RELUCTANCE TO NAME FORMAL TAXA

This contrast between the eagerness to sub-divide T. cruzi and the reluctance to name formal taxa is curious in the light of the comparison with the related trypanosomatid genus Leishmania. For example the phylogenetic diversity in T. cruzi is comparable to that observed in the whole of the genus Leishmania (Tibayrenc 1998a), which is currently divided into nearly 50 species. Even if the comparison is limited to the same geographical area and a single order of reservoir, there are still about twenty mammalian species of New world Leishmania as compared to a single T. cruzi species. Although there is some criticism of the excess number of species in Leishmania, with the level of phylogenetic divergence between some species of Leishmania comparable to lower clades of T. cruzi (Tibayrenc 1998a), the benefit of the named species in clarifying the ecoepidemiology and causes of the diverse clinical manifestations of the leishmaniases is undoubted. Furthermore the studies of Andrade (1974) provided a similar basis for T. cruzi to that of Leishmania for the description of new taxa.

Several reasons can be put forward to explain this reluctance for describing named taxa for T. cruzi. At the time the principal zymodeme divisions were proposed and in the period afterwards several other studies raised questions about the divisions. For example, Brenner (1977) proposed two polar types (Y and CL). These strains were shown later to posses a number of fundamental differences such as differences in the course of infection in a variety of hosts including morphology of blood forms at peak of parasitemia which occurred at different times and differences in infectivity to mouse peritoneal macrophages, tissue culture cells and in vivo infections. These fundamentally different types appeared to belong to the same zymodeme. The zymodemes themselves appeared not to be stable (Romanha et al. 1979) a finding reinforced by apparent instability of isoenzyme profiles in other parasites (Mirelman et al. 1986). The principal zymodemes also appeared to have geographical variations and could be divided into a number of isoenzyme strains (Tibayrenc & Ayala 1988). At the same time the technique of schizodeme analysis (Morel et al. 1980) showed an extensive heterogeneity within T. cruzi, which could not be readily classified into types. These results were supported by many further DNA studies using a variety of techniques demonstrating the genetic variability of T. cruzi (Macedo & Pena 1998).

Morever the use of these techniques indicated the possibility of heterogeneity within the T. cruzi strains, with particular strains or isolates being mixtures of at least two populations (Morel et al. 1980) and the probability of selective isolation of clones or strains (Deane et al. 1984, Macedo & Pena 1998). These and other reasons favoured the view of T. cruzi as a single polytypic species and against a formal subdivsion, as well as illustrating the difficulty of correlating strains with patient morbidity. However the possibility of a strain or even clone having more than one population of parasites was in fact the explanation for the observed instability of the isoenzyme characters and apparent similarity between the enzyme profile of the polar types (Goldberg & Perreira 1983, Gomes et al. 1991, Clark & Diamond 1993).