THE ORIGINS AND EVOLUTIONARY EXPANSION OF THE STRONGYLIDA (NEMATODA)

The Strongylida are thought to have arisen from free-living rhabditoid nematodes, but the relationships between the major groupings within t he Strongylida, the Strongylina, the Metastrongylina, Trichostrongylin a and the Ancylostomatina are far from clear in spite of the abundance of morphological data now available for analysis. Evolutionary mechan isms including co-evolution, host switching, host dispersal, use of in termediate hosts, Various sites of localisation within the definitive host and modifications of life-cycle strategies appear to have been ut ilised in the expansion of the Strongylida, with different mechanisms predominating in different families or superfamilies. Go-evolution app ears to have been a major mode of evolution in the Strongylina, in con trast to the Trichostrongylina, which have used host dispersal and hos t-switching to great advantage. The phylogeny of the Ancylostomatina s hows little association with host evolution, but does match the feedin g preferences of the hosts. The Metastrongylina have utilised intermed iate hosts and life cycle modifications including a shift to extra-int estinal sites as major means of diversification, in contrast to the ot her sub-orders. The review, while indicating much progress in our unde rstanding of the phylogeny of the Strongylida, also reveals that enorm ous gaps still exist, and emphasises the tentative nature of many of t he phylogenetic hypotheses tendered to date.