Fig. 2: Four amino acids determine much of the fitness difference between different ZIKV strains in both mosquitoes and A129 mice.

a The hypothetical diagram of fitness changes following the spread of ZIKV from Africa to Asia and the Americas, including strains used for fitness assays. b Schematic representation of competition fitness assay in mosquito bodies, legs, and saliva. Figure adapted from Fig. 1a in Ref. ¹⁴. c, e Fitness comparison between African and Asian lineage, and the four amino-acid substitution mutants (DK-4M and FSS-4M) in mosquito bodies, legs, and saliva. The mosquitoes acquired the ZIKV mixture through membrane blood-feeding and were assayed at 14 days post-infection. The body (carcass), legs, and saliva were collected from each individual mosquito and subjected to RT-PCR amplification and Sanger sequencing to determine relative fitness values of 4 M over wt; each point represents a single mosquito or mouse sample. d, f Competition between DK-WT and DK-4M (d), FSS-WT, and FSS-4M (f) in mouse blood collected 3 days after infection. g, h Fitness of DK-4M (g) and FSS-4M (h) in mosquitoes after oral infection from viremic mice. The mosquitoes acquired the virus through biting a viremic mouse 3 days after infection and were tested after 14 days of extrinsic incubation. c–h, The distribution of the model-adjusted means is illustrated by catseye plots with shaded ± standard error (SE) overlaid by scatterplots of individual measures, which are shown on the log (base-10) scale such that comparisons are against a null value of 1. P values are calculated for the group (strain) coefficient for each linear regression model. n numbers represent the biologically independent samples and shown on the top of each figure. Source data are provided as a Source Data file. c, e, g The results were pooled from 3 independent repeats. d, f The results were collected from a single experiment. h The results were pooled from 4 independent repeats.