Environmental and population dependency of genetic variability-fitness correlations in Rana temporaria

Importance of Genetic-Fitness Correlations for the Conservation of Amphibians

Endangered animals suffer from isolation of their habitats. Isolation leads to a reduction in population size as well as a decrease in genetic diversity and a concomitant increase in the risk of extinction. Amphibians are the most endangered vertebrate class. Besides habitat loss, fragmentation and isolation, amphibians are threatened by emerging diseases e.g., chytrid fungus or Ranavirus. By employing experiments, researchers investigate whether changes in genetic diversity within or among isolated populations affect amphibian fitness. While genetic diversity estimates are based on molecular markers, typically microsatellites, fitness is mostly measured as tadpole performance in rearing experiments often under varying environmental conditions. Tadpole performances (e.g., body mass, growth rate and survival) have been found to be negatively affected by low genetic diversity, as several studies have found a positive association between genetic diversity and these fitness traits. Moreover, infection with pathogens also seems to be more likely in individuals or populations with lower genetic diversity. Overall, these genetic-fitness correlations seem to be more pronounced or detectable in smaller, declining populations but not in larger populations. Genomic studies, which sample a larger fraction of the genome, are still scarce in the conservation genetic literature on amphibians. These are likely to increase in upcoming years and may reveal adaptive variants that protect against dangerous pathogens or environmental changes. Altogether, genetic-fitness correlation studies should be a priority in order to develop effective management plans for the genetic rescue of isolated, imperilled amphibian populations.

Population structure and genetic diversity of the threatened pygmy newt Triturus pygmaeus in a network of natural and artificial ponds

Pond physical characteristics (connectivity, hydroperiod) have shown to be highly relevant in explaining species presence, reproductive success, and survival in breeding-pond amphibians. However, few studies have addressed the influence that these factors may have on the genetic variability of pond populations. We examined genetic variation at 11 microsatellite loci in Iberian endemic, the pygmy newt (Triturus pygmaeus), from 58 breeding ponds in the Doñana National Park (Andalusia), including both temporary ponds and artificially deepened ponds that remain wet during the whole year. Temporary ponds are located in the North part of the region where the surrounding habitat-wet meadows-facilitates the connectivity among populations, whereas the deepest ponds (‘zacallones’) are located in the southern edge embedded in a matrix of unsuitable habitat (thickets and dry underbrush). We investigated genetic diversity and structure within and among ponds. Our results show that both regions (Doñana-North and Doñana-South) are well-differentiated and form two main clusters. We found higher genetic diversity within ponds from the North region, which also exhibited a higher degree of genetic admixture in comparison with populations from the southern edge. Although we found an isolation-by-distance pattern within each cluster, it arose due to the effect of a few isolated ponds located on the edge of each zone, suggesting the existence of substantial gene flow between ponds in the core area. According to our findings, landscape’s permeability to movement (pond connectivity) may constitute a more important factor than hydroperiod length in determining the genetic diversity and viability of pygmy newt populations in this area. Although Doñana populations show a good state, more peripheral and isolated populations present a more worrisome condition as a result of fragmentation and thus, require conservation efforts. Our study provides key insights that could help guide management practices of this threatened and poorly-studied salamander.

The long-standing significance of genetic diversity in conservation

Since allozymes were first used to assess genetic diversity in the 1960s and 1970s, biologists have attempted to characterize gene pools and conserve the diversity observed in domestic crops, livestock, zoos and (more recently) natural populations. Recently, some authors have claimed that the importance of genetic diversity in conservation biology has been greatly overstated. Here, we argue that a voluminous literature indicates otherwise. We address four main points made by detractors of genetic diversity's role in conservation by using published literature to firmly establish that genetic diversity is intimately tied to evolutionary fitness, and that the associated demographic consequences are of paramount importance to many conservation efforts. We think that responsible management in the Anthropocene should, whenever possible, include the conservation of ecosystems, communities, populations and individuals, and their underlying genetic diversity.

Heterozygous Trees Rebound the Fastest after Felling by Beavers to Positively Affect Arthropod Community Diversity

Although genetic diversity within stands of trees is known to have community-level consequences, whether such effects are present at an even finer genetic scale is unknown. We examined the hypothesis that genetic variability (heterozygosity) within an individual plant would affect its dependent community, which adds a new dimension to the importance of genetic diversity. Our study contrasted foliar arthropod community diversity and microsatellite marker-derived measures of genetic diversity of cottonwood (Populus fremontii) trees that had been felled by beavers (Castor canadensis) and were resprouting, relative to adjacent standing, unfelled trees. Three patterns emerged: 1. Productivity (specific leaf area), phytochemical defenses (salicortin), and arthropod community richness, abundance, and diversity were positively correlated with the heterozygosity of individual felled trees, but not with that of unfelled trees; 2. These relationships were not explained by population substructure, genetic relatedness of the trees, or hybridization; 3. The underlying mechanism appears to be that beaver herbivory stimulates increased productivity (i.e., 2× increase from the most homozygous to the most heterozygous tree) that is the greatest in more heterozygous trees. Salicortin defenses in twigs were also expressed at higher concentrations in more heterozygous trees (i.e., 3× increase from the most homozygous to the most heterozygous tree), which suggests that this compound may dissuade further herbivory by beavers, as has been found for other mammalian herbivores. We suggest that high stress to trees as a consequence of felling reveals a heterozygosity–productivity linkage, which in turn is attractive to arthropods. Although experiments are required to demonstrate causality, these results link the genetic diversity of individual trees to community diversity, supporting the hypothesis that interactions among foundation species (beavers and trees) have community-level effects, and underscores the importance of genetic diversity for biodiversity, conservation, and restoration.

Individual Heterozygosity Influences Arrival Times and Mating Success of Male Red-Breasted Flycatchers Ficedula parva

We examined the relationship between individual heterozygosity of male Red-breasted Flycatchers (Ficedula parva; a small long-distance migratory, socially monogamous bird species) and their mating success, arrival time and age. Using eight polymorphic microsatellite loci, we found that male heterozygosity is related to both mating success and arrival time, but not to age. Mated and earlier arriving males had higher heterozygosity than later arrivals and bachelors, but we did not find a relationship between age and individual heterozygosity of males. To our knowledge, this is the first study to provide evidence about the relationship between individual genetic diversity and arrival time, thus arrival time could be used as a signal of individual heterozygosity and quality.

Population demography and heterozygosity-fitness correlations in natural guppy populations: An examination using sexually selected fitness traits

Heterozygosity-fitness correlations (HFCs) have been examined in a wide diversity of contexts, and the results are often used to infer the role of inbreeding in natural populations. Although population demography, reflected in population-level genetic parameters such as allelic diversity or identity disequilibrium, is expected to play a role in the emergence and detectability of HFCs, direct comparisons of variation in HFCs across many populations of the same species, with different genetic histories, are rare. Here, we examined the relationship between individual microsatellite heterozygosity and a range of sexually selected traits in 660 male guppies from 22 natural populations in Trinidad. Similar to previous studies, observed HFCs were weak overall. However, variation in HFCs among populations was high for some traits (although these variances were not statistically different from zero). Population-level genetic parameters, specifically genetic diversity levels (number of alleles, observed/expected heterozygosity) and measures of identity disequilibrium (g2 and heterozygosity-heterozygosity correlations), were not associated with variation in population-level HFCs. This latter result indicates that these metrics do not necessarily provide a reliable predictor of HFC effect sizes across populations. Importantly, diversity and identity disequilibrium statistics were not correlated, providing empirical evidence that these metrics capture different essential characteristics of populations. A complex genetic architecture likely underpins multiple fitness traits, including those associated with male fitness, which may have reduced our ability to detect HFCs in guppy populations. Further advances in this field would benefit from additional research to determine the demographic contexts in which HFCs are most likely to occur.

Genetic diversity of Morato's Digger Toad, Proceratophrys moratoi: spatial structure, gene flow, effective size and the need for differential management strategies of populations

The Morato's Digger Toad, Proceratophrys moratoi, is a critically endangered toad species with a marked population decline in southern Brazilian Cerrado. Despite this, new populations are being discovered, primarily in the northern part of the distribution range, which raises a number of questions with regard to the conservation status of the species. The present study analyzed the genetic diversity of the species based on microsatellite markers. Our findings permitted the identification of two distinct management units. We found profound genetic structuring between the southern populations, on the left margin of the Tietê River, and all other populations. A marked reduction was observed in the contemporary gene flow among the central populations that are most affected by anthropogenic impacts, such as extensive sugar cane plantations, which presumably decreases habitat connectivity. The results indicated reduced diversity in the southern populations which, combined with a smaller effective population size, may make these populations more susceptible to extinction. We recommend the reclassification of P. moratoi as vulnerable and the establishment of a special protection program for the southern populations. Our results provide important insights about the local extinction of southern populations of this toad.

Local effects drive heterozygosity-fitness correlations in an outcrossing long-lived tree

Heterozygosity-fitness correlations (HFCs) have been used to understand the complex interactions between inbreeding, genetic diversity and evolution. Although frequently reported for decades, evidence for HFCs was often based on underpowered studies or inappropriate methods, and hence their underlying mechanisms are still under debate. Here, we used 6100 genome-wide single nucleotide polymorphisms (SNPs) to test for general and local effect HFCs in maritime pine (Pinus pinaster Ait.), an iconic Mediterranean forest tree. Survival was used as a fitness proxy, and HFCs were assessed at a four-site common garden under contrasting environmental conditions (total of 16 288 trees). We found no significant correlations between genome-wide heterozygosity and fitness at any location, despite variation in inbreeding explaining a substantial proportion of the total variance for survival. However, four SNPs (including two non-synonymous mutations) were involved in significant associations with survival, in particular in the common gardens with higher environmental stress, as shown by a novel heterozygosity-fitness association test at the species-wide level. Fitness effects of SNPs involved in significant HFCs were stable across maritime pine gene pools naturally growing in distinct environments. These results led us to dismiss the general effect hypothesis and suggested a significant role of heterozygosity in specific candidate genes for increasing fitness in maritime pine. Our study highlights the importance of considering the species evolutionary and demographic history and different spatial scales and testing environments when assessing and interpreting HFCs.

The strength of the association between heterozygosity and probability of interannual local recruitment increases with environmental harshness in blue tits

The extent of inbreeding depression and the magnitude of heterozygosity–fitness correlations (HFC) have been suggested to depend on the environmental context in which they are assayed, but little evidence is available for wild populations. We combine extensive molecular and capture–mark–recapture data from a blue tit (Cyanistes caeruleus) population to (1) analyze the relationship between heterozygosity and probability of interannual adult local recruitment and (2) test whether environmental stress imposed by physiologically suboptimal temperatures and rainfall influence the magnitude of HFC. To address these questions, we used two different arrays of microsatellite markers: 14 loci classified as neutral and 12 loci classified as putatively functional. We found significant relationships between heterozygosity and probability of interannual local recruitment that were most likely explained by variation in genomewide heterozygosity. The strength of the association between heterozygosity and probability of interannual local recruitment was positively associated with annual accumulated precipitation. Annual mean heterozygosity increased over time, which may have resulted from an overall positive selection on heterozygosity over the course of the study period. Finally, neutral and putatively functional loci showed similar trends, but the former had stronger effect sizes and seemed to better reflect genomewide heterozygosity. Overall, our results show that HFC can be context dependent, emphasizing the need to consider the role of environmental heterogeneity as a key factor when exploring the consequences of individual genetic diversity on fitness in natural populations.

Microevolution due to pollution in amphibians: A review on the genetic erosion hypothesis

The loss of genetic diversity, due to exposure to chemical contamination (genetic erosion), is a major threat to population viability. Genetic erosion is the loss of genetic variation: the loss of alleles determining the value of a specific trait or set of traits. Almost a third of the known amphibian species is considered to be endangered and a decrease of genetic variability can push them to the verge of extinction. This review indicates that loss of genetic variation due to chemical contamination has effects on: 1) fitness, 2) environmental plasticity, 3) co-tolerance mechanisms, 4) trade-off mechanisms, and 5) tolerance to pathogens in amphibian populations.

Exploring heterozygosity-survival correlations in a wild songbird population: contrasting effects between juvenile and adult stages

The relationship between genetic diversity and fitness, a major issue in evolutionary and conservation biology, is expected to be stronger in traits affected by many loci and those directly influencing fitness. Here we explore the influence of heterozygosity measured at 15 neutral markers on individual survival, one of the most important parameters determining individual fitness. We followed individual survival up to recruitment and during subsequent adult life of 863 fledgling pied flycatchers born in two consecutive breeding seasons. Mark-recapture analyses showed that individual heterozygosity did not influence juvenile or adult survival. In contrast, the genetic relatedness of parents was negatively associated with the offspring’s survival during the adult life, but this effect was not apparent in the juvenile (from fledgling to recruitment) stage. Stochastic factors experienced during the first year of life in this long-distance migratory species may have swamped a relationship between heterozygosity and survival up to recruitment.

Heterozygosity-fitness correlations in a wild mammal population: accounting for parental and environmental effects

HFCs (heterozygosity–fitness correlations) measure the direct relationship between an individual's genetic diversity and fitness. The effects of parental heterozygosity and the environment on HFCs are currently under-researched. We investigated these in a high-density U.K. population of European badgers (Meles meles), using a multimodel capture–mark–recapture framework and 35 microsatellite loci. We detected interannual variation in first-year, but not adult, survival probability. Adult females had higher annual survival probabilities than adult males. Cubs with more heterozygous fathers had higher first-year survival, but only in wetter summers; there was no relationship with individual or maternal heterozygosity. Moist soil conditions enhance badger food supply (earthworms), improving survival. In dryer years, higher indiscriminate mortality rates appear to mask differential heterozygosity-related survival effects. This paternal interaction was significant in the most supported model; however, the model-averaged estimate had a relative importance of 0.50 and overlapped zero slightly. First-year survival probabilities were not correlated with the inbreeding coefficient (f); however, small sample sizes limited the power to detect inbreeding depression. Correlations between individual heterozygosity and inbreeding were weak, in line with published meta-analyses showing that HFCs tend to be weak. We found support for general rather than local heterozygosity effects on first-year survival probability, and g2 indicated that our markers had power to detect inbreeding. We emphasize the importance of assessing how environmental stressors can influence the magnitude and direction of HFCs and of considering how parental genetic diversity can affect fitness-related traits, which could play an important role in the evolution of mate choice.

The effect of drought stress on heterozygosity-fitness correlations in pedunculate oak (Quercus robur)

BACKGROUND AND AIMS

The interaction between forest fragmentation and predicted climate change may pose a serious threat to tree populations. In small and spatially isolated forest fragments, increased homozygosity may directly affect individual tree fitness through the expression of deleterious alleles. Climate change-induced drought stress may exacerbate these detrimental genetic consequences of forest fragmentation, as the fitness response to low levels of individual heterozygosity is generally thought to be stronger under environmental stress than under optimal conditions.

METHODS

To test this hypothesis, a greenhouse experiment was performed in which various transpiration and growth traits of 6-month-old seedlings of Quercus robur differing in multilocus heterozygosity (MLH) were recorded for 3 months under a well-watered and a drought stress treatment. Heterozygosity-fitness correlations (HFC) were examined by correlating the recorded traits of individual seedlings to their MLH and by studying their response to drought stress.

KEY RESULTS

Weak, but significant, effects of MLH on several fitness traits were obtained, which were stronger for transpiration variables than for the recorded growth traits. High atmospheric stress (measured as vapour pressure deficit) influenced the strength of the HFCs of the transpiration variables, whereas only a limited effect of the irrigation treatment on the HFCs was observed.

CONCLUSIONS

Under ongoing climate change, increased atmospheric stress in the future may strengthen the negative fitness responses of trees to low MLH. This indicates the necessity to maximize individual multilocus heterozygosity in forest tree breeding programmes.

Heterozygosity is linked to the costs of immunity in nestling great tits (Parus major)

There is growing evidence that heterozygosity–fitness correlations (HFCs) are more pronounced under harsh conditions. Empirical evidence suggests a mediating effect of parasite infestation on the occurrence of HFCs. Parasites have the potential to mediate HFCs not only by generally causing high stress levels but also by inducing resource allocation tradeoffs between the necessary investments in immunity and other costly functions. To investigate the relative importance of these two mechanisms, we manipulated growth conditions of great tit nestlings by brood size manipulation, which modifies nestling competition, and simultaneously infested broods with ectoparasites. We investigated under which treatment conditions HFCs arise and, second, whether heterozygosity is linked to tradeoff decisions between immunity and growth. We classified microsatellites as neutral or presumed functional and analyzed these effects separately. Neutral heterozygosity was positively related to the immune response to a novel antigen in parasite-free nests, but not in infested nests. For nestlings with lower heterozygosity levels, the investments in immunity under parasite pressure came at the expenses of reduced feather growth, survival, and female body condition. Functional heterozygosity was negatively related to nestling immune response regardless of the growth conditions. These contrasting effects of functional and neutral markers might indicate different underlying mechanisms causing the HFCs. Our results confirm the importance of considering marker functionality in HFC studies and indicate that parasites mediate HFCs by influencing the costs of immune defense rather than by a general increase in environmental harshness levels.

Sex-specific correlations of individual heterozygosity, parasite load, and scalation asymmetry in a sexually dichromatic lizard

Heterozygosity-fitness correlations (HFCs) provide insights into the genetic bases of individual fitness variation in natural populations. However, despite decades of study, the biological significance of HFCs is still under debate. In this study, we investigated HFCs in a large population of the sexually dimorphic lizard Takydromus viridipunctatus (Lacertidae). Because of the high prevalence of parasitism from trombiculid mites in this lizard, we expect individual fitness (i.e., survival) to decrease with increasing parasite load. Furthermore, because morphological asymmetry is likely to influence individuals' mobility (i.e., limb asymmetry) and male biting ability during copulation (i.e., head asymmetry) in this species, we also hypothesize that individual fitness should decrease with increasing morphological asymmetry. Although we did not formally test the relationship between morphological asymmetry and fitness in this lizard, we demonstrated that survival decreased with increasing parasite load using a capture-mark-recapture data set. We used a separate sample of 140 lizards to test the correlations between individual heterozygosity (i.e., standardized mean d(2) and HL based on 10 microsatellite loci) and the two fitness traits (i.e., parasite load and morphological asymmetry). We also evaluated and excluded the possibility that single-locus effects produced spurious HFCs. Our results suggest male-only, negative correlations between individual heterozygosity and parasite load and between individual heterozygosity and asymmetry, suggesting sex-specific, positive HFCs. Male T. viridipunctatus with higher heterozygosity tend to have lower parasite loads (i.e., higher survival) and lower asymmetry, providing a rare example of HFC in reptiles.

Heterozygosity-fitness correlations among wild populations of European tree frog (Hyla arborea) detect fixation load

Quantifying the impacts of inbreeding and genetic drift on fitness traits in fragmented populations is becoming a major goal in conservation biology. Such impacts occur at different levels and involve different sets of loci. Genetic drift randomly fixes slightly deleterious alleles leading to different fixation load among populations. By contrast, inbreeding depression arises from highly deleterious alleles in segregation within a population and creates variation among individuals. A popular approach is to measure correlations between molecular variation and phenotypic performances. This approach has been mainly used at the individual level to detect inbreeding depression within populations and sometimes at the population level but without consideration about the genetic processes measured. For the first time, we used in this study a molecular approach considering both the interpopulation and intrapopulation level to discriminate the relative importance of inbreeding depression vs. fixation load in isolated and non-fragmented populations of European tree frog (Hyla arborea), complemented with interpopulational crosses. We demonstrated that the positive correlations observed between genetic heterozygosity and larval performances on merged data were mainly caused by co-variations in genetic diversity and fixation load among populations rather than by inbreeding depression and segregating deleterious alleles within populations. Such a method is highly relevant in a conservation perspective because, depending on how populations lose fitness (inbreeding vs. fixation load), specific management actions may be designed to improve the persistence of populations.

Genetic diversity in water frog hybrids (Pelophylax esculentus) varies with population structure and geographic location

Pelophylax esculentus is a hybridogenetic frog originating from matings between P. ridibundus (RR) and P. lessonae (LL). Typically, diploid hybrids (LR) live in sympatry with one of their parental species, upon which they depend for successful reproduction. In parts of their range, however, pure hybrid populations can be found. These hybrid populations have achieved reproductive independence from their parental species by using triploid hybrids (LLR, LRR) rather than LL and RR as their sexual hosts. These different breeding systems also entail differences in reproduction (clonal versus sexual) and hence offer the opportunity to study how genetic diversity is affected by reproductive mode, population structure and geographic location. We investigated 33 populations in the Scania region (South Sweden) and 18 additional populations from Northern and Central Europe. Within both genomes (L, R), genetic variability increases with the potential for recombination and declines from the main species distribution area southeast of the Baltic Sea to the fringe populations northwest of the Baltic Sea. Within the main study area in Scania, genetic diversity is low and decreases from a core area to the periphery. Genetic differentiation between Scania populations is small but significant and best explained by 'isolation by distance'. Despite the low genetic variability within the discrete genomes, all-hybrid P. esculentus populations in southern Sweden are apparently not suffering from direct negative fitness effects. This is probably because of its somatic hybrid status, which increases diversity through the combination of genomes from two species.

High-pitched notes during vocal contests signal genetic diversity in ocellated antbirds

Animals use honest signals to assess the quality of competitors during aggressive interactions. Current theory predicts that honest signals should be costly to produce and thus reveal some aspects of the phenotypic or genetic quality of the sender. In songbirds, research indicates that biomechanical constraints make the production of some acoustic features costly. Furthermore, recent studies have found that vocal features are related to genetic diversity. We linked these two lines of research by evaluating if constrained acoustic features reveal male genetic diversity during aggressive interactions in ocellated antbirds (Phaenostictus mcleannani). We recorded the aggressive vocalizations of radiotagged males at La Selva Biological Station in Costa Rica, and found significant variation in the highest frequency produced among individuals. Moreover, we detected a negative relationship between the frequency of the highest pitched note and vocalization duration, suggesting that high pitched notes might constrain the duration of vocalizations through biomechanical and/or energetic limitations. When we experimentally exposed wild radiotagged males to simulated acoustic challenges, the birds increased the pitch of their vocalization. We also found that individuals with higher genetic diversity (as measured by zygosity across 9 microsatellite loci) produced notes of higher pitch during aggressive interactions. Overall, our results suggest that the ability to produce high pitched notes is an honest indicator of male genetic diversity in male-male aggressive interactions.

Genetic variability is unrelated to growth and parasite infestation in natural populations of the European eel (Anguilla anguilla)

Positive correlations between individual genetic heterozygosity and fitness-related traits (HFCs) have been observed in organisms as diverse as plants, marine bivalves, fish or mammals. HFCs are not universal and the strength and stability of HFCs seem to be variable across species, populations and ages. We analysed the relationship between individual genetic variability and two different estimators of fitness in natural samples of European eel, growth rate (using back-calculated length-at-age 1, 2 and 3) and parasite infestation by the swimbladder nematode Anguillicola crassus. Despite using a large data set of 22 expressed sequence tags-derived microsatellite loci and a large sample size of 346 individuals, no heterozygote advantage was observed in terms of growth rate or parasite load. The lack of association was evidenced by (i) nonsignificant global HFCs, (ii) a Multivariate General Linear Model showing no effect of heterozygosity on fitness components, (iii) single-locus analysis showing a lower number of significant tests than the expected false discovery rate, (iv) sign tests showing only a significant departure from expectations at one component, and, (v) a random distribution of significant single-locus HFCs that was not consistent across fitness components or sampling sites. This contrasts with the positive association observed in farmed eels in a previous study using allozymes, which can be explained by the nature of the markers used, with the allozyme study including many loci involved in metabolic energy pathways, while the expressed sequence tags-linked microsatellites might be located in genes or in the proximity of genes uncoupled with metabolism/growth.

Heterozygosity-fitness correlations revealed by neutral and candidate gene markers in roe deer from a long-term study

Heterozygosity-fitness correlations (HFCs) are increasingly reported but the underlying mechanisms causing HFCs are generally poorly understood. Here, we test for HFCs in roe deer (Capreolus capreolus) using 22 neutral microsatellites widely distributed in the genome and four microsatellites in genes that are potentially under selection. Juvenile survival was used as a proxy for individual fitness in a population that has been intensively studied for 30 years in northeastern France. For 222 juveniles, we computed two measures of genetic diversity: individual heterozygosity (H), and mean d(2) (relatedness of parental genomes). We found a relationship between genetic diversity and fitness both for the 22 neutral markers and two candidate genes: IGF1 (Insulin-like Growth Factor I) and NRAMP (natural resistance-associated macrophage protein). Statistical evidence and the size of genetic effects on juvenile survival were comparable to those reported for early development and cohort variation, suggesting a substantial influence of genetic components on fitness in this roe deer population. For the 22 neutral microsatellites, a correlation with fitness was revealed for mean d(2), but not for H, suggesting a possible outbreeding advantage. This heterosis effect could have been favored by introduction of genetically distant (Hungarian) roe deer to the population in recent times and, possibly, by the structuring of the population into distinct clans. The locus-specific correlations with fitness may be driven by growth rate advantages and resistance to diseases known to exist in the studied population. Our analyses of neutral and candidate gene markers both suggest that the observed HFCs are likely mainly due to linkage with dominant or overdominant loci that affect fitness ("local" effect) rather than to a genome-wide relationship with homozygosity due to inbreeding ("general" effect).

Risk of ectoparasitism and genetic diversity in a wild lesser kestrel population

Parasites and infectious diseases are major determinants of population dynamics and adaptive processes, imposing fitness costs to their hosts and promoting genetic variation in natural populations. In the present study, we evaluate the role of individual genetic diversity on risk of parasitism by feather lice Degeeriella rufa in a wild lesser kestrel population (Falco naumanni). Genetic diversity at 11 microsatellite loci was associated with risk of parasitism by feather lice, with more heterozygous individuals being less likely to be parasitized, and this effect was statistically independent of other nongenetic parameters (colony size, sex, location, and year) which were also associated with lice prevalence. This relationship was nonlinear, with low and consistent prevalences among individuals showing high levels of genetic diversity that increased markedly at low levels of individual heterozygosity. This result appeared to reflect a genome-wide effect, with no single locus contributing disproportionably to the observed effect. Thus, overall genetic variation, rather than linkage of markers to genes experiencing single-locus heterosis, seems to be the underlying mechanism determining the association between risk of parasitism and individual genetic diversity in the study host-parasite system. However, feather lice burden was not affected by individual heterozygosity; what suggest that differences in susceptibility, rather than variation in defences once the parasite has been established, may shape the observed pattern. Overall, our results highlight the role of individual genetic diversity on risk of parasitism in wild populations, what has both important evolutionary implications and major consequences for conservation research on the light of emerging infectious diseases that may endanger genetically depauperated populations.

Genetic variability predicts common frog (Rana temporaria) size at metamorphosis in the wild

We investigated associations between genetic variability and two fitness-related traits--size and age at metamorphosis--in two subartic populations of the common frog, Rana temporaria. We found that metamorphic size was positively correlated with individual heterozygosity (as estimated using eight microsatellite loci) and that maternal heterozygosity also explained a significant amount of variation in this trait. In contrast, age at metamorphosis was only explained by environmental factors. Since size at metamorphosis is positively correlated with fitness in amphibians, these results suggest that genetic variability may be an important component of individual fitness in common frogs. The environmental variation underlying timing of metamorphosis may indicate that strong selection pressure on this trait in the Nordic environment is likely to override genetic effects.

Environmental stress and life-stage dependence on the detection of heterozygosity-fitness correlations in the European eel, Anguilla anguilla

Heterozygosity-fitness correlations (HFCs) have been reported in populations of many species, although HFCs can clearly vary across species, conspecific populations, temporal samples, and sexes. We studied (i) the temporal stability of the association between genetic variation and growth rate (length and mass increase) and (ii) the influence of genetic variability on survival in the European eel (Anguilla anguilla L). HFCs were assessed using genotypes from 10 allozyme and 6 microsatellite markers in 22-month-old experimental individuals. The results were compared with those of a previous study carried out in 12-month-old individuals, in which more heterozygous individuals showed a significantly faster growth rate. In contrast, 22-month-old individuals showed no evidence that genetic variability was correlated with growth rate. Additionally, heterozygous individuals did not show a higher survival rate compared with more homozygous individuals after either handling stress or parasite infection. The decrease in HFCs over time is consistent with the general prediction that differences in growth and survival among individuals are maximal early in life and in our case most likely due to the relaxation of environmental conditions related to population-density effects. Alternatively, the decline in HFCs could be attributed to either ontogenetic variance in gene activity between 12- and 22-month-old individuals or differential mortality leaving only the largest individuals.

Demographic and genetic estimates of effective population and breeding size in the amphibian Rana temporaria

Genetic methods for estimating effective population size ( Ne) or the effective number of breeders ( Nb) have become popular, but comparisons of these estimates with demographic estimates of Ne and Nb are rare, especially in anurans. We used three genetic (linkage disequilibrium, temporal moments, Bayesian coalescent-based method) and three demographic models, the latter considering number of breeding individuals, sex ratio, reproductive skew, and other demographic data, to estimate Ne and Nb in two subarctic populations (T and P) of the common frog Rana temporaria, subject to long-term capture-recapture studies. Demographic estimates of Ne based on total population size ( Ne ([T])= 44.5-56.9; Ne ([P])= 68.8-93.7) deviated markedly from the genetic estimates obtained using the linkage disequilibrium method ( Ne ([T])= 97.1; Ne ([P])= 13.2). The demographic estimates of Nb, taking into consideration sex ratio and variance in reproductive success ( Nb ([T])= 10.1-39.7; Nb ([P])= 3.9-21.3), were higher than the genetic estimates ( Nb ([T])= 3.7-5.4; Nb ([P])= 3.5-3.9). The main factors affecting the effective size estimates were sex ratio and reproductive skew. The discrepancies between corresponding Ne and Nb estimates highlight the sensitivity of both demographic and genetic estimates on their underlying assumptions. Yet the ratios of effective or breeding effective size to the census population size were similar to those reported earlier for anurans, reinforcing the view that the discrepancy between actual and effective breeding sizes in anuran populations is typically very large.

The importance of heterozygosity in a frog's life

High genetic variability may increase metabolic efficiency and thus allows responding to environmental challenges as limits to adaptation are approached. Therefore, it has been suggested that high genetic variability contributes strongly to the fitness of an individual. Survival to high age may thus depend on high genetic variability, and genetically variable individuals may have a higher survival rate to high ages in comparison to less variable sympatric conspecifics. Such a heterozygosity x age relationship might be more readily detectable in stressful as compared to benign environments. For testing the relationship between age and heterozygosity, we genetically analyzed 71 individuals of the frog species Rana perezi from a total of seven populations at 13 allozyme loci. The age of the individuals was determined by skeletochronology. We found effects on age of both environment and allozyme heterozygosity, especially in populations with high stress regimes. A significant heterozygosity x age relationship has so far rarely been shown in natural populations. The result of our analysis suggests that more heterozygous individuals have a higher longevity and may be an important source of genetic variability of a population, likely contributing to a stabilization of the effective population size.