GONZALEZ, DOLORES* and ANDREW P. VOVIDES. Instituto de Ecologia, A.C., Apartado postal 63, Xalapa Ver., Mexico, c.p. 91000. - Low interspecific divergence in the genus Ceratozamia (Zamiaceae) detected with nuclear ribosomal DNA ITS, and chloroplast DNA trnL-F non-coding region as indication of recent speciation.
The cycads are generally perceived as a very ancient lineage. In
addition, high extinction rates and very long generation times may
preclude the dynamics of genetic variation. More than 20 of the known
and putative species of the Mexican cycad genus Ceratozamia
were examined for variation at the molecular level in non-coding
regions from the chloroplast and nuclear genomes. Cladistic analyses
of single and combined sequences of ITS and trnL-F non-coding
region supported three main clades within Ceratozamia. The rate
of change of these genomes is considered fast and appropriate to
recover variation at the level of genera. However, only 33 nucleotide
positions were informative out of 2184, among the species. Such low
level of variation suggests that, despite the genus being considered
very old, the species appear very young as inferred by the amount of
sequence divergence. The three clades, the pattern of low divergence,
and restricted distribution of most species has biogeographic
implications. First, it suggests a probable ancestral geographic area
for Ceratozamia in Southeast Mexico, already known as a
biodiversity "hot spot". Second, the time of speciation
within the genus appears to be associated to the post-Pleistocene
spread of floristic communities from proposed Pleistocene tropical
refugia of S. E. Mexico that date back to or even beyond Miocene
times.
Key words: cycads, gymnosperm, nuclear and chloroplast DNA, phylogeny