The Expression of Chemosensory Genes in Male Maxillary Palps of Anopheles coluzzii (Diptera: Culicidae) and An. quadriannulatus

Abstract

Because of its importance as a malaria vector, Anopheles coluzzii's Coetzee & Wilkerson olfactory system has been studied extensively. Among this work is a series of studies comparing the expression of chemosensory genes in olfactory organs in females and/or males of these species. These have identified species- and female-biased chemosensory gene expression patterns. However, many questions remain about the role of chemosensation in male anopheline biology. To pave the way for future work we used RNAseq to compare chemosensory gene expression in the male maxillary palps of An. coluzzii and its sibling species An. quadriannulatus Theobald. As expected, the chemosensory gene repertoire is small in the male maxillary palps. Both species express the tuning receptors Or8 and Or28 at relatively high levels. The CO2 receptor genes Gr22-Gr24 are present in both species as well, although at much lower level than in females. Additionally, several chemoreceptors are species-specific. Gr37 and Gr52 are exclusive to An. coluzzii, whereas Or9 and Gr60 were detected only in An. quadriannulatus. Furthermore, several chemosensory genes show differential expression between the two species. Finally, several Irs, Grs, and Obps that show strong differential expression in the female palps, are absent or lowly expressed in the male palps. While many questions remain about the role of chemosensation in anopheline male biology, these results suggest that the male maxillary palps could have both a sex- and species-specific role in the perception of chemical stimuli. This work may guide future studies on the role of the male maxillary palp in these species.

Keywords: chemosensory gene; malaria vector; male behavior; maxillary palp.

Fig. 1.

Venn diagram indicating differential gene expression between the maxillary palps of male Anopheles coluzzii and Anopheles quadriannulatus. Genes are considered differentially expressed if q > 0.05 and fold change > 2.0.

Fig. 2.

A comparison of the expression in transcripts per million (TPM) of four chemosensory gene families in male maxillary palps of Anopheles coluzzii vs. Anopheles quadriannulatus. A: Odorant receptors (Ors), (B): Ionotropic receptors (Irs), C: Gustatory receptors (Grs), D: Odorant binding proteins (Obps). The line indicates equal expression between the two species. Genes with differential expression (q < 0.05, fold-change >2) are indicated by grey dots. Only genes expressed at >1 TPM are shown.