The analysis of 9 populations of Echeandia nana showed all to be diploid, w
ith 2n = 16, n = 8 (x = 8). The analyzed populations displayed two cytotype
s. Cytotype A = 10m+6sm, having two pairs of chromosomes with a satellite,
was observed in four populations from the eastern flanks of the Pachuca mou
ntain range. The five remaining populations from the western nank of the Pa
chuca and Sierra Nevada mountain ranges (Mexico) showed cytotype B = 6m+-8s
m+2st, having one pair of chromosomes with a satellite. The analysis of mei
osis revealed heterozygotic exchanges. Analysis of MI showed three heteromo
rphic bivalents in cytotypes A and B. Analysis of AI showed sub-chromatid a
berrations, (side arm bridges = SAB), which were more frequent in cytotype
B (39.88-44.84%) than in cytotype A (2.86-31-53%). Cells with two bridges (
SAB aberrations) were observed in cytotype B. The intraspecific cytological
and genical differentiation of cytotypes A and B is probably the result of
geographical isolation between populations of E. nana. This suggests that
this species is undergoing through a major process of genomic differentiati
on involving heterozygotic chromosomal rearrangements; which favors a proce
ss of speciation between both cytotypes without the occurrence of significa
nt morphological changes. This cytological and genical differentiation betw
een cytotypes A and B was evident in the significant differences of the low
number of fruits and viable seeds produced after cross-pollination among c
ytotypes (AxB: 0-16 fruits; 0-448 abortive seeds), relative to the larger v
alues recorded after cross-pollination within cytotypes.