Cycad Literature Database

Karyotype evolution in cycads has been described in 113 species covering all the 10 genera. Cycads are a conservative group in this regard as the genera with low basic numbers i.e. Dioon, Macrozamia, Lepidozamia, Encephalartos, Bowenia (2n = 18) and Ceratozamia and Stangeria (2n = 16) have relatively symmetrical karyotypes while those with higher numbers i.e. Cycas (2n = 22) and Microcycas (2n = 26) have more asymmetric karyotypes. The situation in Zamia is more complicated with different species showing a range of chromosome numbers of 2n = 16–28 while some species even show intraspecific variation. The increase or decrease in number as seen in some Zamia species has occurred mainly by chromosomal fission/fusion. Some variability is however, seen in chromosome banding as well as the position of rDNA loci. The phylogenetic relationships between the genera have been discussed on the basis of karyotype evolution.