A mixed-model approach to association mapping using pedigree information with an illustration of resistance to Phytophthora infestans in potato.

Association or linkage disequilibrium (LD)-based mapping strategies are receiving increased attention for the identification of quantitative trait loci (QTL) in plants as an alternative to more traditional, purely linkage-based approaches. An attractive property of association approaches is that they do not require specially designed crosses between inbred parents, but can be applied to collections of genotypes with arbitrary and often unknown relationships between the genotypes. A less obvious additional attractive property is that association approaches offer possibilities for QTL identification in crops with hard to model segregation patterns. The availability of candidate genes and targeted marker systems facilitates association approaches, as will appropriate methods of analysis. We propose an association mapping approach based on mixed models with attention to the incorporation of the relationships between genotypes, whether induced by pedigree, population substructure, or otherwise. Furthermore, we emphasize the need to pay attention to the environmental features of the data as well, i.e., adequate representation of the relations among multiple observations on the same genotypes. We illustrate our modeling approach using 25 years of Dutch national variety list data on late blight resistance in the genetically complex crop of potato. As markers, we used nucleotide binding-site markers, a specific type of marker that targets resistance or resistance-analog genes. To assess the consistency of QTL identified by our mixed-model approach, a second independent data set was analyzed. Two markers were identified that are potentially useful in selection for late blight resistance in potato.

A gene of Acinetobacter calcoaceticus BD413 encodes a periplasmic peptidyl-prolyl cis-trans isomerase of the cyclophilin sub-class that is not essential for growth.

Downstream of the Acinetobacter calcoaceticus estA gene, encoding a cell-bound esterase, an open reading frame (orf) was identified, which may encode a protein with a mass of 20.4 kDa. This protein shows extensive similarity to both prokaryotic and eukaryotic peptidyl-prolyl cis-trans isomerases (PPIases) of the cyclophilin sub-class, especially to the periplasmic rotamase (RotA) of Escherichia coli. A putative signal sequence suggests that the product of the Acinetobacter gene, we termed rotA, is located outside the cytoplasm. Transcription of the gene is initiated from a promoter, just upstream of the rotA orf. The observation that two A. calcoaceticus rotA deletion mutants display no apparent mutant phenotype, suggests that this PPIase is not essential for growth of the organism. These mutants, to our knowledge, are the first prokaryotic PPIase mutants reported.

Genetic structure in populations of an ancient woodland sedge, Carex sylvatica Hudson, at a regional and local scale.

Wood sedge (Carex sylvatica) is a well-known ancient woodland species with a long-term persistent seed bank and a caespitose growth habit. All thirteen isolated Carex sylvatica populations in the Dutch Rhine floodplain (including the river branches Waal and IJssel) were mapped in detail and analysed for genetic variation at a large number of AFLP loci and one microsatellite locus. Across all populations, only 40 % of the sampled individuals (n=216) represented a unique genotype. A high number of the studied patches (spatial clusters of tussocks, 2-10 m in diameter) within populations contained only one or a few genotypes. Identical plants (tussocks) were also found 20-500 m apart and in one case even 1000 m apart. Observed heterozygosity levels (H(O)=0.029) were low, indicating low levels of gene flow, which is in agreement with the selfing nature of other caespitose sedges. Although the number of genotypes in populations is low, these genotypes are genetically very distinct and variation within populations accounted for 55% of the total variation. The absence of a correlation between genetic and geographic distances among populations, and the scattered distribution of genotypes among patches within woodlands, support our hypothesis of rare establishments and subsequent local dispersal within woodlands in this forest floor species, which may benefit from and partly depend on human land use and forest management activities.

Cloning in Escherichia coli of the gene specifying the DNA-entry nuclease of Bacillus subtilis.

With the aim of cloning genes involved in transformation of Bacillus subtilis, a set of transformation-deficient mutants was isolated by means of insertional mutagenesis with plasmid pHV60 (Vosman et al., 1986). Analysis of these mutants showed that those mapping in the aroI region lacked the DNA-entry nuclease activity. Plasmid pHV60 derivatives, containing flanking chromosomal DNA fragments, were isolated from these mutants and were used to screen a library of B. subtilis chromosomal DNA in phage lambda EMBL4. In Escherichia coli lysates, prepared with the phages that hybridized to the pHV60-based probe, a prominent nuclease activity could be detected. The nuclease encoded by the phage DNA had the same Mr as the B. subtilis DNA-entry nuclease and its activity was strongly stimulated by Mn2+, which is also characteristic for the B. subtilis DNA-entry nuclease. From these results it was concluded that the gene specifying the B. subtilis DNA-entry nuclease had been cloned. It was shown that the nuclease activity was specified by a 700-bp EcoRI-PstI fragment.

Construction and testing of a microsatellite database containing more than 500 tomato varieties.

The aim of this study was to evaluate the suitability of sequence tagged microsatellite site (STMS) markers for varietal identification and discrimination in tomato. For this purpose, a set of 20 STMS primer pairs was used to construct a database containing the molecular description of the most common varieties (>500) of tomato grown in Europe. The database was built and tested by a consortium of five European laboratories each using a different STMS detection system. In this way, it could be demonstrated that the STMS markers and database were suitable for use in network activities where a common database is being established on a continuing basis with data from different laboratories.Microsatellite polymorphism in tomato was found to be relatively low. The number of alleles per locus ranged from 2 to 8 with an average of 4.7 alleles per locus. Nevertheless, more than 90% of the varieties had different microsatellite profiles. A "blind testing" exercise showed that in general, identification of unknown samples (or detecting the most similar variety) with the 20 markers and the database was relatively easy for homogeneous varieties but less certain with heterogeneous varieties when using pools of 6 individuals.

Microsatellite fingerprinting in lettuce (Lactuca sativa L.) and wild relatives.

Southern hybridisation with a single microsatellite probe, (TCT)10, sufficed to discriminate between a representative set of cultivars/accessions of lettuce, Lactuca sativa L., and its wild relatives L. serriola, L. saligna and L. virosa. Variability within cultivars was tested in a relatively modern cultivar (Hector), where no variation was found, and in an older and morphologically more variable cultivar (Madrilene), where heterogeneity was observed in the TCT fingerprint. (TCT)10 fingerprinting should be useful for variety identification and homogeneity testing in lettuce.

QTL identification for early blight resistance (Alternaria solani) in a Solanum lycopersicum x S. arcanum cross.

Alternaria solani (Ellis and Martin) Sorauer, the causal agent of early blight (EB) disease, infects aerial parts of tomato at both seedling and adult plant stages. Resistant cultivars would facilitate a sustainable EB management. EB resistance is a quantitatively expressed character, a fact that has hampered effective breeding. In order to identify and estimate the effect of genes conditioning resistance to EB, a quantitative trait loci (QTL) mapping study was performed in F2 and F3 populations derived from the cross between the susceptible Solanum lycopersicum (syn. Lycopersicon esculentum) cv. 'Solentos' and the resistant Solanum arcanum (syn. Lycopersicon peruvianum) LA2157 and genotyped with AFLP, microsatellite and SNP markers. Two evaluation criteria of resistance were used: measurements of EB lesion growth on the F2 plants in glasshouse tests and visual ratings of EB severity on foliage of the F3 lines in a field test. A total of six QTL regions were mapped on chromosomes 1, 2, 5-7, and 9 with LOD scores ranging from 3.4 to 17.5. Three EB QTL also confer resistance to stem lesions in the field, which has not been reported before. All QTL displayed significant additive gene action; in some cases a dominance effect was found. Additive x additive epistatic interactions were detected between one pair of QTL. For two QTL, the susceptible parent contributed resistance alleles to both EB and stem lesion resistance. Three of the QTL showed an effect in all tests despite methodological and environmental differences.

The distribution of genetic diversity in a Brassica oleracea gene bank collection related to the effects on diversity of regeneration, as measured with AFLPs.

The ex situ conservation of plant genetic resources in gene banks involves the selection of accessions to be conserved and the maintenance of these accessions for current and future users. Decisions concerning both these issues require knowledge about the distribution of genetic diversity within and between accessions sampled from the gene pool, but also about the changes in variation of these samples as a result of regenerations. These issues were studied in an existing gene bank collection of a cross-pollinating crop using a selection of groups of very similar Dutch white cabbage accessions, and additional groups of reference material representing the Dutch, and the global white cabbage gene pool. Six accessions were sampled both before and after a standard regeneration. 30 plants of each of 50 accessions plus 6 regeneration populations included in the study were characterised with AFLPs, using scores for 103 polymorphic bands. It was shown that the genetic changes as a result of standard gene bank regenerations, as measured by AFLPs, are of a comparable magnitude as the differences between some of the more similar accessions. The observed changes are mainly due to highly significant changes in allele frequencies for a few fragments, whereas for the majority of fragments the alleles occur in similar frequencies before and after regeneration. It is argued that, given the changes of accessions over generations, accessions that display similar levels of differentiation may be combined safely.

Unique genomic configuration revealed by microsatellite DNA in polyploid dogroses, Rosa sect. Caninae.

An allopolyploid complex with high genomic integrity has been studied. Dogroses transmit only seven chromosomes (from seven bivalents) through the pollen, whereas 21, 28 or 35 chromosomes (from seven bivalents and 14, 21 or 28 univalents) come from the egg cells. Seedlings derived from two interspecific crosses were analysed with flow cytometry and molecular markers to determine ploidy level, mode of reproduction and genomic constitution. Evidence was obtained for the formation of unreduced male and female gametes, which can take part in fertilization (producing seedlings with higher ploidy than the parental plants) or in apomictic reproduction. Random amplified polymorphic DNA (RAPD) and microsatellite analyses indicated that three seedlings (5%) were derived through apomixis, whereas the other 49 were hybrids. Bivalent formation appears to involve chromosomes that consistently share the same microsatellite alleles. Allele-sharing between the maternally transmitted and highly conserved univalent-forming chromosomes reflected the taxonomic distance between different genotypes. The frequently recombining bivalent-forming chromosomes were taxonomically less informative.

Microsatellite analysis of Rosa damascena Mill. accessions reveals genetic similarity between genotypes used for rose oil production and old Damask rose varieties.

Damask roses are grown in several European and Asiatic countries for rose oil production. Twenty-six oil-bearing Rosa damascena Mill. accessions and 13 garden Damask roses were assayed by molecular markers. Microsatellite genotyping demonstrated that R. damascena Mill. accessions from Bulgaria, Iran, and India and old European Damask rose varieties possess identical microsatellite profiles, suggesting a common origin. At the same time, the data indicated that modern industrial oil rose cultivation is based on a very narrow genepool and that oil rose collections contain many genetically identical accessions. The study of long-term vegetative propagation of the Damask roses also reveals high somatic stability for the microsatellite loci analyzed.

Introduction of a Streptococcus cremoris plasmid in Bacillus subtilis.

Streptococcus cremoris Wg2 plasmid pWV01 was introduced in Bacillus subtilis by protoplast transformation. The yield of pWV01 isolated from B. subtilis was low. pWV01 contains a unique site for the restriction endonuclease MboI.

Molecular characterization of GATA/GACA microsatellite repeats in tomato.

Microsatellite repeats like GATA or GACA display a degree of variability that allows their use in cultivar identification. Southern hybridization with oligonucleotide probes complementary to these microsatellites were used for the detection of polymorphisms. To understand the molecular structure of the detected DNA, fragments hybridizing to GATA and GACA probes were cloned and sequenced. In the four clones analyzed, repeats of GATA and GACA were found intertwined. The GATA and GACA arrays were not perfect but were heavily degenerated, in that they contained many tetranucleotides that might have been derived by a single point mutation from GATA or GACA. Some of these derived sequences, like GGTA and GGAT, were present as relatively long stretches that also contained some point mutations. This supports the hypothesis that long stretches of repeats are stabilized by the accumulation of point mutations. Analysis of the flanking sequences of the fragments obtained with the GACA probe showed that one of them was homologous to a Lilium henryi retrotransposon and the other to a sequence upstream of a potato patatin gene. The two fragments obtained using the GATA probe were flanked by DNA that had no homology to any known sequence but they were highly homologous to each other. This DNA was frequently associated with GATA elements and was present in the tomato genome in approximately 4300 copies. The function of this new class of repetitive DNA, here termed U30, is presently unknown.

Molecular cloning and functional characterization of a recA analog from Pseudomonas stutzeri and construction of a P. stutzeri recA mutant.

A recombinant plasmid carrying the Pseudomonas stutzeri recA gene was isolated by complementation of the Escherichia coli recA13 mutation. Subcloning experiments showed that the gene was located on a 1500 bp PvuII-BglII fragment. The cloned gene complements an E. coli recA mutant for resistance to Methylmethanosulphonate (MMS) and UV irradiation. It was also capable of restoring the recombination proficiency in that mutant. The cloned fragment was used to construct a recA deletion mutant of P. stutzeri. This mutant too was shown to be sensitive towards MMS and UV irradiation. The mutant strain was found to be completely deficient in natural transformation with chromosomal DNA, due to the impairment in homologous recombination.

Cloning and characterization of a Bacillus subtilis transcription unit involved in ATP-dependent DNase synthesis.

By insertional mutagenesis, 36 transformation-deficient, mitomycin C-sensitive Bacillus subtilis mutants were isolated, 16 of which were ATP-dependent DNase (ADD) deficient. PBS1 transduction showed that the mutations were closely linked to the metD marker and weakly linked to the glyB marker. With the aid of one of the mutants, a transcription unit involved in ADD synthesis was cloned. The chromosomal location of the transcription unit was established at the restriction site level by determining the presence or absence of ADD in transformants of wild-type cells obtained with various DNA fragments inserted in pUC derivatives. The transcription unit complemented a mutant in which the add transcription unit had been deleted.

Integration of vector-containing Bacillus subtilis chromosomal DNA by a Campbell-like mechanism.

Using plasmid pHV60, which contains a chloramphenicol resistance (Cmr) gene that is expressed in Bacillus subtilis, a set of transformation-deficient strains of B. subtilis was isolated by insertional mutagenesis. When chromosomal DNA from these mutants was used to transform a transformation-proficient B. subtilis strain, almost all of the Cmr transformants had the mutant phenotype as expected. However, with a frequency of approximately 3 X 10(-4) atypical transformants with the wild-type phenotype were produced. Data concerning amplification of the DNA containing the Cmr marker and duplication of DNA sequences are presented that suggest that these atypical transformants are the result of a Campbell-like integration of the chromosomal DNA containing the integrated plasmid. Transductional mapping showed that in the atypical transformants the vector-containing DNA had a strong tendency to integrate at sites adjacent to the original site of integration, although integration at sites elsewhere on the chromosome was also observed. The production of atypical transformants is explained on the basis of integration of chromosomal DNA by a Campbell-like mechanism. Circularization of vector-containing chromosomal DNA is thought to occur through joining of the extremities of single-stranded DNA molecules by fortuitous base pairing with an independently entered single-stranded DNA molecule.

Transformation in Bacillus subtilis: involvement of the 17-kilodalton DNA-entry nuclease and the competence-specific 18-kilodalton protein.

A protein complex, consisting of a 17-kilodalton (kDa) nuclease and an 18-kDa protein, is believed to be involved in the binding and entry of donor DNA during transformation of Bacillus subtilis (H. Smith, K. Wiersman, S. Bron, and G. Venema, J. Bacteriol. 156:101-108, 1983). In this paper, the nucleotide sequences of the genes encoding both the nuclease and the 18-kDa protein are presented. The genes are encoded by a 904-base-pair PstI-HindIII fragment. The open reading frames encoding both proteins are partly overlapping. A B. subtilis mutant was constructed by insertion of a Cmr marker into the gene encoding the nuclease. This mutant lacked the competence-specific nuclease activity and the 18-kDa protein but retained 5% residual transformation. The total DNA association of the mutant was higher than that of the wild-type cells, and DNA entry was reduced to 30% of the wild-type level. These results suggest that an alternative pathway exists for the internalization of transforming DNA. A mutant, exclusively deficient for the 18-kDa protein, previously suggested to be involved in the binding of transforming DNA, was constructed by insertion of a kanamycin resistance gene into the coding sequence of the gene. Since the mutant showed wild-type DNA-binding activity, the 18-kDa protein is probably not involved in the binding of donor DNA to competent cells. The transforming activity of the mutant was reduced to 25% of the wild-type level, indicating that the 18-kDa protein has a function in the transformation process. In vitro experiments showed that the 18-kDa protein is capable of inhibiting the activity of the competence-specific nuclease. Its possible role in transformation is discussed.

Tissue culture-induced DNA methylation polymorphisms in repetitive DNA of tomato calli and regenerated plants.

The propagation of plants through tissue culture can induce a variety of genetic and epigenetic changes. Variation in DNA methylation has been proposed as a mechanism that may explain at least a part of these changes. In the present study, the methylation of tomato callus DNA was compared with that of leaf DNA, from control or regenerated plants, at MspI/HpaII sites around five middle-repetitive sequences. Although the methylation of the internal cytosine in the recognition sequence CCGG varied from zero to nearly full methylation, depending on the probe used, no differences were found between callus and leaf DNA. For the external cytosine, small differences were revealed between leaf and callus DNA with two probes, but no polymorphisms were detected among DNA samples of calli or DNA samples of leaves of regenerated plants. When callus DNA cut with HindIII was studied with one of the probes, H9D9, most of the signal was found in high-molecular-weight DNA, as opposed to control leaf DNA where almost all the signal was in a fragment of 530 bp. Also, an extra fragment of 630 bp was found in the callus DNA that was not present in control leaf DNA. Among leaves of plants regenerated from tissue culture, the 630-bp fragment was found in 10 of 68 regenerated plants. This 630-bp fragment was present among progeny of only 4 of these 10 plants after selfing, i.e. it was partly inherited. In these cases, the fragment was not found in all progeny plants, indicating heterozygosity of the regenerated plants. The data are interpreted as indicating that a HindIII site becomes methylated in callus tissue, and that some of this methylation persists in regenerated plants and is partly transmitted to their progeny.

Microsatellite markers useful throughout the genus Dianthus.

Using repeats found in sequences from Dianthus species present in the EMBL database, primers for STMS (sequence-tagged microsatellite site) analysis were developed and tested. Five loci were polymorphic and amplified products of sufficient quality in nearly all of the 26 Dianthus species tested, except MS-DINGSTA, which amplified in only one-third of the species. Loci MS-DINMADSBOX and MS-DCDIA30 produced allele series that were mostly two nucleotides (the repeat unit) apart. MS-DCAMCRBSY and MS-DINCARACC also amplified regular series of alleles, but more than two fragments per individual were detected in a number of species. Both loci code for a member of the ACC synthase gene family. The observation that the loci amplified across a wide range of Dianthus species may imply that the different species within the genus are relatively closely related. Alternatively, it may indicate that the regions selected for primer design (some of which are in coding regions) are well conserved. These microsatellites will be useful for the measurement of genetic diversity in natural populations of Dianthus species and the identification of carnation varieties.

Identification of cut rose (Rosa hybrida) and rootstock varieties using robust sequence tagged microsatellite site markers.

In this study a DNA fingerprinting protocol was developed for the identification of rose varieties based on the variability of microsatellites. Microsatellites were isolated from Rosa hybrida L. using enriched small insert libraries. In total 24 polymorphic sequenced tagged microsatellite site (STMS) markers with easily scorable allele profiles, from six different linkage groups, were used to characterize 46 Hybrid Tea varieties and 30 rootstock varieties belonging to different species (Rosa canina L., Rosa indica Thory., Rosa chinensis Jacq., Rosa rubiginosa L., and Rosa rubrifolia glauca Pour.). Clones and known flower color mutants were identified as being identical, all other varieties were differentiated by a unique pattern with as few as three STMS markers. The high discriminating power of the loci suggests that a selection of the most-robust STMS markers may be able to differentiate any two varieties within rootstocks or Hybrid Teas except for mutants. The selected STMS markers will be useful as a tool for reference collection management, for assessing essential derivation of varieties and illegal propagation.