PCR detection of phage genes in the phage-resistant P. plecoglossicida variants. Lanes 1 to 10, phage-resistant variants produced by PPpW-3; lanes 13 to 22, phage-resistant variants produced by PPpW-4; lanes 11 and 23, PTH-9802 (parent strain); lane 12, PPpW-3; lane 24, PPpW-4; M, molecular size marker.