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

Abstract

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 cytotype exclusion principle). I tested this hypothesis using experimental populations of Chamerion angustifolium (Onagraceae) that contained di¡erent proportions of tetraploids and diploids. Fitness, measured as total seed production over the entire £owering season, was calculated from a census of £ower number and estimates of ovule number per £ower and proportion of seed set per fruit. The ¢tness 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 because of a negative relationship between tetraploid frequency and seed set per fruit in diploids. Seed set in tetraploids was independent of cytotype frequency. The frequency-independent e¡ect in tetraploids re£ects higher assortative mating, partly because of non-random patterns of bee visitation. Bees visited a disproportionately high number of diploid in£orescences; however, the proportion of successive £ights between tetraploids increased above random expectations as the frequency of tetraploids decreased. These results provide the ¢rst experimental test of frequency-dependent ¢tness in diploid^polyploid mixtures and suggest an important role for more gradual, population processes governing the evolution of polyploidy in natural populations.

Reproductive isolation of a new hybrid species, Senecio eboracensis Abbott & Lowe (Asteraceae)

“…This is because most offspring will result from crosses with the parents and be sterile or exhibit low fertility (Felber, 1991;Husband, 2000). This disadvantage will be avoided if a prezygotic isolating mechanism prevents or reduces significantly the production of such offspring.…”

The “Polyploid Hop”: Shifting Challenges and Opportunities Over the Evolutionary Lifespan of Genome Duplications

“…Assuming that interploidy matings are more likely to produce inviable or sterile offspring, rare cytotypes should have reduced relative fitness. Such a frequency-dependent mating disadvantage was described from experimental and natural mixed-ploidy populations (Hagberg and Elleström, 1959;Maceira et al, 1993;Husband, 2000;Baack and Stanton, 2005;Mráz et al, 2012), but only a few studies further evaluated its significance for polyploid establishment. Interestingly, studies of mixed-ploidy populations of Chamerion angustifolium indicate a surprising asymmetry in this relationship between ploidies.…”

“…Newly formed polyploids are expected to suffer a mating disadvantage when they are relatively rare in the population (Husband, 2000). This type of frequency-dependent disadvantage is known as minority cytotype exclusion (Levin, 1975).…”

“…Even more, polyploid establishment is substantially constrained by overall low chances of polyploid mutants persisting among their diploid progenitors (Levin, 2002;Ramsey and Schemske, 2002). This stems from both direct competition between the two cytotypes (Yamauchi et al, 2004) and frequency dependent selection (Levin, 1975), which suggests most autopolyploids are likely to go extinct before establishment (Levin, 1975;Husband, 2000).…”

Genetic mapping in polyploids

“…In most cases, neopolyploid plants are usually at an immediate disadvantage, being unadapted and reproductively isolated (what is termed "minority cytotype disadvantage" (Levin, 1975;Husband, 2000)). The speed at which newly-established polyploid lineages prosper and diversify varies, with indications that there may be a significant time lag before this occurs (Schranz et al, 2012).…”