Recurrent evolution of two competing haplotypes in an insect DNA virus

Hosts and viruses are constantly evolving in response to each other: as
hosts attempt to suppress the virus, the virus attempts to evade and
suppress the host’s immune system. This arms race results in the evolution
of novel pathways in both the host and virus to gain the upper hand. Here
we describe the coevolution between Drosophila species and a common and
virulent DNA virus. We identify two distinct viral types that differ
100-fold in viral titer in infected individuals, with similar effects
across multiple species. Our analysis suggests that one of the viral types
appears to have recurrently evolved at least 4 times in the past ~30,000
years, including in another geographically distinct species, due to the
high effective mutation rate which increases with titer. The higher titer
viral type is associated with suppression of the host immune system and an
increased transmission rate compared to the low viral titer type. Both
types are maintained in all populations, likely due to an increased
virulence in the high titer type creating a trade-off between effective
transmission and virulence and resulting in nearly equal reproduction
rates (R0) in both types. Together these results suggest that the
reciprocal selective pressures caused by the co-evolution between host and
virus has resulted in this recurrently evolving relationship.