¹Department of Botany, University of Guelph, Guelph, Ontario, Canada N1G 2W1

Correspondence: Brian C. Husband, E-mail: bhusband@uoguelph.ca

Received 8 May 2001; Accepted 19 July 2001.

Abstract

Models of polyploid evolution indicate that tetraploids are more likely to establish within diploid populations when they are formed recurrently through the union of unreduced (n=2n) gametes. To account for the coexistence of diploids and tetraploids in populations of Chamerion angustifolium, diploid, triploid and tetraploid plants were crossed in all possible combinations and fecundity and ploidy using flow cytometry of the resulting progeny were measured. Combined with previous data on cytotype fitness, these data were used in a simulation to examine the impact of unreduced gametes on tetraploid evolution. Seed set per fruit was highest in 2x 2x crosses (69%), intermediate in 4x 4x, 2x 4x and 3x 4x crosses (range, 11–35%) and lowest in 3x 2x and 3x 3x crosses (range, 1–10%). Offspring were diploid (94%) or triploid (6%) in 2x 2x crosses, diploid (17.5%), triploid (56%) or tetraploid (26.5%) in 3x 2x crosses, and triploid (53%) and tetraploid (44%) in all others (4x 4x, 3x 4x, 2x 4x), indicating that some gametes are unreduced, particularly in triploids. Forty-two percent of offspring, from three different crosses, had DNA contents greater than tetraploids. Computer simulations based on these results showed that unreduced gamete formation via triploids in C. angustifolium can promote the coexistence of diploids and tetraploids, but, due to law triploid fitness, is insufficient to overcome tetraploid minority disadvantage.