PRIMARY PHYLOGENETIC DIVISIONS

Lumsden (1977) defined three classes of nomenclature, (i) operational, without any indication of characterization, which included terms such as population, sample, isolate, clone, stock and (ii) Linnean, including genus, species and subspecies. The third class he called "a new nomenclature to designate the manifold new subspecific categories which are being discovered by new methods of characterization _ the multiplicity of functionally different populations which exist within the same morphological species". Although he did not formally name this class we can refer to it as infraspecific, however as pointed out by Lumsden for many microrganisms, non-contentious recognition is more often at the level of genus and subgenus. This third class has proved very popular in molecular studies of T. cruzi as the profusion of names in the Table demonstrates.

Attention has again been recently focused on two primary phylogenetic divisions within T. cruzi (Tibayrenc 1995, Souto et al. 1996, Nunes et al. 1997). While there are differences of opinion about the significance of this division (Brisse et al. 1998, Souto et al. 1998, Macedo & Pena 1998) the basis for the division is well supported (Table). The discovery that microbial lineages maintain their genetic integrity over long time intervals and over great distances, that is, their genomes are not rapidly broken down or reshuffled by recurrent mutation and recombination is known as the clone concept (Orskov & Orskov 1983). Tibayrenc et al. (1986) have proposed this model as the main population genetic structure for T. cruzi . The application of this model with the presence of a primary infraspecific division in T. cruzi means that Chagas disease can no longer be considered as a single disease entity. At least two diseases corresponding to the two divisions must be considered with obvious implications for clinical and experimental studies as well as control of the disease. Results of many investigations need now to be reinterpreted based on the classification of the strains used. This may also be an explanation for differences in observations among researchers in many studies such as the use of diagnostic techniques reported in the literature.