Cytonuclear Epistasis Controls the Density of Symbiont in Nongonadal Tissues of Mosquito .

, a bacterial symbiont infecting arthropods and nematodes, is vertically transmitted through the female germline and manipulates its host's reproduction to favor infected females. also infects somatic tissues where it can cause nonreproductive phenotypes in its host, including resistance to viral pathogens. -mediated phenotypes are strongly associated with the density of in host tissues. Little is known, however, about how density is regulated in native or heterologous hosts. Here, we measure the broad-sense heritability of density among families in field populations of the mosquito , and show that densities in ovary and nongonadal tissues of females in the same family are not correlated, suggesting that density is determined by distinct mechanisms in the two tissues. Using introgression analysis between two different strains of the closely related species , we show that densities in ovary tissues are determined primarily by cytoplasmic genotype, while densities in nongonadal tissues are determined by both cytoplasmic and nuclear genotypes and their epistatic interactions. Quantitative-trait-locus mapping identified two major-effect quantitative-trait loci in the genome explaining a combined 23% of variance in density, specifically in nongonadal tissues. A better understanding of how density is regulated will provide insights into how density can vary spatiotemporally in insect populations, leading to changes in -mediated phenotypes such as viral pathogen resistance.