Genetic variation of geminiviruses: comparison between sexual and asexual host plant populations

One of the most promising hypotheses for the evolution of sex is that sexua l reproduction is advantageous because it increases the rate of adaptive ev olution in response to parasites. To investigate this advantage of sex, we compared genetic variation of geminiviruses infecting sexual and asexual po pulations of Eupatorium (Asteraceae). The infection frequency was 37.5% in the sexual population and 87.8% in the asexual population. The lower infect ion frequency in the sexual population might be the result of higher geneti c diversity of host plants. If geminiviruses have diverged to counter defen ce systems of genetically variable hosts, genetic diversity of viruses is e xpected to be higher in sexual host populations than in asexual host popula tions. To test this expectation, we used single-strand conformation polymor phism (SSCP) analysis to examine genetic diversity of the geminiviruses in a DNA region containing the open-reading frame (ORF) C4 gene, which is know n to function as a host range determinant. As predicted, higher genetic div ersity of viruses was observed in the sexual population: three SSCP types w ere found in the asexual population while six types were found in the sexua l population. Sequencing of the polymerase chain reaction (PCR) products re vealed further genetic diversity. Phylogenetic analysis of the sequences sh owed that the SSCP types belonged to four different clades. Several SSCP ty pes from the same clade were found in the sexual population, whereas the as exual population included only one SSCP type from each clade. Amino acid re placements of ORF C4 are suggested to be accelerated in the sexual populati on. This evidence supports the hypothesis that sexual reproduction is advan tageous as a defence against epidemic disease.