Studies On The Comparative Mitochondrial Genomics And Phylogeny Of Megascolecidae In China

Megascolecidae is the largest family of the earthworms,however,there is much debate about its taxonomy systematics,relationships and monophyly of genera units and species groups.Mitochondrial genome studies have been widely applied in the field of molecular evolution.Phylogenetic analysis based on the mitochondrial genome level,can reduce the analytical problems caused by individual genes with more or less conservation and stochastic errors.Thus far,only three worms have been determined.In this study,we conducted a large scale sequencing of Chinese megascolecoid earthworm mitochondrial genomes,and determined the mitochondrial genomes of 21 megascolecoid earthworms,and one earthworm in Moniligastridae(Drawida japonica).Based on these dataset,the characteristics and evolution of megascolecoid mitochondrial genomes were analyzed,by using comparative genomic and bioinformatics methods;we reconstructed the phylogeny of Megascolecidae,Conclusions were drawn as follows:(1)The general earthworm mitogenomic features revealed to be conservative: all genes encoded on the same strand,gene orders fo all species were the same,all the protein coding loci shared the same initiation codon(ATG).(2)The D.japonica mitogenome displayed the highest A + T content,reversed AT/GC-skews and the highest genetic diversity.(3)Genetic distances among protein coding genes displayed their maximum and minimum interspecific values in the ATP8 and CO1 genes,respectively.COX genes and Cytb were relatively conserved,while NADH and ATP genes evolved faster.(4)Amynthas longisiphonus contained one extra abbreviated copy of Leucine(CUN)tRNA.The 22 tRNAs showed variable substitution patterns between the considered earthworm mitogenomes.(5)Regardless of patitioning or not,PCGs datasets alone could not get stable tree topologies,which implied that the phylogetnetic information of PCGs dataset was not sufficient to stand for the whole mitogenome levels.(6)The inclusion of rRNAs positively increased phylogenetic support and the stability of tree topology,there is thus no justification for the exclusion of rRNA genes from mitochondrial phylogenomic analyses.(7)PartitionFinder maked biological sense,and the ML tree support of PR dataset increased obviously by using PartitionFinder,indicating that it heped mining phylogenetic information.We recommended using PartitionFinder to select a partitioning scheme when in muti-locus reconstruction.(8)Our analyses rejected reciprocal monophyly among Amynthas and Metaphire and indicated that the two genera should be systematically classified into one.