The haplotype distribution of two genes of citrus tristeza virus is altered after host change or aphid transmission

Genetic variability of citrus tristeza virus (CTV) was studied using the ha plotypes detected by single-strand conformation polymorphism (SSCP) analysi s of genes p18 and p20 in six virus populations of two origins. The Spanish group included a CTV isolate and subisolates obtained by graft-transmissio n to different host species. The other included two subisolates aphid-trans mitted from a single Japanese isolate. The homozygosity observed for gene p 20 was always significantly higher than that expected under neutral evoluti on, whereas only three populations showed high homozygosity for p18 suggest ing stronger host constraints for p20 than for p18. Sequential transmission s of a Spanish isolate to new host species increased the difference between its population and that of the successive subisolates for gene p18 as esti mated by the F statistic. Analysis of molecular variance indicated that var iation between both groups of populations was not statistically significant , whereas variations between populations of the same group or within popula tions were significant for both genes studied. Our data indicate that selec tion affects the haplotype distribution and that adaptation to a new host c an be as important or more as the geographical origin. Variation of the CTV populations after host change or aphid transmission may explain in part th e wide biological variability observed among CTV isolates. (C) 1999 Academi c Press.