Genome-material costs and functional trade-offs in the autopolyploid Solidago gigantea (giant goldenrod) series.

PREMISE: Increased genome-material costs of N and P atoms inherent to organisms with larger genomes have been proposed to limit growth under nutrient scarcities and to promote growth under nutrient enrichments. Such responsiveness may reflect a nutrient-dependent diploid versus polyploid advantage that could have vast ecological and evolutionary implications, but direct evidence that material costs increase with ploidy level and/or influence cytotype-dependent growth, metabolic, and/or resource-use trade-offs is limited. METHODS: We grew diploid, autotetraploid, and autohexaploid Solidago gigantea plants with one of four ambient or enriched N:P ratios and measured traits related to material costs, primary and secondary metabolism, and resource-use. RESULTS: Relative to diploids, polyploids invested more N and P into cells, and tetraploids grew more with N enrichments, suggesting that material costs increase with ploidy level. Polyploids also generally exhibited strategies that could minimize material-cost constraints over both long (reduced monoploid genome size) and short (more extreme transcriptome downsizing, reduced photosynthesis rates and terpene concentrations, enhanced N-use efficiencies) evolutionary time periods. Furthermore, polyploids had lower transpiration rates but higher water-use efficiencies than diploids, both of which were more pronounced under nutrient-limiting conditions. CONCLUSIONS: N and P material costs increase with ploidy level, but material-cost constraints might be lessened by resource allocation/investment mechanisms that can also alter ecological dynamics and selection. Our results enhance mechanistic understanding of how global increases in nutrients might provide a release from material-cost constraints in polyploids that could impact ploidy (or genome-size)-specific performances, cytogeographic patterning, and multispecies community structuring.

Exploring environmental selection on genome size in angiosperms.

Angiosperms show a remarkable range in genome size (GS), yet most species have small genomes, despite the frequency of polyploidy and repeat amplification in the ancestries of most lineages. It has been suggested that larger genomes incur costs that have driven selection for GS reduction, although the nature of these costs and how they might impact selection remain unclear. We explore potential costs of increased GS encompassing impacts on minimum cell size with consequences for photosynthesis and water-use efficiency and effects of greater nitrogen and phosphorus demands of the nucleus leading to more severe trade-offs with photosynthesis. We suggest that nutrient-, water-, and/or CO2-stressed conditions might favour species with smaller genomes, with implications for species' ecological and evolutionary dynamics.

Impacts of soil nitrogen and phosphorus levels on cytotype performance of the circumboreal herb Chamerion angustifolium: implications for polyploid establishment.

PREMISE: Although polyploidy commonly occurs in angiosperms, not all polyploidization events lead to successful lineages, and environmental conditions could influence cytotype dynamics and polyploid success. Low soil nitrogen and/or phosphorus concentrations often limit ecosystem primary productivity, and changes in these nutrients might differentially favor some cytotypes over others, thereby influencing polyploid establishment. METHODS: We grew diploid, established tetraploid, and neotetraploid Chamerion angustifolium (fireweed) in a greenhouse under low and high soil nitrogen and phosphorus conditions and different competition treatments and measured plant performance (height, biomass, flower production, and root bud production) and insect damage responses. By comparing neotetraploids to established tetraploids, we were able to examine traits and responses that might directly arise from polyploidization before they are modified by natural selection and/or genetic drift. RESULTS: We found that (1) neopolyploids were the least likely to survive and flower and experienced the most herbivore damage, regardless of nutrient conditions; (2) both neo- and established tetraploids had greater biomass and root bud production under nutrient-enriched conditions, whereas diploid biomass and root bud production was not significantly affected by nutrients; and (3) intra-cytotype competition more negatively affected diploids and established tetraploids than it did neotetraploids. CONCLUSIONS: Following polyploidization, biomass and clonal growth might be more immediately affected by environmental nutrient availabilities than plant survival, flowering, and/or responses to herbivory, which could influence competitive dynamics. Specifically, polyploids might have competitive and colonizing advantages over diploids under nutrient-enriched conditions favoring their establishment, although establishment may also depend upon the density and occurrences of other related cytotypes in a population.