Isozyme Evidence for Ancient Polyploidy in Primitive Angiosperms

Abstract

Although the original chromosome number for angiosperms is thought to have been low, many primitive angiosperms actually have high chromosome numbers. Two hypotheses have been offered to explain this paradox: 1) the original basic chromosome number for angiosperms was actually high (x = 12-14) and other numbers were derived via descending aneuploidy; 2) the original basic chromosome number for angiosperms was low (x = 6 or 7) and high basic chromosome numbers were derived via ancient polyploidy. Using enzyme electrophoresis, we determined that species from families with high basic chromosome numbers exhibit increases in isozyme number compared to the numbers of isozymes typical of diploid seed plants. Although the possibility exists that isozyme increase occurred due to chromosomal rearrangement, the most parsimonious expla- nation appears to be ancient polyploidy. Genetic data therefore are in agreement with the hypothesis that these plants are ancient polyploids with diploid progenitors that are now extinct. Our data suggest that considerable speciation has occurred at the polyploid level in large families such as Magnoliaceae and Lauraceae. Lastly, isozyme data support the hypothesis that basic chromosome numbers near x = 11 or 12 may mark a convenient point for demarcating polyploids.