Chromosome Number Patterns in Primitive Angiosperms

A survey was made of the chromosome numbers in the Magnoliales sensu lato. An extensive aneuploid series is present in this group. Polyploid series occur but do not dominate the chromosome-number pattern. Secondary aneuploidy at polyploid levels is common. No single basic number has a really widespread systematic distribution in primitive dicots. The most widespread basic numbers among 19 families of Magnoliales and seven primitive dicot families belonging to other orders are x = 8, 12, 14, and 19. In this group of 26 families, n = 7 is not particularly common and x = 7 is not widespread. The currently prevailing hypothesis that the original basic number of the dicots and of the angiosperms was x = 7 is not supported by the present survey but is not ruled out either. An ancestral x = 8 is an equally or more attractive possibility on the present evidence. The paleopolyploid hypothesis holds that the generally high basic numbers in woody dicots (n = 12 and upward) are old polyploid numbers. The chromosome-number pattern in primitive dicots indicates that this hypothesis is applicable mainly to the higher basic numbers (n = 18, 19, etc.). Ascending aneuploidy probably accounts for most cases of n = 12 and 13 and for many or most cases of n = 14 and 15 in primitive woody dicots.