Multiple introductions of the dengue vector, Aedes aegypti, into California

Abstract

The yellow fever mosquito Aedes aegypti inhabits much of the tropical and subtropical world and is a primary vector of dengue, Zika, and chikungunya viruses. Breeding populations of A. aegypti were first reported in California (CA) in 2013. Initial genetic analyses using 12 microsatellites on collections from Northern CA in 2013 indicated the South Central US region as the likely source of the introduction. We expanded genetic analyses of CA A. aegypti by: (a) examining additional Northern CA samples and including samples from Southern CA, (b) including more southern US populations for comparison, and (c) genotyping a subset of samples at 15,698 SNPs. Major results are: (1) Northern and Southern CA populations are distinct. (2) Northern populations are more genetically diverse than Southern CA populations. (3) Northern and Southern CA groups were likely founded by two independent introductions which came from the South Central US and Southwest US/northern Mexico regions respectively. (4) Our genetic data suggest that the founding events giving rise to the Northern CA and Southern CA populations likely occurred before the populations were first recognized in 2013 and 2014, respectively. (5) A Northern CA population analyzed at multiple time-points (two years apart) is genetically stable, consistent with permanent in situ breeding. These results expand previous work on the origin of California A. aegypti with the novel finding that this species entered California on multiple occasions, likely some years before its initial detection. This work has implications for mosquito surveillance and vector control activities not only in California but also in other regions where the distribution of this invasive mosquito is expanding.

Fig 1. Maps of geographic locations of Aedes aegypti populations used in these analyses.

(A) Geographic locations of the California populations. Letters refer to each city: A = San Mateo, B = Madera, C = Fresno, D = Exeter, E = Clovis, F = Anaheim, G = Orange, H = Garden Grove, I = Santa Ana, J = Mission Viejo, K = San Diego, L = Los Angeles. (B) Geographic locations of all populations included in the analyses. Colors correspond to regions: red = Southern California, blue = Northern California, green = Southwest, purple = South Central, and yellow = Southeast.

Fig 2. Colonization scenarios tested by DIYABC.

Time is indicated on the y-axis. Time 0 corresponds to the present and time increases towards the past. The four figures show four different colonization scenarios, as described in the methods with their corresponding support values (“p” indicates the posterior probability of each scenario). Lineages are drawn in different colors. Changes in line color suggest possible changes in effective population size through time.

Fig 3. Genetic structure of California and North American populations using microsatellite loci.

In these plots, each vertical bar represents an individual. The proportion of each color assigned to an individual represents the proportion of the individual’s ancestry attributable to each of K theoretical genetic clusters. Letters within the plot refer to city as in Fig 1 and Table 1. (A) California populations (K = 2). (B) California populations (K = 8). (C) California and North American populations (K = 2).

Fig 4. Genetic structure within California and North American populations using SNP data.

The fraction of each vertical bar assigned to each color represents the proportion of that individual’s ancestry attributable to each of 6 K theoretical genetic clusters.

Fig 5. Population structure of California populations using SNP data and Principle Component Analysis.

Data were transformed using PCA and plotted as a function of the first two principle components.