Wolbachia : evolutionary significance in nematodes

Our knowledge of the symbiosis between Wolbachia and filarial nematodes has grown rapidly in recent years. Phylogenetic analyses, which highlight a coevolutionary pattern for filarial nematodes and their wolbachiae, and molecular evolutionary analyses, showing no evidence for positive selection in a surface protein, are both concordant with the idea that Wolbachia has evolved a mutualistic association with its hosts. There are, however, several open questions regarding the biology and evolution of this symbiotic association. In particular, the actual distribution of Wolbachia in filariae and the overall molecular diversity of these bacteria have not yet been completely uncovered. Wolbachia is apparently at fixation in positive species of filariae. Other filariae, which are in some cases phylogenetically related to positive species, lack this symbiosis. This picture is intriguing if we consider that Wolbachia is thought to be beneficial in positive filariae. If filariae rely on Wolbachia for some key biological functions, why should some species have renounced to its presence? We could perhaps suggest that the association between Wolbachia and filariae, while being of some usefulness to the nematode, had not evolved to a state of complete dependence of the host, at least in those nematodes that have renounced to this symbiosis.