Pathogenicity of two Candidatus Xenohaliotis californiensis genetic variants against three abalone species (the genus Haliotis)

Candidatus Xenohaliotis californiensis, a Rickettsia-like organism (RLO) that causes 'withering syndrome' (WS) in abalone, has been recently detected in Japan. We analyzed partial nucleotide sequence (113 bp) of 16S rRNA of WS-RLOs (n = 335) from Japanese black abalone Haliotis discus discus, Ezo abalone H. discus hannai, giant abalone H. gigantea, tokobushi abalone H. diversicolor aquatilis and fukutokobushi abalone H. diversicolor diversicolor. All the sequences from Japanese black, Ezo and giant abalone were identical, but different from those from tokobushi and fukutokobushi at one nucleotide position. We also conducted cohabitation challenge to determine whether the WS-RLO in fukutokobushi infects Japanese black and giant abalone or the agent in Japanese black abalone infects fukutokobushi. Twenty fukutokobushi naturally infected with WS-RLO were cohabited with 10 healthy individuals of each of Japanese black, giant, and fukutokobushi abalone for a total of 84 days. At the end of the experiment, surviving fukutokobushi abalone were positive in PCR test for WS-RLO, but negative for that of Japanese black and Ezo abalone. In the reverse combination experiment, in which naturally infected Japanese black abalone were cohabited with these three species, WS-RLO transmissions were found in Japanese black and Ezo, but not in fukutokobushi abalone. These results suggest that two genetic variants of WS-RLO have a different host specificity.