Heterosis increases the effective migration rate

Individuals coming from the same subpopulation are more likely to share deleterious mutations at any given locus than hybrids formed between parents from different populations. Offspring of migrants therefore may experience heterosis and have higher fitness than resident individuals. This will, in turn, result in the immigrant alleles being present in higher frequencies than predicted from neutral expectations and thus a higher effective migration rate. In this paper we derive a formula to calculate the effective migration rate in the presence of heterosis. It is shown that the effect of heterosis on the migration rate can be substantial when fitness reduction within local populations is severe. The effect will be more pronounced in species with relatively short map lengths. Furthermore the heterosis effect will be highly variable throughout the genome, with the largest effect seen near selected genes and in regions of high gene density.     

作物杂种后代基因型值和杂种优势的预测方法

本文提出了利用作物亲本和F_1预测杂种后代基因型值和杂种优势的统计分析方法.该方法运用加性-显性遗传模型,分析双列杂交试验资料,用MINQUE(1)法估算方差分量以及预测遗传效应值.由加性和显性效应预测值可进一步预测F_1,F_2,BC_1,BC_2,等不同世代的基因型值,在预测F_1群体平均优势和群体超亲优势的基础上,可以推导出其它各世代的杂种优势.提出了预测杂种后代保持超亲优势世代数的简单公式,根据杂交组合F_1群体平均优势和双亲相对遗传差异,便可预测该组合能在生产上直接利用的世代数.以棉花六个品种完全双列杂交试验资料为例,分析了各组合F_1和F_2的基因型值、超亲优势和保持5％超亲优势的世代数.     

A genomic selection component analysis characterizes migration‐selection balance

The genetic differentiation of populations in response to local selection pressures has long been studied by evolutionary biologists, but key details about the process remain obscure. How rapidly can local adaptation evolve, how extensive is the process across the genome, and how strong are the opposing forces of natural selection and gene flow? Here, we combine direct measurement of survival and reproduction with whole‐genome genotyping of a plant species (Mimulus guttatus) that has recently invaded a novel habitat (the Quarry population). We renovate the classic selection component method to accommodate genomic data and observe selection at SNPs throughout the genome. SNPs showing viability selection in Quarry exhibit elevated divergence from neighboring populations relative to neutral SNPs. We also find that nonsignificant SNPs exhibit a subtle, but still significant, change in allele frequency toward neighboring populations, a predicted effect of gene flow. Given that the Quarry population is most probably only 30–40 generations old, the alleles conferring local advantage are almost certainly older than the population itself. Thus, local adaptation owes to the recruitment of standing genetic variation.     

Rapid evolutionary responses in a translocated population of intertidal snail (<Emphasis Type="Italic">Bembicium vittatum</Emphasis>) utilise variation from different source populations

The artificial movement of individuals between populations (translocation) can be an effective way to increase genetic diversity within populations, but few studies have undertaken long term genetic monitoring to determine if variation introduced by translocation is maintained over many generations or whether it can be used to adapt to local conditions. Here, we report on the changes in morphological and molecular variation over a 12-year period in a population of an intertidal littorine snail (Bembicium vittatum) that was created by mixing individuals from three geographically disjunct populations. These source populations differ genetically in shell shape and in allele frequency at several allozyme loci. We found that the translocated population had higher allozyme diversity than any of the source populations and that this pattern was maintained over multiple generations. Variation in shell shape also increased, but this declined over time as shells became taller. Some allozyme loci also showed significant changes in frequency over time. These changes were not consistently towards the genetic makeup of a single source population, and in the case of shell shape, were towards a phenotype that was most suited to the local environment. Our results suggest that genetic variation introduced into a population by translocation can be rapidly incorporated and used to adapt to local conditions without domination by a single source population’s genome. However, more studies are needed before generalisations on the benefits of mixing individuals from disjunct populations can be made.     

Outbreeding effects in an inbreeding insect,Cimex lectularius

In some species, populations with few founding individuals can be resilient to extreme inbreeding. Inbreeding seems to be the norm in the common bed bug, Cimex lectularius, a flightless insect that, nevertheless, can reach large deme sizes and persist successfully. However, bed bugs can also be dispersed passively by humans, exposing inbred populations to gene flow from genetically distant populations. The introduction of genetic variation through this outbreeding could lead to increased fitness (heterosis) or be costly by causing a loss of local adaptation or exposing genetic incompatibility between populations (outbreeding depression). Here, we addressed how inbreeding within demes and outbreeding between distant populations impact fitness over two generations in this re‐emerging public health pest. We compared fitness traits of families that were inbred (mimicking reproduction following a founder event) or outbred (mimicking reproduction following a gene flow event). We found that outbreeding led to increased starvation resistance compared to inbred families, but this benefit was lost after two generations of outbreeding. No other fitness benefits of outbreeding were observed in either generation, including no differences in fecundity between the two treatments. Resilience to inbreeding is likely to result from the history of small founder events in the bed bug. Outbreeding benefits may only be detectable under stress and when heterozygosity is maximized without disruption of coadaptation. We discuss the consequences of these results both in terms of inbreeding and outbreeding in populations with genetic and spatial structuring, as well as for the recent resurgence of bed bug populations.     

Heterosis and outbreeding depression in descendants of natural immigrants to an inbred population of song sparrows (Melospiza melodia)

We studied heterosis and outbreeding depression among immigrants and their descendants in a population of song sparrows on Mandarte Island, Canada. Using data spanning 19 generations, we compared survival, seasonal reproductive success, and lifetime reproductive success of immigrants, natives (birds with resident-hatched parents and grandparents), and their offspring (F1s, birds with an immigrant and a native parent, and F2s, birds with an immigrant grandparent and resident-hatched grandparent in each of their maternal and paternal lines). Lifetime reproductive success of immigrants was no worse than that of natives, but other measures of performance differed in several ways. Immigrant females laid later and showed a tendency to lay fewer clutches, but had relatively high success raising offspring per egg produced. The few immigrant males survived well but were less likely to breed than native males of the same age that were alive in the same year. Female F1s laid earlier than expected based on the average for immigrant and native females, and adult male F1s were more likely to breed than expected based on the average for immigrant and native males. The performance differences between immigrant and native females and between F1s and the average of immigrants and natives are consistent with the hypothesis that immigrants were disadvantaged by a lack of site experience and that immigrant offspring benefited from heterosis. However, we could not exclude the possibility that immigrants had a different strategy for optimizing reproductive success or that they experienced ecological compensation for life-history parameters. For example, the offspring of immigrants may have survived well because immigrants laid later and produced fewer clutches, thereby raising offspring during a period of milder climatic conditions. Although sample sizes were small, we found large performance differences between F1s and F2s, which suggested that either heterosis was associated with epistasis in F1s, that F2s experienced outbreeding depression, or that both phenomena occurred. These findings indicate that the performance of dispersers may be affected more by fine-scale genetic differentiation than previously assumed in this and comparable systems.     

Do assortative mating and immigrant inviability help maintain population genetic structuring of an herbivore on a crop and a wild relative?

Population genetic structuring is common among herbivorous insects and frequently is associated with divergent host plants, such as crops and their wild relatives. Previous studies showed population genetic structuring in corn leafhopper Dulbulus maidis in Mexico, such that the species consists of two sympatric, host plant-associated populations: an abundant and widespread "pestiferous” population on maize (Zea mays mays), and a small and localized "wild" population on perennial teosinte (Zea diploperennis). a maize wild relative with a limited distribution. This study addressed whether assortative mating and immigrant inviability mediate genetic structuring of corn leafliopper by comparing the mating and reproductive successes of pestiferous and wild females that colonize their nonassociated host plants against the successes of females colonizing their associated host plants. Assortative mating was assessed by comparing mating frequencies and premating and mating times among females of each population on each host plant: immigrant inviability was assessed by comparing, across two generations, the fecundity, survival, development time, sex ratio, and population growth rate among leafhopper populations and host plants. Our results showed that on maize, and compared to resident, pestiferous females, wild females were more likely to mate, and greater proportions of their offspring survived to adult stage and were daughters;consequently, the per-generation population growth rate on maize was greater for immigrant, wild leafhoppers compared to resident, pestiferous leafhoppers. Our results suggested that wild leafhoppers emigrating to maize have a fitness advantage over resident, pestiferous leafhoppers, while immigrant pestiferous and resident wild leafhoppers on teosinte have similar fitnesses.     

Genome-Wide High-Resolution Mapping by Recurrent Intermating Using Arabidopsis Thaliana as a Model

We demonstrate a method for developing populations suitable for genome-wide high-resolution genetic linkage mapping, by recurrent intermating among F(2) individuals derived from crosses between homozygous parents. Comparison of intermated progenies to F(2) and ``recombinant inbred' (RI) populations from the same pedigree corroborate theoretical expectations that progenies intermated for four generations harbor about threefold more information for estimating recombination fraction between closely linked markers than either RI-selfed or F(2) individuals (which are, in fact, equivalent in this regard). Although intermated populations are heterozygous, homozygous ``intermated recombinant inbred' (IRI) populations can readily be generated, combining additional information afforded by intermating with the permanence of RI populations. Intermated populations permit fine-mapping of genetic markers throughout a genome, helping to bridge the gap between genetic map resolution and the DNA-carrying capacity of modern cloning vectors, thus facilitating merger of genetic and physical maps. Intermating can also facilitate high-resolution mapping of genes and QTLs, accelerating map-based cloning. Finally, intermated populations will facilitate investigation of other fundamental genetic questions requiring a genome-wide high-resolution analysis, such as comparative mapping of distantly related species, and the genetic basis of heterosis.     

Population-based resequencing analysis of improved wheat germplasm at wheat leaf rust resistance locus Lr21

Little is known about the genetic control of heterosis in the complex polyploid crop species oilseed rape (Brassica napus L.). In this study, two large doubled-haploid (DH) mapping populations and two corresponding sets of backcrossed test hybrids (THs) were analysed in controlled greenhouse experiments and extensive field trials for seedling biomass and yield performance traits, respectively. Genetic maps from the two populations, aligned with the help of common simple sequence repeat markers, were used to localise and compare quantitative trait loci (QTL) related to the expression of heterosis for seedling developmental traits, plant height at flowering, thousand seed mass, seeds per silique, siliques per unit area and seed yield. QTL were mapped using data from the respective DH populations, their corresponding TH populations and from mid-parent heterosis (MPH) data, allowing additive and dominance effects along with digenic epistatic interactions to be estimated. A number of genome regions containing numerous heterosis-related QTL involved in different traits and at different developmental stages were identified at corresponding map positions in the two populations. The co-localisation of per se QTL from the DH population datasets with heterosis-related QTL from the MPH data could indicate regulatory loci that may also contribute to fixed heterosis in the highly duplicated B. napus genome. Given the key role of epistatic interactions in the expression of heterosis in oilseed rape, these QTL hotspots might harbour genes involved in regulation of heterosis (including fixed heterosis) for different traits throughout the plant life cycle, including a significant overall influence on heterosis for seed yield.     

Modeling problems in conservation genetics using captive Drosophila populations: Improvement of reproductive fitness due to immigration of one individual into small partially inbred populations

Immigration into small isolated captive and wild populations is recommended to alleviate inbreeding depression. The effects on reproductive fitness of introducing one immigrant into 10 small partially inbred captive populations of D. melanogaster were evaluated. The relative reproductive fitness of the immigrant populations (0.628) was approximately double that of the isolated populations (0.294) and about halfway between the isolated populations and the outbred base population (1.00). Every replicate population increased in fitness following the introduction of an immigrant. The improvements in reproductive fitness shown by the immigrant populations were not due to F₁ hybrid vigor, as the experimental populations underwent three generations of random mating prior to the fitness tests. These results indicate substantial benefits can be gained by the translocation of as few as a single animal between small, partially inbred populations. © 1992 Wiley-Liss, Inc.     

Source population characteristics affect heterosis following genetic rescue of fragmented plant populations

Understanding the relative importance of heterosis and outbreeding depression over multiple generations is a key question in evolutionary biology and is essential for identifying appropriate genetic sources for population and ecosystem restoration. Here we use 2455 experimental crosses between 12 population pairs of the rare perennial plant Rutidosis leptorrhynchoides (Asteraceae) to investigate the multi-generational (F₁, F₂, F₃) fitness outcomes of inter-population hybridization. We detected no evidence of outbreeding depression, with inter-population hybrids and backcrosses showing either similar fitness or significant heterosis for fitness components across the three generations. Variation in heterosis among population pairs was best explained by characteristics of the foreign source or home population, and was greatest when the source population was large, with high genetic diversity and low inbreeding, and the home population was small and inbred. Our results indicate that the primary consideration for maximizing progeny fitness following population augmentation or restoration is the use of seed from large, genetically diverse populations.     

Genetic load,inbreeding depression and heterosis in an age‐structured metapopulation

In a metapopulation, the process of recurrent local extinction and recolonization gives rise to an age structure among demes. Recently established demes will tend to differ from older demes in terms of the levels of genetic diversity found within them and the way this diversity is distributed among demes in the same and different ages. The effects of population turnover on average levels of genetic diversity among demes in a metapopulation have been the focus of much attention, both for neutral and nonneutral loci, but much less is known about the distribution of nonneutral genetic diversity among demes of different ages. In this paper, we used computer simulations to study the distribution of genetic load, inbreeding depression and heterosis in an age‐structured metapopulation. We found that, for mildly deleterious mutations, within‐deme inbreeding depression increased, whereas heterosis and genetic load decreased with deme age following severe colonization bottlenecks. In contrast, recessive lethal alleles tended to be purged during colonization, with older populations showing higher genetic load and higher within‐deme inbreeding depression. Heterosis caused by recessive lethal alleles and resulting from gene flow among different demes tended to be greatest for young demes, because the mutations responsible tended to be purged in the first few generations after colonization, but its effects increased again as populations grow older as a result of immigration. Our results point to a need for estimates of genetic diversity, genetic load, within‐deme inbreeding depression and heterosis in demes of different age classes separately.     

Heterosis in an isolated, effectively small, and self-fertilizing population of the flowering plant Leavenworthia alabamica

Mildly deleterious mutations are thought to play a major role in the extinction of natural populations, especially those that are small, isolated, or inbred. Self-fertilization should reduce the effective size of populations and simultaneously reduce migration between populations. A history of self-fertilization should therefore cause a population to harbor a substantial "local drift load" caused by the fixation of mildly deleterious mutations. This hypothesis was tested in Leavenworthia alabamica, which contains large, self-incompatible populations and smaller self-compatible populations with adaptations for self-fertilization. The fitness of offspring from within- and between-population crosses was compared to quantify heterosis caused by the masking of deleterious alleles in the heterozygous state. Little heterosis was observed in crosses between five large, self-incompatible populations and two of the three small, self-fertilizing populations of L. alabamica. However, the most geographically isolated and genetically divergent self-fertilizing population (Tuscumbia) exhibited a 110.2% increase in germination and a 73.6% increase in fitness, which is consistent with a sizeable local drift load. The finding of substantial heterosis for fitness supports the idea that small effective size, reproductive isolation, and self-fertilization can make populations particularly vulnerable to mutation accumulation.     

Genomic selection of purebred animals for crossbred performance in the presence of dominant gene action

Background

Genomic selection is an appealing method to select purebreds for crossbred performance. In the case of crossbred records, single nucleotide polymorphism (SNP) effects can be estimated using an additive model or a breed-specific allele model. In most studies, additive gene action is assumed. However, dominance is the likely genetic basis of heterosis. Advantages of incorporating dominance in genomic selection were investigated in a two-way crossbreeding program for a trait with different magnitudes of dominance. Training was carried out only once in the simulation.

Results

When the dominance variance and heterosis were large and overdominance was present, a dominance model including both additive and dominance SNP effects gave substantially greater cumulative response to selection than the additive model. Extra response was the result of an increase in heterosis but at a cost of reduced purebred performance. When the dominance variance and heterosis were realistic but with overdominance, the advantage of the dominance model decreased but was still significant. When overdominance was absent, the dominance model was slightly favored over the additive model, but the difference in response between the models increased as the number of quantitative trait loci increased. This reveals the importance of exploiting dominance even in the absence of overdominance. When there was no dominance, response to selection for the dominance model was as high as for the additive model, indicating robustness of the dominance model. The breed-specific allele model was inferior to the dominance model in all cases and to the additive model except when the dominance variance and heterosis were large and with overdominance. However, the advantage of the dominance model over the breed-specific allele model may decrease as differences in linkage disequilibrium between the breeds increase. Retraining is expected to reduce the advantage of the dominance model over the alternatives, because in general, the advantage becomes important only after five or six generations post-training.

Conclusion

Under dominance and without retraining, genomic selection based on the dominance model is superior to the additive model and the breed-specific allele model to maximize crossbred performance through purebred selection.     

Spatiotemporal variation in local adaptation of a specialist insect herbivore to its long‐lived host plant

Local adaptation of interacting species to one another indicates geographically variable reciprocal selection. This process of adaptation is central in the organization and maintenance of genetic variation across populations. Given that the strength of selection and responses to it often vary in time and space, the strength of local adaptation should in theory vary between generations and among populations. However, such spatiotemporal variation has rarely been explicitly demonstrated in nature and local adaptation is commonly considered to be relatively static. We report persistent local adaptation of the short‐lived herbivore Abrostola asclepiadis to its long‐lived host plant Vincetoxicum hirundinaria over three successive generations in two studied populations and considerable temporal variation in local adaptation in six populations supporting the geographic mosaic theory. The observed variation in local adaptation among populations was best explained by geographic distance and population isolation, suggesting that gene flow reduces local adaptation. Changes in herbivore population size did not conclusively explain temporal variation in local adaptation. Our results also imply that short‐term studies are likely to capture only a part of the existing variation in local adaptation.     

Heterosis for viability,fecundity, and male fertility in Drosophila melanogaster: comparison of mutational and standing variation.

If genetic variation for fitness traits in natural populations ("standing" variation) is maintained by recurrent mutation, then quantitative-genetic properties of standing variation should resemble those of newly arisen mutations. One well-known property of standing variation for fitness traits is inbreeding depression, with its converse of heterosis or hybrid vigor. We measured heterosis for three fitness traits, pre-adult viability, female fecundity, and male fertility, among a set of inbred Drosophilia melanogaster lines recently derived from the wild, and also among a set of lines that had been allowed to accumulate spontaneous mutations for over 200 generations. The inbred lines but not the mutation-accumulation (MA) lines showed heterosis for pre-adult viability. Both sets of lines showed heterosis for female fecundity, but heterosis for male fertility was weak or absent. Crosses among a subset of the MA lines showed that they were strongly differentiated for male fertility, with the differences inherited in autosomal fashion; the absence of heterosis for male fertility among the MA lines was therefore not caused by an absence of mutations affecting this trait. Crosses among the inbred lines also gave some, albeit equivocal, evidence for male fertility variation. The contrast between the results for female fecundity and those for male fertility suggests that mutations affecting different fitness traits may differ in their average dominance properties, and that such differences may be reflected in properties of standing variation. The strong differentiation among the MA lines in male fertility further suggests that mutations affecting this trait occur at a high rate.     

Discriminating the impact of recent human mediated stock transfer from historical gene flow on genetic structure of European grayling Thymallus thymallus L.

Microsatellite markers were first used to partition individuals of European grayling Thymallus thymallus , from the Danube, Rhine and Main, and Elbe drainage systems into subpopulations and to estimate individual immigrant ancestries over the last few generations. Subsequently, the studied populations were 'purged' from recent immigrants and the proportions of evolutionary lineages within the 'purged' populations were re-estimated by applying mtDNA markers. The results confirmed a high level of admixture of the divergent mtDNA lineages ( i.e. natural secondary contact) in populations sampled at the contact zones of the drainages. In addition, a substantial amount of introgression was observed for several populations that were known to be affected by stocking of European grayling from different origins.     

The Yank of Dobzhansky’s Bequest

Dobzhansky studied mechanisms of balancing selection using systems of inversions in Drosophila and he soon found that changes in inversion frequencies along generations in experimental populations conformed to the expectation for a simple model of heterosis. However, other more complex modes of selection, like rare male advantage, were later found to affect the maintenance of inversion polymorphisms. Here we show that a more realistic (and complex) model than heterosis—integrating all known fitness component estimates obtained in independent experiments for the ST/CH system of inversions in Drosophila pseudoobscura—not only conforms to but actually also predicts the inversion frequencies. This concludes this line of work and points to other selection mechanisms than heterosis that were also considered by Dobzhansky—frequency- and sex-dependent selection—as potential mechanisms of balancing selection responsible for the maintenance of the inversion polymorphisms in Drosophila.     

Prediction of heterosis in crosses between inbred lines of Drosophila melanogaster

Summary The aim of the experiment was to determine if the estimated genetic distance between two populations could be used to predict the amount of heterosis that would occur when they were crossed. Eight lines of known relatedness to each other were produced by eight generations of sib mating and sub-lining. This produced lines that varied in coefficient of coancestry from zero to 0.78. Fourteen reciprocal crosses of these lines were used to measure heterosis for larval viability and adult fecundity. Gene frequencies at six polymorphic enzyme loci were used to estimate the genetic distances between lines, which were then compared with the known degrees of coancestry. The estimated genetic differences were poorly correlated with the known coancestry coefficients (r=0.4), possibly due to the small number of loci typed. Also genetic distances were only about 1/3 of what was expected. Selection acting on blocks of genes linked to the enzyme loci probably prevented the expected increase in homozygosity. Coancestry coefficient was correlated with heterosis (r=0.44–0.71). This level of correlation implied differences in heterosis among parent lines with the same level of coancestry. This variability is expected if a small number of loci explain most of the heterosis. The average level of heterosis was less than expected after eight generations of sib mating. This is most likely due to selection opposing the increase in homozygosity caused by inbreeding. The combination of these two imperfect correlations resulted in no significant correlation between genetic distance estimated from markers and heterosis.     

Local adaptation to higher temperatures reduces immigration success of genotypes from a warmer region in the water flea Daphnia

There is growing awareness that microevolutionary dynamics may alter ecological processes. Rising temperatures under global change are expected to open windows for establishment of species and genotypes from warmer regions. Yet, microevolutionary tracking of temperature change by local populations may reduce establishment success of these immigrants. We exposed a UK population of the water flea, Daphnia magna , to two temperature regimes during a 1.5-year experimental evolution trial, and subsequently compared competitive strength of non-warm-adapted and warm-adapted D. magna in competition with French genotypes. Our results indicate that local microevolutionary responses to global warming may reduce establishment success of immigrant genotypes that are preadapted to warmer climate. Simulation modeling shows that microevolution results in a reduced likelihood and speed of displacement of local populations by immigrant genotypes under realistic immigrant/resident ratios. We conclude that local evolutionary dynamics may shift the relative impact of local and regional processes in response to global change.