Cytological and genical differentiation between cytotypes of Echeandia nana (Anthericaceae)

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.