Molecular variability of banana mild mosaic virus

Banana mild mosaic virus (BanMMV), a member of the #Flexiviridae# family, is currently one of the major viral constraints for #Musa# germplasm exchanges, since the virus is propagated vegetatively. Current data show that synergistic infections with Banana streak virus (BSV) or Cucumber mosaic virus (CMV) can lead to severe necrotic symptoms, although single infections by BanMMV cause no or only very mild symptoms. Very little information is currently available on the epidemiology of BanMMV, which has no known vector. In order to gain insights into the biology of BanMMV, we have undertaken studies on its molecular variability. An IC-RT-nested PCR-based detection method was developed and amplification products within the RdRp gene were cloned and sequenced for 69 distinct accessions originating from CIRAD-Guadeloupe's #Musa# collection. A total of 154 sequences were analysed. Sequence comparisons showed an overall exceptionally high molecular variability between sequences originating from distinct accessions, while intra-plant variability was generally low. In a significant number of cases however, very high intra-plant diversity was observed. In other cases, very closely related sequences were isolated from different accessions suggesting transmission of BanMMV between plants. These results were confirmed by sequence comparisons of the 3' end of BanMMV genomic RNA covering part of the CP gene and the 3' untranslated region, performed with 30 more sequences originating from a selection of 10 out of the 69 above-mentioned accessions. No conclusive correlation between the geographical or genetic origin of the accessions could be established. These results tend to indicate that BanMMV could be preferentially but not exclusively transmitted vegetatively. They show that BanMMV is subject to a very active viral evolution process within isolated phyla. They also provide compelling evidence for movement of BanMMV isolates between accessions. Possible correlations between the very high variability observed and plant defence mechanisms will be discussed, as well as consequences of this variability on the development of efficient molecular tools for the detection of BanMMV (Texte intégral)