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Summary Polyploidization is pervasive in plants, but little is known about the niche divergence of wild allopolyploids (species that harbor polyploid genomes originating from different diploid species) relative to their diploid progenitor species and the gene expression patterns that may underlie such ecological divergence. We conducted a fine‐scale empirical study on habitat and gene expression of an allopolyploid and its diploid progenitors.We quantified soil properties and light availability of habitats of an allotetraploid Cardamine flexuosa and its diploid progenitors Cardamine amara and Cardamine hirsuta in two seasons. We analyzed expression patterns of genes and homeologs (homeologous gene copies in allopolyploids) using RNA sequencing.We detected niche divergence between the allopolyploid and its diploid progenitors along water availability gradient at a fine scale: the diploids in opposite extremes and the allopolyploid in a broader range between diploids, with limited overlap with diploids at both ends. Most of the genes whose homeolog expression ratio changed among habitats in C. flexuosa varied spatially and temporally.These findings provide empirical evidence for niche divergence between an allopolyploid and its diploid progenitor species at a fine scale and suggest that divergent expression patterns of homeologs in an allopolyploid may underlie its persistence in diverse habitats.

Empirical evidence is limited on whether allopolyploid species combine or merge parental adaptations to broaden habitats. The allopolyploid Arabidopsis kamchatica is a hybrid of the two diploid parents Arabidopsis halleri and Arabidopsis lyrata. A. halleri is a facultative heavy metal hyperaccumulator, and may be found in cadmium (Cd) and zinc (Zn) contaminated environments, as well as non-contaminated environments. A. lyrata is considered non-tolerant to these metals, but can be found in serpentine habitats. Therefore, the parents have adaptation to different environments. Here, we measured heavy metals in soils from native populations of A. kamchatica. We found that soil Zn concentration of nearly half of the sampled 40 sites was higher than the critical toxicity level. Many of the sites were near human construction, suggesting adaptation of A. kamchatica to artificially contaminated soils. Over half of the A. kamchatica populations had >1,000 μg g–1 Zn in leaf tissues. Using hydroponic treatments, most genotypes accumulated >3,000 μg g–1 Zn, with high variability among them, indicating substantial genetic variation in heavy metal accumulation. Genes involved in heavy metal hyperaccumulation showed an expression bias in the A. halleri-derived homeolog in widely distributed plant genotypes. We also found that two populations were found growing on serpentine soils. These data suggest that A. kamchatica can inhabit a range of both natural and artificial soil environments with high levels of ions that either of the parents specializes and that it can accumulate varying amount of heavy metals. Our field and experimental data provide a compelling example of combining genetic toolkits for soil adaptations to expand the habitat of an allopolyploid species.

Plants sense light and temperature changes to regulate flowering time. Here, we show that expression of the Arabidopsis florigen gene, FLOWERING LOCUS T (FT), peaks in the morning during spring, a different pattern than we observe in the laboratory. Providing our laboratory growth conditions with a red/far-red light ratio similar to open-field conditions and daily temperature oscillation is sufficient to mimic the FT expression and flowering time in natural long days. Under the adjusted growth conditions, key light signalling components, such as phytochrome A and EARLY FLOWERING 3, play important roles in morning FT expression. These conditions stabilize CONSTANS protein, a major FT activator, in the morning, which is probably a critical mechanism for photoperiodic flowering in nature. Refining the parameters of our standard growth conditions to more precisely mimic plant responses in nature can provide a powerful method for improving our understanding of seasonal response.

Allopolyploids possess complete sets of genomes derived from different parental species and exhibit a range of variation in various traits. Reproductive traits may play a key role in the reproductive isolation between allopolyploids and their parental species, thus affecting the thriving of allopolyploids. However, empirical data, especially in natural habitats, comparing reproductive trait variation between allopolyploids and their parental species remain rare. Here, we documented the flowering phenology and floral morphology of the allopolyploid wild plantCardamine flexuosaand its diploid parentsC. amaraandC. hirsutain their native range in Switzerland. The flowering ofC. flexuosastarted at an intermediate time compared with those of the parents and the flowering period ofC. flexuosaoverlapped with those of the parents.Cardamine flexuosaresembledC. hirsutain the size of flowers and petals and the length/width ratio of petals, while it resembledC. amarain the length/width ratio of flowers. These results provide empirical evidence of the trait-dependent variation of allopolyploid phenotypes in natural habitats at the local scale. They also suggest that the variation in some reproductive traits inC. flexuosais associated with self-fertilization. Therefore, it is helpful to consider the mating system in furthering the understanding of the processes that may have shaped trait variation in polyploids in nature.

• Background and Aims Plants have the ability to compensate for damage caused by herbivores. This is important to plant growth, because a plant cannot always avoid damage, even if it has developed defence mechanisms against herbivores. In previous work, we elucidated the herbivory-induced compensatory response of Quercus (at both the individual shoot and whole sapling levels) in both low- and high-nutrient conditions throughout one growing season. In this study, we determine how the compensatory growth of Quercus serrata saplings is achieved at different nutrient levels. • Methods Quercus serrata saplings were grown under controlled conditions. Length, number of leaves and percentage of leaf area lost on all extension units (EUs) were measured. • Key Results Both the probability of flushing and the length of subsequent EUs significantly increased with an increase in the length of the parent EU. The probability of flushing increased with an increase in leaf damage of the parent EU, but the length of subsequent EUs decreased. This indicates that EU growth is fundamentally regulated at the individual EU level. The probabilities of a second and third flush were significantly higher in plants in high-nutrient soil than those in low-nutrient soil. The subsequent EUs of damaged saplings were also significantly longer at high-nutrient conditions. • Conclusions An increase in the probability of flushes in response to herbivore damage is important for damaged saplings to produce new EUs; further, shortening the length of EUs helps to effectively reproduce foliage lost by herbivory. The probability of flushing also varied according to soil nutrient levels, suggesting that the compensatory growth of individual EUs in response to local damage levels is affected by the nutrients available to the whole sapling.

Background: The effect of herbivory on plant fitness varies widely. Understanding the causes of this variation is of considerable interest because of its implications for plant population dynamics and trait evolution. We experimentally defoliated the annual herb Arabidopsis thaliana in a natural population in Sweden to test the hypotheses that (a) plant fitness decreases with increasing damage, (b) tolerance to defoliation is lower before flowering than during flowering, and (c) defoliation before flowering reduces number of seeds more strongly than defoliation during flowering, but the opposite is true for effects on seed size. Methodology/Principal Findings: In a first experiment, between 0 and 75% of the leaf area was removed in May from plants that flowered or were about to start flowering. In a second experiment, 0, 25%, or 50% of the leaf area was removed from plants on one of two occasions, in mid April when plants were either in the vegetative rosette or bolting stage, or in mid May when plants were flowering. In the first experiment, seed production was negatively related to leaf area removed, and at the highest damage level, also mean seed size was reduced. In the second experiment, removal of 50% of the leaf area reduced seed production by 60% among plants defoliated early in the season at the vegetative rosettes, and by 22% among plants defoliated early in the season at the bolting stage, but did not reduce seed output of plants defoliated one month later. No seasonal shift in the effect of defoliation on seed size was detected. Conclusions/Significance: The results show that leaf damage may reduce the fitness of A. thaliana, and suggest that in this population leaf herbivores feeding on plants before flowering should exert stronger selection on defence traits than those feeding on plants during flowering, given similar damage levels.