Direct vs. Indirect effects of whole-genome duplication on prezygotic isolation in chamerion angustifolium: Implications for rapid speciation

Abstract

PREMISE OF THE STUDY: The depiction of polyploid speciation as instantaneous implies that strong prezygotic and postzygotic isolation form as a direct result of whole-genome duplication. However, the direct vs. indirect contributions of genome duplication to phenotypic divergence and prezygotic isolation are rarely quantifi ed across multiple reproductive barriers. METHODS: We compared the phenotypic diff erences between diploid and both naturally occurring and synthesized tetraploids (neotetraploids) of the plant Chamerion angustifolium. Using this information and additional published values for this species, we compared the magnitude of isolation (ecological, fl owering, pollinator, and gametic) between diploids and natural-occurring tetraploids to that between diploids and neotetraploids. KEY RESULTS: Diff erences among ploidy cytotypes were observed for eight of 12 vegetative and reproductive traits measured. Neotetraploids resembled diploids but differed from natural tetraploids with respect to four traits, including fl owering time and plant height. Diploid–neotetraploid (2 x –4 xneo) experimental arrays exhibited lower pollinator fidelity to cytotype and seed set compared with 2 x –4 xnat arrays. Based on these results and published evidence, reproductive isolation between diploids and neotetraploids across all four life stages averaged 0.48 and deviated signifi cantly from that between diploids and natural tetraploids (RI = 0.96). CONCLUSIONS: Genome duplication causes phenotypic shifts and contributes directly to prezygotic isolation for some barriers (gametic isolation) but cannot account for the cumulative isolation from diploids observed in natural tetraploids. Therefore, conditions for species formation through genome duplication are not necessarily instantaneous and selection to strengthen prezygotic barriers in young polyploids is critical for the establishment of polyploid species in sympatry.