Cellular and Molecular Basis of Variability in Entamoeba histolytica 
zoites. However, the so-called P trophozoites 
form a heterogeneous group of E. histolytica iso- 
lates, which show virulence in a broad spectrum. 
This indicates that virulence-involved genes are 
differentially regulated. 
On the other hand, amebic strains are consti- 
tuted by trophozoites displaying different pheno- 
types. Clones from a given strain express viru- 
lence in the same broad spectrum as amebic 
strains. Some clones are avirulent (are they NP?); 
others are highly virulent, and specific monoclo- 
nal and polyclonal antibodies are able to discrimi- 
nate between them. 
The simplest interpretation for this is that 
strains are a mixture of genetically unrelated tro- 
phozoites living together in the human intestine. 
However, clones derived from cloned popula- 
tions differ phenotypically and genotypically. 
The differences include virulence, antigens, and 
genetic divergence. Clones isolated from clone A 
(strain HM1:IMSS) show the P zymodeme but 
differ in virulence and in the expression of the 
1 12-kDa adhesin, involved in both phagocytosis 
and virulence of E. histolytica. 
Detection and Cloning of DNA Sequences 
Present Only in Pathogenic Trophozoites 
Several E. histolytica genes show high similar- 
ity among P strains but differ from their homolo- 
gous genes in NP strains. Molecular studies have 
given the strongest evidence to suppon the exis- 
tence of two E. histolytica species. To date, there 
is no explanation for the genomic divergence 
shown by P and NP trophozoites. However, reti- 
cence to accept that there are two species based 
on their polymorphism is generated by the re- 
markable genomic plasticity of this parasite, even 
among clones derived from cloned populations. 
Genomic divergence is showed by strains and 
clones cultured under different conditions. Ge- 
netically related clones obtained from a cloned 
population show polymorphism in several genes 
of the same clone cultured in different laborato- 
ries (clone A, strain HM1:IMSS). They also differ 
in their molecular karyotype and in the chromo- 
somal location of some genes, indicating genome 
rearrangements. Finally, we have identified, 
cloned, and sequenced a 1.6-kilobase DNA frag- 
ment from clone A (virulent) that is altered 
in nonvirulent (NP?) clones derived also from 
clone A. 
We are currently studying genetic mechanisms 
underlying this intriguing phenomenon. One hy- 
pothesis, highly speculative, is that there are 
genes in E. histolytica that, under pressure, are 
selectively rearranged, modified, or amplified 
before being transcribed. If certain genes were 
amplified in P trophozoites and not in NP, single- 
copy or slightly modified genes could not be de- 
tected in comparative experiments. If this were 
true, it would explain, at least in part, the poly- 
morphism found in P and NP strains. 
The presence of "cassettes" carrying different 
genes that could be amplified and expressed se- 
lectively is another possibility. We are trying to 
develop a genetic transformation system in E. his- 
tolytica to introduce foreign genes as landmarks 
to follow up the genome modifications that will 
explain the erratic behavior of this parasite, 
or at least the differences between P and NP 
trophozoites. 
This work is supported by a grant from the Na- 
tional Council for Science and Technology, 
Mexico. 
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