Sequencing, characterization and phylogenomics of the complete mitochondrial genome of Dactylogyrus lamellatus (Monogenea: Dactylogyridae).

Despite the worldwide distribution and pathogenicity of monogenean parasites belonging to the largest helminth genus, Dactylogyrus, there are no complete Dactylogyrinae (subfamily) mitogenomes published to date. In order to fill this knowledge gap, we have sequenced and characterized the complete mitogenome of Dactylogyrus lamellatus, a common parasite on the gills of grass carp (Ctenopharyngodon idella). The circular mitogenome is 15,187 bp in size, containing the standard 22 tRNA genes, 2 rRNA genes, 12 protein-encoding genes and a long non-coding region (NCR). There are two highly repetitive regions in the NCR. We have used concatenated nucleotide sequences of all 36 genes to perform the phylogenetic analysis using Bayesian inference and maximum likelihood approaches. As expected, the two dactylogyrids, D. lamellatus (Dactylogyrinae) and Tetrancistrum nebulosi (Ancyrocephalinae), were closely related to each other. These two formed a sister group with Capsalidae, and this cluster finally formed a further sister group with Gyrodactylidae. Phylogenetic affinity between Dactylogyrinae and Ancyrocephalinae was further confirmed by the similarity in their gene arrangement. The sequencing of the first Dactylogyrinae, along with a more suitable selection of outgroups, has enabled us to infer a much better phylogenetic resolution than recent mitogenomic studies. However, as many lineages of the class Monogenea remain underrepresented or not represented at all, a much larger number of mitogenome sequences will have to be available in order to infer the evolutionary relationships among the monogeneans fully, and with certainty.

Characterization and expression of Megalobrama amblycephala toll-like receptor 22 involved in the response to Aeromonas hydrophila.

The toll-like receptors (TLR) tlr22 was identified and characterized for the first time in one of the economically most important freshwater fish species in China, Megalobrama amblycephala. The full-length cDNA (4039 bp) of M. amblycephala tlr22 contains an open reading frame of 2706 bp, encoding a 901 amino-acid long polypeptide. The putative polypeptide contains 16 leucine-rich repeat (LRR) motifs, an LRR C-terminal, a transmembrane region and a cytoplasmic toll-interleukin-1 receptor (TIR) domain. Phylogenetic analyses revealed that M. amblycephala Tlr22 shared the closest relationship with a grass carp ortholog. tlr22 was constitutively expressed in nine tissues and during 10 developmental stages studied, albeit with varying expression levels. Along with many pathological changes observed after Aeromonas hydrophila bacterium infection, tlr22 and myd88 mRNA were significantly upregulated in blood, head kidney, spleen and intestine, indicating that tlr22 is involved in the immune response. These results provide an insight into tlr22 regulation mechanisms in the innate immune response to bacterial infection.