| Hyriopsis cumingii is the most important freshwater pearl mussel for producing non-nucleated pearls in China, followed by Hyriopsis schlegelii. Five strains were collected, including four strains of H. cumingii: a selected strain, a cultured strain, two wild strains from Poyang Lake and Dongting Lake, and a strain of H. schlegelii. The important strain factors that influence the quality of pearls were studied to provide a theoretical basis for screening the best pearl mussels, and technical support to improve the quality of pearls. The results could be summarized as follows: 1. A preliminary study of the correlation between shell nacre color and non-nucleated pearl color among five strainsFive strains were bred synchronously using early-bred technology. The shell nacre color was compared using CIELAB colorimetric measurements. The color parameters of the shell nacre(L*, a*, dE*) in the selected strain were significantly different from those in the cultured strain, Poyang Lake strain and the strain of H. schlegelii(P<0.05). The shell nacre color did not differ significantly between the left and right sides of the shells within the same strain(P>0.05). The selected strain had a relatively darker nacre at the posterior end of the shell, and the color parameters(L*, a*, b*, and dE*) differed significantly from those at the front end(P<0.05). The neighbor-joining(NJ) tree based on shell nacre color parameters was divided into two clusters, which showed that selected strain was separated from other strains.The color parameters of the pearl(L*, dE*) in the selected strain were significantly different from those in the cultured strain, Poyang Lake strain, Dongting Lake strain and the strain of H. schlegelii(P<0.05), which showed that the pearl color of the selected strain was deeper. The color parameters of the pearl(a*, dE*) in the strain of H. schlegelii were significantly different from four strains of H. cumingii, which showed that the pearl color of the strain of H. schlegelii was shallow. The percentage of purple pearls from the selected strain was the highest(74.46%). The strain of H. schlegelii mainly produced white pearls(92.49%).The pearl color parameter L* value of five strains were very significantly positively correlated with the shell color parameter L* value of the donor mussel(P<0.01), and the degree of correlation coefficients between the pearl color parameter L* value of five strains and the shell color parameter L* value of the donor mussel was 0.461. The pearl color parameter dE* value of five strains were very significantly positively correlated with the shell color parameter dE* value of the donor mussel(P<0.01), and the degree of correlation coefficients between the pearl color parameter dE* value of five strains and the shell color parameter dE* value of the donor mussel was 0.443. The pearl color parameter L* value of five strains were significantly positively correlated with the shell color parameter L* value of the host mussel(P<0.05), and the degree of correlation coefficients between the pearl color parameter L* value of five strains and the shell color parameter L* value of the donor mussel was 0.194. The pearl color parameter dE* value of five strains were very significantly positively correlated with the shell color parameter dE* value of the host mussel(P<0.01), and the degree of correlation coefficients between the pearl color parameter dE* value of five strains and the shell color parameter dE* value of the host mussel was 0.229. It could be concluded that the pearl color could probably be improved by improving the shell nacre color of the donor mussels. 2. A preliminary study of correlation between growth traits and non-nucleated pearl size among five strainsThe cultivation performances of the five strains were compared and the correlations between growth traits and the size of the pearls were determined. The results showed that the growth performances of five strains, from high to low were: the strain of H. schlegelii, the selected strain, the Poyang Lake strain, Dongting Lake strain, and the cultured strain. The correlations for the growth indices were extremely significant(P<0.01), and the degrees of association with body quality, from large to small, were: the shell length, shell width and shell height. At 1 year-old, the impact of pearl mussel body mass index(BMI) mainly affected the shell length, and at 2-years old, the BMI mainly affected the shell length and shell width. The shell width and body mass correlation increased in the late pearl mussels, and the shell height and body mass correlation was significantly reduced. Three kinds of multivariate analysis showed that the external morphology of 1 year-old strain of H. schlegelii, the Poyang Lake strain, the Dongting Lake strain and the cultured strain were very similar, whereas the external morphology of the selected strain varied significantly compared with the other four strains.The pearl weight of the selected strain was 51.21 g, which was the highest among four strains of H. cumingii. The total pearl weight of the strain of H. schlegelii was 54.95 g, the average weight of each pearl was 0.0778 g, and was significantly higher than the four freshwater pearl mussel strains(P<0.05). The pearl weight was not significant(P>0.05) between left and right shells. The results showed that the pearl weight of five strains was highly significantly positive correlated with the growth traits of the host mussel(P<0.01), from high to low were body weight, shell height and shell width, and the degree of correlation coefficients between the pearl weight and body weight was 0.452. The pearl weight of five strains was significantly positive correlated with the shell length of the honor mussel(P<0.05), and the degree of correlation coefficients between the pearl weight of five strains and shell length of the honor mussel was 0.192. In consideration of all the correlations above, it can be concluded that the non-nucleated pearl quality can be greatly enhanced by promoting growth traits of the host mussels. 3. Evaluation of the selection effect of shell nacre color and growth traits for selected strain of Hyriopsis cumingiiTo evaluate the selection effect, families were established in the selected strain and cultured stain. Comparisons were made of the shell nacre color(using CIELAB colorimetric measurements) and the growth traits of the two trains. The shell nacre color of the selected strain was deeper and richer than that of the cultured strain. The L*, C*, and dE* values of the selected strain were significantly different from those of the cultured strain(P<0.01), being 17.13% lower, 20.55% higher, and 22.56% higher, respectively, compared with the cultured strain. The-L* and dE* values of the selected train gradually increased from the front to the back of the shell nacre, indicating that the shell nacre color deepened gradually, whereas those did not change significantly in the cultured strain. The color parameters did not differ significantly at the same positions on the mussel shells in each strain(P>0.05).The differences in the growth traits between the two strains were also highly significant(P<0.01), and the shell length, shell height, shell width, and body weight of the selected strain were 12.30%, 9.95%, 8.60%, and 36.34% higher than those of the cultured strain, respectively. A combined analysis of the shell nacre color parameters and the growth traits showed that B3, B2, B4, B5, and B6 displayed superior color and growth traits, and could be used to further develop the strain. The correlation indices between the shell nacre color and growth traits were low, such that the growth traits could not be selected indirectly based on the shell nacre color. 4. Genetic variation analysis of five strains using mitochondrial DNA(mtDNA) and microsatellite markersGenetic differentiation and the structure of the five strains were analyzed using 16 S rRNA, COI, ITS1 and ND1 gene sequences of the mtDNA and microsatellite markers. The results showed that no haplotype was shared among the selected strain, Poyang lake strain, Dongting lake strain and the strain of H. schlegelii. One or two haplotypes were identified between the cultured strain and the other strains. The results showed that the genetic diversity in selected strain was similar to the two wild strains. In this study, the optimal strains of all tested individuals were four theoretical strains based on microsatellite markers. The cultured strain and the Poyang Lake strain were divided into one group because of their similar genetic structures, indicating that cultured strain of H. cumingii was mainly from the Poyang Lake wild strain. The base strain of selected strain was derived from the strains of Poyang Lake and Dongting Lake, and the genetic structure of selected strain was obviously different from the base strains of the two wild strains. In conclusion, the five strains had relatively high genetic diversity, with the potential for further genetic improvement. |
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