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  • Author or Editor: Antonio J. Monforte x
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The study of the genetic control of natural variation in the root architecture of Cucumis melo L. is complex due to the difficulties of root phenotyping and to the quantitative nature of root traits and their plasticity. A library of near-isogenic lines (NILs), constructed by introgressing the genome of the exotic Korean accession Shongwan Charmi [SC (PI161375)] into the genetic background of the cultivar Piel de Sapo (PS) has recently become available. In this work, we used this population to identify quantitative trait loci (QTLs) controlling variation in root growth and architecture. We studied separately the primary root and the secondary and tertiary root systems during a 15-day period. Heritabilities for the root traits were moderate. Correlation and principal component analysis showed independence among traits measuring root length and root branching level, indicating the possibility of modifying both traits independently. PS and SC clearly differed in plant size. Significant allometric relationships between vine biomass and some root traits were identified. The use of NILs with similar plant size of PS allowed us to avoid the inaccuracies caused by size-dependent variation of root traits. A total of 17 QTLs for root traits in seven linkage groups were identified: three QTLs for primary root length, three QTLs for the diameter of the primary root, three QTLs for secondary root density, three QTLs for the average length of the secondary roots, three QTLs for the percentage of secondary roots bearing tertiary roots, and two QTLs for tertiary root density. In most of these traits, transgressive variation was observed.

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Melon (Cucumis melo) is one of the principal vegetable crops for fresh market, for which a large number of breeding programs, oriented to generate inbred pure lines and hybrids, is established worldwide. The process to obtain and select these lines has been highly accelerated by the use of biotechnological techniques such as the generation of doubled haploid line (DHL) populations and molecular markers. Moreover, the use of DHLs in genetic studies is a useful tool because of their complete homozygosity and the permanent availability of plant material perpetuated by seed. In this work, the parthenogenetic response of 17 melon genotypes and the F1 hybrid PI 161375 × Spanish cultivar Piel de Sapo (PS) was studied considering three stages along the in vitro DHL generation process. The response of the analyzed melon cultivars was heterogeneous through the DHL generation with different limiting steps for each genotype. The response of the PI 161375 × PS hybrid was more similar to the male (PS) than the female parent (PI 161375), although the response of the maternal genotype was higher for some stages. This points to the important role of alleles from both parents in the different steps of the DHL generation process, and it could explain the identification of six genomic regions with distorted allelic segregation skewed toward PS or PI 161375. This hybrid was used to generate a population of 109 DHLs, the gametophytic origin of which was confirmed by flow cytometry and molecular markers.

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Seventeen simple sequence repeat (SSR) markers were used to assess the genetic diversity and population structure among traditional Greek and Cypriot melon cultigens (Cucumis melo L.). All SSR markers were polymorphic with a total number of 81 alleles, whereas all cultigens could be distinguished with at least one SSR, except cultigens 43 and 41. Reference accessions showed larger genetic variability with an average of four alleles per locus and 0.65 gene of diversity compared with an average of 2.47 alleles per locus and 0.30 of gene diversity for the Greek/Cypriot cultigens. Observed heterozygosity was very low, indicating a lack of outcrossing, at least in recent times. Unrooted neighbor-joining tree analysis and population structure analysis clustered the cultigens and the reference genotypes into five groups. All cultigens could be distinguished; the Cypriot cultigens were more closely related to the inodorus ‘Piel de Sapo’, whereas the Greek cultigens were located in an intermediate position between the inodorus ‘Piel de Sapo’ and the cantalupensis ‘Védrantais’. The cultigen ‘Kokkini’ was the most divergent among the Greek and Cypriot cultigens. This association between geographic origin and genetic similarity among Greek and Cypriot cultigens indicates geographic isolation. Most of the cultivars from the same cultivar group (i.e., inodorus, cantalupensis) clustered together, but some exceptions were found, suggesting that former inodorus landraces would have been transformed to cantalupensis as a result of intercrossing and further selection by farmers. Results of population structure analysis support mixing between cantalupensis and inodorus. ‘Agiou Basileiou’, an inodorus cultigen, was assigned to the subpopulation IV/II of which II is a pure cantalupensis subpopulation. Greek and Cypriot melon cultigens were developed from a broader germplasm base than western Mediterranean cultivars and exhibited useful for melon breeding programs genetic variability.

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