Showing papers in "Heredity in 1998"
TL;DR: It is concluded that local adaptation is an average phenomenon that requires adequate replication at the appropriate level, that at which the local processes occur, and variable selection pressure and stochasticity may obscure local processes or change the level at which local adaptation occurs.
Abstract: In host-parasite coevolutionary arms races, parasites probably have an evolutionary advantage. Parasite populations should be locally adapted, having higher mean fitness on sympatric than allopatric hosts. Here we assess evidence for local parasite advantage. Further we investigate how adaptation and counter-adaptation of parasites and hosts, necessarily occurring in sympatry, can generate a pattern of local adaptation. Already simple frequency-dependent selection models generate complex patterns of parasite performance on sympatric and allopatric populations. In metapopulations, with extinction, recolonization, and gene flow, variable selection pressure and stochasticity may obscure local processes or change the level at which local adaptation occurs. Alternatively, gene flow may introduce adaptive variation, so differential migration rates can modify the asymmetry of host and parasite evolutionary rates. We conclude that local adaptation is an average phenomenon. Its detection requires adequate replication at the appropriate level, that at which the local processes occur.
498 citations
Abstract: We have briefly reviewed the methods currently available for QTL analysis in segregating populations and summarized some of the conclusions arising from such analyses in plant populations We show that the analytical methods locate QTL with poor precision (10-30 cM), unless the heritability of an individual QTL is high Also the estimates of the QTL effects, particularly the dominance effects tend to be inflated because only large estimates are significant Estimates of numbers of QTL per trait are generally low ( 1, but seldom significantly greater These latter cases need further analysis Many QTL map close to candidate genes, and there is growing evidence from synteny studies of corresponding chromosome regions carrying similar QTL in different species However, unreliability of QTL location may suggest false candidates
433 citations
TL;DR: This issue might soon be resolved provided clear hypotheses and definitions are used and the problem of the neutrality of allozyme variation is not identified with the related issue of HFC, as well as new empirical & theoretical tools.
Abstract: Heterozygosity–fitness correlations (HFC) have been studied in various organisms for more than two decades, but they are not universal. Although their detectability is limited by several factors (null alleles, inaccuracy of the phenotypic description of fitness, small sample sizes) the correlations appear intrinsically weak and often inconsistent across samples. Determining the origins of HFC is therefore a complex task. However, this issue might soon be resolved provided clear hypotheses and definitions are used (especially, if the problem of the neutrality of allozyme variation is not identified with the related issue of HFC), as well as new empirical (molecular markers) & theoretical (statistical models) tools.
353 citations
TL;DR: It is argued that rDNA homogeneity over the species range of T. urticae results from the high colonization potential of this species, preventing long-term differentiation.
Abstract: We compared patterns of intraspecific polymorphism of two markers with contrasted modes of evolution, nuclear ribosomal DNA (rDNA) and mitochondrial DNA (mtDNA), in the phytophagous mite Tetranychus urticae Koch. The second internal transcribed spacer (ITS2) of rDNA and a fragment in the mtDNA gene coding for Cytochrome Oxidase I (COI), were PCR-amplified and sequenced in samples of various geographical origins distributed worldwide. The 15 COI haplotypes found fell into two major phylogenetic lineages differing by an average of 5% nucleotide divergence. Samples from the Mediterranean basin were represented in both lineages, and showed no phylogeographical structure. The other samples, from temperate regions of the northern hemisphere, were clustered in one of the lineages and displayed little variation, indicating a recent colonization of this region. In contrast, no variation at all was found at the ITS2 in this species. We sequenced both COI and ITS2 in four other species of the genus Tetranychus and found that, despite the absence of intraspecific polymorphism, ITS appears to evolve 2.5 times faster than COI. We argue that rDNA homogeneity over the species range of T. urticae results from the high colonization potential of this species, preventing long-term differentiation. Preliminary data on two other mite species (Amphitetranychus viennensis Zacher and Mononychellus progresivus Doreste) with stricter ecological requirements and more restricted colonization potential revealed substantial and concordant geographical differentiation for both ITS2 and COI.
272 citations
TL;DR: The theory and data required to predict sex allocation patterns (let alone sex ratio patterns) in vertebrates are complex and hard to obtain and the investigation of the role of constraint vs. adaptation in evolution is investigated.
Abstract: Sex allocation theory, and its success in predicting sex ratios in such taxa as parasitoid wasps, is often cited as one of the crowning achievements of theoretical evolutionary biology. Its success in some vertebrate taxa, particularly birds, has been more modest. I discuss two reasons for this. First, it is difficult to obtain avian sex ratio data before substantial offspring mortality has occurred. Second, the theory and data required to predict sex allocation patterns (let alone sex ratio patterns) in vertebrates are complex and hard to obtain. Recently developed molecular genetic techniques allowing sex identification from DNA samples have largely solved the first problem and there have been several striking empirical demonstrations of sex ratio biases consistent with sex allocation theory in wild bird populations. Solution of the second problem may come with the incorporation of realistic life history data into models and the use of experimental manipulations to reveal the fitness consequences of allocation strategies. Further data concerning sex ratio variation in taxa such as birds, with chromosomal sex determination, are valuable because they allow the investigation of the role of constraint vs. adaptation in evolution.
268 citations
TL;DR: Patterns of chloroplast DNA (cpDNA) variation are used to investigate the postglacial colonization history of the European oak species Quercus robur and Q. petraea and reveal four polymorphic sites which identify four cytotypes with characteristic geographical distributions.
Abstract: Refugial differentiation and routes of postglacial migration are major determinants of the patterns of geographical variation we see in natural populations today. We used patterns of chloroplast DNA (cpDNA) variation to investigate the postglacial colonization history of the European oak species Quercus robur and Q. petraea. By sequencing two cpDNA segments using universal primers, we revealed four polymorphic sites which identify four cytotypes with characteristic geographical distributions. Of these, the principal eastern, central and western cytotypes divide the range into three longitudinal zones, each extending from the south to the north of Europe. This corroborates the idea that the postglacial colonization started from three distinct southerly refugia. The fourth cytotype, restricted to East Anglia, was probably derived from the western type postglacially. As a special problem, we addressed the controversial origin of Q. robur at its northern limits in south-western Finland, where it currently occupies a narrow coastal zone disjunct from the remaining oak range. Using a PCR-RFLP assay that discriminates the eastern cytotype, a contact zone of two cytotypes was identified in the region of the Salpausselka ridges. This suggests that the marginal northern occurrence was independently colonized both from the east and from the west, across the Baltic Sea.
196 citations
TL;DR: Two quantitative trait loci (QTL) were mapped by detecting a significant association between variance in upper temperature tolerance and alleles at the microsatellite loci Omy325UoG and Ssa14DU suggesting that the effects of the QTL are additive.
Abstract: We searched for linkage among 24 polymorphic loci (allozymes, RAPD, microsatellites) in three half-sib backcross families of rainbow trout (Oncorhynchus mykiss) produced by crossing strains divergent for the quantitative trait of upper temperature tolerance. Seven significant and two suggestive pairwise linkage associations between molecular marker loci were observed involving 14 loci clustered into four linkage groups. The association between a pair of allozyme loci (sIDHP-3* and mMEP-2 *) has been reported previously. Recombination rates varied greatly between the sexes and families. Two quantitative trait loci (QTL) were mapped by detecting a significant association between variance in upper temperature tolerance and alleles at the microsatellite loci Omy325UoG and Ssa14DU. The two QTL appear to reside in different linkage groups and account for ≈13 per cent and 9 per cent of the overall additive genetic variance in upper temperature tolerance. No significant interaction was detected between Omy325UoG and Ssa14DU suggesting that the effects of the QTL are additive.
173 citations
TL;DR: Vertical transmission is identified as a key route to host specialization on effective symbionts, despite variation in symbiont effectiveness.
Abstract: Various animals and plants benefit from symbiotic micro-organisms, but the effectiveness of the microbial symbionts (i.e. the amount of benefit that the animal/plant host derives from the symbiosis) varies among symbiont genotypes in natural populations. Recent studies on symbioses with horizontal transmission suggest that three factors may contribute to this variation: (a) selection pressure on the microbial symbionts to exploit the host, resulting in reduced host benefit (b) variation with environmental circumstances in the amount of benefit derived by a host from different symbiont genotypes and (c) unpredictable or low abundance of the microbial partner available to infect hosts from the free-living environment. The latter two factors would counter the selection pressure on hosts to specialize (i.e. to form symbioses exclusively with highly effective symbiont genotypes), despite variation in symbiont effectiveness. Vertical transmission is identified as a key route to host specialization on effective symbionts.
173 citations
TL;DR: There appears to be a general preference expressed for M HC-dissimilar mates, and such MHC-disassortative mating may be involved in maintaining MHC and/or genome-wide diversity in natural populations.
Abstract: A long series of studies on mice have shown that mate choice decisions can be made on the basis of individual genotype at the major histocompatibility complex (MHC), which accords well with the importance of immunocompetence in some theories of sexual selection. Recent work on other vertebrate species, including humans, indicates that MHC-based mate choice is not restricted to the genus Mus. However, its importance may vary among species as a result of differences in social and mating system structure, and perhaps genome structure. There appears to be a general preference expressed for MHC-dissimilar mates, and such MHC-disassortative mating may be involved in maintaining MHC and/or genome-wide diversity in natural populations. The strength and direction of MHC-based mating preference can vary, and may be modulated by factors such as genetic background, sex, and early life experience.
172 citations
TL;DR: Mating system variation in colonizing, self-incompatible species is valuable for understanding the evolution ofSelf-incompatibility systems, as well as microevolutionary changes of mating system following founder events.
Abstract: The mating system of Centaurea solstitialis L. was investigated in relation to its colonization of North America. A preliminary investigation of its reproductive biology suggested that the species is a pollinator-dependent outbreeder, and probably self-incompatible. Quantitative analysis of the mating system parameters was performed using progeny arrays assayed for nine allozyme markers. Multilocus outcrossing rates (tm) ranged from 0.948 to 0.990 among eight populations. Moderate levels of biparental inbreeding (≈6 per cent apparent selfing) were detected in most populations. The correlation of outcrossed paternity within progeny arrays (rp) ranged from 0.05 to 0.64 among populations, indicating differences in modes of outcross pollination. A geographically marginal population, San Diego, showed the only significant parental inbreeding coefficient (F=0.27), as well as highest rp, suggesting microevolutionary changes of mating system following founder events. One other population exhibited significant variation of individual plant outcrossing rate, with a correlation of selfing within progeny arrays of rs=0.65, indicating variation of self-incompatibility. Mating system variation in colonizing, self-incompatible species is valuable for understanding the evolution of self-incompatibility systems.
152 citations
TL;DR: The results of field experiments indicate that, where pollinator number limits seed-set, flowers with conical epidermal cells receive more pollinator attention than do those with flat cells.
Abstract: The nature of the selective pressures which have resulted in the conical-papillate shape of the cells of the adaxial epidermis of many petals has been a matter for considerable speculation. One suggestion is that this shape focuses light within epidermal cells resulting in an increase in the amount of light absorbed by the floral pigments, intensifying the colour of the petals and possibly enhancing their attractiveness to potential pollinators; another is that conical cells aid pollinator orientation on the flower, either visually or when touched. The recent identification of a mutation at the MIXTA locus of Antirrhinum majus (which blocks the formation of conical petal cells) has allowed us to test this hypothesis. We report the results of field experiments indicating that, where pollinator number limits seed-set, flowers with conical epidermal cells receive more pollinator attention than do those with flat cells. Through the use of double mutants we have examined whether preferences for flowers with conical cells operate through the perception of flavonoid pigments. We have also examined the appearance of flowers with and without conical cells under ultraviolet light to determine whether differences in absorption or reflectance of light at these wavelengths may influence pollinator preference.
TL;DR: It was determined that traits belonging to the same suite of characters were more highly genetically and phenotypically correlated than traits from different suites.
Abstract: A survey of the agricultural and evolutionary literature was undertaken to determine the extent to which phenotypic correlations reflect their genetic counterparts in plants. More than 4000 phenotypic and genetic correlations representing 27 different plant species and over 40 years of research were analysed. In 74 per cent of the comparisons, the arrangement of elements of different magnitudes in genetic and phenotypic correlation matrices was more similar than would be expected by chance alone. In addition, the overall magnitude of correlation was greater in genetic correlation matrices than in phenotypic correlation matrices in 85 per cent of the comparisons. Several studies which reported correlations within and among distinct suites of traits provided the opportunity to evaluate correlations among functionally or developmentally related characters. It was determined from these studies that traits belonging to the same suite of characters were more highly genetically and phenotypically correlated than traits from different suites.
TL;DR: The simulations suggest that RAPD markers may contain substantially higher levels of inherent, but hidden, diversity than that of allozymes, and that estimates of GST using RAPDs should not be significantly biased at the population sizes employed.
Abstract: Thirty-six nuclear-encoded RAPD loci and 20 allozyme loci were studied to compare levels of diversity and differentiation among populations and races of the widespread North American conifer, Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco]. RAPD assays used diploid seed embryo DNA from 22 to 36 trees in each of six populations that sampled the three major races (two populations per race). A comparable allozyme data set for nearby populations was constructed from a published study. RAPDs of organelle origin were excluded by hybridization of blotted RAPD gels with chloroplast and mitochondrial DNA-enriched probes. RAPD and allozyme markers had similar levels of diversity within populations (Hs = 0.22±0.03 and 0.16±0.03, respectively) and differentiation among populations (GsT=0.34±0.07 and 0.29±0.07, respectively). When the allozyme data set was transformed into dominant, biallelic markers to study how RAPDs may bias diversity estimates, resampling studies showed that simulated HS and HT were reduced by half regardless of sample size. Because observed diversity for RAPDs was equivalent to, or higher than, that of allozymes, our simulations suggest that RAPD markers may contain substantially higher levels of inherent, but hidden, diversity. In contrast, the simulations showed that estimates of GST using RAPDs should not be significantly biased at the population sizes we employed.
TL;DR: It was found that, although there are significant consistent differences in the level of asymmetry among different characters within individuals and populations, there was no evidence of significant concordance among individuals or populations.
Abstract: Patterns of correlation of asymmetry values among characters among individuals and populations are equivocal. In general, no significant correlation between characters is found among individuals, yet there are often significant correlations among populations. That is, if an individual is more symmetrical than another for one character, there is no tendency for it to be more symmetrical for any other character, yet if a given population is more symmetrical than another for one character, there is a tendency for it also to be more symmetrical for other characters. However, previous results have been heterogeneous. Here, existing data sets from 50 samples representing 11 invertebrate species are examined for correlation patterns within individuals and populations. Using Kendall’s coefficient of concordance, it was found that, although there are significant consistent differences in the level of asymmetry among different characters within individuals and populations, there was no evidence of significant concordance among individuals or populations. The results indicate that the genetic basis of developmental stability is character, population and taxon specific.
TL;DR: The results suggest that introgression of transgenes from oilseed rape to B. rapa will be slowed down, but not hindered, by the low fitness of second generation hybrids.
Abstract: With the cultivation of genetically modified crops, transgenes may spread by introgression from crops into weedy and wild populations of related species. The likelihood of this depends in part on the fitness of first and later generation hybrids. We here present results on the fitness of F2 and backcross hybrids between oilseed rape (Brassica napus) and weedy B. rapa. Two populations of B. rapa, two varieties of B. napus, and their F1 hybrids were used for controlled crosses, and seed development, survival in the field, pollen viability, pod- and seed-set were estimated for the offspring. Offspring from F2 and backcrosses had a reduced fitness relative to their parents for most of the fitness components and for a combined estimate of fitness, with F2 offspring suffering the lowest fitness. Despite their lower fitness on average, some of the hybrids were as fit as the parents. Significant fitness differences were detected between backcross and F2 offspring from different B. rapa populations, B. napus varieties, and parental plants. Our results suggest that introgression of transgenes from oilseed rape to B. rapa will be slowed down, but not hindered, by the low fitness of second generation hybrids.
TL;DR: The analysis revealed the tetrasomic nature of the inheritance of the chromosomal segment controlling apomixis, which contradicts the usually accepted hypothesis of an allopolyploid origin of apomictic species.
Abstract: Polyploids in Tripsacum, a wild relative of maize, reproduce through the diplosporous type of apomixis, an asexual mode of reproduction through seeds. Diplosporous apomixis involves both the failure of meiosis and the parthenogenetic development of the unreduced gametes, resulting in progenies that are exact genetic copies of the mother plant. Apomixis is believed to be controlled by one single dominant allele, responsible for the whole developmental process. Construction of a linkage map for the chromosome controlling diplosporous apomixis in Tripsacum was carried out in both tetraploid-apomictic and diploid-sexual Tripsacum species using maize restriction fragment length polymorphism (RFLP) probes. A high level of collinearity was observed between the Tripsacum chromosome carrying the control of apomixis and a duplicated segment in the maize genome. In the apomictic tetraploid, there was a strong restriction to recombination, as compared to the corresponding genomic segment in sexual plants and maize. This suggests that apomixis, although inherited as a single Mendelian allele, might really be controlled by a cluster of linked loci. The analysis also revealed the tetrasomic nature of the inheritance of the chromosomal segment controlling apomixis, which contradicts the usually accepted hypothesis of an allopolyploid origin of apomictic species. The implications of these data for the transfer of apomixis into cultivated crops are discussed, and a new approach to studying the genetics of apomixis, based on comparative mapping, is proposed.
TL;DR: It is suggested here that intraspecific variation may be a widespread phenomena, further examination of which could contribute considerably to the authors' understanding of sperm evolution.
Abstract: Interspecific variation in quantitative characters of sperm is considerable and can sometimes be related to aspects of sperm competition. In contrast, continuous intraspecific variation is often ignored; for example, many studies concentrate on measuring a species-typical sperm size. However, statistically significant variation amongst males in a variety of sperm size characters, particularly total length and a number of head characters, has been reported in at least 13 species. Furthermore, sperm size has been shown to influence the outcomes of processes involved in sperm competition in two species, with larger sperm being more successful in both cases. It is suggested here that intraspecific variation may be a widespread phenomena, further examination of which could contribute considerably to our understanding of sperm evolution. Size differences between the sperm of competing males could be used in controlled matings to investigate the fates of sperm within female tracts and storage organs, but care should be taken that size characters may not be neutral markers. Furthermore, the genetic determination of sperm size is considerable (h2=0.56–0.92 for total length and a number of head characters) and determinants are at least sometimes sex-linked, with the result that particular care must go into experimental design if genetic effects are to be fully elucidated. Studies combining genetical and behavioural effects of intraspecific variation in sperm morphology should be particularly rewarding.
TL;DR: It is reported that all-female lineages are associated with low egg-hatching rates and that the trait is cured by antibiotic treatment, and it is proposed that this sex ratio bias is associated with a maternally inherited bacterium that kills males.
Abstract: Females of the butterfly Acraea encedon produce either entirely female offspring or males and females in an almost 1:1 sex ratio. The sex ratio produced is maternally inherited and was previously attributed to sex chromosome meiotic drive. We report that all-female lineages are associated with low egg-hatching rates and that the trait is cured by antibiotic treatment. We thus reject the hypothesis that this sex ratio bias is caused by a meiotically driven sex chromosome and, instead, propose that it is associated with a maternally inherited bacterium that kills males.
TL;DR: The analysis of mtDNA variation in L. calcarifer confirms that population genetic models can detect population structure of not only evolutionary significance but also of demographic significance, and demonstrates the power of inferring such structure from hypervariable markers, which correspond to small effective population sizes.
Abstract: Patterns of population subdivision and the relationship between gene flow and geographical distance in the tropical estuarine fish Lares calcarifer (Centropomidae) were investigated using mtDNA control region sequences. Sixty-three putative haplotypes were resolved from a total of 270 individuals from nine localities within three geographical regions spanning the north Australian coastline. Despite a continuous estuarine distribution throughout the sampled range, no haplotypes were shared among regions. However, within regions, common haplotypes were often shared among localities. Both sequence-based (average Phi(ST)=0.328) and haplotype-based (average Phi(ST)=0.182) population subdivision analyses indicated strong geographical structuring. Depending on the method of calculation, geographical distance explained either 79 per cent (sequence-based) or 23 per cent (haplotype-based) of the variation in mitochondrial gene flow. Such relationships suggest that genetic differentiation of L. calcarifer has been generated via isolation-by-distance, possibly in a stepping-stone fashion. This pattern of genetic structure is concordant with expectations based on the life history of L. calcarifer and direct studies of its dispersal patterns. Mitochondrial DNA variation, although generally in agreement with patterns of allozyme variation, detected population subdivision at smaller spatial scales. Our analysis of mtDNA variation in L. calcarifer confirms that population genetic models can detect population structure of not only evolutionary significance but also of demographic significance. Further, it demonstrates the power of inferring such structure from hypervariable markers, which correspond to small effective population sizes.
TL;DR: Dominance relationships were studied for 249 out of 276 possible pair-wise combinations between 24 S-alleles of Brassica campestris that had been isolated from two natural populations from Turkey and Japan.
Abstract: Dominance relationships were studied for 249 out of 276 possible pair-wise combinations between 24 S-alleles of Brassica campestris that had been isolated from two natural populations from Turkey and Japan. Each F1 hybrid was test-crossed reciprocally against its respective parental S-homozygotes to determine the dominance relationships between the pair of S-alleles it contained. The 24 S-alleles were classified into two groups on the stigma side and three groups on the pollen side. In the stigma, codominance occurred frequently, and dominance or recessiveness seemed to appear according to the combination of S-alleles. In the pollen, codominance was less frequent, and there seemed to be a certain hierarchy of the dominance relationships as a whole, although dominance appeared with certain specific combinations of S-alleles. Interactions among 24 S-alleles were different in the stigma and in the pollen. Independent weakening of S-alleles was found between 20 pairs in the pollen, but only two in the stigma. This interaction seems to be correlated with recessiveness of S-alleles.
TL;DR: It is concluded that although some of the case studies are tantalizing, particularly those associated with Wolbachia, the role of selfish genetic elements in speciation remains unproven.
Abstract: This review concerns the importance of selfish genetic elements (SGEs) in speciation. We assess the importance of medea genes, meiotic drive elements, transposable elements and the bacterium Wolbachia in the creation of postzygotic isolation. Although all of these elements can contribute to postzygotic isolation, their contribution will often disappear if there is gene flow between the populations. Further, there is the possibility that incompatibilities produced by SGEs may lessen over time. We conclude that although some of the case studies are tantalizing, particularly those associated with Wolbachia, the role of selfish genetic elements in speciation remains unproven.
TL;DR: Examining inbreeding depression in the cricket Gryllus firmus, an inhabitant of ephemeral habitats such as sand dunes, finds that growth rate and fecundity show very high levels of inbreeding Depression.
Abstract: Inbreeding depression can cause substantial decreases in trait values. For colonizing organisms, which are likely to suffer relatively high levels of inbreeding at frequent intervals, inbreeding depression could significantly influence the evolution of traits and their genetic architecture. In the present paper, I examine inbreeding depression in the cricket Gryllus firmus, an inhabitant of ephemeral habitats such as sand dunes. Two questions are addressed: (i) do some traits show high levels of inbreeding depression; and (ii) do life history traits show higher levels of inbreeding depression than morphological traits? Growth rate and fecundity show very high levels of inbreeding depression (8–16% for the former, 14% for the latter). These rates of depression could potentially have significant effects on the survival probability of newly established populations. Overall, life history traits show significantly higher inbreeding depression (5.3%) than morphological traits (0.4%).
TL;DR: In this paper, the authors compared the fitness of three weedy B. rapa populations and three varieties of B. napus, and found that the hybrid offspring were intermediate to their parents, and significantly more fit than the non-hybrid offspring.
Abstract: As part of an ongoing study of the hybridization biology of cultivated oilseed rape (Brassica napus) and weedy B. rapa, we studied the fitness of hybrids between three weedy B. rapa populations and three varieties of B. napus. Reciprocal pollinations were performed, and the resulting offspring were scored for seed development, survival in the field, pod- and seed-set. Seeds from heterospecific crosses developed within pods in lower proportions than seeds from conspecific crosses. Hybrid offspring survived in the field as frequently as conspecific offspring, and produced many more pods that contained fewer seeds. Combining the fitness components into a multiplicative estimate, we found the hybrids to be intermediate to their parents, and significantly more fit than B. rapa. Significant genotypic differences were detected between offspring produced by different parental plants, populations and varieties for some of the fitness components scored. Our results on hybrid fitness are discussed with respect to the possibility that transgenes in oilseed rape may introgress spontaneously into weedy B. rapa.
TL;DR: Overall, the ostracods provide an excellent system in which to study the evolution of reproductive modes and molecular data suggest that some clonal lineages are surprisingly old.
Abstract: Asexual reproduction has evolved repeatedly in nonmarine ostracods and takes a variety of forms from ancient asexuals to species in which sexual and asexual lineages coexist. Clonal diversity is highly variable. There is evidence that some of this diversity is maintained by ecological differentiation. Hybridization between asexual females and males, of the same or related species, contributes to clonal diversity. Molecular data suggest that some clonal lineages are surprisingly old (more than 5 Myr). In the ancient asexual Darwinula stevensoni, from a lineage that has apparently been without sex for more than 100 Myr, a remarkable lack of sequence variation in ITS1 may be explained by occasional automixis, gene conversion or somatic recombination, or by efficient DNA repair. Overall, the ostracods provide an excellent system in which to study the evolution of reproductive modes.
TL;DR: It is concluded that strong microclimatic natural selection overrides migration in Drosophila at this microsite, and the multivariate fitness complex analysed in D. melanogaster included oviposition temperature preferences, viability and longevity changes, caused by short-term and lifetime temperature treatments, and resistance to drought stress at different temperatures.
Abstract: Temperature and humidity are among the most important environmental factors affecting insect adaptive strategies and evolution. Here, we report multiple adaptive differences between Drosophila melanogaster isofemale lines derived from the opposite slopes of Lower Nahal Oren canyon at Mount Carmel, Israel. The slopes are separated by 100 m at the bottom and 400 m at the top, and contrast sharply in physical and biotic factors. The multivariate fitness complex analysed in D. melanogaster included oviposition temperature preferences, viability and longevity changes, caused by short-term and lifetime temperature treatments, and resistance to drought stress at different temperatures. Some of these measures were obtained for the sibling species, D. simulans, and gave results that paralleled those of D. melanogaster. We conclude that strong microclimatic natural selection overrides migration in Drosophila at this microsite.
TL;DR: Two single-locus VNTR DNA fingerprint markers are used in conjunction with a maximum likelihood method to infer relatedness among pairs of individuals in a captive population of Pacific chinook salmon to estimate heritability.
Abstract: Heritability is usually estimated with individuals of known relatedness generated using a controlled breeding programme or through response to selection. In this paper, we use two single-locus VNTR DNA fingerprint markers in conjunction with a maximum likelihood method to infer relatedness among pairs of individuals in a captive population of Pacific chinook salmon (Oncorhynchus tshawytscha). Patterns of relatedness inferred from the two DNA fingerprint markers were used to estimate heritability for, and genetic correlations among, several economically and ecologically important traits (weight, length, flesh colour and precocious male maturation). Heritabilities ranged from 0.20 for weight, 0.38 for length, 0.67 for precocious male maturation (‘jacking’) to 0.76 for flesh colour, which are in good agreement with estimates for salmonids generated using classical quantitative genetic methods. This molecular marker-based method allows for the estimation of heritability in wild, long-lived species not easily manipulated for study using controlled breeding programmes.
TL;DR: Nuclear and mtDNA data both indicate that bumble bees from the Canaries and Madeira do not share a common colonization history, and suggest that haplotypes ancestral with respect to B. terrestris occur on the Canary Islands, whereas derived haplotypes were found on the European continent.
Abstract: The bumble bee Bombus terrestris L. is a geographically variable species with a wide distribution in Europe, the near East, northern Africa, Mediterranean islands, the Canary Islands and Madeira. Based on morphological and coat colour pattern differences, the bumble bee populations of the Canary Islands and Madeira are currently treated as separate species, B. canariensis and B. maderensis, respectively. To analyse the phylogeographical associations of these bees with continental B. terrestris, one population each from four islands of the Canaries and one population from Madeira were studied. Genetic variability was assessed at nine microsatellite loci and a fragment of the mitochondrial gene cytochrome b. Genetic differentiation among islands, and between islands and the continent, was extensive. A NJ-tree based on microsatellites strongly supported the distinctness of the Canary Island populations, whereas the Madeira sample was genetically more similar to the continental populations of B. terrestris from Europe. MtDNA sequence data were in good agreement with nuclear markers. They suggest that haplotypes ancestral with respect to B. lucorum occur on the Canary Islands, whereas derived haplotypes were found on the European continent. The Madeira population shares the most common haplotype of continental B. terrestris. Nuclear and mtDNA data both indicate that bumble bees from the Canaries and Madeira do not share a common colonization history.
TL;DR: Levels of genetic subdivision between geographical regions in Europe were assessed using the KST statistic and revealed patterns indicative of the postglacial history of the European meadow grasshopper Chorthippus parallelus.
Abstract: A 300 bp portion of the mitochondrial cytochrome oxidase subunit I (COI) gene was used to investigate the intraspecific genetic structure of the European meadow grasshopper Chorthippus parallelus. Levels of genetic subdivision between geographical regions in Europe were assessed using the KST statistic and revealed patterns indicative of the postglacial history of this organism. Northern European populations are shown to share a very recent common ancestor with, and hence originate from, a Balkan expansion. Genetic distances between these areas and both southern Italy and central Spain are shown to be approximately equal, even though only the Spanish form is recognized as a distinct subspecies. Distance estimates indicate that the common ancestor of the two subspecies lived more than one glacial cycle ago, and the refugial populations have probably been diverging in isolation for five or six glacial cycles (approximately equal to 550,000 years). Comparisons are made to the analysis of a similar data set using an anonymous nuclear DNA marker (Cooper et al., 1995). Although the structure revealed is generally very similar, differences in the location of transition zones between different genetic forms of C. parallelus may reflect the different historical assortment of these molecules.
TL;DR: The data indicate that the population structure within the Atlantic is more complex than the discrete northern and southern stocks proposed by ICES and support present management based on distinct Atlantic and Mediterranean populations.
Abstract: Allozyme analysis of tissue samples of 910 European hake Merluccius merluccius from six North Atlantic Ocean and four Mediterranean Sea locations revealed polymorphism at 21 out of 34 protein-coding loci examined. A major subdivision between Atlantic and Mediterranean collections (mean Nei's genetic distance within regions 0.005; between regions 0.014) was particularly evident from allelic differences at the GAPDH-1* and GR-2* loci. Further population subdivision was indicated within both regions, and gene flow from adjacent Atlantic to proximal Mediterranean populations was suggested by allele frequencies and relevant oceanographic and geological information. The data support present management based on distinct Atlantic and Mediterranean populations and indicate that the population structure within the Atlantic is more complex than the discrete northern and southern stocks proposed by ICES.
TL;DR: The results suggest that the genotype–environment interactions for flowering time are controlled by many minor genes, whose effects are below the detection limit in most mapping experiments.
Abstract: Many plant traits are phenotypically plastic in response to resource levels that vary continuously among environments. To be able to predict phenotypes in new environments, it is useful to model reaction norms as functions, rather than as a collection of discrete character states. Flowering date and rosette leaf number were measured in 100 recombinant inbred lines of Arabidopsis thaliana, grown on a gradient of light intensity. The results show that there is genetic variation among the recombinant inbred lines for parameters of the reaction norm functions. Genetic variances for leaf number and flowering date are highest under low light conditions. Underlying quantitative trait loci (QTLs) affecting the shape of the reaction norm functions were mapped by modifying Haley & Knott (1992) regressions to include polynomial effects of the environment. Quantitative trait loci of large effect were generally insensitive to the resource gradient. Seven QTLs affecting flowering date and eight QTLs for rosette leaf number were identified, of which only two had significant effects on the linear and quadratic components of the reaction norm function. These results suggest that the genotype–environment interactions for flowering time are controlled by many minor genes, whose effects are below the detection limit in most mapping experiments.