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Background  Chemical senses are one of the foremost means by which organisms make sense of their environment, among them the olfactory and gustatory sense of vertebrates and arthropods. Both senses use large repertoires of receptors to achieve perception of complex chemosensory stimuli. High evolutionary dynamics of some olfactory and gustatory receptor gene families result in considerable variance of chemosensory perception between species. Interestingly, both  ora/v1r  genes and the closely related  t2r  genes constitute small and rather conserved families in teleost fish, but show rapid evolution and large species differences in tetrapods. To understand this transition, chemosensory gene repertoires of earlier diverging members of the tetrapod lineage, i.e. lobe-finned fish such as  Latimeria  would be of high interest.    Results  We report here the complete T2R repertoire of L atimeria chalumnae , using thorough data mining and extensive phylogenetic analysis. Eighty  t2r  genes were identified, by far the largest family reported for any species so far. The genomic neighborhood of  t2r  genes is enriched in repeat elements, which may have facilitated the extensive gene duplication events resulting in such a large family. Examination of non-synonymous  vs . synonymous substitution rates (dN/dS) suggests pronounced positive Darwinian selection in  Latimeria  T2Rs, conceivably ensuring efficient neo-functionalization of newly born  t2r  genes. Notably, both traits, positive selection and enrichment of repeat elements in the genomic neighborhood, are absent in the twenty  v1r  genes of  Latimeria . Sequence divergence in  Latimeria  T2Rs and V1Rs is high, reminescent of the corresponding teleost families. Some conserved sequence motifs of Latimeria T2Rs and V1Rs are shared with the respective teleost but not tetrapod genes, consistent with a potential role of such motifs in detection of aquatic chemosensory stimuli.    Conclusions  The singularly large T2R repertoire of Latimeria may have been generated by facilitating local gene duplication  via  increased density of repeat elements, and efficient neofunctionalization  via  positive Darwinian selection.  The high evolutionary dynamics of tetrapod  t2r  gene families precedes the emergence of tetrapods, i.e. the water-to-land transition, and thus constitutes a basal feature of the lobe-finned lineage of vertebrates.    Electronic supplementary material  The online version of this article (doi:10.1186/1471-2164-15-650) contains supplementary material, which is available to authorized users.

Positive Darwinian selection in the singularly large taste receptor gene family of an ‘ancient’ fish, Latimeria chalumnae