The root-knot nematode resistance gene (Mi) in tomato: construction of a molecular linkage map and identification of dominant cDNA markers in resistant genotypes

Abstract

A dominant allele at the Mi locus on chromosome 6 of tomato (Lycopersicon esculentum Mill) confers resistance to three species of root-knot nematodes (Meloidogyne). The resistance, which is associated with a localized necrotic response, was originally introduced into tomato from the wild species Lycopersicon peruvianum. As a step towards the molecular cloning of Mi, we have identified closely linked DNA markers from both cDNA and genomic DNA libraries as restriction fragment length polymorphisms (RFLPs). DNA from tomato populations segregating for nematode resistance was analyzed to generate a high-resolution genetic map of this region. Additional information on gene order was obtained by comparing the size of the introgressed L. peruvianum chromosomal segment within a collection of nematode-resistant tomato lines. Among the four cDNA markers that are tightly linked to Mi, three are dominant, i.e. L. peruvianum-specific. One cDNA marker corresponds to a gene family comprising 20-30 members, one of which is diagnostic for all nematode-resistant genotypes tested. The presence of non-homologous sequences around the Mi gene may contribute to the suppression of recombination in this region of the genome in crosses heterozygous for Mi. The potential of 'walking' from closely linked markers to Mi is discussed.

Plant-nematode interactions

“…Pure virulent and avirulent lines of G. rostochiensis for the H 1 resistance gene in potato have been produced by Janssen et al [57] and with the help of these pure lines, they provided the only clear example of Mendelian proof for a gene-for-gene relation between a plant and a nematode [3,56]. Breeding attention has focussed on dominant resistance genes for economically important crops and a select number of them are currently the subject of RFLP-mapping programs as a prelude to the isolation of the actual resistance genes [4,32,46,63,65,76,86]. A potentially faster source of resistance genes may come from the Arabidopsis genome although an extensive screen with 74 ecotypes yielded no resistance against H. schachtii [ 102].…”

Long-range physical maps of two loci (Aps-1 and GP79) flanking the root-knot nematode resistance gene (Mi) near the centromere of tomato chromosome 6

“…To translate the genetic distances of the markers into physical distances we have constructed physical maps of the two closest markers bracketing Mi, GP79 and Aps-1, using a set of rarely cutting restriction enzymes and pulsed-field gel electrophoresis (PFGE). It is shown that these two [21]). markers and two other markers (H3F8 and H4H 10) which map genetically to the same locus as Aps-1 [21, see Fig.…”

“…Recently a variety of tightly linked molecular markers have been identified including the acid phosphatase-1 (Aps-1)gene [1,12,33,39] and various RFLP [21,23] and RAPD [24] markers. These markers have been ordered into a highresolution molecular linkage map of the Mi region [21,28].…”

“…These markers have been ordered into a highresolution molecular linkage map of the Mi region [21,28]. Due to strong suppression of recombination accurate genetic mapping in the Mi region proved rather cumbersome, with genetic distances ranging from 0.3 to 2 cM between the closest Miflanking markers, Aps-1 and GP79 [21,28].…”

“…These markers have been ordered into a highresolution molecular linkage map of the Mi region [21,28]. Due to strong suppression of recombination accurate genetic mapping in the Mi region proved rather cumbersome, with genetic distances ranging from 0.3 to 2 cM between the closest Miflanking markers, Aps-1 and GP79 [21,28]. To translate the genetic distances of the markers into physical distances we have constructed physical maps of the two closest markers bracketing Mi, GP79 and Aps-1, using a set of rarely cutting restriction enzymes and pulsed-field gel electrophoresis (PFGE).…”