Resolving tylenchid evolutionary relationships through multiple gene analysis derived from EST data

Sequence-based phylogenetic analyses typically are based on a small number of character sets and report gene trees which may not reflect the true species tree. We employed an EST mining strategy to suppress such incongruencies, and recovered the most robust phylogeny for five species of plant-parasitic nematode (Meloidogyne arenaria, M. chitwoodi, M. hapla, M. incognita, and M. javanica), three closely related tylenchid taxa (Heterodera glycines, Globodera pallida, and G. rostochiensis) and a distant taxon, Caenorhabditis elegans. Our multiple-gene approach is based on sampling more than 80,000 publicly available tylenchid EST sequences to identify phylum-wide orthologues. Bayesian inference, minimum evolution, maximum likelihood and protein distance methods were employed for phylogenetic reconstruction and hypothesis tests were constructed to elucidate differential selective pressures across the phylogeny for each gene. Our results place M. incognita and M. javanica as sister taxa, with M. arenaria as the next closely related nematode. Significant differences in selective pressure were revealed for some genes under some hypotheses, though all but one gene are exclusively under purifying selection, indicating conservation across the orthologous groups. This EST-based multi-gene analysis is a first step towards accomplishing genome-wide coverage for tylenchid evolutionary analyses.