Infection with the skin fluke Benedenia seriolae is a serious threat of yellowtail aquaculture. Development of genomic resources and molecular markers for B. seriolae is an essential infrastructure to develop control strategies against B. seriolae infections. As a seminal study to establish the genomic resources of the Japanese B. seriolae populations, we sequenced the complete mitochondrial genome (mitogenome) sequences of 12 B. seriolae specimens collected from four prefectures in Japan. The Japanese B. seriolae mitogenomes were mutually 99% identical, while the identity was 85% when compared to that of an Australian specimen. Furthermore, the gene arrangement in the Japanese B. seriolae mitogenome was slightly different from that in the Australian specimen. The substantial mitogenomic divergence between the Australian and Japanese specimens indicates that B. seriolae is composed of molecularly divergent populations. Variable sites were dispersed in the mitogenome of the Japanese B. seriolae. Phylogenetic relationships among the Japanese B. seriolae specimens did not reflect the host species or geographic distributions within Japan.
In a previous field survey of Perkinsus olseni in Manila clams in a tidal flat contaminated with the parasite, the parasite was not detected in spats smaller than 2 mm in shell length collected between April and June. To understand the reason, we undertook an experimental challenge, exposing clam spats to the zoospores of parasite. The spats became infected and began to die when mean infection intensity reached about 106 cells/g soft tissue. Our results, in conjunction with previously reported filtration rates of Manila clams, suggest the absence of infected spats was attributable to clams' low filtration and low zoospore densities.
A novel PCR-RFLP typing method was developed using PCR primers specific for the B subunit of DNA gyrase gene (gyrB) of Flavobacterium psychrophilum and restriction endonuclease Bsp119I. With this PCR-RFLP, 300 isolates of F. psychrophilum were classified into two genotypes (197 and 103 isolates), and the respective genotypes were correlated with the host fish species from which the isolates were derived. These isolates were further subdivided into 12 genotypes by combining with three previous PCR-RFLPs. It is suggested that this PCR-RFLP typing system can be a useful tool in epizootiological studies on bacterial cold-water disease.