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.