Constraints on polyploid evolution: a test of the minority cytotype exclusion principle

Polyploid evolution is often considered a mechanism of instant speciation; yet the establishment of rare tetraploids within diploid populations may be constrained by a frequency-dependent mating disadvantage (minority cytotyp e exclusion principle). I tested this hypothesis using experimental populat ions of Chamerion angustifolium (Onagraceae) that contained different propo rtions of tetraploids and diploids. Fitness: measured as total seed product ion over the entire flowering season, was calculated from a census of flowe r number and estimates of ovule number per flower and proportion of seed se t per fruit. The fitness of tetraploids relative to diploids was frequency dependent, increasing from 0.4, when tetraploids were rare, to 0.7 when at 50% and 1.15 when they were in the majority (67%). This pattern exists beca use of a negative relationship between tetraploid frequency and seed set pe r fruit in diploids. Seed set in tetraploids was independent of cytotype fr equency. The frequency -independent effect in tetraploids reflects higher a ssortative mating, partly because of non-random patterns of bee visitation. Bees visited a disproportionately high number of diploid inflorescences; h owever, the proportion of successive flights between tetraploids increased above random expectations as the frequency of tetraploids decreased. These results provide the fir st experimental test of frequency-dependent fitness in diploid-polyploid mixtures and suggest an important role for more gradu al, population processes governing the evolution of polyploidy in natural p opulations.