Ecological mechanisms of reproductive isolation between diploid and tetraploid Chamerion angustifolium

1 Theoretical models indicate that coexistence of diploid and polyploid ind ividuals in sympatric populations is unlikely when mating is random among c ytotypes and hybrids are inviable. However, coexistence may be facilitated by prezygotic isolating mechanisms that reduce intercytotype mating and inc rease assortative mating. 2 We examined the ecological factors regulating intercytotype mating in Cha merion (formerly Epilobium) angustifolium by measuring floral morphology, f lowering synchrony and insect foraging preferences in a mixed population wi thin a diploid-tetraploid contact zone. We also calculated the minimum rate of between-cytotype mating from estimates of the frequency of triploid off spring. 3 Diploids had significantly shorter and narrower petals as well as shorter styles than tetraploids. Inflorescences were significantly taller in tetra ploids than diploids, but the mean number of open flowers per inflorescence did not differ. Diploid and tetraploid flowering periods overlapped by 51% , with diploid stems flowering earlier. 4 In a plot of 20 diploid and 28 tetraploid stems that were flowering simul taneously, only 26% of all bee flights were between flowers on different st ems. Of the total flights between inflorescences, only 15% were between dif ferent cytotypes. The combined effects of flowering asynchrony and insect f oraging reduced the opportunities for intercytotype mating from 49% expecte d if mating is random to about 2% of the total number of pollinator flights . 5 A computer simulation indicated that the deficiency of pollinator flights between cytotypes was due largely to the spatial structure of cytotypes an d limited pollinator flight distances within the observation plot. The freq uency of triploid offspring produced during the period when both cytotypes were in flower was 6.6%, similar to the proportion of flights observed betw een flowers on different cytotypes (4%). 6 The results indicate that flowering phenology and insect behaviour las in fluenced by clonal structure) have a significant effect on prezygotic matin g isolation and triploid production, and may contribute to the maintenance of mixed cytotype populations in Chamerion angustifolium.