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Low probability of chloroplast movement from oilseed rape (Brassica napus) into wild Brassica rapa.

Pollen-mediated movement of transgenes from transplastomic oilseed rape (Brassica napus) into wild relatives will be avoided if chloroplasts are maternally transmitted. We assess the probability of chloroplast exchange between conventional oilseed rape and wild Brassica rapa to model the future behavior of transplastomic cultivars. Primers specific to cpDNA were used to demonstrate maternal inheritance of chloroplasts in 47 natural hybrids between cultivated B. napus and wild B. rapa. We conclude that there will be no or negligible pollen-mediated chloroplast dispersal from oilseed rape. Transgene introgression could still occur in mixed populations, however, if B. napus acted as the recurrent female parent. Rate of transfer would then depend on the abundance of mixed populations, their persistence as mixtures, and hybridization frequency within stands. A low incidence of sympatry (0.6-0.7%) between wild B. rapa and cultivated B. napus along the river Thames, UK, in 1997 and 1998, suggests mixed stands will form only rarely. Eighteen feral populations of B. napus also showed a strong tendency toward rapid decline in plant number, seed return, and ultimately, extinction within 3 years. Conversely, hybrid production is significant in mixed stands, and the absence of control practices means that oilseed rape will have slightly greater persistence. We infer that some introgression from transplastomic B. napus into B. rapa is inevitable in mixed populations even though such populations will occur infrequently and will tend to lose B. napus plants relatively quickly. Chloroplast exchange will be extremely rare and scattered.

Brassica↗

The ecological genetics of introduced populations of the giant toad, Bufo marinus. IV. Gene flow estimated from admixture in Australian populations.

Allele frequency variation is described at nine polymorphic enzyme loci in 21 samples from populations of the introduced Giant Toad, Bufo marinus, in the region of Townsville in north Queensland, Australia. Some of these populations appear to have been established through the introgression of other populations that previously had been isolated. Comparisons of allele frequencies at three polymorphic loci between the introgressed populations and the original populations are used to obtain admixture estimates. These are used to estimate a rate of gene flow among the populations of approximately 2 km/year. This is consistent with an estimate based on the rate at which Bufo marinus has colonised new areas in Australia when discontinuities in the pattern of this colonisation are taken into account. The estimate of gene flow is combined with published data on population density to estimate neighbourhood size. The estimate obtained is substantially greater than the effective population size estimate determined previously from data on allele frequency variances in other populations. This discrepancy is most likely due to inaccuracies in the population density estimates, to underestimates of the extent of offspring number variance and perhaps to occasional departures from sex ratio parity. It has important implications for the study of the genetic structure of populations which are discussed.

Alleles↗

Biodiversity (Communications arising): suspect evidence of transgenic contamination.

Quist and Chapela claim that transgenic DNA constructs have been introgressed into a traditional maize variety in Mexico, and furthermore suggest that these constructs have been reassorted and introduced into different genomic backgrounds. However, we show here that their evidence for such introgression is based on the artefactual results of a flawed assay; in addition, the authors misinterpret a key reference to explain their results, concluding that reassortment of integrated transgenic DNA occurs during transformation or recombination.

Artifacts↗

Congenic BB.SHR (D4Mit6-Npy-Spr) rats: a new aid to dissect the genetics of obesity.

OBJECTIVE: The phenotypic characterization of congenic BB.LL rats recombining a segment of the SHR chromosome 4 (D4Mit6-Npy-Spr; 12 cM) into the BB/OK background indicated that these rats were not lymphopenic and did not develop diabetes, but they were significantly heavier (at 16 weeks of age) and showed higher serum triglycerides and total cholesterol concentration. RESEARCH METHODS AND PROCEDURES: BB.LL rats were longitudinally studied for facets of metabolic syndrome (body mass index, blood glucose, serum lipids, insulin, leptin, and systolic and diastolic blood pressure) from 2 to 12 months of age. RESULTS: In this study, it was shown that BB.LL are obese, hyperleptinemic, hyperinsulinemic, and dyslipidemic compared with their parental BB/OK rats. DISCUSSION: It can be concluded that there is a gene(s) in the introgressed segment causing incomplete metabolic syndrome, because they do not develop hypertension and diabetes. To identify the gene(s), the introgressed chromosomal segment must be systematically whittled down to generate recombinants and new subcongenic lines carrying a much smaller segment of the SHR/Mol rat to increase the chance of identification of the appropriate gene(s).

Animals↗

Alleles on rat chromosome 4 (D4Got41-Fabp1/Tacr1) regulate subphenotypes of obesity.

OBJECTIVE: The use of inbred animal models is an essential component of the genetic dissection of complex diseases. Because quantitative trait loci for serum triglycerides, total cholesterol, and body weight were mapped on chromosome 4 in a cross of BioBreeding/OttawaKarlsburg (BB/OK) and spontaneously hypertensive (SHR) rats, we established a congenic BB.SHR rat strain by introgressing a SHR segment of chromosome 4 (D4Got41-Tacr1) into a BB/OK background. The phenotype of these BB.SHR rats (BB.4S) confirmed the quantitative trait loci. To discover whether the phenotype of BB.4S can only be attributed to the SHR segment per se, we established an additional congenic BB.WOKW strain by introgressing a similar segment of chromosome 4 (D4Got41-Fabp1) of the Wistar Ottawa Karlsburg RT1(u) rat into a BB/OK background, termed briefly BB.4W. RESEARCH METHODS AND PROCEDURES: Male normoglycemic BB/OK (20), BB.4S (20), and BB.4W (16) rats were longitudinally studied for body weight, serum triglycerides, total and high-density lipoprotein-cholesterol, and glucose tolerance. At the end of the observation period (32 weeks), serum insulin, leptin, and adiposity index (AI) were determined. RESULTS AND DISCUSSION: Congenic BB.4S and BB.4W were significantly heavier, and AI, serum triglycerides, and total cholesterol values were significantly elevated in BB.4S and BB.4W compared with BB/OK but more pronounced in BB.4S. The highest serum insulin was found in BB.4W and highest leptin in BB.4S. Because the body weight gain and AI were comparable between BB.4S and BB.4W, the obviously higher insulin levels in BB.4W and higher leptin values in BB.4S suggest that the two congenics most probably define two subphenotypes of obesity and provide the unique opportunity to study their genetics.

Alleles↗

Hybridization as driving force for cryptic species diversity in the Caribbean coral genus Madracis.

Species boundaries in scleractinian corals remain highly elusive due to conflicting patterns between morphological and molecular phylogenies, often caused by morphological plasticity, occurrence of cryptic species, incomplete lineage sorting or introgressive hybridization. Here, we use an integrated systematics approach, which combines reduced representation genome sequencing (nextRAD), micro-morphometric characterization, SEM analyses and compilation of life history traits, to infer phylogenetic relationships among closely related species in the Caribbean coral genus Madracis. In total, we analyzed 235 Madracis specimens from Curaçao and Bermuda collected from 10-90 m depth. Sequence- and SNP-based analyses for 115 samples generated unprecedented species resolution in Madracis, greatly supporting the morphology-based taxonomy of the current, accepted Caribbean species M. senaria, M. decactis, M. formosa, M. carmabi and M. mirabilis (M. auretenra). The exception was M. pharensis, in which we found evidence for three separate lineages, and for which we found signatures of admixture and introgression. These three M. pharensis lineages showed distinct depth distributions (thus classified as shallow, deep and very deep) and were partially distinguishable on the basis of fine microstructural elements of the collumella, septa and coenosteum. Further taxonomic comparisons are needed to formalize these putative cryptic species. Overall, our integrated systematics approach further resolves species relationships in the Caribbean genus Madracis, supports the morphological descriptions for most of the recognized species, but also reveals the existence of cryptic diversity in groups marked by high admixture, thus suggesting hybridization as a driving force in coral species diversity.

Animals↗

Contact zones between chromosomal races of Mus musculus domesticus. 3. Molecular and chromosomal evidence of restricted gene flow between the CD race (2n = 22) and the ACR race (2n = 24).

The contact zone between the Ancarano (ACR; 2n = 24) and Cittaducale (CD; 2n = 22) races of Mus musculus domesticus was studied. We used chromosomes and mitochondrial DNA (mtDNA) sequences of the control region as genetic markers to detect introgression between races. In total, 76 mice were trapped at 17 localities. Cytogenetic analysis was performed on 73 mice. A segment of the control region (468 bp) was sequenced in 41 specimens. The two races are distributed parapatrically and the contact zone was identified inside a village (Pizzoli). No mixed population was found in the study area. The contact zone does not correspond to any geographical or ecological barrier but is located in a zone of potentially high density of mice. The sequence analysis clearly demonstrates genetic differentiation between races (1.4% of sequence divergence). Hybridization is rare. Evidence of introgression was found in two individuals in the contact zone: one individual of the ACR race carries a metacentric belonging to the CD race, while another ACR individual carries a CD-like haplotype. In these ecological conditions, the observed distribution pattern and the very low level of hybridization suggest the presence of a premating mechanism of reproductive isolation.

Animals↗

Hybridization of banteng (Bos javanicus) and zebu (Bos indicus) revealed by mitochondrial DNA, satellite DNA, AFLP and microsatellites.

Hybridization between wild and domestic bovine species occurs worldwide either spontaneously or by organized crossing. We have analysed hybridization of banteng (Bos javanicus) and zebu (Bos indicus) in south-east Asian cattle using mitochondrial DNA (PCR-RFLP and sequencing), AFLP, satellite fragment length polymorphisms (SFLP or PCR-RFLP of satellite DNA) and microsatellite genotyping. The Indonesian Madura zebu breed is reputed to be of hybrid zebu-banteng origin, but this has never been documented and Bali cattle are considered to be a domesticated form of banteng. The banteng mitochondrial type was found in all animals sampled on the isle of Bali, Indonesia, but only in 35% of the animals from a Malaysian Bali-cattle population. The Madura animals also carried mitochondrial DNA of either zebu and banteng origin. In both populations, zebu introgression was confirmed by AFLP and SFLP. Microsatellite analysis of the Malaysian Bali population revealed for 12 out of 15 loci screened, Bali-cattle-specific alleles, several of which were also found in wild banteng animals. The tools we have described are suitable for the detection of species in introgression studies, which are essential for the genetic description of local breeds and the preservation of their economic and cultural value.

Animals↗

Invasion dynamics of two alien Carpobrotus (Aizoaceae) taxa on a Mediterranean island: II. Reproductive strategies.

This study compares sexually and asexually produced fruit set, seed production, biomass, germination, and seedling size in Carpobrotus acinaciformis and C. edulis following controlled pollination experiments in order to evaluate the potential role of reproductive traits with respect to the invasive potential of these taxa. C. edulis is slightly agamospermic, completely self-fertile, slightly preferentially self-compatible, experiences no inbreeding depression, and has low hybrid vigour. In contrast, C. acinaciformis does not have reliable agamospermy, is only slightly self-fertile and self-compatible, experiences a slight inbreeding depression, and has a strong hybrid vigour. Both taxa have relatively low, although significantly different germination frequencies, and insignificantly different seedling sizes. Owing to the high performance in hybridisation as compared to all other controlled pollinations in C. acinaciformis, as well as a large amount of previously demonstrated introgression, we refer to the population studied on the island of Bagaud (France) as C. affine acinaciformis. We conclude that both C. edulis and C. affine acinaciformis should be considered as harmful invasive plants in the Mediterranean Basin, the former because of the flexibility of its mating system and high seed production, and the latter because of its strong clonality, high hybrid vigour, and potential for continued introgression from C. edulis genes. These differences require different control strategies, while the avoidance of sympatry is a distinct priority.

Aizoaceae↗

A secondary hybrid zone between diploid Dactylorhiza incarnata ssp. cruenta and allotetraploid D. lapponica (Orchidaceae).

Secondary hybrid zones are not uncommon in Dactylorhiza, but knowledge of ecological and evolutionary consequences of hybridization are scarce. Here, we assess interploidal gene flow and introgression in a hybrid zone between diploid Dactylorhiza incarnata ssp. cruenta (2n = 2x = 40) and its putative allotetraploid derivative D. lapponica (2n = 4x = 80). Photometric quantification of DNA content and morphology confirmed that triploids are abundant in sympatric populations in our study area. Allozyme segregation patterns in D. lapponica supported an allopolyploid origin, although unbalanced genotypes suggested rare pairings between homoeologous chromosomes. Photometric data and chromosome counts suggest backcrossing between the triploid hybrid and D. lapponica, and hence some hybrid fertility. Triploids are morphologically more similar to the tetraploids than the diploids, maybe owing to the hybrid origin of both triploids and tetraploids. The diploids and tetraploids were not more similar in the parapatric populations compared to when they occur in allopatry. This indicates that backcrossing rarely leads to introgression, or alternatively that allopatric populations are not isolated enough to prevent influx of pollen from the other species. Despite some evidence of backcrossing, our study gives few indications that widespread hybridization entails local breakdown of species boundaries. Rather, the hybrid zone may be a transient phenomenon due to intensive mowing, resulting in the opening of habitats and hence bringing the parental species into close contact.

Chimera↗

GISH/FISH mapping of genes for freezing tolerance transferred from Festuca pratensis to Lolium multiflorum.

The first backcross breeding programme for the transfer of freezing-tolerance genes from winter hardy Festuca pratensis to winter-sensitive Lolium multiflorum is described. A partly fertile, triploid F(1) hybrid F. pratensis (2n=2x=14) x L. multiflorum (2n=4x=28) was employed initially, and after two backcrosses to L. multiflorum (2x) a total of 242 backcross two (BC(2)) plants were generated. Genomic in situ hybridisation (GISH) was performed on 61 BC(2) plants selected for their good growth and winter survival characters in the spring following one Polish winter (2000-2001). Among the winter survivors, diploid chromosome numbers were present in 80% of plants. An appropriate single Festuca introgression in an otherwise undisturbed Lolium genome could provide increased freezing tolerance without compromise to the good growth and plant vigour found in Lolium. Among all the diploids, a total of 20 individuals were identified, each with a single F. pratensis chromosome segment. Another diploid plant contained 13 Lolium chromosomes and a large metacentric F. pratensis chromosome, identified as chromosome 4, with two large distal Lolium introgressions on each chromosome arm. Three of the diploid BC(2), including the genotype with Festuca chromosome 4 DNA sequences, were found to have freezing tolerance in excess of that of L. multiflorum, and in one case in excess of the F. pratensis used as control. A detailed cytological analysis combining GISH and fluorescence in situ hybridisation analyses with rDNA probes revealed that the other two freezing-tolerant genotypes carried a Festuca chromosome segment at the same terminal location on the non-satellite arm of Lolium chromosome 2.

Acclimatization↗

RAPD of controlled crosses and clones from the field suggests that hybrids are rare in the Salix alba-Salix fragilis complex.

The polyploid Salix alba-Salix fragilis hybrid complex is rather difficult to study when using only morphological characters. Most of the features have a low diagnostic value for unambiguously identifying the hybrids, introgression patterns and population structures, though morphological traits have proved to be useful in making a hybrid index. Morphology and molecular variation from RAPDs were investigated in several case studies on willows from Belgium. A thorough screening of full-sib progenies of interspecific controlled crosses was made to select homologous amplification products. The selected amplified products proved to be useful in a principal coordinate analysis for the estimation of variability of hybrid progenies. On the basis of genetic similarities and ordination analysis, a method for the identification of clones in the field was established using presumed pure species and presumed introgressants. The chosen reference clones were checked against additional European samples of putative pure species to ensure the reliability of the method beyond a regional scale. The RAPDs suggested that both species have kept their gene pools well separated and that hybridization actually does not seem to be a dominating process. The observation that molecular markers do not always follow the morphological traits or allozyme data is discussed.

Belgium↗

The zone of sympatry and hybridization of Mytilus edulis and M. galloprovincialis, as described by intron length polymorphism at locus mac-1.

Intron-size variation at the actin gene locus mac-1 was used to characterize mussel, Mytilus spp., populations in the approximately 2000-km wide zone of contact and hybridization ('hybrid zone') between M. edulis and M. galloprovincialis in western Europe. Twenty-five samples were collected in 1995-99 in locations within the hybrid zone and from reference populations of each species. We used correspondence analysis on the matrix of allelic frequencies to determine which alleles are characteristic of each species, and to characterize samples along the genetic gradient between M. edulis and M. galloprovincialis. In the hybrid zone, some samples exhibited mac-1 allele frequencies that were typical of M. edulis; other samples were distributed along the M. edulis/M. galloprovincialis gradient and displayed variable levels of intergradation that were not correlated with geography. Some of the latter samples exhibited significant heterozygote deficiencies. The simple admixture hypothesis (Wahlund effect) could not be rejected for two-fifths of the samples. The hybrid zone thus appeared as a mosaic of populations which are either pure M. edulis, or hybrid between M. galloprovincialis and M. edulis, or a mixture of the foregoing with M. galloprovincialis individuals. These results were consistent with published allozyme data, suggesting that they can be extended to the entire nuclear genome. M. edulis mac-1 alleles were present at moderate frequency in Atlantic M. galloprovincialis, and at significantly lower frequency in some Mediterranean samples. This pattern was homogeneous over a broad geographical range within each basin. It was not evident that introgression of M. edulis into M. galloprovincialis presently occurs south of the zone of contact. We propose that the distinctness of the Atlantic M. galloprovincialis population results from past introgression by M. edulis alleles.

Actins↗

Geographic distribution and frequency of a taurine Bos taurus and an indicine Bos indicus Y specific allele amongst sub-saharan African cattle breeds.

We report for the first time, and for the whole of sub-Saharan Africa, the geographical distribution and the frequency of an indicine and a taurine Y specific allele amongst African cattle breeds. A total of 984 males from 69 indigenous African populations from 22 countries were analysed at the microsatellite locus INRA 124. The taurine allele is probably the oldest one on the continent. However, the taurine and the indicine alleles were present in 291 males (30%), and 693 males (70%), respectively. More particularly, 96% of zebu males (n = 470), 50% of taurine males (n = 263), 29% of sanga males (crossbreed Bos taurus x Bos indicus, n = 263) and 95% of zebu x sanga crossbred males (n = 56) had the indicine allele. The Borgou, a breed classified as zebu x taurine cross showed only the zebu allele (n = 12). The indicine allele dominates today in the Abyssinian region, a large part of the Lake Victoria region and the sahelian belt of West Africa. All the sanga males (n = 64) but only one from the Abyssinian region had the indicine allele. The taurine allele is the commonest only among the sanga breeds of the southern African region and the trypanotolerant taurine breeds of West Africa. In West Africa and in the southern Africa regions, zones of introgression were detected with breeds showing both Y chromosome alleles. Our data also reveal a pattern of male zebu introgression in Mozambique and Zimbabwe, probably originating from the Mozambique coast. The sanga cattle from the Lake Victoria region and the Kuri cattle of Lake Chad, cattle populations surrounded by zebu breeds were, surprisingly, completely devoid of the indicine allele. Human migration, phenotypic preferences by the pastoralists, adaptation to specific habitats and to specific diseases are the main factors explaining the present-day distribution of the alleles in sub-Saharan Africa.

Africa↗

The role of hybridization in evolution.

Hybridization may influence evolution in a variety of ways. If hybrids are less fit, the geographical range of ecologically divergent populations may be limited, and prezygotic reproductive isolation may be reinforced. If some hybrid genotypes are fitter than one or both parents, at least in some environments, then hybridization could make a positive contribution. Single alleles that are at an advantage in the alternative environment and genetic background will introgress readily, although such introgression may be hard to detect. 'Hybrid speciation', in which fit combinations of alleles are established, is more problematic; its likelihood depends on how divergent populations meet, and on the structure of epistasis. These issues are illustrated using Fisher's model of stabilizing selection on multiple traits, under which reproductive isolation evolves as a side-effect of adaptation in allopatry. This confirms a priori arguments that while recombinant hybrids are less fit on average, some gene combinations may be fitter than the parents, even in the parental environment. Fisher's model does predict heterosis in diploid F1s, asymmetric incompatibility in reciprocal backcrosses, and (when dominance is included) Haldane's Rule. However, heterosis arises only when traits are additive, whereas the latter two patterns require dominance. Moreover, because adaptation is via substitutions of small effect, Fisher's model does not generate the strong effects of single chromosome regions often observed in species crosses.

Alleles↗

A comparison of nuclear and mitochondrial cline shapes in a hybrid zone in the Sceloporus grammicus complex (Squamata; Phrynosomatidae).

The F5 and FM2 chromosome races of the Sceloporus grammicus complex form a hybrid zone in the Mexican state of Hidalgo. Previous studies of this zone have assessed genetic structure by averaging estimates of shape and width across three diagnostic chromosome markers. This approach is likely to mask subtle differences in cline shape among loci (e.g. selected vs. neutral), and obscure any displacement of cline centres (if present). Here we use maximum likelihood methods to construct the best fitting individual clines for three chromosomal markers, and also add two new markers; the mitochondrial DNA (mtDNA) locus, and the nuclear ribosomal DNA (rDNA) repeat. For each locus, hybrid zone models were fitted by cline shape and width, and the position and number of segments describing the centre of the zone. Pairwise comparisons between all clines revealed concordance between chromosomes 2 and 6, but significant discordance in cline structure among all other paired combinations. The concordance of chromosomes 2 and 6 suggests that these clines are maintained by genome-wide forces. The discordance of the chromosome 1 cline suggests an influence of asymmetric introgression, while the mtDNA cline is probably influenced by selection and drift. The rDNA locus reveals a pattern best explained by either extreme asymmetric introgression or gene conversion. The structure of zone indicates that genome-wide processes and locus specific selective forces as well as drift, are operating to different degrees on different loci. The locus-by-locus approach used here permits a finer discrimination among possible mechanisms responsible for the maintenance of the individual clines.

Animals↗

Population history of Manihot esculenta (Euphorbiaceae) inferred from nuclear DNA sequences.

The nature of gene flow in plants -- including the propensity for interspecific introgression -- makes them interesting candidates for phylogeographical analysis. Plant phylogeography studies have been limited, however, by the availability of suitable intraspecific variation. In this study, DNA sequence variation from a nuclear gene [Glyceraldehyde 3-phosphate dehydrogenase; (G3pdh)] was used to examine the population history of Manihot esculenta ssp. flabellifolia and a potentially hybridizing species, M. pruinosa. These species occur in the rainforest-savanna ecotone adjoining the Amazon basin, a region believed to have undergone major habitat shifts since the Pleistocene. Geographical distributions of the G3pdh haplotypes indicate genetic isolation-by-distance across the range of M. esculenta ssp. flabellifolia. However, there is greater genetic similarity between northeastern and western populations than would be expected given the present species distribution. A nested clade analysis suggests that northeastern and western populations were connected by gene flow until relatively recently, when they became fragmented. This inferred fragmentation event is consistent with post-Pleistocene habitat shifts proposed for the Amazon basin. At the interspecific level, haplotype sharing with M. pruinosa may reflect either recent interspecific introgression or incomplete lineage sorting between these closely related species.

Brazil↗

Estimating the long-term effects of stocking domesticated trout into wild brown trout (Salmo trutta) populations: an approach using microsatellite DNA analysis of historical and contemporary samples.

Indigenous salmonid fish gene pools are affected by domesticated conspecifics, derived from aquaculture escapes and deliberate releases. Variability was examined at nine microsatellite loci in order to assess the long-term impact of stocking domesticated trout in two brown trout populations. The study was based on analysis of two historical samples (1945-56), represented by old scale collections, and seven contemporary samples (1986-2000). In one population historical and contemporary samples were remarkably genetically similar despite more than a decade of intense stocking. Estimation of admixture proportions showed a small genetic contribution from domesticated trout (approximately 6%), and individual admixture analysis demonstrated a majority of nonadmixed individuals. The expected genetic contribution by domesticated trout was 64%, assessed from the number of stocked trout and assuming equal survival and reproductive performance of wild and domesticated trout. This demonstrates poor performance and low fitness of domesticated trout in the wild. In another population there was a strong genetic contribution from domesticated trout (between 57% and 88% in different samples), both in samples from a broodstock thought to represent the indigenous population and in a sample of wild spawners. Survival of domesticated trout and admixture with indigenous fish in the broodstock and subsequent stocking into the river, combined with a low population size of native trout relative to the number of stocked trout, could explain the observed introgression. Few nonadmixed individuals remained in the introgressed population, and I discuss how individual admixture analysis can be used to identify and conserve nonintrogressed remains of the population.

Alleles↗