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PubMed1993

Mitochondrial DNA polymorphism and feminizing sex factors dynamics in a natural population of Armadillidium vulgare (Crustacea, Isopoda).

Sex determination in Armadillidium vulgare may be under the control of two parasitic sex factors that reverse genetic males into functional neo-females. The first feminizing factor (F) is a Wolbachia and the other (f) is probably a sequence of the F bacterial DNA unstably integrated into the host genome. Both of these feminizing factors are mainly maternally transmitted. Here we investigate the mitochondrial DNA polymorphism of wild iso-female lineages harbouring either F or f. Among the four haplotypes present in the population, two were the f-harbouring lineages, while two were common to the F- and f-harbouring lineages. This result suggests that there has been an introgression of the f factor into lineages infected by F Wolbachia. Based on previous data, we propose two different ways to account for such introgression. Given the particular dynamics of feminizing factors (f-harbouring lineages increase in populations at the expense of F-harbouring lineages), such an introgression should prevent the replacement of F-linked mitochondrial types by f-linked mitochondrial types in wild populations.

Animals↗

PubMed1996

A three-locus system of interspecific incompatibility underlies male inviability in hybrids between Drosophila buzzatii and D. koepferae.

In hybrids between the sibling species D. buzzatii and D. koepferae, both sexes are more or less equally viable in the F1. However, backcross males to D. buzzatii are frequently inviable, apparently because of interspecific genetic incompatibilities that are cryptic in the F1. We have performed a genetic dissection of the effects of the X chromosome from D. koepferae. We found only two cytological regions, termed hmi-1 and hmi-2, altogether representing 9% of the whole chromosome, which when introgressed into D. buzzatii cause inviability of hybrid males. Observation of the pattern of asynapsis of polytene chromosomes (incomplete pairing, marking introgressed material) in females and segregation analyses were the technique used to infer the X chromosome regions responsible for this hybrid male inviability. The comparison of these results with those previously obtained with the same technique for hybrid male sterility in this same species pair indicate that in the X chromosome of D. koepferae there are at least seven times more regions that produce hybrid male sterility than hybrid male inviability. We have also found that the inviability brought about by the introgression of hmi-1 is suppressed by the cointrogression of two autosomal sections from D. koepferae. Apparently, these three regions conform to a system of species-specific complementary factors involved in an X-autosome interaction that, when disrupted in backcross hybrids by recombination with the genome of its sibling D. buzzatii, brings about hybrid male inviability.

Animals↗

PubMed1992

Genetic and physical analysis of the rice bacterial blight disease resistance locus, Xa21.

Nearly isogenic lines (NILs) of rice (Oryza sativa) differing at a locus conferring resistance to the pathogen Xanthomonas oryzae pv. oryzae were surveyed with 123 DNA markers and 985 random primers using restriction fragment length plymorphism (RFLP) and random amplified polymorphic DNA (RAPD) analysis. One chromosome 11 marker (RG103) detected polymorphism between the NILs that cosegregated with Xa21. All other chromosome 11 DNA markers tested were monomorphic between the NILs, localizing the Xa21 introgressed region to an 8.3 cM interval on chromosome 11. Furthermore, we identified two polymerase chain reaction (PCR) products (RAPD2148 and RAPD818) that detected polymorphisms between the NILs. Genomic sequences hybridizing with RAPD818, RAPD248 and RG103 were duplicated specifically in the Xa21 NIL. All three markers cosegregated with the resistance locus, Xa21, in a F2 population of 386 progeny. Based on the frequency with which we recovered polymorphic Xa21-linked markers, we estimated the physical size of the introgressed region to be approximately 800 kb. This estimation was supported by physical mapping (using pulsed field gel electrophoresis) of the sequences hybridizing with the three Xa21-linked DNA markers. The results showed that the three Xa21-linked markers are physically close to each other, with one copy of the RAPD818 sequences located within 60 kb of RAPD248 and the other copy within 270 kb of RG103. None of the enzymes tested generated a DNA fragment that hybridized with all three of the markers indicating that the introgressed region containing the resistance locus Xa21 is probably larger than 270 kb.

Chromosome Mapping↗

PubMed1979

A comparison of chromosomal and allozymal variation across a narrow hybrid zone in the grasshopper Caledia captiva.

A hydrid zone between the Moreton and Torresian taxa of the grasshopper Caledia captiva in S.E. Queensland has been characterised in terms of allozyme and chromosome variation within the same individuals.--On chromosomal criteria (pericentric rearrangements), the zone is asymmetrical with evidence of high levels of introgression of Torresian chromosomes into the Moreton taxon. This is apparent from the analysis of two independent transects across the hydrid zone. Major changes in chromosomal frequency occur over distances of less than 0.5 km. and the level of introgression differs between the two transects, with much higher levels in the northern Moreton populations, characterised by an acrocentric X-chromosome, when compared with the southern metacentric-X Moreton populations. Chromosome analysis of samples taken from the same transect over two years has revealed no major changes in the structure of the zone. Moreover, a Moreton population located only 0.5 km. from the null point was found to be stable over 6 generations with evidence for a new balanced genome having originated following the differential incorportation of Torresian chromosomes.--Contrary to the chromosomal situation, the same hybrid zone was found to be symmetrical with respect to allozyme variation with evidence of movement of diagnostic alleles in both directions across the zone. The alleles are independent and not tightly linked to any of the pericentric rearrangements. Thus these 5 alleles are acting as markers of the background genome and reveal the relatively free movement of genes which are located outside the pericentric rearrangements.--It is proposed that the hybrid zone in Caledia captiva is unstable and is moving slowly in a westerly direction into the Torresian territory. This is due to the ability of the Moreton taxon to incorporate more readily into its genome those Torresian chromosomes or chromosome segments which increase the fitness of the Moreton taxon. On chromosomal criteria, the Torresian taxon does not share the same capacity.--It is suggested that, so long as the two taxa retain their ability to hybridise with subsequent asymmetrical introgression, the zone will continue to move westwards and eventually lead to the selective incorporation of the Torresian genome into the Moreton taxon. This will result in a polymorphic situation with clinal variation in chromosomal frequencies. The structure of the zone is dependent upon a fine balance between genomic reorganisation in recombinant genotypes and the relative dispersal capacities of the two hydridising taxa.

Animals↗

PubMed2003

Hybridization, transgressive segregation and evolution of new genetic systems in Drosophila.

Introgressive hybridization facilitates incorporation of genes from one species into the gene pool of another. Studies on long-term effects of introgressive hybridization in animal systems are sparse. Drosophila nasuta (2n = 8) and D. albomicans (2n = 6)-a pair of allopatric, morphologically almost identical, cross-fertile members of the nasuta subgroup of the immigrans species group-constitute an excellent system to analyse the impact of hybridization followed by transgressive segregation of parental characters in the hybrid progeny. Hybrid populations of D. nasuta and D. albomicans maintained for over 500 generations in the laboratory constitute new recombinant hybrid genomes, here termed cytoraces. The impact of hybridization, followed by introgression and transgressive segregation, on chromosomal constitution and karyotypes, some fitness parameters, isozymes, components of mating behaviour and mating preference reveals a complex pattern of interracial divergence among parental species and cytoraces. This assemblage of characters in different combinations in a laboratory hybrid zone allows us to study the emergence of new genetic systems. Here, we summarize results from our ongoing studies comparing these hybrid cytoraces with the parental species, and discuss the implications of these findings for our understanding of the evolution of new genetic systems.

Animals↗

PubMed1997

Amelioration of bacterial genomes: rates of change and exchange.

Although bacterial species display wide variation in their overall GC contents, the genes within a particular species' genome are relatively similar in base composition. As a result, sequences that are novel to a bacterial genome-i.e., DNA introduced through recent horizontal transfer-often bear unusual sequence characteristics and can be distinguished from ancestral DNA. At the time of introgression, horizontally transferred genes reflect the base composition of the donor genome; but, over time, these sequences will ameliorate to reflect the DNA composition of the new genome because the introgressed genes are subject to the same mutational processes affecting all genes in the recipient genome. This process of amelioration is evident in a large group of genes involved in host-cell invasion by enteric bacteria and can be modeled to predict the amount of time required after transfer for foreign DNA to resemble native DNA. Furthermore, models of amelioration can be used to estimate the time of introgression of foreign genes in a chromosome. Applying this approach to a 1.43-megabase continuous sequence, we have calculated that the entire Escherichia coli chromosome contains more than 600 kb of horizontally transferred, protein-coding DNA. Estimates of amelioration times indicate that this DNA has accumulated at a rate of 31 kb per million years, which is on the order of the amount of variant DNA introduced by point mutations. This rate predicts that the E. coli and Salmonella enterica lineages have each gained and lost more than 3 megabases of novel DNA since their divergence.

Bacteria↗

PubMed2002

Brassica napus DNA markers linked to white rust resistance in Brassica juncea.

White rust, caused by Albugo candida, is an economically important disease of Brassica juncea mustard. The most efficient and cost effective way of protecting mustard plants from white rust is through genetic resistance. The development of canola quality B. juncea through interspecific crosses of B. juncea with Brassica napus has lead to the introgression of white rust resistance from B. napus into B. juncea. The objective of this study was to identify DNA markers for white rust resistance, derived from the introgressed B. napus chromosome segment, in a BC(3)F(2) population of condiment B. juncea mustard. This segregating population was phenotyped for white rust reaction and used to screen for AFLP markers associated with white rust resistance using bulked segregant analysis. Segregation data indicated that a single dominant gene controlled resistance to white rust. Eight AFLP markers linked to white rust resistance were identified, all derived from B. napus. The B. napus chromosome segment, carrying the white rust resistance gene ( Ac2V(1)), appeared to have recombined with the B. juncea DNA since recombinant individuals were identified. Comparative mapping of the eight B. napus-derived AFLP markers in a typical B. napus mapping population was inconclusive; therefore, the size of the introgressed B. napus fragment could not be determined.

Journal Article↗

PubMed2004

Pollination between maize and teosinte: an important determinant of gene flow in Mexico.

Gene flow between maize [Zea mays (L.)] and its wild relatives does occur, but at very low frequencies. Experiments were undertaken in Tapachula, Nayarit, Mexico to investigate gene flow between a hybrid maize, landraces of maize and teosinte (Z. mays ssp. mexicana, races Chalco and Central Plateau). Hybridization, flowering synchrony, pollen size and longevity, silk elongation rates, silk and trichome lengths and tassel diameter and morphology were measured. Hybrid and open-pollinated maize ears produced a mean of 8 and 11 seeds per ear, respectively, when hand-pollinated with teosinte pollen, which is approximately 1-2% of the ovules normally produced on a hybrid maize ear. Teosinte ears produced a mean of 0.2-0.3 seeds per ear when pollinated with maize pollen, which is more than one-fold fewer seeds than produced on a maize ear pollinated with teosinte pollen. The pollination rate on a per plant basis was similar in the context of a maize plant with 400-500 seeds and a teosinte plant with 30-40 inflorescences and 9-12 fruitcases per inflorescence. A number of other factors also influenced gene-flow direction: (1) between 90% and 95% of the fruitcases produced on teosinte that was fertilized by maize pollen were sterile; (2) teosinte collections were made in an area where incompatibility systems that limit fertilization are present; (3) silk longevity was much shorter for teosinte than for maize (approx. 4 days vs. approx. 11 days); (4) teosinte produced more pollen on a per plant basis than the landraces and commercial hybrid maize; (5) teosinte frequently produced lateral branches with silks close to a terminal tassel producing pollen. Collectively these factors tend to favor crossing in the direction of teosinte to maize. Our results support the hypothesis that gene flow and the subsequent introgression of maize genes into teosinte populations most probably results from crosses where teosinte first pollinates maize. The resultant hybrids then backcross with teosinte to introgress the maize genes into the teosinte genome. This approach would slow introgression and may help explain why teosinte continues to co-exist as a separate entity even though it normally grows in the vicinity of much larger populations of maize.

Breeding↗

PubMed2005

Construction of a Festuca pratensis BAC library for map-based cloning in Festulolium substitution lines.

Introgression in Festulolium is a potentially powerful tool to isolate genes for a large number of traits which differ between Festuca pratensis Huds. and Lolium perenne L. Not only are hybrids between the two species fertile, but the two genomes can be distinguished by genomic in situ hybridisation and a high frequency of recombination occurs between homoeologous chromosomes and chromosome segments. By a programme of introgression and a series of backcrosses, L. perenne lines have been produced which contain small F. pratensis substitutions. This material is a rich source of polymorphic markers targeted towards any trait carried on the F. pratensis substitution not observed in the L. perenne background. We describe here the construction of an F. pratensis BAC library, which establishes the basis of a map-based cloning strategy in L. perenne. The library contains 49,152 clones, with an average insert size of 112 kbp, providing coverage of 2.5 haploid genome equivalents. We have screened the library for eight amplified fragment length polymorphism (AFLP) derived markers known to be linked to an F. pratensis gene introgressed into L. perenne and conferring a staygreen phenotype as a consequence of a mutation in primary chlorophyll catabolism. While for four of the markers it was possible to identify bacterial artificial chromosome (BAC) clones, the other four AFLPs were too repetitive to enable reliable identification of locus-specific BACs. Moreover, when the four BACs were partially sequenced, no obvious coding regions could be identified. This contrasted to BACs identified using cDNA sequences, when multiple genes were identified on the same BAC.

Blotting, Southern↗

PubMed2005

Genetic and physical mapping of the grapevine powdery mildew resistance gene, Run1, using a bacterial artificial chromosome library.

Resistance to grapevine powdery mildew is controlled by Run1, a single dominant gene present in the wild grapevine species, Muscadinia rotundifolia, but absent from the cultivated species, Vitis vinifera. Run1 has been introgressed into V. vinifera using a pseudo-backcross strategy, and genetic markers have previously been identified that are linked to the resistance locus. Here we describe the construction of comprehensive genetic and physical maps spanning the resistance locus that will enable future positional cloning of the resistance gene. Physical mapping was performed using a bacterial artificial chromosome (BAC) library constructed using genomic DNA extracted from a resistant V. vinifera individual carrying Run1 within an introgression. BAC contig assembly has enabled 20 new genetic markers to be identified that are closely linked to Run1, and the position of the resistance locus has been refined, locating the gene between the simple sequence repeat (SSR) marker, VMC4f3.1, and the BAC end sequence-derived marker, CB292.294. This region contains two multigene families of resistance gene analogues (RGA). A comparison of physical and genetic mapping data indicates that recombination is severely repressed in the vicinity of Run1, possibly due to divergent sequence contained within the introgressed fragment from M. rotundifolia that carries the Run1 gene.

Ascomycota↗

PubMed2005

Molecular dissection of interspecific variation between Gossypium hirsutum and Gossypium barbadense (cotton) by a backcross-self approach: I. Fiber elongation.

The current study is the first installment of an effort to explore the secondary gene pool for the enhancement of Upland cotton (Gossypium hirsutum L.) germplasm. We developed advanced-generation backcross populations by first crossing G. hirsutum cv. Tamcot 2111 and G. barbadense cv. Pima S6, then independently backcrossing F(1) plants to the G. hirsutum parent for three cycles. Genome-wide mapping revealed introgressed alleles at an average of 7.3% of loci in each BC(3)F(1) plant, collectively representing G. barbadense introgression over about 70% of the genome. Twenty-four BC(3)F(1) plants were selfed to generate 24 BC(3)F(2) families of 22-172 plants per family (totaling 2,976 plants), which were field-tested for fiber elongation and genetically mapped. One-way analysis of variance detected 22 non-overlapping quantitative trail loci (QTLs) distributed over 15 different chromosomes. The percentage of variance explained by individual loci ranged from 8% to 28%. Although the G. barbadense parent has lower fiber elongation than the G. hirsutum parent, the G. barbadense allele contributed to increased fiber elongation at 64% of the QTLs. Two-way analysis of variance detected significant (P<0.001) among-family genotype effects and genotypexfamily interactions in two and eight regions, respectively, suggesting that the phenotypic effects of some introgressed chromosomal segments are dependent upon the presence/absence of other chromosomal segments.

Analysis of Variance↗

PubMed2006

Pyramiding and dissecting disease resistance QTL to barley stripe rust.

Quantitative resistance (QR) to disease is usually more durable than qualitative resistance, but its genetic basis is not well understood. We used the barley/barley stripe rust pathosystem as a model for the characterization of the QR phenotype and associated genomic regions. As an intermediate step in the preparation of near-isogenic lines representing individual QTL alleles and combinations of QTL alleles in a homogeneous genetic background, we developed a set of QTL introgression lines in a susceptible background. These intermediate barley near-isogenic (i-BISON) lines represent disease resistance QTL combined in one-, two-, and three-way combinations in a susceptible background. We measured four components of disease resistance on the i-BISON lines: latent period, infection efficiency, lesion size, and pustule density. The greatest differences between the target QTL introgressions and the susceptible controls were for the latter three traits. On average, however, the QTL introgressions also had longer latent periods than the susceptible parent (Baronesse). There were significant differences in the magnitudes of effects of different QTL alleles. The 4H QTL allele had the largest effect, followed by the alleles on 1H and 5H. Pyramiding multiple QTL alleles led to higher levels of resistance in terms of all components of QR except latent period.

Alleles↗

PubMed2001

Genetic and physical analysis of a single Festuca pratensis chromosome segment substitution in Lolium perenne.

Molecular marker analysis and genomic in situ hybridisation (GISH) were used to examine the process of chromosome segment introgression in BC2 diploid hybrids (2n=2x=14) between Lolium perenne and Festuca pratensis. Two genotypes having what appeared to be the same, single, introgressed chromosome segment of F. pratensis in the L. perenne background were crossed with diploid L. perenne to produce a recombinant series for the introgressed region. Physical and genetic analysis of this series showed that, while recombination seemed to be possible at all points along the chromosome arm, the rate of recombination varied depending on relative position: more recombination was detected in the interstitial region as compared with the centromeric or telomeric regions. The implications of these results for the use of GISH and molecular marker analysis in the measurement of linkage drag in backcross breeding programmes is discussed.

Alleles↗

PubMed2006

Natural hybridization in primates: one evolutionary mechanism.

The role and importance of natural hybridization in the evolutionary histories of animal taxa is still debated. This results largely from a history of zoological investigations that assumed, rather than documented, a limited evolutionary role for this process. However, it is now becoming apparent that, just as for plants, the creative effects of reticulate evolution are widespread in animal taxa as well. This conclusion is supported by the documentation of numerous instances of the formation of new taxa and the genetic enrichment through introgressive hybridization. In the present review, we use primates as a paradigm for how natural hybridization can affect the evolution of species complexes and remains a footprint on genomes. Findings for a number of groups, including basal (e.g. lemurs) and derived (e.g. Old World apes) lineages, demonstrate that introgression and hybrid speciation have caused a reticulate pattern that is still detectable in the, often mosaic, genomes of primates. For example, results from genetic analyses of our own species demonstrate the process of past introgressive hybridization with the progenitors of our sister taxa (i.e. chimpanzees and gorillas) and most likely also our extinct, close relatives in the hominid lineage.

Animals↗

PubMed2002

Identification of malodorous, a wild species allele affecting tomato aroma that was aelected against during domestication.

Vegetable cultivation favored the inclusion of pleasant aromas in the produce, whereas unpleasant aromas were selected against. Introgression lines, generated by hybridization of a cultivated tomato (Lycopersicon esculentum) to its wild relative L. pennellii, were used to map quantitative trait loci (QTL) that influence tomato aroma. A marked undesirable flavor was detected by taste panelists in L. pennellii fruits and was related to an introgressed segment from the short arm of chromosome 8. Analysis of the ripe fruits' volatiles of chromosome 8 introgressed lines revealed an up to 60-fold increase in the levels of 2-phenylethanol and phenylacetaldehyde, as compared to the cultivated tomato. This effect was associated with a 10 cM segment originating from the wild species. Although 2-phenylethanol and phenylacetaldehyde have favorable contribution to tomato aroma when present at low levels, phenylacetaldehyde has a nauseating objectionable aroma when present in levels >0.005 ppm. The loss of the ability to produce high levels of phenylacetaldehyde contributed to the development of desirable aroma of the cultivated tomato. The findings provide a genetic explanation for one of the aroma changes that occurred during the domestication of the tomato.

Acetaldehyde↗

PubMed2003

Variability on the dot chromosome in the Drosophila simulans clade.

A recent study suggested that recent nuclear gene introgression between Drosophila simulans and D. mauritiana may have obscured efforts to estimate the phylogeny of the species of the D. simulans clade, which includes these two species and D. sechellia. Here, we report sequence variation of an intron of the eyeless gene in this species group. This gene should introgress freely between these species because it is not linked to any known barriers to gene exchange. We have also reevaluated levels of sequence divergence among species in this clade, noting differences between loci in regions of low recombination (as in all chromosome 4 loci) relative to other loci. Overall, none of the data analyzed were consistent with recent introgression exclusively between D. simulans and D. mauritiana.

Animals↗

PubMed2003

Development of cost-effective Hordeum chilense DNA markers: molecular aids for marker-assisted cereal breeding.

Hordeum chilense is a potential source of useful genes for wheat breeding. The use of this wild species to increase genetic variation in wheat will be greatly facilitated by marker-assisted introgression. In recent years, the search for the most suitable DNA marker system for tagging H. chilense genomic regions in a wheat background has lead to the development of RAPD and SCAR markers for this species. RAPDs represent an easy way of quickly generating suitable introgression markers, but their use is limited in heterogeneous wheat genetic backgrounds. SCARs are more specific assays, suitable for automatation or multiplexing. Direct sequencing of RAPD products is a cost-effective approach that reduces labour and costs for SCAR development. The use of SSR and STS primers originally developed for wheat and barley are additional sources of genetic markers. Practical applications of the different marker approaches for obtaining derived introgression products are described.

DNA, Plant↗

PubMed2003

Transferability of microsatellite and sequence tagged site markers in Oryza species.

The genus Oryza comprises 22 species which are potentially useful as a source of genetic variability that can be introgressed into the worldwide cultivated rice, Oryza sativa. Molecular markers are useful tools for monitoring gene introgressions and for detecting polymorphism among species. In this study, cross-amplification was estimated among 28 accessions of 16 Oryza species, representing the genomes AA, BB, CC, BBCC and CCDD, using 59 microsatellite (OG, OS and RM series) and 15 STS (Sequence Tagged Sites) markers. All markers amplified at least one Oryza species, indicating different levels of transferability across species. Markers based on microsatellite sequences amplified 37 % of the accessions, with an average of 6.58 alleles per locus and an average polymorphism information content (PIC) of 70 %. For STS markers, the amplification level was 53.3 %, and the average number of alleles and PIC values were 1.6 and 10 %, respectively. These Results showed that although the STS markers detected a reduced level of genetic diversity, the transferability was higher, indicating that they can be used for genetic analysis when evaluating less genetically related species of Oryza. Among the microsatellite markers, an analysis of species with an AA genome showed that the OG markers produced the highest level of polymorphic loci (54.6 %), followed by RM markers (48 %). Highly polymorphic and transferable molecular markers in Oryza can be useful for exploiting the genetic resources of this genus, for detecting allelic variants in loci associated with important agronomic traits, and for monitoring alleles introgressed from wild relatives to cultivated rice.

Agriculture↗