XVI International Botanical Congess

Polyploidy is a common mechanism of adaptation and speciation, yet pathways and rates of polyploid formation are poorly understood. I examined gametic non-reduction and neopolyploid formation in natural populations of Achillea millefolium, a perennial with tetraploid and hexaploid populations. Tetraploids were found to produce variable frequencies of unreduced pollen (range 0-16%, mean 0.4%), which could generate neohexaploids through union with reduced eggs. To test this, I used flow cytometry to compare the frequency of neopolyploid progeny from maternal parents which were either open pollinated or crossed to individuals producing large amounts of unreduced pollen. Neohexaploids and aneuploids were encountered at low frequencies (<1%) in both classes of progeny. Neohexaploids are vigorous and morphologically and phenologically distinct from established hexaploids.