Effect of citrus hosts on the generation , maintenance and 1 evolutionary fate of genetic variability of Citrus exocortis 2 viroid ( CEVd )

22 23 2 SUMMARY 23 Citrus exocortis viroid (CEVd) populations are composed by closely related 24 haplotypes whose frequencies in the population result from the equilibrium 25 between mutation, selection and genetic drift. The genetic diversity of CEVd 26 populations infecting different citrus hosts was studied by comparing 27 populations recovered from infected trifoliate orange and sour orange seedling 28 trees after 10 years of evolution and with the ancestral population maintained 29 for the same period in the original host, Etrog citron. Furthermore, populations 30 isolated from these trifoliate orange and sour orange trees were transmitted 31 back to Etrog citron plants and the evolution of their mutant spectra studied. 32 The results indicate that (i) the amount and composition of the within-plant 33 genetic diversity generated varies between these two hosts and is markedly 34 different from that characteristic of the original Etrog citron host and (ii) the 35 genetic diversity found after transmitting back to Etrog citron is 36 undistinguishable from that characteristic of the ancestral Etrog citron 37 population regardless the citrus plant from where the evolved populations 38 were isolated. The relationship between the CEVd populations from Etrog 39 citron and trifoliate orange, both sensitive hosts, and those from sour orange, 40 which is a tolerant host, is discussed.