| Melon(Cucumis melo L.) is a diploid species with 2n=2x=24 chromosomes, which is a worldwide fruit ranked 9th. Melon is well liked by people because of its nutritious, flavor and good-shape. Our nation is the secondary center of thick-skinned melon and the original and secondary center of thin-skinned melon. Shen Nong herbal classic reported that the pedicel of melon was the best medication and melon seed was discovered in a female cadaver in the West-Han tomb at Mawangdui, Hunan province. So melon has been used for eating and medicinal purposes for over two thousand years in our country. Cucumis melo L. is the most variable species of the Cucurbitaceae family except Cucurbita. The plentiful genetic resources of melon provided massive materials for genetic researches and breeding work. Problems arose when huge melon resources were collected and only small parts of resources could be used effectively in the breeding practice. Otherwise there was a large amount of synonyms and homonyms in the germplasm. So this brought many difficulties for the germplasm preservation, appraisal, research and utilization. Therefore, it’s very necessary to strengthen the management of germplasm resources and make effective use of melon germplasm. The effective method was core germplasm construction. The core collection represents the maximum of the genetic diversity of the whole collection with the minimum of materials and the minimum of redundancy by selecting a subset from the whole germplasm by certain methods. So far, core collection was established for more than 10 species in our nation, such as sesame, cotton, tobacco, tea, plum. Based on the 191 melon primary core collection built previously, this paper employed genetic distance method(Least distance stepwise sampling, LDSS and Stepwise cluster based on random sampling,SCR) to construct melon core collection and assessed the diversity and representative of core collection using main phenotypic traits in field and molecular markers(EST-SSR and SRAP). Main results were achieved as follows:1. Genetic diversity analysis of melon primary core collection based on 19 phenotypic traits. The average genetic diversity of melon qualitative characters and quantitative characters was 1.102 and 1.869 separately. The qualitative characters owned much richer genetic variance than quantitative characters. In total, the phenotypic traits of the primary core collection had substantial variance.2. Selection of ESR-SSR and SRAP markers and clustering analysis of primary core collection. The text used 20 materials which showed obvious morphological difference to select EST-SSR primers and SRAP primers. As a result, we choose 14 pairs of EST-SSR primers and 10 pairs of SRAP primers which bands were clear and great polymorphic from 18 pairs of EST-SSR primers and 19 pairs of SRAP primers. Using optimum molecular primers to analysis the 191 melon germplasm, the results showed that the average allele numbers was 4.6 and the average PIC was 0.817 based on EST-SSR primers; the average polymorphic loci number was 14.8, the average percentage of polymorphic locus was 0.980 and the average PIC was 0.952 based on SRAP primers. The clustering analysis of 191 melon germplasm molecular primers data showed that the total germplasm was grouped into 5 by ESR-SSR markers or SRAP markers. Although the materials of each group had widely origin areas and showed phenotypic differences, they had certain similarity on DNA level. Therefore, we can use molecular primers to condense the primary core collection.3. Constructed candidate core collection using different sampling strategies. According to geographic origin, the melon primary core collection was grouped into 19. LDSS method and SCR method were used to construct candidate core collections in 10%、15%、20%、25%、30% of sampling ratios. Using random sampling method constructed a candidate core collection as contrast. Based on test parameter of ESR-SSR and SRAP data and subsidiary data of 19 phenotypic traits, one core collection of best diversity and representative was selected. The results indicated that the LDSS sampling strategy and 25% sampling ratio constructed the best core collection. The core collection had 50 germplasm which originated from 18 countries. After the experiment we collected another 14 India originated melon and 13 Iran originated melon. Because melon had district diversity and India and Iran were one of the secondary origin, we selected 4 materials respectively from India and Iran through compared their phenotypic characters as core collection germplasm. The text eventually constructed the core collection contained 58 germplasm.4. Evaluation of the constructed core collections based on molecular data using genetic parameters. Compared core collection with primary core collection by t-test with 4 genetic parameters( Na、 Ne、 Nei′s、 I), the results showed that the four parameters were no significant difference. These indicated that the four parameters could good represent the primary core collection. Compared core collection with primary core collection by χ2-test, the results showed two(CM16 and ECM52) of the fourteen primers were significant difference(P<0.05), but P value of the others was greater than 0.05; likewise, based on SRAP primers, only Me9:Em10 and Me2:Em1 of the ten SRAP primers were significant difference by χ2-test, P value of the others was greater than 0.05. These indicated that the core collection loci distributed basically the same as the primary core collection based on EST-SSR and SSR markers. So the core collection greatly represented the total germplasm.5. Ensure the core collection constructed by molecular markers. Compared the core collection with the primary core collection by Principal Coordinate Analysis(PCo A)based on EST-SSR and SRAP data, the results showed that the core collection evenly distributed in the primary core collection no matter SSR data or SRAP data.The results of analysis of the phenotypic data of core collection and primary core collection by molecular markers showed that MD、VD、VR、CR were 5.07%、0.53%、103.44% and 89.97% respectively. These indicated that the core collection can good represent the primary core collection on main phenotypic traits. |
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