COMPETITIVE DOMINANCE AMONG STRAINS OF LUMINOUS BACTERIA PROVIDES AN UNUSUAL FORM OF EVIDENCE FOR PARALLEL EVOLUTION IN SEPIOLID SQUID-VIBRIO SYMBIOSES

One of the principal assumptions in symbiosis research is that associa ted partners have evolved in parallel. We report here experimental evi dence for parallel speciation patterns among several partners of the s epiolid squid-luminous bacterial symbioses. Molecular phylogenies for 14 species of host squids were derived from sequences of both the nucl ear internal transcribed spacer region and the mitochondrial cytochrom e oxidase subunit I; the glyceraldehyde phosphate dehydrogenase locus was sequenced for phylogenetic determinations of 7 strains of bacteria l symbionts, Comparisons of trees constructed for each of the three lo ci revealed a parallel phylogeny between the sepiolids and their respe ctive symbionts. Because both the squids and their bacterial partners can be easily cultured independently in the laboratory, we were able t o couple these phylogenetic analyses with experiments to examine the a bility of the different symbiont strains to compete with each other du ring the colonization of one of the host species. Our results not only indicate a pronounced dominance of native symbiont strains over nonna tive strains, but also reveal a hierarchy of symbiont competency that reflects the phylogenetic relationships of the partners, For the first time, molecular systematics has been coupled with experimental coloni zation assays to provide evidence for the existence of parallel specia tion among a set of animal-bacterial associations.