Studies On The Heterosis Mechanism Of Scallop Chlamys Farreri (♀)×Patinopecten Yesoensis (♂)

The phylum Mollusca is one of the largest, most diverse and important groups in the animal kingdom, bivalve culture is an efficient way to produce animal protein and that it improves the income and nutrition of people. The external shape of the hybrids of intercrosses Chlamys farreri (♀)×Patinopecten yessoensis (♂) was similar to female parent, but the hybrid had a high survival rate and the growth rate was improved obviously, while there was large scale death of C. farreri in high water temperature season, the heterosis was very apparent. The mechanisms underlying heterosis remain unknown regardless that heterosis has been proven a powerful tool for agricultural development. Many hypotheses have been developed to explain heterosis, such as dominance hypothesis, overdominance hypothesis, epistatic effects hypothesis, genetic equilibrium hypothesis, active gene hypothesis, genetic vibration theory and so on. With the rapid development of molecular biology, scientists began to explore this area at molecular level from many angles.1. The amplified fragment length polymorphism (AFLP) technology was used to analyze the genetic variations and genetic structure among the three populations of C.farreri (♀), P.yessoensis (♂) and their offspring F₁. In total, 315 loci were amplified with 5 primer pairs, and 311 of these loci were polymorphic, accounting for 98.73%. The Shannon genetic diversity indices were 0.3291±0.1850, 0.2897±0.1959 and 0.3828±0.1612, respectively for the female parent, the male parent and F₁, while Nei's gene diversities were 0.3291±0.1850, 0.2897±0.1959 and 0.3828±0.1612. The genetic distance between C. farreri and P. yessoensis was 0.4116, as it was 0.1316 for F₁ and their female parents, and 0.2780 for F₁ and their male parents. The dendrograms showed that the three population gathered independently, which indicated that each population of the three populations was independent. AMOVA analysis indicated that 30.47% of variance was among populations and 69.53% of variance was within populations, and it suggested the rich genetic diversity level within populations. In all, the expression of heterosis has a correlation with the genetic variation.2. In order to examine the genetic diversity and genetic structure of three scallop populations, including Chlamys farreri, Patinopecten yessoensis and their hybrid (F₁), the mitochondrial DNA cytochrome oxidase subunit I(COI) and cytochrome b(Cytb) gene fragments were amplified and analyzed. Fragments with 781bp of the partial COI gene and 725bp of the partial Cytb gene were obtained after alignment and emendation, and 47 haplotypes were found in the sequences of the 74 samples. The genetic diversity indexes, including the haplotype number, haplotype diversity, nucleotide diversity and average number of nucleotide differences, showed that, the genetic diversity of the F₁ were higher than the parents. The genetic distance between the F₁ and C.farreri was the nearest, and that between the F₁ and P.yessoensis was the farthest. The level of gene flow between F₁ and P.yessoensis was lower than the level between F₁ and C.farreri, while the population differentiation values between F₁ and P.yessoensis was higher than the value between F₁ and C.farreri, it suggested that the differentiation between C.farreri and P.yessoensis had happened long ago. UPGMA tree and reduced median network of haplotypes got the same result, the population of C. farreri and F₁ gathered together, and the population of P. yessoensis gathered together independently. The PCR re-amplification found that, the special primers of C.farreri could be amplified in the F₁ population, but the P.yessoensis'special primers could not be amplified in the F₁ population, it indicated that the mitochondrial DNA genetic model of the F₁ was matrilinear inheritance.3. DNA methylation is related to a number of epigenetic phenomena and plays an important role in gene expression. In this study, the technique of methylation-sensitive amplified polymorphism (MSAP) was firstly used to analyze the DNA cytosine methylation of Chlamys farreri, Patinopecten yessoensis, F₁ and F₂, and we made a concrete analysis between the methylation ratio and phenotypic traits, assessed the extent and pattern of cytosine methylation among the parents and the offspring, and emphasized the discussion on the molecular basis of heterosis in terms of the DNA methylation. The results showed that, (1)DNA methylation was negatively related to the shell length, shell height, soft body weight and adductor muscle weight, but positively related to the shell wideness and gross weight, and there was great correlativity between the DNA methylation and the adductor muscle weight(P<0.01); (2) there was significant difference between the parents and the offspring: the methylation ratio of 5'-CCGG sites was 32.79% in P. yessoensis, 24.13% in C. farreri, 19.98% in F₁, and 20.18% in F₂ respectively, the methylation ratio of F₁ was less than the parents and there was no significant difference between F₁ and F₂, the alteration of the DNA methylation was the result of the cross of the parents; four classes of MSAP patterns, A, B, C and D, among hybrids and their parents were detected: pattern A, methylation levels were not changed in both parents and F₁; pattern B, demethylation, both or one of the parent was methylated, but there was no methylation in the same site in Fl; pattern C, hypermethylation, the methylation level in F₁ was higher than those in both parents; pattern D, hypomethylation, methylation ratio in F₁ was lower than those in both parents, The patterns of the F₁ DNA methylation experienced change and adjustments in order to coordinate the expression of the gene from the parents, the DNA methylation have something to do with the expression of heterosis.