A comparison of serological and nucleic acid techniques was used to investigate whether the PCRpositives were sequences amplified from badnavirus particles or putative endogenous badnavirus sequences in the yam genome. Protein A sandwich-enzyme-linked immunosorbent assay (PAS-ELISA) with badnavirus polyclonal antisera detected cross-reacting viral particles in only 60% (92 of 153) of the CIRAD collection samples analyzed, in contrast to the aforementioned 81% by PCR.Immunosorbent electron microscopy (ISEM) of virus preparations of a select set of 16 samples, representing different combinations of positive and negative PCR and PAS-ELISA results, identified bacilliform particles in 11 of these samples. Three PCR-positive yam samples from Burkina Faso (cv. Pilimpikou) were identified in which no viral particles were detected by either PAS-ELISA or ISEM.Southern hybridisation results using a yam badnavirus RT-RNaseH sequence (Gn155Dr) as probe, supported a lack of badnavirus particles in the cv. Pilimpikou and identified their equivalent sequences to be of plant genome origin. Probe Gn155Dr, however, hybridised to viral particles and plant genomic DNA in three D. rotundata samples from Guinea. These results represent the first data demonstrating the presence of integrated sequences of badnaviruses in yam. The implications of this for virus-indexing, and breeding and multiplication of seed yams are discussed.
IntroductionYam (Dioscorea species) is the fourth most important food tuber crop in the world after potato, sweet potato, and cassava (FAO, 2012). In West Africa, it is the second most important food crop after cassava by value and production (FAO, 2012;Scarcelli et al., 2006). It plays an essential role in food security and income generation for smallholders, particularly in West Africa which produces about 95% of the world's total yam production (Asiedu and Sartie, 2010; IITA, 2012;Mignouna et al., 2008 Several surveys on yam viruses suggest that badnaviruses are the most prevalent globally (Bousalem et al., 2009; Eni et al., 2008a, b;Galzi et al., 2013;Kenyon et al., 2008). Badnavirus particles were first reported in yam in association with a flexuous virus, causing internal brown spot disease in D. alata and D. cayenensis in the Caribbean (Harrison and Roberts, 1973;Mantell and Haque, 1978). Two decades later yam badnaviruses were characterised by their nucleic acid and serological properties; particles isolated from D. alata and D. bulbifera were partially characterised and named informally as Dioscorea alata bacilliform virus (DaBV) and Dioscorea bulbifera bacilliform virus (DbBV) Phillips et al., 1999). These viruses were reported to induce leaf distortions and veinal chlorosis , although others found that often infected plants show no marked symptoms (Kenyon et al., 2008;Seal and Muller, 2007). Sensitive virus diagnostic tests are required to enable the identification of virus-free seed yams, and will underpin current efforts in West Africa to generate and multiply disease-free yam planting material (I...