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Results for “Genetic Introgression”

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PubMed1999

Hybrid zones and the genetic architecture of a barrier to gene flow between two sunflower species.

Genetic analyses of reproductive barriers represent one of the few methods by which theories of speciation can be tested. However, genetic study is often restricted to model organisms that have short generation times and are easily propagated in the laboratory. Replicate hybrid zones with a diversity of recombinant genotypes of varying age offer increased resolution for genetic mapping experiments and expand the pool of organisms amenable to genetic study. Using 88 markers distributed across 17 chromosomes, we analyze the introgression of chromosomal segments of Helianthus petiolaris into H. annuus in three natural hybrid zones. Introgression was significantly reduced relative to neutral expectations for 26 chromosomal segments, suggesting that each segment contains one or more factors that contribute to isolation. Pollen sterility is significantly associated with 16 of these 26 segments, providing a straightforward explanation of why this subset of blocks is disadvantageous in hybrids. In addition, comparison of rates of introgression across colinear vs. rearranged chromosomes indicates that close to 50% of the barrier to introgression is due to chromosomal rearrangements. These results demonstrate the utility of hybrid zones for identifying factors contributing to isolation and verify the prediction of increased resolution relative to controlled crosses.

Alleles↗

PubMed2002

Alps, genes, and chromosomes: their role in the formation of species in the Sorex araneus group (Mammalia, Insectivora), as inferred from two hybrid zones.

During the Pleistocene glaciations, the Alps were an efficient barrier to gene flow between isolated populations, often leading to allopatric speciation. Afterwards, the Alps strongly influenced the post-glacial recolonization of Europe and represent a major suture zone between differentiated populations. Two hybrid zones in the Swiss and French Alps between genetically and chromosomally well-differentiated species-the Valais shrew, Sorex antinorii, and the common shrew, S. araneus-were studied karyotypically and by analyzing the distribution of seven microsatellite loci. In the center of the Haslital hybrid zone the two species coexist over a distance of 900 m. Hybrid karyotypes, among them the most complex known in Sorex, are rare. F-statistics based on microsatellite data revealed a strong heterozygote deficit only in the center of the zone, due to the sympatric distribution of the two species with little hybridization between them. Structuring within the species (both F(IS) and F(ST)) was low. An hierarchical analysis showed a high level of interspecific differentiation. Results were compared with those previously reported in another hybrid zone located at Les Houches in the French Alps. Genetic structuring within and between species was comparable in both hybrid zones, although chromosomal incompatibilities are more important in Haslital, where a linkage block of the race-specific chromosomes should additionally impede gene flow. Evidence for a more restricted gene flow in Haslital comes from the genetically intermediate hybrid karyotypes, whereas in Les Houches, hybrid karyotypes are genetically identical to individuals of the pure karyotypic races. Genic and chromosomal introgression was observed in Les Houches, but not in Haslital. The possible influence of a river, separating the two species at Les Houches, on gene flow is discussed.

Altitude↗

PubMed2005

High-resolution mapping of a new brown planthopper (BPH) resistance gene, Bph18(t), and marker-assisted selection for BPH resistance in rice (Oryza sativa L.).

Brown planthopper (BPH) is a destructive insect pest of rice in Asia. Identification and the incorporation of new BPH resistance genes into modern rice cultivars are important breeding strategies to control the damage caused by new biotypes of BPH. In this study, a major resistance gene, Bph18(t), has been identified in an introgression line (IR65482-7-216-1-2) that has inherited the gene from the wild species Oryza australiensis. Genetic analysis revealed the dominant nature of the Bph18(t) gene and identified it as non-allelic to another gene, Bph10 that was earlier introgressed from O. australiensis. After linkage analysis using MapMaker followed by single-locus ANOVA on quantitatively expressed resistance levels of the progenies from an F2 mapping population identified with marker allele types, the Bph18(t) gene was initially located on the subterminal region of the long arm of chromosome 12 flanked by the SSR marker RM463 and the STS marker S15552. The corresponding physical region was identified in the Nipponbare genome pseudomolecule 3 through electronic chromosome landing (e-landing), in which 15 BAC clones covered 1.612 Mb. Eleven DNA markers tagging the BAC clones were used to construct a high-resolution genetic map of the target region. The Bph18(t) locus was further localized within a 0.843-Mb physical interval that includes three BAC clones between the markers R10289S and RM6869 by means of single-locus ANOVA of resistance levels of mapping population and marker-gene association analysis on 86 susceptible F2 progenies based on six time-point phenotyping. Using gene annotation information of TIGR, a putative resistance gene was identified in the BAC clone OSJNBa0028L05 and the sequence information was used to generate STS marker 7312.T4A. The marker allele of 1,078 bp completely co-segregated with the BPH resistance phenotype. STS marker 7312.T4A was validated using BC2F2 progenies derived from two temperate japonica backgrounds. Some 97 resistant BC2F2 individuals out of 433 screened completely co-segregated with the resistance-specific marker allele (1,078 bp) in either homozygous or heterozygous state. This further confirmed a major gene-controlled resistance to the BPH biotype of Korea. Identification of Bph18(t) enlarges the BPH resistance gene pool to help develop improved rice cultivars, and the PCR marker (7312.T4A) for the Bph18(t) gene should be readily applicable for marker-assisted selection (MAS).

Alleles↗

PubMed2004

Distinguishing human ethnic groups by means of sequences from Helicobacter pylori: lessons from Ladakh.

The history of mankind remains one of the most challenging fields of study. However, the emergence of anatomically modern humans has been so recent that only a few genetically informative polymorphisms have accumulated. Here, we show that DNA sequences from Helicobacter pylori, a bacterium that colonizes the stomachs of most humans and is usually transmitted within families, can distinguish between closely related human populations and are superior in this respect to classical human genetic markers. H. pylori from Buddhists and Muslims, the two major ethnic communities in Ladakh (India), differ in their population-genetic structure. Moreover, the prokaryotic diversity is consistent with the Buddhists having arisen from an introgression of Tibetan speakers into an ancient Ladakhi population. H. pylori from Muslims contain a much stronger ancestral Ladakhi component, except for several isolates with an Indo-European signature, probably reflecting genetic flux from the Near East. These signatures in H. pylori sequences are congruent with the recent history of population movements in Ladakh, whereas similar signatures in human microsatellites or mtDNA were only marginally significant. H. pylori sequence analysis has the potential to become an important tool for unraveling short-term genetic changes in human populations.

DNA, Bacterial↗

PubMed2004

Hybrid zone structure and the potential role of selection in hybridizing populations of native westslope cutthroat trout (Oncorhynchus clarki lewisi) and introduced rainbow trout (O. mykiss).

Introgressive hybridization is a common feature of many zones of contact between divergent lineages of fishes. This is particularly common when taxa that are normally allopatric come into artificial (human-induced) secondary contact. We examined 18 native populations of westslope cutthroat trout (Oncorhynchus clarki lewisi, WCT) to determine the extent of introgressive hybridization with introduced rainbow trout (O. mykiss, RBT) and the genetic structure of hybridizing populations in the upper Kootenay River, southeastern British Columbia, Canada. Using four diagnostic nuclear loci we calculated a hybrid index, inbreeding coefficient, FIS, and the linkage disquilibrium correlation coefficient, Rij, for each locality to determine the distribution of genotypes in each population. We also categorized the 142 hybrid individuals found across localities into four hybrid classes based on their genotypes. The majority of localities (11/18) showed a unimodal distribution of genotypes skewed towards genotypes of WCT. Two localities, however (lower Gold Creek and Lodgepole Creek) showed a flat to bimodal distribution and one site (lower Bull River) showed a unimodal distribution skewed towards RBT genotypes. The majority of hybrid individuals were classified genotypically as WCT backcrosses (59%) and post-F1 individuals (24%). We found a skewed ratio of pure WCT to pure RBT (17:1) and only four F1 hybrids (3%), suggesting that the spread of RBT alleles may be facilitated by hybrids straying to neighbouring populations. We also tested for the action of selection in one population using cohort analyses, but found little evidence of differential selection between pure WCT and hybrid individuals. Pooled across age classes there were significant differences in genotypic frequencies among loci suggesting differential introgression. There was no asymmetry to the hybridization between rainbow trout and westslope cutthroat trout because both species' mitochondrial DNA haplotypes were observed at similar frequencies in the hybrids. Our analyses suggest that hybrid swarms are likely to form in the upper Kootenay River drainage and that certain native WCT populations in British Columbia are at risk of local genomic extinction.

Age Factors↗

PubMed1994

Genetics of reproductive isolation in the Drosophila simulans clade: complex epistasis underlying hybrid male sterility.

We have analyzed the sterility associated with introgressions of the distal one-fourth of the X chromosome from either Drosophila mauritiana or Drosophila sechellia into the genome of Drosophila simulans using a series of visible and DNA markers. Because in Drosophila hybrids, male sterility is usually complete and is often tightly linked with each of several markers used in crosses, a simple genetic basis has generally been assumed. In our low resolution mapping experiment, we were not able to reject the null hypothesis that a single gene, introgressed from either D. mauritiana or D. sechellia, is the cause of male sterility. High resolution mapping, however, reveals a much more complex picture. At least three distinct factors from D. mauritiana, or two from D. sechellia, were identified that need to be jointly present to confer full sterility. Each individual factor by itself is relatively ineffective in causing sterility, or even a partial spermatogenic defect. Moreover, there appear to be more sterility factors on comparable introgressions from D. mauritiana than from D. sechellia. On the basis of these observations, we propose a model which suggests that multilocus weak allele interactions are a very common cause of reproductive incompatibility between closely related species. We also present theoretical argument and empirical evidence against extrapolating the results of within-species analysis to interpret the genetic basis of species differences. The implications of this model on the theories of evolution of species differences and the attempt to understand the mechanisms of hybrid sterility/inviability at the molecular level are discussed.

Animals↗

PubMed2024

QTL mapping for seed vigor-related traits under artificial aging in common wheat in two introgression line (IL) populations.

BACKGROUND: Seed vigor recognized as a quantitative trait is of particular importance for agricultural production. However, limited knowledge is available for understanding genetic basis of wheat seed vigor. METHODS: The aim of this study was to identify quantitative trait loci (QTL) responsible for 10 seed vigor-related traits representing multiple aspects of seed-vigor dynamics during artificial aging with 6 different treatment times (0, 24, 36, 48, 60, and 72 h) under controlled conditions (48 °C, 95% humidity, and dark). The mapping populations were two wheat introgression lines (IL-1 and IL-2) derived from recipient parent (Lumai 14) and donor parent (Shaanhan 8675 or Jing 411). RESULTS: A total of 26 additive QTLs and 72 pairs of epistatic QTLs were detected for wheat seed-vigor traits. Importantly, chromosomes 1B and 7B contained several co-located QTLs, and chromosome 2A had a QTL-rich region near the marker Xwmc667, indicating that these QTLs may affect wheat seed vigor with pleiotropic effects. Furthermore, several possible consistent QTLs (hot-spot regions) were examined by comparison analysis of QTLs detected in this study and reported previously. Finally, a set of candidate genes for wheat seed vigor were predicted to be involved in transcription regulation, carbohydrate and lipid metabolism. CONCLUSION: The present findings lay new insights into the mechanism underlying wheat seed vigor, providing valuable information for wheat genetic improvement especially marker-assisted breeding to increase seed vigor and consequently achieve high grain yield despite of further investigation required.

Triticum↗

PubMed2001

Frequent cytoplasmic exchanges between oak species that are not closely related: Quercus suber and Q. ilex in Morocco.

Chloroplast (cp) and mitochondrial (mt) DNA variation were studied in 97 populations of cork oak (Quercus suber) in Morocco; in 31 of these populations, holm oak (Quercus ilex), a clearly distinct species, also occurred and was compared with Q. suber. Three cpDNA and one mtDNA primer pairs were used in the survey, each in combination with one restriction enzyme. Six haplotypes belonging to two very divergent lineages were detected; one lineage predominates in each species, and is probably ancestral, as inferred from comparisons with other oak species. In the mixed-species populations, cytoplasmic genomes were frequently shared across species, as indicated by an introgression ratio of 0.63. This index is a new measure of the propensity of species to share locally genetic markers, varying from zero (complete differentiation) to one (no differentiation). By contrast, more closely related deciduous oak species (Q. robur, Q. petraea and Q. pubescens) have introgression ratios varying from 0.82 to 0.97. The introgression events appear to have been more frequent in the direction Q. ilex (female) x Q. suber (male), a finding which seems attributable to the flowering phenology of these two species. This asymmetry may have favoured immigration of Q. suber beyond its main range, in regions already colonized by Q. ilex. There, rare hybridization and further introgression through long distance pollen flow have established populations that are morphologically indistinguishable from Q. suber but that have cytoplasmic genomes originating from the local Q. ilex populations.

DNA, Chloroplast↗

PubMed2002

Incipient speciation across a depth gradient in a scleractinian coral?

A few marine cases have demonstrated morphological and genetic divergence in the absence of spatial barriers to gene flow, suggesting that the initial phase of speciation is possible without geographic isolation. In the Bocas del Toro Archipelago of the Atlantic Coast of Panama, we found two morphotypes of the scleractinian coral Favia fragum with opposing depth distributions. One morphotype fit the classical description of F. fragum and was most abundant at 3 m depth. A second morphotype was distinguished by raised corallites and was restricted to < or = 1 m depth. The two morphotypes overlapped in distribution at 1 m depth. Multivariate analysis of polyp-level characters (shape and distribution of septa within corallites) divided samples into two groups corresponding to initial qualitative observations of colony shape and corallite relief. To determine whether reduced gene flow maintains morphological variation, we measured the frequencies of alleles at five allozyme loci in both morphotypes at three sites 1-2 km distant. While there were significant differences in allele frequencies between morphotypes within sites, there were also frequency differences among sites at most loci, with the exception of nearly fixed alleles at the PGM locus. Extremely low heterozygosity permitted us to use haplotypes to compare genetic distance between morphotypes and among sites. Comparisons between haplotype data and a null model assuming gene flow between morphotypes showed that the two morphotypes shared significantly fewer haplotypes than expected, and average genetic distance between morphotypes was significantly greater than expected. Partitioning haplotype variation with analysis of molecular variance demonstrated that 35% of the variation was explained by morphotype, whereas 28% of the variation was explained by site. Two PGM heterozygotes and several individuals homozygous for rare PGM alleles are consistent with hybridization, and perhaps introgression by selfing within morphotypes. We consider three hypotheses for this morphological and genetic divergence in F. fragum: (1) intraspecific polymorphism, (2) incipient species, (3) biological species; and discuss the role of reproductive characters in a divergence-with-gene flow mechanism of speciation.

Animals↗

PubMed1996

Marker-assisted introgression in backcross breeding programs.

The efficiency of marker-assisted introgression in backcross populations derived from inbred lines was investigated by simulation. Background genotypes were simulated assuming that a genetic model of many genes of small effects in coupling phase explains the observed breed difference and variance in backcross populations. Markers were efficient in introgression backcross programs for simultaneously introgressing an allele and selecting for the desired genomic background. Using a marker spacing of 10-20 cM gave an advantage of one to two backcross generations selection relative to random or phenotypic selection. When the position of the gene to be introgressed is uncertain, for example because its position was estimated from a trait gene mapping experiment, a chromosome segment should be introgressed that is likely to include the allele of interest. Even for relatively precisely mapped quantitative trait loci, flanking markers or marker haplotypes should cover approximately 10-20 cM around the estimated position of the gene, to ensure that the allele frequency does not decline in later backcross generations.

Alleles↗

PubMed1995

Direct isolation of cDNA sequences from specific chromosomal regions of the tomato genome by the differential display technique.

The differential display technique was originally developed for the isolation of differentially expressed genes from eukaryotic tissues. We have adapted this technique for the isolation of cDNA markers from specific regions of the tomato genome. For this purpose, differential display was performed on RNA extracted from leaf tissue of nearly isogenic lines for the Tm-2a gene of tomato. On average, one out of 20 primer combinations resulted in a polymorphism at the cDNA level. When used as hybridization probes, all of these cDNA fragments were single or low copy and all of them were polymorphic on Southern hybridizations using DNA from the isogenic lines. Genetic mapping revealed in each case at least one locus in the introgressed segment on chromosome 9 of tomato. Thus, this technique might provide a way for the direct isolation of transcribed sequences from specific regions of any animal or plant genome for which such lines exist.

Chromosome Mapping↗

PubMed1994

Mitochondrial control region and protein coding genes sequence variation among phenotypic forms of brown trout Salmo trutta from northern Italy.

The Pô River basin of northern Italy is the home of distinctive and endemic morphological forms of brown trout Salmo trutta. We used PCR-direct sequencing and RFLP techniques to study variation in the mitochondrial control region of 225 trout in order to assess genetic relatedness among 18 populations from that region. The distribution analysis of these genotypes among north Italian populations confirmed the phylogenetic differentiation of marbled trout Salmo trutta marmoratus populations and the postglacial origin of S. t. carpio. Extensive genetic heterogeneity was observed among morphologically identical S. t. fario populations. Introgression with domestic strains of Atlantic basin origin was detected in all forms. In order to assess the phylogenetic congruence detected in coding and noncoding regions of the mitochondrial genome, we also analysed sequence variation in segments of the cytochrome b and ATPase subunit VI genes among representatives of all variants detected in the analysis of the control region. Variation in protein coding genes was only slightly less than that observed in the control region of the same individuals, both in terms of number of variants detected and of pairwise sequence divergence estimates among variants. Phylogenetic analysis based on protein coding genes sequences identified the same phylogenetic groupings defined by the control region analysis and also allowed a partial resolution of their phyletic relationships that was previously unresolved. However, coding and noncoding segments differed substantially in the transition-transversion ratio (17:0 in coding segments vs. 17:6 in control region segments).

Animals↗

PubMed2005

The recessive potyvirus resistance gene pot-1 is the tomato orthologue of the pepper pvr2-eIF4E gene.

The translation initiation factor 4E (eIF4E) has been implicated in naturally occurring resistance to Potato virus Y (PVY) determined by the pvr2 locus in pepper (Capsicum annuum). Here, the molecular basis of the recessive resistance to PVY and Tobacco etch virus (TEV) controlled by the pot-1 locus in tomato (Lycopersicon esculentum; now Solanum lycopersicum) was investigated. On the basis of genetic mapping data that indicated that pot-1 and pvr2 are located in syntenic regions of the tomato and pepper genomes, the possible involvement of eIF4E in pot-1-mediated resistance was assessed. Genetic mapping of members of the eIF4E multigenic family in tomato introgression lines revealed that an eIF4E locus indeed maps in the same genomic region as pot-1. By comparing eIF4E coding sequences between resistant and susceptible Lycopersicon genotypes, a small number of polymorphisms that co-segregate with the pot-1 locus were identified, suggesting that this gene could be involved in resistance to potyviruses. Functional complementation experiments using Potato virus X-mediated transient expression of eIF4E from a susceptible genotype in a resistant pepper genotype confirmed that a small number of amino acid substitutions in the eIF4E protein indeed account for resistance/susceptibility to both the PVY and TEV, and consequently that pot-1 and pvr2 are orthologues. Taken together, these results support the role of this eIF4E gene as a key component of recessive resistance to potyviruses, and validate the comparative genomic approach for the molecular characterization of recessive resistance genes.

Amino Acid Sequence↗

PubMed2005

Low level of gene flow from cultivated beets (Beta vulgaris L. ssp. vulgaris) into Danish populations of sea beet (Beta vulgaris L. ssp. maritima (L.) Arcangeli).

Gene flow from sugar beets to sea beets occurs in the seed propagation areas in southern Europe. Some seed propagation also takes place in Denmark, but here the crop-wild gene flow has not been investigated. Hence, we studied gene flow to sea beet populations from sugar beet lines used in Danish seed propagation areas. A set of 12 Danish, two Swedish, one French, one Italian, one Dutch, and one Irish populations of sea beets, and four lines of sugar beet were analysed. To evaluate the genetic variation and gene flow, eight microsatellite loci were screened. This analysis revealed hybridization with cultivated beet in one of the sea beet populations from the centre of the Danish seed propagation area. Triploid hybrids found in this population were verified with flow cytometry. Possible hybrids or introgressed plants were also found in the French and Italian populations. However, individual assignment test using a Bayesian method provided 100% assignment success of diploid individuals into their correct subspecies of origin, and a Bayesian Markov chain Monte Carlo (MC MC) approach revealed clear distinction of individuals into groups according to their subspecies of origin, with a zero level of genetic admixture among subspecies. This underlines that introgression beyond the first hybridization is not extensive. The overall pattern of genetic distance and structure showed that Danish and Swedish sea beet populations were closely related to each other, and they are both more closely related to the population from Ireland than to the populations from France, the Netherlands, and Italy.

Bayes Theorem↗

PubMed2004

[Effect of an introgression from Aegilops cylindrica host on manifestation of productivity traits in winter common wheat F2 plants].

The effect of introgression of a chromosome 1D segment from Aegilops cylindrica to winter common wheat on productivity traits in F2 plants was studied using storage protein loci as genetic markers. An allele of the gliadin-coding Gli-D1 locus served as a marker of the introgression. Using of two- and three-locus interaction models, it was shown that the introgression tagged with Gli-D1 affected the manifestation of productivity traits (productive tillering, grain weight per plant and grain number per plant) through interaction with other marker storage protein loci: Glu-B1, Glu-D1, and Gli-B2.

Chimera↗

PubMed2005

A library of Solanum lycopersicoides introgression lines in cultivated tomato.

A set of introgression lines (ILs), containing individual chromosome segments from the wild nightshade Solanum lycopersicoides bred into the genetic background of cultivated tomato (Lycopersicon esculentum), has been developed. A primary group of 56 lines was selected for maximum representation of the S. lycopersicoides genome (approximately 96% of the total map units), homozygosity, and a minimum number of introgressed segments per line. A secondary set of 34 lines provides increased map resolution in certain regions. Approximately 34% of the lines were sterile in the homozygous condition, but could be maintained by heterozygotes. To facilitate identification of segregating ILs, restriction fragment length polymorphism probes were converted to higher throughput cleaved amplified polymorphic sequence markers, which supplement allozyme and morphological loci. Strong segregation distortion was observed in F2 progeny of heterozygous ILs, with an excess of L. esculentum alleles in most regions. For introgressions on distal chromosome 1L, a preferential transmission of S. lycopersicoides alleles was observed in the male germ line. Homozygous ILs generally yielded less seed from self pollination than corresponding heterozygotes, indicating that sterility effects were recessive. This IL library provides a novel resource for genetic studies of traits found in S. lycopersicoides.

Chromosomes, Plant↗

PubMed1999

An Alu insertion polymorphism in a baboon hybrid zone.

A novel polymerase chain reaction (PCR) primer pair was used to analyze the frequency of insertion of the first described, nonhuman, baboon-specific Alu repetitive element in populations from the Papio hamadryas anubis and the Papio hamadryas hamadryas subspecies, and from a number of anubis-hamadryas hybrids. The Alu insertion is found in intron 7 of the baboon lipoprotein lipase (LPL) gene. Each of the populations had different frequencies for the insertion, and the hybrids examined had a frequency intermediate to that of the parental populations. All hybrids and all P. h. anubis groups except the group of anubis sampled in 1973 exhibited higher-than-expected heterozygosity, while P. h. hamadryas and 1973 P. h. anubis showed lower-than-expected heterozygosity, supporting behavioral and other genetic observations of greater anubis outbreeding relative to hamadryas. This may include asymmetric introgression of the Alu insertion from hamadryas to the anubis population due to hybridization.

Alu Elements↗

PubMed2002

Evidence for multiple genetic forms with similar eyeless phenotypes in the blind cavefish, Astyanax mexicanus.

A diverse group of animals has adapted to caves and lost their eyes and pigmentation, but little is known about how these animals and their striking phenotypes have evolved. The teleost Astyanax mexicanus consists of an eyed epigean form (surface fish) and at least 29 different populations of eyeless hypogean forms (cavefish). Current alternative hypotheses suggest that adaptation to cave environments may have occurred either once or multiple times during the evolutionary history of this species. If the latter is true, the unique phenotypes of different cave-dwelling populations may result from convergence of form, and different genetic changes and developmental processes may have similar morphological consequences. Here we report an analysis of variation in the mitochondrial NADH dehydrogenase 2 (ND2) gene among different surface fish and cavefish populations. The results identify a minimum of two genetically distinctive cavefish lineages with similar eyeless phenotypes. The distinction between these divergent forms is supported by differences in the number of rib-bearing thoracic vertebrae in their axial skeletons. The geographic distribution of ND2 haplotypes is consistent with roles for multiple founder events and introgressive hybridization in the evolution of cave-related phenotypes. The existence of multiple genetic lineages makes A. mexicanus an excellent model to study convergence and the genes and developmental pathways involved in the evolution of the eye and pigment degeneration.

Animals↗