n° 385 - July 2000

The development of molecular tools has revitalized systematic research. In recent years this increased interest has brought attention to two points*. Firstly, the comparison of several species gives information on adaptive processes. Secondly, phylogenetic relationships can confound the interpretation of observations and the validity of correlations. That is, two species can share traits not because of a common adaptation, but due to inheritance from a common ancestor. Comparative analysis must therefore take into account phylogeny.
In the case of host-parasite relationships, numerous studies have shown that host evolution is not independent of the history of its parasites. Two types of historical relationship between hosts and parasites have been established.
Firstly, most studies have addressed models of co-speciation which are based on the hypothesis that each speciation event in the host is accompanied by parasite speciation. This results in congruence between the phylogenetic trees of the host and the parasite. Examples of such co-speciation are models which potentially allow the study of the co-evolution of host defense genes, and parasite counter-defense genes.
Secondly, it has been observed that the constitution of parasite communities is not independent of host phylogeny. It is therefore possible to reconstruct the degree of relationship between hosts using parasites as markers even in the absence of co-speciation. Hosts which are phylogenetically close share common parasites because of a shared recent evolutionary history.
Phylogenetic reconstructions are necessary for both hosts and parasites. Comparison of these phylogenies will be used to test hypotheses concerning co-speciation and co-evolution in biogeographical contexts. Finally, the hypothesis that parasites are a major force in evolution remains to be tested. If it turns out to be true, it would prove the influence of parasites on the diversification of host species.

* The Laboratory "Biologie des populations d'helminthes parasites" (CNRS-Université de Perpignan) does these researches.

For further information :

 

R. Poulin and S. Morand. (2000). The Diversity of Parasites. Quart. Review of Biology (à paraître).

Morand S. (2000). Wormy word : comparative tests of theoretical hypotheses on parasite species richness. In: Evolutionary biology of host-parasite relationships : reality meets models. Éds Poulin R., S. Morand and A. Skorping. Elsevier (à paraître).