Sequence polymorphism of 2 pioneer genes expressed in phytoparasitic nematodes showing different host ranges.

In order to identify genes involved in pathogenicity, we compared the closely related species Globodera pallida (GP) and Globodera "mexicana" (GM) that have different host ranges and are able to produce viable and fertile hybrids. Three pioneer genes were previously identified as differentially expressed between GP and GM: GPLIA7 and GPLIB3 were found to be more highly expressed in GP, whereas GMLIVG9 was found more highly expressed in GM. In this study, we showed that Ia7 and IVg9 genes probably encode products secreted by the subventral oesophageal glands and the dorsal oesophageal gland, respectively. No Blast homolog was found in the databases, but a metridin-like ShK (Stichodactyla helianthus) toxin domain was identified in the Ia7 sequence. Analysis of the full-length sequences of these 2 genes between GP and GM revealed a high level of interspecies variability (8% for the Ia7 transcript and 17% for the IVg9 transcript) and a high proportion (90%) of nonsynonymous mutations among the substitutions observed. This suggested that these 2 pioneer genes are under strong diversifying selection pressures and therefore may be involved in pathogenicity. Further investigations of the sequence polymorphism of Ia7 and IVg9 genes were conducted in GP x GM hybrid lines that were selected in laboratory conditions for their different ability to develop on potato and black nightshade. As similar sequences were obtained for all the hybrid lines tested independently of their pathogenicity status, no correlation could be established between IA7 and IVG9 amino acid changes and the host range differences observed between GP and GM.