Back to Basics: Are Begomoviruses Whitefly Pathogens?

Begomoviruses and whiteflies have interacted for geological times. An assumed long-lasting virus-vector intimate relationship of this magnitude implies that the partners have developed co-evolutionary mechanisms that insure on one hand the survival and the efficient transmission of the virus, and on the other hand the safeguard of the insect host from possible deleterious effects of the virus. Several studies have indicated that viruses belonging to the Tomato yellow leaf curl virus (TYLCVs) family from China, Israel and Italy are reminiscent of insect pathogens. TYLCVs like all begomoviruses are transmitted in a circulative manner by the whitefly Bemisia tabaci. The survival of the virus in the haemolymph of B. tabaci is ensured by a GroEL homologue produced by a whitefly secondary endosymbiont. Following acquisition and transfer to non-host plants, the virus may remain associated with the insect for its entire 4-5 wk-long adult life. During this period, the ability of the insects to inoculate plants steadily decreased, but did not disappear. The long-term presence of TYLCVs in B. tabaci was associated with a decrease in the insect longevity and fertility. Viral DNA was transmitted to progeny, but seldom infectivity. TYLCV transcripts were found associated with the insects, raising the possibility of replication and expression in the vector. TYLCVs may spread amidst whiteflies during copulation. Functional genomics tools such as microarrays, deep sequencing, quantitative PCR and gene silencing allow revisiting the proposition that TYLCVs have retained, or acquired, some characteristics of an insect pathogen.