Simple-sequence repeat markers used in merging linkage maps of melon (Cucumis melo L.).

A set of 118 simple sequence repeat (SSR) markers has been developed in melon from two different sources: genomic libraries (gSSR) and expressed sequence-tag (EST) databases (EST-SSR). Forty-nine percent of the markers showed polymorphism between the 'Piel de Sapo' (PS) and PI161375 melon genotypes used as parents for the mapping populations. Similar polymorphism levels were found in gSSR (51.2%) and EST-SSR (45.5%). Two populations, F2 and a set of double haploid lines (DHLs), developed from the same parent genotypes were used for map construction. Twenty-three SSRs and 79 restriction fragment length polymorphisms (RFLPs), evenly distributed through the melon genome, were used to anchor the maps of both populations. Ten cucumber SSRs, 41 gSSRs, 16 EST-SSR, three single nucleotide polymorphism (SNP) markers, and the Nsv locus were added in the DHL population. The maps developed in the F2 and DHL populations were co-linear, with similar lengths, except in linkage groups G1, G9, and G10. There was segregation distortion in a higher proportion of markers in the DHL population compared with the F2, probably caused by selection during the construction of DHLs through in vitro culture. After map merging, a composite genetic map was obtained including 327 transferable markers: 226 RFLPs, 97 SSRs, three SNPs, and the Nsv locus. The map length is 1,021 cM, distributed in 12 linkage groups, and map density is 3.11 cM/marker. SSR markers alone cover nearly 80% of the map length. This map is proposed as a basis for a framework melon map to be merged with other maps and as an anchor point for map comparison between species of the Cucurbitaceae family.

Single-nucleotide polymorphisms detected in expressed sequence tags of melon (Cucumis melo L.).

A search was performed for single-nucleotide polymorphisms (SNP) and short insertions-deletions (indels) in 34 melon (Cucumis melo L.) expressed sequence tag (EST) fragments between two distantly related melon genotypes, a group Inodorus 'Piel de sapo' market class breeding line T111 and the Korean accession PI 161375. In total, we studied 15 kb of melon sequence. The average frequency of SNPs between the two genotypes was one every 441 bp. One indel was also found every 1666 bp. Seventy-five percent of the polymorphisms were located in introns and the 3'untranslated regions. On average, there were 1.26 SNPs plus indels per amplicon. We explored three different SNP detection systems to position five of the SNPs in a melon genetic map. Three of the SNPs were mapped using cleaved amplified polymorphic sequence (CAPS) markers, one SNP was mapped using the single primer extension reaction with fluorescent-labelled dideoxynucleotides, and one indel was mapped using polyacrilamide gel electrophoresis separation. The discovery of SNPs based on ESTs and a suitable system for SNP detection has broad potential utility in melon genome mapping.

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

Two populations [an F(2) 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 ( R(2)>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.

Genetic control and evolution of sesquiterpene biosynthesis in Lycopersicon esculentum and L. hirsutum.

Segregation analysis between Lysopersicon esculentum (cultivated tomato) and L. hirsutum (wild form) in conjunction with positional verification by using near-isogenic lines demonstrated that biosynthesis of two structurally different classes of sesquiterpenes in these species is controlled by loci on two different chromosomes. A locus on chromosome 6, Sesquiterpene synthase1 (Sst1), was identified for which the L. esculentum allele is associated with the biosynthesis of beta-caryophyllene and alpha-humulene. At this same locus, the L. hirsutum allele is associated with biosynthesis of germacrene B, germacrene D, and an unidentified sesquiterpene. Genomic mapping, cDNA isolation, and heterologous expression of putative sesquiterpene synthases from both L. esculentum and L. hirsutum revealed that Sst1 is composed of two gene clusters 24 centimorgans apart, Sst1-A and Sst1-B, and that only the genes in the Sst1-A cluster are responsible for accumulation of chromosome 6-associated sesquiterpenes. At a second locus, Sst2, on chromosome 8, the L. hirsutum allele specified accumulation of alpha-santalene, alpha-bergamotene, and beta-bergamotene. Surprisingly, the L. esculentum allele for Sst2 is not associated with the expression of any sesquiterpenes, which suggests that cultivated tomato may have a nonfunctional allele. Sesquiterpene synthase cDNA clones on chromosome 6 do not cross-hybridize on genomic DNA gel blots with putative sesquiterpene synthases on chromosome 8, an indication that the genes in Sst1 and Sst2 are highly diverged, each being responsible for the biosynthesis of structurally different sets of sesquiterpenes.

Development of a set of near isogenic and backcross recombinant inbred lines containing most of the Lycopersicon hirsutum genome in a L. esculentum genetic background: a tool for gene mapping and gene discovery.

A novel population consisted of a set of 99 near isogenic lines (NILs) and backcross recombinant inbred lines (BCRILs) derived from a cross between the cultivated tomato Lycopersicon esculentum cv. E6206 and L. hirsutum accession LA1777 is presented. Most of the lines contain a single defined introgression from L. hirsutum in the L. esculentum genetic background and together, the lines provide a coverage of more than the 85% of the L. hirsutum genome. These lines represent a new tool to uncover the genetic resources hidden in L. hirsutum as well as to study the genes responsible of its unique biology. Furthermore, the study of the allelic frequency and heterozygosity among BCRILs showed that specific genomic regions were likely subjected to unintentional selection pressures during the stock development. Genes involved in the reproductive behavior and (or) pollen viability are hypothesized to be responsible for these alterations.

Citrus and Prunuscopia-like retrotransposons.

Many of the world's most important citrus cultivars ("Washington Navel", satsumas, clementines) have arisen through somatic mutation. This phenomenon occurs fairly often in the various species and varieties of the genus.The presence of copia-like retrotransposons has been investigated in fruit trees, especially citrus, by using a PCR assay designed to detect copia-like reverse transcriptase (RT) sequences. Amplification products from a genotype of each the following species Citrus sinensis, Citrus grandis, Citrus clementina, Prunus armeniaca and Prunus amygdalus, were cloned and some of them sequenced. Southern-blot hybridization using RT clones as probes showed that multiple copies are integrated throughout the citrus genome, while only 1-3 copies are detected in the P. armeniaca genome, which is in accordance with the Citrus and Prunus genome sizes. Sequence analysis of RT clones allowed a search for homologous sequences within three gene banks. The most similar ones correspond to RT domains of copia-like retrotransposons from unrelated plant species. Cluster analysis of these sequences has shown a great heterogeneity among RT domains cloned from the same genotype. This finding supports the hypothesis that horizontal transmission of retrotransposons has occurred in the past. The species presenting a RT sequence most similar to citrus RT clones is Gnetum montanum, a gymnosperm whose distribution area coincides with two of the main centers of origin of Citrus spp. A new C-methylated restriction DNA fragment containing a RT sequence is present in navel sweet oranges, but not in Valencia oranges from which the former originated suggesting, that retrotransposon activity might be, at least in part, involved in the genetic variability among sweet orange cultivars. Given that retrotransposons are quite abundant throughout the citrus genome, their activity should be investigated thoroughly before commercializing any transgenic citrus plant where the transgene(s) is part of a viral genome in order to avoid its possible recombination with an active retroelement. Focusing on other strategies to control virus diseases is recommended in citrus.

Salt tolerance in Lycopersicon species. IV. Efficiency of marker-assisted selection for salt tolerance improvement.

The usefulness of marker-assisted selection (MAS) to develop salt-tolerant breeding lines from a F2 derived from L. esculentum x L. pimpinellifolium has been studied. Interval mapping methodology of quantitative trait locus (QTL) analysis was used to locate more precisely previously detected salt tolerance QTLs. A new QTL for total fruit weight under salinity (TW) near TG24 was detected. Most of the detected QTLs [3 for TW, 5 for fruit number, (FN) and 4 for fruit weight (FW)] had low R (2) values, except the FW QTL in the TG180-TG48 interval, which explains 36.6% of the total variance. Dominant and overdominant effects were detected at the QTLs for TW, whereas gene effects at the QTLs for FJV and FW ranged from additive to partial dominance. Phenotypic selection of F2 familes and marker-assisted selection of F3 families were carried out. Yield under salinity decreased in the F2 generation. F3 means were similar to those of the F1 as a consequence of phentoypic selection. The most important selection response for every trait was obtained from the F3 to F4 where MAS was applied. While F3 variation was mainly due to the within-family component, in the F4 the FN and FW between-family component was larger than the within-family one, indicating an efficient compartmentalization and fixation of QTLs into the F4 families. Comparison of the yield of these families under control versus saline conditions showed that fruit weight is a key trait to success in tomato salt-tolerance improvement using wild Lycopersicon germplasm. The QTLs we have detected under salinity seem to be also working under control conditions, although the interaction family x treatment was significant for TW, thereby explaining the fact that the selected families responded differently to salinity.