (iv) the spatial pattern of the color polymorphism seems to be adaptively selected by at least two factors: visual predation and climatic determinants.     

Genetic variation and climatic selection in the lizard Agama stellio in Israel and Sinai

Summary Allozymic variation in proteins encoded by 25 loci was analyzed electrophoretically in 242 adult specimens representing nine populations of the Levantine lizard, Agama stellio, comprising two subspecies: the Mediterranean A. stellio subsp., and the desert-inhabiting A. stellio brachydactyla from the Negev and Sinai. Likewise, four body traits were measured in the same populations. The nine populations were sampled along a general southward transect of increasing aridity. Agama stellio is above average in both polymorphism, P, and heterozygosity, H, as compared to other reptiles and vertebrates in general, displaying levels of genetic variation characterizing habitat generalist vertebrates. In the populations studied no fixation of alternative alleles was found in any of the 25 loci: rather the commonest allele was either fixed or predominated in 23 of 25 loci examined. Eleven loci (44%) were monomorphic in all nine populations. However, of the remaining 14 polymorphic loci, eight were strongly polymorphic displaying distinct genetic differentiation between populations. Genetic diversity (indexed by P and H) displayed geographic variation and was slightly higher in A.s. brachydactyla than in A. stellio subsp. Nevertheless, genic similarity between populations was high. A statistically significant amount of morphological variation between localities was found for all body characters. In general, body size increased southwards and eastwards with aridity.Selection at some loci is suggested by significant deviation from Hardy-Weinberg expectations and possibly by excess heterogeneity of effective inbreeding coefficients, F_e. Furthermore, allozymic variation at seven loci (Ldh-1, Idh-1, 6-Pgd-1, Aat-1, Pgm-2, Pept-1, and Trf) and geographic variation in body size and weight were significantly correlated with, and predictable by, climatic variables, primarily by water availability and secondarily by temperature. Finally, allozymic and morphological variations were partly correlated.The spatial patterns and ecological correlates of genic and morphological variations in Agama stellio in Israel and Sinai suggest that at least some proteins and body size differentiate geographically and appear to be adaptive, presumably with respect to factors affecting the availability of water.     

Genetic benefits of a female mating preference in gray tree frogs are context-dependent

Abstract "Good genes" models of sexual selection predict that male courtship displays can advertise genetic quality and that, by mating with males with extreme displays, females can obtain genetic benefits for their offspring. However, because the relative performance of different genotypes can vary across environments, these genetic benefits may depend on the environmental context; in which case, static mating preferences may not be adaptive. To better understand how selection acts on the preference that female gray tree frogs ( Hyla versicolor ) express for long advertisement calls, I tested for genetic benefits in two realistic natural environments, by comparing the performance of half-sibling offspring sired by males with long versus short calls. Tadpoles from twelve such maternal half-sibships were raised in enclosures in their natal pond at two densities. In the low-density treatment, offspring of long-call males were larger at metamorphosis than were offspring of short-call males, whereas in the high-density treatment, offspring of males with long calls tended to metamorphose later than offspring of males with short calls. Thus, although the genes indicated by long calls were advantageous under low-density conditions, they were not beneficial under all conditions, suggesting that a static preference for long calls may not be adaptive in all environments. Such a genotype-by-environment interaction in the genetic consequences of mate choice predicts that when the environment is variable, selection may favor plasticity in female preferences or female selectivity among environments to control the conditions experienced by the offspring.     

Environmental and climatic determinants of molecular diversity and genetic population structure in a coenagrionid damselfly

Identifying environmental factors that structure intraspecific genetic diversity is of interest for both habitat preservation and biodiversity conservation. Recent advances in statistical and geographical genetics make it possible to investigate how environmental factors affect geographic organisation and population structure of molecular genetic diversity within species. Here we present a study on a common and wide ranging insect, the blue tailed damselfly Ischnuraelegans, which has been the target of many ecological and evolutionary studies. We addressed the following questions: (i) Is the population structure affected by longitudinal or latitudinal gradients?; (ii) Do geographic boundaries limit gene flow?; (iii) Does geographic distance affect connectivity and is there a signature of past bottlenecks?; (iv) Is there evidence of a recent range expansion and (vi) what is the effect of geography and climatic factors on population structure? We found low to moderate genetic sub-structuring between populations (mean F_ST = 0.06, D_est = 0.12), and an effect of longitude, but not latitude, on genetic diversity. No significant effects of geographic boundaries (e.g. water bodies) were found. F_ST-and D_est-values increased with geographic distance; however, there was no evidence for recent bottlenecks. Finally, we did not detect any molecular signatures of range expansions or an effect of geographic suitability, although local precipitation had a strong effect on genetic differentiation. The population structure of this small insect has probably been shaped by ecological factors that are correlated with longitudinal gradients, geographic distances, and local precipitation. The relatively weak global population structure and high degree of genetic variation within populations suggest that I. elegans has high dispersal ability, which is consistent with this species being an effective and early coloniser of new habitats.     

Genetic diversity is related to climatic variation and vulnerability in threatened bull trout

Understanding how climatic variation influences ecological and evolutionary processes is crucial for informed conservation decision‐making. Nevertheless, few studies have measured how climatic variation influences genetic diversity within populations or how genetic diversity is distributed across space relative to future climatic stress. Here, we tested whether patterns of genetic diversity (allelic richness) were related to climatic variation and habitat features in 130 bull trout (Salvelinus confluentus) populations from 24 watersheds (i.e., ~4–7th order river subbasins) across the Columbia River Basin, USA. We then determined whether bull trout genetic diversity was related to climate vulnerability at the watershed scale, which we quantified on the basis of exposure to future climatic conditions (projected scenarios for the 2040s) and existing habitat complexity. We found a strong gradient in genetic diversity in bull trout populations across the Columbia River Basin, where populations located in the most upstream headwater areas had the greatest genetic diversity. After accounting for spatial patterns with linear mixed models, allelic richness in bull trout populations was positively related to habitat patch size and complexity, and negatively related to maximum summer temperature and the frequency of winter flooding. These relationships strongly suggest that climatic variation influences evolutionary processes in this threatened species and that genetic diversity will likely decrease due to future climate change. Vulnerability at a watershed scale was negatively correlated with average genetic diversity (r = ?0.77; P < 0.001); watersheds containing populations with lower average genetic diversity generally had the lowest habitat complexity, warmest stream temperatures, and greatest frequency of winter flooding. Together, these findings have important conservation implications for bull trout and other imperiled species. Genetic diversity is already depressed where climatic vulnerability is highest; it will likely erode further in the very places where diversity may be most needed for future persistence.     

A tree of tree frogs around the Black Sea

Speciation, the process by which one species evolves into two or more, is a major focus of ongoing debate, particularly regarding the geographic context in which it occurs. Geographic models of speciation tend to fall into discrete categories, typically referred to as allopatric, parapatric and sympatric speciation, according to whether two groups evolve reproductive isolation while geographically isolated, differentiated but connected by gene flow, or completely co‐occurring. Yet molecular studies indicate that full development of reproductive isolation can take very long compared with the timescale at which climatic oscillations occur, such that the geographic context of differentiating forms might change often during the long process to full species. Studies of genetic relationships across the ranges of organisms with low‐dispersal distances have the potential to reveal these complex histories. In a particularly elegant example in this issue, Dufresnes et al. ( 2016 ) use genetic variation and ecological niche modelling to show that a ring of populations of the eastern tree frog (Hyla orientalis) surrounding the Black Sea had a complex history of geographic differentiation. Alternating phases of geographic fragmentation and phases of gene flow between neighbouring populations have produced a pattern of gradual genetic change connecting the western, southern and eastern sides of the ring, with the northwestern and northeastern forms being most differentiated. In the north, a population in Crimea appears to have been produced through mixture of the two extreme forms. The overall genetic relationships are reminiscent of those found in ring species, which have been used as prime demonstrations of the process of speciation. The difference, however, is that the terminal forms appear to have mixed rather than be reproductively isolated, although more research is needed to infer whether there might be some reproductive isolation on the northern side of the ring.     

Scale specific determinants of tree diversity in an old growth temperate forest in China

The major processes generating pattern in plant community composition depend upon the spatial scale and resolution of observation, therefore understanding the role played by spatial scale on species patterns is of major concern. In this study, we investigate how well environmental (topography and soil variables) and spatial variables explain variation in species composition in a 25-ha temperate forest in northeastern China. We used new variation partitioning approaches to discover the spatial scale (using multi-scale spatial PCNM variables) at which environmental heterogeneity and other spatially structured processes influence tree community composition. We also test the effect of changing grain of the study (i.e. quadrat size) on the variation partitioning results. Our results indicate that (1) species composition in the Changbai mixed broadleaf-conifer forest was controlled mainly by spatially structured soil variation at broad scales, while at finer scales most of the explained variation was of a spatial nature, pointing to the importance of biotic processes. (2) These results held at all sampling grains. However, reducing quadrat size progressively reduced both spatially and environmentally explained variance. This probably partly reflects increasing stochasticity in species abundances, and the smaller proportion of quadrats occupied by each species, when quadrat size is reduced. The results suggest that environmental heterogeneity (i.e. niche processes) and other biotic processes such as dispersal work together, but at different spatial scales, to build up diversity patterns.     

Genetic diversity and environmental associations of wild wheat,Triticum dicoccoides,in Israel

Summary Allozyme variation in the tetraploid wild progenitor of wheat, Triticum dicoccoides, was studied for the proteins encoded by about 50 gene loci in 457 individuals representing 12 populations from Israel. Six spikelet morphological traits were measured in the same populations. The results indicate that: (a) 16 loci (= 32%) were monomorphic in all 12 populations, 15 loci (= 30%) were locally polymorphic, and 19 loci (= 38%) were regionally polymorphic. All polymorphic loci (but one) displayed high levels of polymorphism ( 10%). In Israel, the proportion of polymorphic loci per population, P, in wild wheat averaged 0.25 (range, 0.16–0.38), and the genetic diversity index, H_e averaged 0.07, (range, 0.03 – 0.12). (b) Altogether there were 110 alleles at the 50 putative loci tested (c) Genetic differentiation of populations included regional and local patterns: (i) The coefficients of genetic distance between populations were high (mean D = 0.10 range, 0.02 – 0.25), and indicated sharp genetic differentiation over short distances, (ii) Common ( 10%) but sporadic and localized alleles were frequent (76%), and (iii) Rare alleles were few (only 5 alleles). (d) The patterns of allozyme and spikelet variation in the wild gene pool were significantly correlated with, and partly predictable by, water factors, including those of precipitation, evaporation, and relative humidity as well as of soil type, (e) All six spikelet characters showed statistically significant variation among localities and (f) Allozymic variation was correlated with spikelet variation.These results suggest in T. dicoccoides: (i) the operation of natural selection in population genetic structure, (ii) local adaptive genetic differentiation caused by diversifying selection through climate and soil, and (iii) the guidelines for sampling these resources for use in wheat breeding programs.     

Ever-young sex chromosomes in European tree frogs

Non-recombining sex chromosomes are expected to undergo evolutionary decay, ending up genetically degenerated, as has happened in birds and mammals. Why are then sex chromosomes so often homomorphic in cold-blooded vertebrates? One possible explanation is a high rate of turnover events, replacing master sex-determining genes by new ones on other chromosomes. An alternative is that X-Y similarity is maintained by occasional recombination events, occurring in sex-reversed XY females. Based on mitochondrial and nuclear gene sequences, we estimated the divergence times between European tree frogs (Hyla arborea, H. intermedia, and H. molleri) to the upper Miocene, about 5.4–7.1 million years ago. Sibship analyses of microsatellite polymorphisms revealed that all three species have the same pair of sex chromosomes, with complete absence of X-Y recombination in males. Despite this, sequences of sex-linked loci show no divergence between the X and Y chromosomes. In the phylogeny, the X and Y alleles cluster according to species, not in groups of gametologs. We conclude that sex-chromosome homomorphy in these tree frogs does not result from a recent turnover but is maintained over evolutionary timescales by occasional X-Y recombination. Seemingly young sex chromosomes may thus carry old-established sex-determining genes, a result at odds with the view that sex chromosomes necessarily decay until they are replaced. This raises intriguing perspectives regarding the evolutionary dynamics of sexually antagonistic genes and the mechanisms that control X-Y recombination.     

Genetic diversity and structure of the invasive tree Miconia calvescens in Pacific islands

Aim This study investigates the amount and distribution of genetic variation within and among populations of the highly invasive tree, Miconia calvescens (Melastomataceae; hereafter miconia), in tropical island habitats that are differently impacted (distribution and spread) by this weed. Location Invasive populations were included from northern and southern Pacific islands including the Hawaiian Islands (Hawaii, Kauai and Maui), Marquesas Islands (Nuku Hiva), Society Islands (Tahiti, Tahaa, Moorea, Raiatea) and New Caledonia. Methods We used 9 codominant microsatellite and 77 highly variable dominant intersimple sequence repeat markers (ISSRs) to characterize and compare genetic diversity among and within invasive miconia populations. For the codominant microsatellite data we calculated standard population genetic estimates (heterozygosity, number of alleles, inbreeding coefficients, etc.) and described population genetic structure using AMOVA, Mantel tests (to test for isolation by distance), unweighted pair‐group method with arithmetic averages (UPGMA) cluster analysis and principal components analysis (PCA). We also tested for the presence of a population bottleneck and used a Bayesian analysis of population structure in combination with individual assignment tests. For the dominant ISSR data we used AMOVA, PCA, upgma and a Bayesian approach to investigate population genetic structure. Results Both markers types showed little to no genetic differentiation among miconia populations from northern and southern Pacific hemispheres (AMOVA: microsatellite, 3%; ISSR, 0%). Bayesian and frequency‐based analysis also failed to support geographical genetic structure, confirming considerable low genetic differentiation throughout the Pacific. Molecular data furthermore showed that highly successful miconia populations throughout the Pacific are currently undergoing severe bottlenecks and high levels of inbreeding (f = 0.91, ISSR; F_IS = 0.27, microsatellite). Main conclusions The lack of population genetic structure is indicative of similar geographical sources for both hemispheres and small founding populations. Differences in invasive spread and distribution among Pacific islands are most likely the result of differences in introduction dates to different islands and their accompanying lag phases. Miconia has been introduced to relatively few tropical islands in the Pacific, and the accidental introduction of a few or even a single seed into favourable habitats could lead to high invasive success.     

昆明城郊中国树鼩群体线粒体DNA遗传多样性

由于树鼩是灵长类动物的近亲,且具有体型小、繁殖周期短、饲养管理成本低等优点,长期以来被认为有望替代灵长类动物用于人类疾病的动物模型研究.然而,目前对树鼩的群体遗传结构还知之甚少,这极大地限制了其在疾病动物模型研究的应用,也是其品系资源创制的瓶颈.本研究通过分析80只采自于云南省昆明周边地区的野生树鼩(Tupaia belangeri chinensis)线粒体DNA(mtDNA)多态性,结合国外报道的2个树鼩(Tupaia belangeri)序列比较后发现,在604 bp的mtDNA控制区片段中兵检测到29个核苷酸替代变异,这些变异共界定了13种单倍型,表现较高的群体遗传多样度.另外,昆明地区的树鼩与国外报道的2个树鼩间存在较大的遗传分化,mtDNA控制区单倍型之间的核苷酸替换数大于18个,远高于昆明地区树鼢群体内部不同单倍型之间的差异.选择含有代表性的mtDNA控制区单倍型的17个昆明地区树鼩个体进一步测定了细胞色素b基因片段(1134 bp),结合前人报道的数据分析,结果进一步支持mtDNA控制区数据反映的遗传格局及揭示的昆明地区树询与国外报道树鼩之间的明显差异.本研究结果提示,昆明地区树鼩与国外树鼩之间存在较大遗传差异,在将树鼩用于人类疾病动物模型研究中要注意这些遗传差别.昆明城郊的树鼩群体具有较高的遗传多样度,在开展近交系建立等工作时须考虑选取群体内部具有代表性的mtDNA世系.

Abstract：

Due to their special phylogenetic position in the Euarchontoglires and close affinity to primates, tree shrews have been proposed as an alternative experimental animal to primates in biomedical research. However, the population genetic structure of tree shrews has largely remained unknown and this has hindered the development of tree shrew breeding and selection. Here we sampled 80 Chinese tree shrews (Tupaia belangeri chinensis) in Kunming, China, and analyzed partial mtDNA control region sequence variation. Based on our samples and two published sequences from northern tree shrews (T. belangeri), we identified 29 substitutions in the mtDNA control region fragment (～ 604 bp)across 82 individuals and defined 13 hapiotypes. Seventeen samples were selected for sequencing of the cytochrome b (Cyt b; 1134 bp) gene based on control region sequence variation and were analyzed in combination with 34 published sequences to solidify the phylogenetic pattern obtained from control region data. Overall, tree shrews from Kunming have high genetic diversity and present a remarkable long genetic distance to the two reported northern tree shrews outside China. Our results provide some caution when using tree shrews to establish animal models because of this apparent genetic difference. In addition, the high genetic diversity of Chinese tree shrews inhabiting Kunming suggests that systematic genetic investigations should be conducted before establishing an inbred strain for medical and biological research.     

Extreme heterochiasmy and nascent sex chromosomes in European tree frogs

We investigated sex-specific recombination rates in Hyla arborea, a species with nascent sex chromosomes and male heterogamety. Twenty microsatellites were clustered into six linkage groups, all showing suppressed or very low recombination in males. Seven markers were sex linked, none of them showing any sign of recombination in males (r=0.00 versus 0.43 on average in females). This opposes classical models of sex chromosome evolution, which envision an initially small differential segment that progressively expands as structural changes accumulate on the Y chromosome. For autosomes, maps were more than 14 times longer in females than in males, which seems the highest ratio documented so far in vertebrates. These results support the pleiotropic model of Haldane and Huxley, according to which recombination is reduced in the heterogametic sex by general modifiers that affect recombination on the whole genome.     

Genetic differentiation of the Iberian tree frogs Hyla arborea molleri and Hyla meridionalis (Amphibia: Anura)

Genetic differentiation of Hyla arborea molleri and H. meridionalis was studied through allozyme electrophoresis and through essays of artificial hybridization. Samples were taken at several localities in Portugal, where these species occur either in sympatry or allopatry, and at one locality in France. Of the 21 presumptive loci analyzed, 6 were monoallelic, while 9 loci were discriminative between both species. A relatively high level of genetic differentiation was found between the two species (NEI's D = 0.725), contrasting with their morphological similarity. Remarkably low levels of genetic variability were found within species, with greater differentiation found among samples of H. meridionalis than of H. a. molleri. Although the capacity for interspecific hybridization was low, hybrid embryos resulted from both reciprocal crosses, but with much greater success in the cross ♂H. a. molleri x δH. meridionalis. Hybrids showed greater similarity with the male parent, and showed heterozygote allozymic phenotypes for discriminative loci. Results are compared with those of similar studies on the same and different species of Hyla, and differentiation of H. a. molleri and H. meridionalis is discussed in relation to their taxonomic status and origin.     

Genetic diversity of MHC class I loci in six non-model frogs is shaped by positive selection and gene duplication

Comparative studies of major histocompatibility complex (MHC) genes across vertebrate species can reveal the evolutionary processes that shape the structure and function of immune regulatory proteins. In this study, we characterized MHC class I sequences from six frog species representing three anuran families (Hylidae, Centrolenidae and Ranidae). Using cDNA from our focal species, we amplified a total of 79 unique sequences spanning exons 2-4 that encode the extracellular domains of the functional alpha chain protein. We compared intra- and interspecific nucleotide and amino-acid divergence, tested for recombination, and identified codon sites under selection by estimating the rate of non-synonymous to synonymous substitutions with multiple codon-based maximum likelihood methods. We determined that positive (diversifying) selection was acting on specific amino-acid sites located within the domains that bind pathogen-derived peptides. We also found significant signals of recombination across the physical distance of the genes. Finally, we determined that all the six species expressed two or three putative classical class I loci, in contrast to the single locus condition of Xenopus laevis. Our results suggest that MHC evolution in anurans is a dynamic process and that variation in numbers of loci and genetic diversity can exist among taxa. Thus, the accumulation of genetic data for more species will be useful in further characterizing the relative importance of processes such as selection, recombination and gene duplication in shaping MHC loci among amphibian lineages.     

Genetic diversity of reintroduced tree populations in restoration plantations of the Brazilian Atlantic Forest

Long‐term ecological success of large‐scale restoration programs planned for the next decades will rely on genetic diversity (GD) of reintroduced or colonizing species, a limiting factor in highly fragmented landscapes. In small and isolated natural remnants or restoration areas, substantial reduction in population's size or connectivity may lead to local extinctions due to the accumulation of deleterious recessive alleles and ongoing reduction of fecundity, plant vigor, recruitment success, and adaptive potential. Despite the paramount role of GD for species persistence, its levels in restoration programs are poorly known. We assessed the GD of four model tree species (different succession stages, dispersal, and pollination syndromes) from the Brazilian Atlantic Forest, comparing two high‐diversity restoration plantations, one forest fragment and one conserved remnant. Contrary to the expectation that the plantation strategies adopted in the restoration programs could result in genetic composition homogenization, we found that restoration areas established heterogeneous genetic groups with similar levels of neutral GD and inbreeding to those observed in natural forest remnants. This pattern was consistent across the four functionally different tree species, despite some species idiosyncrasies. For instance, we observed lower allelic richness in early successional species in restoration sites, suggesting that some species may be more prone to reintroduction with lower GD. Thus, we advocate the use of high GD levels in restoration to support biodiversity conservation in human‐modified landscapes, thus reinforcing the role of ecological restoration for recovering the diversity of genes—the basic constituent of biodiversity.     

A comparison of photolyase activity in three Australian tree frogs

Ultraviolet-B (UV-B) irradiation of DNA generates mutagenic photoproducts such as cyclobutane pyrimidine dimers (CPDs) which can affect the growth and development of amphibian embryos. Differential ability to repair UV-B-induced DNA damage may be␣responsible for differences in population stability between␣some amphibian species. Photoreactivation via the enzyme photolyase is a major mechanism used to remove CPDs from DNA. The aim of this study was to determine if photolyase activity differed in three sympatric Australian amphibian species, one of which has suffered marked population declines (Litoria aurea) and two whose populations do not appear to be in decline (L. dentata and L. peronii). The specific activity of photolyase was measured in each species and compared to the hatching success of their eggs under unfiltered summer sunlight. The mean specific activities of photolyase were 1.10 ± 0.18 × 10¹¹, 5.76 ± 1.01 × 10¹¹, and 2.66 ± 0.15 × 10¹¹ CPDs repaired per hour per microgram of egg protein extract, for L. aurea, L. dentata and L. peronii, respectively. When intrinsic differences in hatching success between species were controlled for, the relative percentage hatching success under unfiltered sunlight of L. aurea (77%) was lower than that of L.␣peronii (91%) and L. dentata (98%); however, these values did not differ significantly. L. aurea had the lowest photolyase activity of the three species and showed a non-significant trend of reduced hatching success under UV-B exposure. Received: 15 December 1997 / Accepted: 9 March 1998     

Genetic diversity and fitness in peripheral and central populations of the European tree frog Hyla arborea

Genetic diversity is expected to decrease in small and isolated populations as a consequence of founder effects, bottlenecks, inbreeding and genetic drift. In this study we analyse temporal and spatial effects on genetic variation and progeny viability of the European tree frog (Hyla arborea) at two scales. First, the Swedish distribution has been isolated from the continental distribution for more than 8000 thousand years, and secondly, within Sweden, recent habitat alterations that have taken place during this century have increased isolation between local populations. Genetic variation and progeny survival in relation to isolation was studied within the entire Swedish distribution of the tree frog. Allozyme electrophoresis analysis of froglets, sampled across the Swedish distribution, revealed a low overall genetic variation (1.06 alleles/locus) at the protein level in comparison with continental populations (1.54-1.68 alleles/locus). However, egg hatchability (97%) and early larval survival (95%) were not lower than in other parts of the tree frog distribution or in other anuran species. Within the Swedish distribution, early larval survival was lower in isolated breeding ponds than in more central ones. However, no differences in genetic variation were found in relation to isolation. Polymorphism was detected only at a single locus, and was restricted geographically to the eastern part of the Swedish distribution. Bottlenecks due to climatic changes and fragmentation of suitable habitat (primarily natural pastures with ponds) are suggested as possible causes of the low genetic diversity of the Swedish tree frog population.