| The clam (Meretrix meretrix) is an important commercial marine mollusk. Theincreasing seeds demand, over-fishing, disease, and coastal environmental pollutionhave made the natural resources decreased rapidly, which limited the development ofthe clam aquaculture in China. Therefore, in order to protect the clam germplasmresources and to promote its stable and healthy development, this paper wasperformed to achieve the conservation and improvement for clam germplasm bymeans of molecular biology methods, based on genetic breeding technologies.Firstly,260polymorphic microsatellite markers were developed from the twomethods of enriched libraries (G-SSR)and EST database (EST-SSR) screening. Theresults showed that the G-SSRs have higher polymorphism than the EST-SSRs, so theG-SSRs were more suitable for genetic analysis. However, the EST-SSRs were fromcandidate genes, which made them useful for identification of functional markersassociated with QTLs. The large number of SSR markers developed in this studywould lay a foundation for genetic linkage mapping, QTL identification andmarker-assisted breeding.To apply the microsatellite markers into the clam breeding, the SSR markerswere combined with COI marker for pedigree identification. In the practice, both inrelative and distant families, the probabilities of offspring to find the correct paternaland maternal parents using5microsatellites were significantly lower than thecomputer simulation. After determining the maternal parents using COI marker, boththe probabilities were increased largely. The results showed that the combination ofmicrosatellite markers and COI marker in pedigree identification of the clam is an effective method.To guide the breeding program, the microsatellites were used for the geneticanalysis of the populations from a diallele cross with two geographical populations, aswell as the growth performance. The results showed that the genetic diversity of thereciprocal populations was higher than the pure populations; the reciprocalpopulations have significant heterosis, and there were significant maternal effect inthis study. It suggested that hybridization between different geographical clampopulations could obtain large production performance and genetic improvement.To clarify the influence of the process of the current seeds production on the clamgermplasm, four successive generations were produced with closed breeding tosimulate this process. After the genetic monitoring for these generations and theirnatural population with microsatellite markers, it showed that the natural populationhas similar genetic diversity with the generations and without genetic differentiation.In addition, the generations have stable and high level of genetic diversity and lowinbreeding coefficient. It showed that the closed breeding could not lead togermplasm degradation in the clam, and the cultured populations could be used for thegenetic conservation of nature population.Three EST-SSR markers were detected to be associated with the growth QTLswithin a growth-faster population and a control population. After the furtherconfirmation in an independent population and the functional matches for theSSR-containing EST sequences in the NCBI database, the three significant markerswere again revealed to be associated with the growth QTLs. The three identified andconfirmed EST-SSR markers might be used for marker-assisted breeding in the clam.Associated analysis for SNPs from the superoxide dismutase gene wasperformed between a vibrio-susceptible population and a vibrio-resistant population.Four SNPs were found in the SOD exons, and allele frequencies of two loci, genotypefrequencies in three loci, and one haplotype were significantly different between thetwo groups. In addition, one of the three nSNPs influences the effect of the protein.After the further confirmation between an independent vibrio-resistant population anda control population, the results consist with the above. It indicated that the significant SNPs in the superoxide dismutase gene were involving in the vibrio-resistance trait,which could be used for the resistance breeding of the clam.Genetic linkage map construction and growth QTLs identification wereperformed using microsatellites in a F1family with significantly different growthperformance. Nineteen linkage groups were identified with84SSR markers, whichwas consistent with the clam karyotype2n=38. The genetic map spanned a totalgenetic distance of1499.7cM, with an average interval of19.9cM. Genomecoverage of the linkage map was approximately71.9%. One shell length QTL and oneweight QTL were identified, which could explain54.7%and50.6%phenotypicvariance, respectively.
|
PDF Full Text Request