In this article, we describe two melon (Cucumis melo L.) breeding lines with the genetic background of a traditional nonclimacteric inodorus cultivar that show fruit characteristics similar to climacteric types. The breeding lines described here were identified during a breeding program aimed at introgressing the tolerance to melon vine decline (MVD) of ‘Pat 81’ (C. melo ssp. agrestis var. conomon) into the genetic background of ‘Piel de Sapo’ (C. melo ssp. melo var. inodorus; Fita et al., 2009). MVD is a melon soilborne disease caused primarily (but not only) by the fungus M. cannonballus Pollack et Uecker, which rots the roots. Melon vine decline and/or collapse results from a damaged root that is unable to supply the vine's demands for nutrients and water. The occurrence and intensity of collapse is highly dependent on environmental conditions and cultural practices (Fita et al., 2007). ‘Pat 81’ is one of the most tolerant melon entries reported to date. This is because of its long and branched root system, which is able to explore vast soil areas, and its low level of infection by the MVD-causing fungus. The combination of these two characteristics is crucial to overcoming the disease (Dias et al., 2002, 2004; Fita et al., 2006, 2008). After the introgression of genetic tolerance of ‘Pat 81’ into the ‘Piel de Sapo’ genome, the first MVD-tolerant ‘Piel de Sapo’ breeding lines were recently released (Fita et al., 2009). However, there is a lack of genetic tolerance to MVD in other melon types.
During the introgression of the tolerance into ‘Piel de Sapo’, some plants with new, unexpected transgressive fruit phenotypes appeared. The fruits of these plants were round and showed a ripening process with certain traits similar to those of climacteric melons, namely changes in rind color, flesh softening, fruit abscission, and subtle aroma (Pech et al., 2008). However, neither ‘Piel de Sapo’ nor ‘Pat 81’ are considered climacteric. ‘Piel de Sapo’ is a cultivar group that belongs to the large inodorus group and is a model of nonclimacteric ripening, like other inodorus types (honeydew, amarillo, piel de sapo, tendral, kirkagac, yuva, and casaba). It lacks the ethylene peak associated with ripening and does not exhibit the changes associated with climacteric maturation, which is typical of cantalupensis types. ‘Pat 81’ belongs to the same botanical group as PI 161375, whose ripening process has been studied in detail and is considered to be nonclimacteric (Périn et al., 2002). Other experiments made by crossing two nonclimacteric melons have generated offspring with climacteric ripening phenotypes with increased respiration and ethylene levels that results in flesh softening, changes in the fruit color, abscission, and aroma production (e.g., the segregant populations derived from crosses between ‘Piel de Sapo’ and PI 161375). Those parents are supposed to be nonclimacteric phenotypes caused by mutations in several different genes (Monforte et al., 2004; Moreno et al., 2008). In fact, when we phenotypically characterized ‘Pat 81’ and PI 161375 along with different nonclimacteric inodorus types, including ‘Piel de Sapo’, we observed differences in the ripening process between the inodorus and the conomon types such as late-season fruit abscission for the latter group.
The breeding lines presented here could be the starting point for the development of new cultivars with different characteristics from the inodorus group and more similarities to muskmelons or other climacteric cultivars and possessing the advantage of being tolerant to MVD, a soilborne disease of increasing significance worldwide.
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