José María Alvarez

Identification of quantitative trait loci involved in fruit quality traits in melon (Cucumis melo L.)

Two populations [an F2 and a set of 77 double haploid lines (DHLs)] developed from a cross between a ‘Piel de Sapo’ cultivar (PS) and the exotic Korean accession PI 161375 were used to detect QTLs involved in melon fruit quality traits: earliness (EA), fruit shape (FS), fruit weight (FW) and sugar content (SSC); and loci involved in the colour traits: external colour (ECOL) and flesh colour (FC). High variation was found, showing transgressive segregations for all traits. The highest correlation among experiments was observed for FS and the lowest for FW and SSC. Correlations among traits within experiments were, in general, not significant. QTL analysis, performed by Composite Interval Mapping, allowed the detection of nine QTLs for EA, eight for FS, six for FW and five for SSC. Major QTLs (R2>25%) were detected for all traits. QTLs for different traits were no clearly co-localised, suggesting low pleiotropic effects at QTLs. Sixty-one per cent of them were detected in two or more experiments. QTLs for FS were detected in more trials than QTLs for FW and SSC, confirming that FS is under highly hereditable polygenic control. ECOL segregated as yellow:green in both experimental populations. The genetic control of ECOL was found to be complex, probably involving more than two loci with epistatic interactions. One of these loci was mapped on linkage group 9, but the other loci could not be clearly resolved. FC segregated as white:green:orange. The locus responsible for the green FC was mapped on linkage group 1, and it was proposed to correspond to the previously described locus gf. The genetic control of orange FC was complex: two loci in linkage groups 2 and 12 were associated with orange flesh, but larger population sizes would be necessary to elucidate completely the genetic control of orange flesh in this cross. Exotic alleles from PI161375 showed beneficial effects on EA, FW and SSC, indicating the usefulness of PI 161375 as a new source of genetic variability to improve European and American cultivars.

Occurrence, Distribution, and Relative Incidence of Mosaic Viruses Infecting Field-Grown Melon in Spain

The main areas for field-grown melon (Cucumis melo) production in Spain were surveyed for the occurrence and relative incidence of cucumber mosaic virus (CMV), papaya ringspot virus-watermelon strain (PRSV-W), watermelon mosaic virus-2 (WMV-2), and zucchini yellow mosaic virus (ZYMV) during the growing seasons of 1995 and 1996. Samples from 1,152 plants showing symptoms of virus infection were collected from commercial melon fields and analyzed by enzyme-linked immunosorbent assay (ELISA). CMV and WMV-2 were the most frequently found viruses, both by the number of locations and by their incidence in each location. In contrast, PRSV-W and ZYMV were detected in fewer sites and at lower incidences. PRSV-W was not found in 1996. In 79% of the samples, only one virus was detected; 15% of the samples were doubly infected. Both the incidence of plants showing symptoms of viral infection and the relative incidence of each of the four viruses varied according to the region, while the main trends of virus distribution were similar for 1995 and 1996.

Epidemics of Aphid-transmitted Viruses in Melon Crops in Spain

A survey of mosaic diseases in fields of open air grown melon was done in three provinces of Spain over 3 years. The incidence of the mosaic-inducing viruses Cucumber mosaic virus (CMV), Watermelon mosaic virus-2 (WMV-2), Papaya ringspot virus watermelon strain (PRSV-W) and Zucchini yellow mosaic virus (ZYMV) was assessed. The only viruses present in the three provinces during the 3 years studied were CMV and WMV-2, but their incidence differed according to year and region. For each epidemic, disease progress curves (DPC) were obtained, all of them well described by the Gompertz model. With the five descriptive variables of the model, a principal component analysis was done and two principal components, representing a severity factor and a time factor were found. Cluster analyses done with these two principal components, grouped the epidemics into 6 clusters that did not correlate with year, virus or province. Correlation analyses between epidemics caused by WMV-2 and CMV and climatic variables were done. Although the temperature in the spring months was the main factor associated with the severity of the epidemics caused by these two viruses, other differential correlations were found. Spatial evolution of CMV and WMV-2 epidemics was also different suggesting different aphid species acting as vectors for the viruses.

Morphological and molecular characterization of melon accessions resistant to Fusarium wilts

Fusarium wilt incited by Fusarium oxysporum f. sp. melonis (F.o.m) is one of the most widespread and devastating melon diseases. While resistance to physiological races 0, 1, and 2 is relatively frequent in different botanical varieties, sources of resistance to race 1,2 are restricted to a few Far-Eastern accessions. In this work, the results of a screening for resistance to F.o.m. race 1,2 among 32 accessions are presented. Three Japanese accessions (‘Kogane Nashi Makuwa’, ‘C-211’, and ‘C-40’) showed the highest resistance levels, but useful levels of resistance were also detected in one Russian ‘C-160’ and two Spanish (‘C-300’ and ‘Mollerusa-7’) accessions. These resistant materials, together with other accessions previously described as resistant to F.o.m. races 0, 1, and/or 2 have been morphologically and molecularly characterized. Based on cluster analysis, these accessions have been grouped according to the botanical subspecies they belong to. Assessment of genetic diversity indicated that the resistant accessions to races 0, 1 and 2, are scattered along the established clusters. On the other hand, high levels of resistance to the race 1,2 could be found only among accessions belonging to Cucumis melo subsp. agrestis, nevertheless, a certain degree of resistance to this race could also be found within some accessions belonging to subsp. melo. As far as we know, this is the first report of resistance to F.o.m race 1,2 found out from the Far-Eastern melon material. Based on fruits characteristics, it appears that several inodurus and cantalupensis accessions could be exploited in breeding programs as resistance sources to F.o.m races 0, 1 and/or 2 for the improvement of these melon types. The accessions with the highest levels of resistance to the race 1,2 appeared to be very distant both molecularly and morphologically from the commercial types. Nevertheless ‘C-160’, ‘C-300’, and ‘Mollerusa-7’ classified as var. inodorus are morphologically very similar to the Spanish commercial types and might be used as resistant sources in breeding these melon types.

Breeding melon for resistance to Fusarium wilt: recent developments

Melon Fusarium wilt (MFW), caused by Fusarium oxysporum f. sp. melonis (Fom), is one of the most destructive diseases of melon (Cucumis melo L.). The development and deployment of resistant cultivars is generally considered to be the best approach to control MFW. Based on the host resistance genes associated with variants of this pathogen, Fom isolates were classified into four physiological races designated 0, 1, 2, and 1,2. Two dominant resistance genes, Fom-1 and Fom-2, control resistance to races 0 and 2, and 0 and 1, respectively. Fom isolates classified as race 1,2 are able to induce disease in melon lines carrying the above resistance genes. Many sources of resistance to Fom races 0, 1, and 2 have been reported. Partial resistance to race 1,2 controlled by polygenic recessive genes was only detected in a few Far Eastern melon accessions, except for the breeding line BIZ where complete resistance was described. Identification of DNA markers tightly linked to genes conferring resistance to Fom has immediate application in MFW resistance breeding programs. The Fom-2 gene has been cloned, and it encodes a protein with a nucleotide binding site (NBS) and leucine-rich repeats domain (LRR). Based on the sequence of this domain, some molecular markers linked to this gene were developed. Several DNA markers linked to Fom-1 have also been described. However, the usefulness of these markers was variety-dependent. Therefore, their combined use would be very useful in marker assisted selection for introducing resistance to Fom races 0 and 2 in melon. Recently, these markers were used for the positional cloning of this gene, which encoded a protein with a NBS–LRR domains that shows similarity to the toll and interleukin-1 receptores (TIR). Regarding Fom race 1,2, nine QTL were detected on five linkage groups by composite interval mapping. In this paper we review the current knowledge of MFW disease, and focus on genetic resistance to Fom and marker-assisted selection for resistance.

Molecular characterization of Fom-1 gene and development of functional markers for molecular breeding of resistance to Fusarium race 2 in melon

Melon Fusarium wilt disease caused by the soil-borne pathogen Fusarium oxysporum f. sp. melonis (Fom) is one of the most devastating melon diseases worldwide. Recently, the Fom-1 gene responsible for resistance against Fom races 0 and 2 was cloned. In this study we amplified, cloned and sequenced full genomic DNA and cDNA of Fom-1 from several melon resistant and susceptible accessions using three pairs of primers designed within this gene. Sequence analyses showed that this gene contains four exons interrupted by three introns. The comparative sequence analysis of the cloned cDNA amplicons from resistant and susceptible genotypes revealed eight nucleotide substitutions, within Fom-1 coding regions, among which four were non-synonymous. RT-PCR revealed that the Fom-1 expression is induced by Fom race 2 inoculation. The Fom-1 predicted protein (FOM-1) exhibits a tripartite modular structure composed of an N-terminal TIR domain, a central NB–ARC domain and a C-terminal LRR domain. FOM-1 from resistant melon accessions share four amino acid differences relative to the FOM-1 protein in susceptible ones. Two amino acid substitutions N56K and R103H were located at the FOM-1 TIR domain and the substitution E385K in the NB–ARC domain. Based on single nucleotide polymorphisms within the coding region of the Fom-1 locus, we have generated two CAPS markers, Fom-1R and Fom-1S. Results from screening various melon accessions clearly demonstrated the usefulness of both functional CAPS markers in the marker-assisted selection for melon breeding programs.

Breeding and Genetics of Resistance to Fusarium Wilt in Melon

Melon Fusarium wilt (MFW), caused by Fusarium oxysporum f. sp. melonis (Fom), and is an important soil-borne disease of melon worldwide. The four known races 0, 1, 2, and 1,2 of this pathogen can be distinguished by infection on appropriate cultivars. Effective control can be achieved only through host resistance. Two major genes, Fom-1 and Fom-2, control resistance to races 0 and 2, and 0 and 1, respectively; whereas partial polygenic resistance to race 1,2 has been described. Fom-2 gene has been cloned, and the information generated from the LRR region sequences allowed the development of useful functional markers. Also, several molecular markers linked to Fom-1 gene have been reported; nevertheless, their usefulness was variety-dependent. These markers were used for the positional cloning of this gene. More recently this gene was isolated by a map-based cloning strategy. The sequence analysis revealed that Fom-1 belongs to the TIR-NB-LRR type. Resistance to Fom race 1,2 is complex and appears to be under polygenic control. Partial resistance to this race was detected in a few Far Eastern melon accessions, except for the breeding line BIZ where near-complete resistance was described. To date, quantitative trait loci (QTLs) associated with resistance to race 1,2 have been reported only in the line Isabelle and two other breeding lines BIZ and 03MFR001795. This chapter summarizes findings reported in the literature on genetic resources of resistance, molecular markers and quantitative trait loci for resistance to Fom.