Chromosome patterns and nuclear phenomena in the cycad families Stangeriaceae and Zamiaceae

Abstract

The chromosome karyotypes are described and scale diagrams prepared of 35 species representing 8 genera of the “cycad” families Stangeriaceae and Zamiaceae. The karyotype patterns, chromosome types and characteristics of the nucleolar organisers and heterochromatin are discussed in relation to their evolution in this ancient group of plants. A possible path of chromosome evolution is suggested by the concurrence in some genera of terminal heterochromatic knobs on chromosome arms and a single pair of telocentric homologues which in the remaining genera are replaced by complex partially heterochromatic nucleolar-organising chromosomes. — It is suggested that telocentric chromosomes may be of ancient origin in the cycads and that there may have been a progression from telocentric to mesocentric karyotypes. The paucity of genera today and the taxonomic isolation of the cycads from the remainder of the present-day flora renders impossible the corroboration of this theory by direct cytological comparisons with other groups.