Measuring the fitness cost of type I resistance breaking by CpGV

Abstract: In a previous work we have measured virus productivity and pathogenicity of two virus isolates:CpGV-M, and CpGV-R5 (a type I resistance breaking isolate). We were not able to find significative differences between them. Another way of testing fitness differences is to analyse genotype replacement in mixed genotype experimental populations over successive generations. We constructed five experimental populations with various proportions of each isolate and followed them through six succesive passages on insects. In absence of selection, that is when replicating in an insect colony susceptible to both virus isolates, the ability to kill resistant larvae is not lost. After 6 passages, the pathogenicities of the 5 experimental virus populations on the resistant insect colony RGV converge to a common level χ2 = 13.35, 8 dof, P > 0.05). In addition, markers specific to both isolates are detected. Our results suggest again that, at least in laboratory conditions, the cost for the virus to break type I resistance -if it exists- is under the level of detection. All these observations, on the virus and on the host, suggest the existence of multiple choices with no cost for the players (virus and insect).