Ommochrome pathway components in mosquito vectors: As similar as different

Mosquitoes are vectors of many diseases, such as malaria or dengue, which cause extensive morbidity and mortality worldwide. Anopheles, Aedes and Culex are the main mosquito genera involved in the transmission of mosquito-borne diseases.

The ommochrome pathway is mainly involved in the synthesis of eye pigment in mosquitoes. The deficiency of some of the enzymes of this pathway, such as kynurenine 3-monooxygenase (kmo) and cardinal (cd), results in a distinctive eye-colour phenotype. Because this trait can be used as a phenotypic marker, these genes are widely employed in mosquito research.

Using CRISPR/Cas9 to modify these genes in Culex quinquefasciatus, Aedes aegypti and Anopheles stephensi we have observed varying effects on fitness. In C. quinquefasciatus we observed slow growth in a kmo knock-out (KO), and mortality of the homozygous larvae before pupation in kmo knock-in (KI) lines. In Ae. aegypti, little fitness cost was observed in either KO or KI lines, even when homozygous. Finally, for An. stephensi, a decrease in male fertility was detected in cd KI lines and a cd KO line. Some lethality in larvae and adult female mortality after bloodfeeding were also observed in homozygous cd KI lines.

These observations reveal that the enzymes involved in the development of eye pigmentation of mosquitoes are also involved in processes related to other aspects of fitness. Moreover, different effects have been observed in the different genus of mosquitoes indicating that observations made in one genus cannot reliably be transferred to another.