Genetic linkage maps of the European pear ( Pyrus communis L.) cultivar 'Bartlett' and the Japanese pear ( Pyrus pyrifolia Nakai) cultivar 'Housui' were constructed based on AFLPs, SSRs from pear, apple and Prunus, isozymes and phenotypic traits by using their F(1) progenies. The map of the female parent Bartlett consisted of 226 loci including 175 AFLPs, 49 SSRs, one isozyme and one S locus on 18 linkage groups over a total length of 949 cM, while that for 'Housui' contained 154 loci including 106 AFLPs, 42 SSRs, two phenotypic traits and the other four markers on 17 linkage groups encompassing a genetic distance of 926 cM. These maps were partially aligned using 20 codominant markers which showed segregating alleles in both parents. Compared with the reports of apple genetic maps, these pear maps were not saturated but were near saturation. Distorted segregation was observed in two and one regions of the genome of Bartlett and Housui, respectively. The position of 14 SSRs originating from apple could be successfully determined in pear maps, which enabled us to compare the two maps. Some SSRs developed from Prunus (peach, cherry) were also mapped. The relationships between pear and the other species belonging to the Rosaceae were discussed based on the position of SSRs.
Thirteen polymorphic microsatellite loci were developed in the Japanese pear (Pyrus pyrifolia Nakai) by using an enriched genomic library. The obtained microsatellite loci showed a high degree of polymorphism in the Japanese pear with 3–6 alleles per locus. The average values of observed and expected heterozygosities among these 13 loci were 0.69 and 0.71, respectively. Ten microsatellites could successfully amplify loci in the European pear (Pyrus communis L.), which were highly polymorphic as well.
Sixty Asian pear accessions from 6 Pyrus species were genetically identified by 9 SSR markers with a total of 133 putative alleles. Among them, 58 varieties could be successfully differentiated except for 2 pairs of synonymous or clonal varieties. All the SSR markers produced 1 or 2 discrete amplified fragments for all the diploid accessions, whereas a triploid variety showed 3 fragments with some SSRs. The number of putative alleles ranged from 7 to 20, with an average value of 14.8. The observed heterozygosity and the power of discrimination were 0.63 and 0.91, respectively. A phenogram based on the SSR genotypes was obtained, showing 3 major groups corresponding to the Japanese, Chinese and European groups. The SSR markers were highly polymorphic and could be utilized as a reliable tool for cultivar identification in Asian pears.
Pear scab (caused by Venturia nashicola) is one of the most harmful diseases of pears, especially Japanese and Chinese pear species. The molecular identification and early selection of resistant plants could greatly improve pear breeding. We have identified the position of the scab resistance gene, designated Vnk in an indigenous Japanese pear cultivar Kinchaku, within the pear genome by using simple sequence repeat (SSR) markers derived from pear and apple. The position of Vnk was identified in the central region of linkage group 1 of Kinchaku. Several amplified fragment length polymorphism (AFLP) markers linked to Vnk were obtained by bulked segregant analysis. Among them, the AFLP marker closest to Vnk was converted into a sequence tagged site (STS) marker. Four random amplified polymorphic DNA (RAPD) markers previously found to be loosely associated with Vnk (Iketani et al. 2001) were successfully converted into STS markers. Six markers (one SSR Hi02c07 and five STSs converted from AFLP and RAPD) showed tight linkages to Vnk, being mapped with distances ranging from 2.4 to 12.4 cM. The SSR CH-Vf2, which was isolated from a BAC clone of the contig containing the apple scab gene Vf, was mapped at the bottom of linkage group 1 in Kinchaku, suggesting that the Vnk and Vf loci are located in different genomic regions of the same homologous linkage group.
Generally the main component of fishy flavor is considered to be trimethylamine. On the other hand, carbonyl compounds, produced from oxidation of polyunsaturated fatty acid by lipoxygenase or by autoxidation, might have some contribution to the fishy flavor. Since sardine skin contains high levels of polyunsaturated fatty acids and lipoxygenase, carbonyl compounds may be generated more easily than trimethylamine. In this study, volatile flavor compounds of sardine were analyzed by gas chromatograph-mass spectrometry and gas chromatograph-olfactometry combined with solid phase microextraction. Then, the flavor components that contribute to fishy flavor were identified. At normal pH (6.2), trimethylamine was not detected or sensed from the fresh sardines. When the pH was raised, the amount of trimethylamine became higher. Trimethylamine flavor was weak at pH 9 and strongly sensed at pH 11 or higher. On the other hand, 33 other compounds were positively or tentatively identified, including 8 hydrocarbons, 5 ketones, 1 furan, 1 sulfur compound, 12 aldehydes, and 6 alcohols in fresh sardines. Among them, 2,3-pentanedione, hexanal, and 1-penten-3-ol were the main components. Forty-seven flavors were detected by gas chromatograph-olfactometry. Among them, paint-like (1-penten-3-one), caramel-like (2,3-pentanedione), green-like (hexanal), shore-like ((Z)-4-heptenal), citrus note (octanal), mushroom-like (1-octen-3-one), potato-like (methional), insect-like ((E,Z)-2,6-nonadienal), and bloody note (not identified) were strongly sensed. From the aforementioned results, it can be concluded that these compounds rather than trimethylamine contributed to fresh sardine flavor.
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