Photobacterium comprises several species in Vibrionaceae, a large family of Gram-negative, facultatively aerobic, bacteria that commonly associate with marine animals. Members of the genus are widely distributed in the marine environment and occur in seawater, surfaces, and intestines of marine animals, marine sediments and saline lake water, and light organs of fish. Seven Photobacterium species are luminous via the activity of the lux genes, luxCDABEG. Much recent progress has been made on the phylogeny, genomics, and symbiosis of Photobacterium. Phylogenetic analysis demonstrates a robust separation between Photobacterium and its close relatives, Aliivibrio and Vibrio, and reveals the presence of two well-supported clades. Clade 1 contains luminous and symbiotic species and one species with no luminous members, and Clade 2 contains mostly nonluminous species. The genomes of Photobacterium are similar in size, structure, and organization to other members of Vibrionaceae, with two chromosomes of unequal size and multiple rrn operons. Many species of marine fish form bioluminescent symbioses with three Photobacterium species: Photobacterium kishitanii, Photobacterium leiognathi, and Photobacterium mandapamensis. These associations are highly, but not strictly species specific, and they do not exhibit symbiont-host codivergence. Environmental congruence instead of host selection might explain the patterns of symbiont-host affiliation observed from nature.
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