The antennal transcriptome analysis and characterizations of odorant-binding proteins in Megachile saussurei (Hymenoptera, Megachilidae)

Background
Odorant-binding proteins (OBPs) are essential in insect’s daily behaviors mediated by olfactory percep‑
tion. Megachile saussurei Radoszkowski (Hymenoptera, Megachilidae) is a principal insect pollinating alfalfa (Medicago
sativa) in Northwestern China. The olfactory function have been less conducted, which provides a lot of possibilities
for our research.
Results
Our results showed that 20 OBPs were identifed in total. Multiple sequence alignment analysis indicated
MsauOBPs were highly conserved with a 6-cysteine motif pattern and all belonged to the classic subfamily, coding
113-196 amino acids and sharing 41.32%-99.12% amino acid identity with known OBPs of other bees. Phylogenetic
analysis indicated there were certain homologies existed among MsauOBPs and most sequences were clustered
with that of Osmia cornuta (Hymenoptera, Megachilidae). Expression analysis showed the identifed OBPs were mostly
enriched in antennae instead of other four body parts, especially the MsauOBP2, MsauOBP3, MsauOBP4, MsauOBP8,
MsauOBP11 and MsauOBP17, in which the MsauOBP2, MsauOBP4 and MsauOBP8 presented obvious tissue-biased
expression pattern. Molecular docking results indicated MsauOBP4 might be the most signifcant protein in recogniz‑
ing alfalfa fower volatile 3-Octanone, while MsauOBP13 might be the most crucial protein identifying (Z)-3-hexenyl
acetate. It was also found the lysine was a momentous hydrophilic amino acid in docking simulations.
Conclusion
In this study, we identifed and analyzed 20 OBPs of M. saussurei. The certain homology existed
among these OBPs, while some degree of divergence could also be noticed, indicating the complex functions
that diferent MsauOBPs performed. Besides, the M. saussurei and Osmia cornuta were very likely to share similar physi‑
ological functions as most of their OBPs were clustered together. MsauOBP4 might be the key protein in recognizing
3-Octanone, while MsauOBP13 might be the key protein in binding (Z)-3-hexenyl acetate. These two proteins might
contribute to the alfalfa-locating during the pollination process. The relevant results may help determine the highly
specifc and efective attractants for M. saussurei in alfalfa pollination and reveal the molecular mechanism of odorevoked pollinating behavior between these two species.