Karyotype evolution by centromeric fission in Zamia (Cycadales)

The chromosome numbers of several species of Zamia from Mexico are reported. Z. paucijuga, distributed from central Oaxaca to Nayarit, has been found to have 2n = 23, 25, 26, 27 and 28. 2n = 28 is the highest chromosome number yet found in the cycads. Karyotypes of this species differ principally in the number of telocentric and metacentric chromosomes present in each; 2n = 23, 25, 26, 27 and 28 were found to have 5, 3, 2, 1 and 0 metacentric and 8, 12, 14, 16 and 18 telocentric chromosomes, respectively. Z. fischeri has been found to be 2n = 16, Z. furfuracea and Z. loddigesii 2n = 18. Zamia paucijuga on the basis of morphological and ecological characteristics, is considered to be an advanced member of this genus. Chromosome and karyotype evolution in Z. paucijuga may have occurred by centromeric fission of metacentric chromosomes; the karyotypes of Z. paucijuga are strongly asymmetrical, suggesting that they evolved recently.