Gene copy number and function of the APL1 immune factor changed during Anopheles evolution

Area of Science:

• Genomics
• Immunology
• Evolutionary Biology

Background:

• Anopheles stephensi possesses a single APL1 immune gene, contrasting with the three paralogs found in Anopheles gambiae and Anopheles coluzzii.
• The evolutionary history and functional significance of the unique APL1 gene in An. stephensi remain largely unexamined.

Purpose of the Study:

• To investigate the evolutionary trajectory and functional role of the APL1 gene in Anopheles mosquitoes.
• To determine the impact of APL1 gene variations on mosquito immunity and survival against pathogens.

Main Methods:

• Manual annotation of the APL1 locus in An. stephensi.
• Phylogenomic analysis to trace APL1 evolution across Anopheles species.
• Gene silencing experiments in An. stephensi and An. coluzzii, followed by survival, Plasmodium infection, and expression analyses.

Main Results:

• APL1 expanded to three paralogs in an African lineage (An. gambiae complex, An. christyi), while remaining a single gene in most Anopheles, including An. stephensi.
• Silencing the single APL1 gene in An. stephensi caused significant mortality, rescued by antibiotics, indicating essentiality for bacterial infection survival.
• An. stephensi's Plasmodium development relies on APL1 for protection against bacterial-induced mortality; An. coluzzii's APL1 paralogs showed no such effect.
• The single An. stephensi APL1 gene is regulated by Imd and Toll pathways, which differentially regulate paralogs in the expanded An. coluzzii locus.

Conclusions:

• APL1 gene function and copy number evolved through loss and gain, with expansion to three paralogs in a specific African Anopheles lineage.
• The unique APL1 gene in An. stephensi is crucial for longevity, providing protection against bacterial pathology.
• The expanded APL1 gene family in the An. gambiae complex may contribute to their high efficiency in malaria transmission.