Genome-Wide Analysis of the Odorant Receptor Gene Family in Solenopsis invicta, Ooceraea biroi, and Monomorium pharaonis (Hymenoptera: Formicidae)

Abstract

Olfactory systems in eusocial insects play a vital role in the discrimination of various chemical cues. Odorant receptors (ORs) are critical for odorant detection, and this family has undergone extensive expansion in ants. In this study, we re-annotated the OR genes from the most destructive invasive ant species Solenopsis invicta and 2 other Formicidae species, Ooceraea biroi and Monomorium pharaonis, with the aim of systematically comparing and analyzing the evolution and the functions of the ORs in ant species, identifying 356, 298, and 306 potential functional ORs, respectively. The evolutionary analysis of these ORs showed that ants had undergone chromosomal rearrangements and that tandem duplication may be the main contributor to the expansion of the OR gene family in S. invicta. Our further analysis revealed that 9-exon ORs had biased chromosome localization patterns in all three ant species and that a 9-exon OR cluster (SinvOR4-8) in S. invicta was under strong positive selection (Ka/Ks = 1.32). Moreover, we identified 5 S. invicta OR genes, namely SinvOR89, SinvOR102, SinvOR352, SinvOR327, and SinvOR135, with high sequence similarity (>70%) to the orthologs in O. biroi and M. pharaonis. An RT-PCR analysis was used to verify the antennal expression levels of these ORs, which showed caste-specific expression. The subsequent analysis of the antennal expression profiles of the ORs of the S. invicta workers from the polygyne and monogyne social forms indicated that SinvOR35 and SinvOR252 were expressed at much higher levels in the monogyne workers than in the polygyne workers and that SinvOR21 was expressed at higher levels in polygyne workers. Our study has contributed to the identification and analysis of the OR gene family in ants and expanded the understanding of the evolution and functions of the ORs in Formicidae species.