Wolbachia strain wPip yields a pattern of cytoplasmic incompatibility enhancing a Wolbachia-based suppression strategy against the disease vector Aedes albopictus

Abstract

Background: Cytoplasmic incompatibility (CI) is induced in nature by Wolbachia bacteria, resulting in conditional male sterility. Previous research demonstrated that the two Wolbachia strains (wAlbA and wAlbB) that naturally co-infect the disease vector mosquito Aedes albopictus (Asian tiger mosquito) can be replaced with the wPip Wolbachia strain from Culex pipiens. Since Wolbachia-based vector control strategies depend upon the strength and consistency of CI, a greater understanding is needed on the CI relationships between wPip, wAlbA and wAlbB Wolbachia in Ae. albopictus.

Methods: This work consisted of a collaborative series of crosses carried out in Italy and in US to study the CI relationships between the "wPip" infected Ae. albopictus strain (ARwP) and the superinfected SR strain. The Ae. albopictus strains used in Italian tests are the wPip infected ARwP strain (ARwPIT), the superinfected SR strain and the aposymbiotic AR strain. To understand the observed pattern of CI, crossing experiments carried out in USA focused on the study of the CI relationships between ARwP (ARwPUS) and artificially-generated single infected lines, in specific HTA and HTB, harbouring only wAlbA and wAlbB Wolbachia respectively.

Results: The paper reports an unusual pattern of CI observed in crossing experiments between ARwP and SR lines. Specifically, ARwP males are able to induce full sterility in wild type females throughout most of their lifetime, while crosses between SR males and ARwP females become partially fertile with male aging. We demonstrated that the observed decrease in CI penetrance with SR male age, is related to the previously described decrease in Wolbachia density, in particular of the wAlbA strain, occurring in aged superinfected males.

Conclusions: The results here reported support the use of the ARwP Ae. albopictus line as source of "ready-made sterile males", as an alternative to gamma radiation sterilized males, for autocidal suppression strategies against the Asian tiger mosquito. In addition, the age dependent CI weakening observed in the crosses between SR males and ARwP females simplifies the downstream efforts to preserve the genetic variability within the laboratory ARwP colonies, to date based on the antibiotic treatment of wild captured superinfected mosquitoes, also reducing the costs.

Figure 1

Estimated CI levels in function of male ageing, CI_corr (%) [41] was estimated from egg hatching data observed in incompatible and compatible crossing types (female x male) involving SR (superinfected), AR (aposymbiotic) and ARwP_IT (transinfected) Ae. albopictus lines. Different letters at each male age interval indicate significant differences (P<0.05) (Anova-Newman-Keuls Multiple Comparison Test).

Figure 2

CI relationships among wPip and Ae. albopictus native Wolbachia strains. The Wolbachia strain wPip is a strong CI inducer towards wAlbA and wAlbB (both unable to rescue its mod function) While wPip it is not able to rescue the wAlbA mod function, it can partially rescue wAlbB (arrows indicate CI intensity).