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).