Fitness Differences Among Diploids, Tetraploids, and Their Triploid Progeny in Chamerion Angustifolium: Mechanisms of Inviability and Implications for Polyploid Evolution

Abstract

Abstract. Theoretical models indicate that the evolution of tetraploids in diploid populations will depend on both the relative fitness of the tetraploid and that of the diploid-tetraploid hybrids. Hybrids are believed to have lower fitness due to imbalances in either the ploidy (endosperm imbalance) or the ratio of maternal to paternal genomes in their endosperm (genomic imprinting). In this study we created diploids, tetraploids, and hybrid triploids of Chamerion angustifolium from crosses between field-collected diploid and tetraploid plants and evaluated them at six life stages in a greenhouse comparison. Diploid offspring (from 2x ϫ 2x crosses) had significantly higher seed production and lower biomass than tetraploid offspring (from 4x ϫ 4x crosses). Relative to the diploid, the cumulative fitness of tetraploids was 0.67. In general, triploids (from 2x ϫ 4x, 4x ϫ 2x crosses) had significantly lower seed production, lower pollen viability, and higher biomass than diploid individuals. Triploid offspring derived from diploid maternal parents had lower germination rates, but higher pollen production than those with tetraploid mothers. Relative to diploids, the cumulative fitness of 2x ϫ 4x triploids and 4x ϫ 2x triploids was 0.12 and 0.06, respectively, providing some support for effect of differing maternal:paternal ratios and endosperm development as a mechanism of hybrid inviability. Collectively, the data show that tetraploids exhibit an inherent fitness disadvantage, although the partial viability and fertility of triploids may help to reduce the barrier to tetraploid establishment in sympatric populations.

Pollinator-mediated assortative mating in mixed ploidy populations of Chamerion angustifolium (Onagraceae)

“…This will lessen the benefits of assortative pollination. However, the fitness of selfed offspring is still higher than the relative fitness of triploid hybrids (9%; Burton and Husband 2000), which would result from between-ploidy pollination. Therefore, we conclude that the presence of assortative pollination, reinforced by pollinator activity, may be the single most consistent mechanism allowing polyploid speciation to occur in sympatry with its diploid ancestors.…”

Habitat Differentiation between Diploid and Tetraploid Galax urceolata (Diapensiaceae)

“…Alternatively, spatial segregation between diploids and polyploids may be maintained by environmentally independent selection. Such selection results from the inherently low fitness of triploid hybrids formed from diploid-tetraploid matings (Levin 1975;Ramsey and Schemske 1998;Burton and Husband 2000), as well as the transmission disadvantage experienced by the minority cytotype (Levin 1975;Husband 2000). Selection against triploids, as with homoploid hybrids, will restrict or prevent the invasion of a population of one cytotype by another and thereby maintain any geographical structure in cytotype distribution that has arisen from past dispersal or colonization events (Barton and Hewitt 1985).…”

“…This study builds on recent research on the distribution of diploid (2np2xp12) and autopolyploid (2np3xp18; 2np4xp24) cytotypes of Galax urceolata (Poiret) Brummitt (Diapensiaceae). Galax urceolata is a clonal herbaceous perennial plant that is endemic to the Blue Ridge Mountains of southeastern United States (Nesom 1983;Burton and Husband 1999). Populations occur in forested habitats and range in size from fewer than 10 to several thousand plants (Baldwin 1941;Nesom 1983;Burton and Husband 1999).…”

“…Galax urceolata is a clonal herbaceous perennial plant that is endemic to the Blue Ridge Mountains of southeastern United States (Nesom 1983;Burton and Husband 1999). Populations occur in forested habitats and range in size from fewer than 10 to several thousand plants (Baldwin 1941;Nesom 1983;Burton and Husband 1999). In a sample of 1570 individuals from 42 locations, Burton and Husband (1999) showed that G. urceolata comprises discrete populations that form a mosaic of cytotype frequencies across the species' range.…”

Gametes with somatic chromosome number and their significance in interspecific hybridization in Fuchsia

“…The production of gametes with somatic chromosome number, referred to subsequently as 2n gametes has been reported in 85 angiosperm genera from across the taxonomic spectrum including Archillea (Ramsey and Schemske 1998), Boechera (Taskin et al 2009), Chamerion (Burton and Husband 2000, Husband and Schemske 2000, Husband 2004, Fragaria Senanayake 1966, Bringhurst andGill 1970), Gilia (Grant 1952), Lolium (Jansen and Den Nijs 1993), Rosa (Crespel et al 2006) and Trifolium (Parrott and Smith 1984). The frequency of 2n gamete production differs between plant species (Bretagnolle and Thompson 1995) and even within the individuals of a species as found in Medicago sativa among the flowers of the same plant (McCoy 1982) and even between anthers of a single bud as in Solanum tuberosum (Ramanna 1983).…”