While evolvability of genes and traits may promote specialization during species diversification, how ecology subsequently restricts such variation remains unclear. Chemosensation requires animals to decipher a complex chemical background to locate fitness-related resources, and thus the underlying genomic architecture and morphology must cope with constant exposure to a changing odorant landscape; detecting adaptation amidst extensive chemosensory diversity is an open challenge. In phyllostomid bats, an ecologically diverse clade that evolved plant-visiting from an insectivorous ancestor, the evolution of novel food detection mechanisms is suggested to be a key innovation, as plant-visiting species rely strongly on olfaction, supplementarily using echolocation. If this is true, exceptional variation in underlying olfactory genes and phenotypes may have preceded dietary diversification. We compared olfactory receptor (OR) genes sequenced from olfactory epithelium transcriptomes and olfactory epithelium surface area of bats with differing diets. Surprisingly, although OR evolution rates were quite variable and generally high, they are largely independent of diet. Olfactory epithelial surface area, however, is relatively larger in plant-visiting bats and there is an inverse relationship between OR evolution rates and surface area. Relatively larger surface areas suggest greater reliance on olfactory detection and stronger constraint on maintaining an already diverse OR repertoire. Instead of the typical case in which specialization and elaboration are coupled with rapid diversification of associated genes, here the relevant genes are already evolving so quickly that increased reliance on smell has led to stabilizing selection, presumably to maintain the ability to consistently discriminate among specific odorants — a potential ecological constraint on sensory evolution.

Specimens were collected over the course of five field expeditions: two to the Dominican Republic in 2014 and 2015 (collection permit VAPB-01436), one to Belize in 2014 (Belize Forestry Department Scientific Research and Collecting Permit CD/60/3/14), one to Peru in 2015 (collection permit 0002287), and one to Costa Rica in 2017 (collection permit R-041-2017-OT-CONAGEBIO). All genetic tissue and morphological specimens were exported in accordance with research permit and country guidelines. Samples were imported in accordance with U.S. Center for Disease Control and U.S. Fish & Wildlife guidelines. All specimens were collected, handled, and euthanized in accordance with Stony Brook University IACUC permit 614763-3 for Peru, and 448712-3 for Costa Rica, and Brown University IACUC 1205016 and 1504000134, University of Georgia IACUC AUP A2009-10003-0 and A2014 04-016-Y3-A5 for Belize.

The main olfactory epithelium was dissected, RNA was extracted and sequenced, olfactory receptor reading frames were identified, and receptors were aligned.

These can be opened in a text editor or any phylogenetic alignment handling program.