Chondrichthyes Chitinase: Molecular Cloning, Distribution, and Phylogenetic Analysis

We have previously reported the presence of three types of chitinase (acidic fish chitinase-1: AFCase-1, acidic fish chitinase-2: AFCase-2, fish chitinase-3: FCase-3) in Actinopterygii. In the present research, we report the identification of the novel chitinase genes HjChi (ORF: 1380 bp) and DkChi (ORF: 1440 bp) from the stomach of Chondrichthyes, Japanese bullhead shark (Heterodontus japonicas) and Kwangtung skate (Dipturus kwangtungensis), respectively. Organ-specific expression analysis identified the stomach-specific expression of HjChi, whereas DkChi was expressed widely in all organs. Chitinase activity was measured using pNP-(GlcNAc)n (n = 2, 3) as a substrate and β-N-acetylhexosaminidase (Hex) activity was measured using pNPGlcNAc. Relatively high values of chitinase activity were observed in the stomach, spleen, and gonads of the Japanese bullhead shark, H. japonicas , compared with that observed in the stomach of the Kwangtung skate D. kwangtungensis . However, Hex activity was detected throughout the body of both species. The optimal pH of chitinase in both the Japanese bullhead shark, H. japonicas, and the Kwangtung skate, D. kwangtungensis, were 3.5 - 5.5 and 3.5 - 4.0, respectively, and 4.0 for Hex in both species. Phylogenetic analysis revealed that Chondrichthyes chitinase forms a unique group (Chondrichthyes chitinase). These results suggested that the possibility of the formation of chitinase groups for each class in the phylogenetic analysis based on the observation of class-specific chitinase.

The complete mitochondrial genome of the grey bamboo shark (Chiloscyllium griseum) (Orectolobiformes: Hemiscylliidae): genomic characterization and phylogenetic application

The complete mitochondrial genome of the grey bamboo shark, Chiloscyllium griseum （Orectolobiformes： HemiscyUiidae） was obtained, with a total of 16 755 base pairs （bp）. The mitogenome encoded 13 protein- coding genes, two ribosomal RNAs, 22 transfer RNAs and a noncoding control region, a gene arrangement identical to the most common type found in vertebrates. All tRNA sequences were folded into typical clover- leaf secondary structure with the exception of tRNA-Ser2, in which the DHU arm stem was replaced with 12 unpaired nucleotides and formed a simple loop. In the control region, the putative termination-associated sequences （TAS） with hairpin-loop structure and the conserved sequence blocks （CSB） I-III were identified and considered to be associated with the replication and transcription of mtDNA. No repeat motifs were found in the mitogenome of C. griseum. On the basis of the mitogenomic data available in the Class Chon- drichthyes from both Genbank and this study, and the phylogenetic analyses using maximum likelihood （ML） and Bayesian inference （BI） methods, the results strongly support the basal division between batoids and sharks and the monophyly of the Superorder Galeomorphii, and confirm the phylogenetic position of C. griseum in the Subclass Elasmobranchii. This study suggests that the mitogenomic data are more robust for reconstructing phylogenetic relationships than individual genes in the Elasmobranchii.

Hox Gene Clusters of Early Vertebrates:Do They Serve as Reliable Markers for Genome Evolution?

Hox genes, responsible for regional specification along the anteroposterior axis in embryogenesis, are found as clusters in most eumetazoan genomes sequenced to date. Invertebrates possess a single Hox gene cluster with some exceptions of secondary cluster breakages, while osteichthyans （bony vertebrates） have multiple Hox clusters. In tetrapods, four Hox clusters, derived from the so-called two-round whole genome duplications （2R-WGDs）, are observed. Overall, the number of Hox gene clusters has been regarded as a reliable marker of ploidy levels in animal genomes. In fact, this scheme also fits the situations in teleost fishes that experienced an additional WGD. In this review, I focus on cyclostomes and cartilaginous fishes as lineages that would fill the gap between invertebrates and osteichthyans. A recent study highlighted a possible loss of the HoxC cluster in the galeomorph shark lineage, while other aspects of cartilaginous fish Hox clusters usually mark their conserved nature. In contrast, existing resources suggest that the cyclostomes exhibit a different mode of Hox cluster organization. For this group of species, whose genomes could have differently responded to the 2R-WGDs from jawed vertebrates, therefore the number of Hox clusters may not serve as a good indicator of their ploidy level.