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Abstract

The insect odorant receptor (Or) gene family is among the largest multigene families in insect genomes, but its evolutionary origin and mode of expansion is still a matter of debate. We performed
transcriptomic surveys of two wingless insect species, the silverfish Lepisma saccharina and
Tricholepidion gertschi, and identified multiple Or gene family members in both species. A
phylogenetic analysis suggests that the silverfish Ors do not fall into the clade comprised of more
derived flying insect ligand-binding Ors, but, along with bristletail, firebrat and some mayfly Ors, are
consistently resolved as a distinct set of genes that may constitute an evolutionary intermediate
between gustatory receptors and the more derived Ors of flying insects. We propose to consider these
“primitive Ors” separately from higher insect Ors until their cellular expression patterns and function
are resolved and suggest a multistep evolutionary scenario ultimately leading to the highly sensitive,
rapidly evolving and physiologically diverse Or gene family observed in higher insects.