Abstract:Lamprotula leai is an important freshwater mussel for pearl production. Doubly Uniparental Inheritance (DUI) is one of the most striking exceptions to the general rule of strict maternal transmission of mitochondrial DNA (mtDNA) in animals, and characterized by the presence of gender-associated mtDNA lineages that are inherited through male (male-transmitted or M type) or female (female-transmitted or F type) respectively. The complete mitochondrial genome of F type of Lamprotula leai was obtained using long and accurate polymerase chain reaction (LA-PCR), shotgun sequencing. The genome contains 16,530 base pairs and 13 protein-coding genes, 22 transfer RNA genes including 2 tRNASer and 2 tRNALeu, anticodons are tRNALeu1(UUR)=(TAA), tRNALeu2(CUN)=(TAG), tRNASer1(AGN)=(TCT), tRNASer2(UCN)=(TGA), 2 ribosomal RNA genes, and 27 non-coding regions. The base composition for is 36.34%A, 23.94%T, 27.17% C, 12.55%G and 60.28% (A+T). Most genes are encoded on the L strand while ND3-ND5, ND4L, COⅠ-COⅢ, ATP8, ATP6, tRNAAsp, tRNAHis are encoded on the H strand. The structure and organization of mitochondrial genomes of L. leai and other six freshwater mussels were analyzed using comparative genomics and bioinformatics methods. Results showed that: (ⅰ) Strong bias is toward A+T for the F type genome of L.leai. (ⅱ) The striking mitochondrial genome difference in the size performed on the non-coding regions in all these freshwater mussels. (ⅲ) The gene arrangement of L. leai is identical to that of Hyriopsis cumingii, but is different from that of Cristaria plicata, Lampsilis ornate, Pyganodon grandis, Quadrula quadrulaand and Venustaconcha ellipsiformis between COⅡ and 12S rRNA. Therefore, the F type of freshwater mussels exist two gene orders between COⅡ and 12S rRNA: one is COⅡ-ND3-H-A-S1-S2-E-ND2-M-W-R-rrnS, another is COⅡ-H-S1-ND2- M-ND3-A-S2-E- W-R-rrnS. The difference is caused by rearrangement of 8 genes, including ND3, tRNAHis, tRNAAla, tRNASer1, tRNASer2, tRNAGlu, ND2 and tRNAMet. (ⅳ) 13 protein genes contain 4 initiation codons which are I (AUU, AUC), V (GUG), M(AUA), L(UUG) and the stop codons of UAA or UAG with the exception of ND4 with incomplete T. (ⅴ) Analysis show that most of 22 tRNAs have typical cloverleaf structures, the acceptor of tRNALys and tRNAThr have bulge loop for T and A base unpairing, the anticodon loop of tRNAS2 has 9 bases. 5 kinds of base unpairing are showed in the secondary structure of tRNA, such as A-C, A-A, G-T, T-T and T-C, in acceptor, D-loop and TψC loop. (ⅵ) 27 noncoding regions exist in L. leai, ranging in size from 1 to 969 bp. 4 largest noncoding regions are found between ND5-tRNAGln (969bp), tRNAGlu-tRNATrp (288 bp), ND3-tRNATrp (116 bp), and tRNAHis-tRNASer (114 bp), only the 116 bp noncoding regions has an high A+T content (A+T=74.14%). (ⅶ) Phylogenetic trees show that L. leai is clustered together with other freshwater mussels and far away from marine bivalves. The results of this study provide basic information for genetic resources and rule of DUI transmission of mtDNA in freshwater mussel, also provide the basis and model for gene rearrangement of mitochondrial genome in bivalves.
