Ecological niche differences between two polyploid cytotypes of Saxifraga rosacea

Local adaptation, recombination, and the fate of neopolyploids

Polyploidy is widely recognized as an important speciation mechanism because it isolates tetraploids from their diploid progenitors. Polyploidy also provides new genetic material that may facilitate adaptive evolution. However, new mutations are more likely to arise after a neopolyploid has already successfully invaded a population. Thus, the role of adaptive forces in establishing a polyploid remains unclear. One solution to this apparent paradox may lie in the capacity of polyploids to suppress recombination among preexisting locally adapted alleles. The local adaptation mechanism requires that spatially heterogeneous selection acts on multiple loci and that gene flow introduces maladapted alleles to the population where the polyploid forms. The mechanism requires neither strong genetic drift nor any intrinsic benefit of genome doubling and can accommodate any mode of gene action. A unique prediction of the mechanism is that adaptive alleles should predate polyploidization, a pattern consistent with observations from a few well-studied polyploids. The mechanism is also consistent with the coexistence of both diploid and tetraploid cytotypes, fitness heterogeneity among independently derived polyploids, and the prevalence of outcrossing among older polyploids. The local adaptation mechanism also makes novel predictions about circumstances favoring polyploid invasions that can be tested using molecular genetic or comparative approaches.

Polyploid goldback and silverback ferns (Pentagramma) occupy a wider, colder, and wetter bioclimatic niche than diploid counterparts

PREMISE

The western North American fern genus Pentagramma (Pteridaceae) is characterized by complex patterns of ploidy variation, an understanding of which is critical to comprehending both the evolutionary processes within the genus and its current diversity.

METHODS

We undertook a cytogeographic study across the range of the genus, using a combination of chromosome counts and flow cytometry to infer ploidy level. Bioclimatic variables and elevation were used to compare niches.

RESULTS

We found that diploids and tetraploids are common and widespread, and triploids are rare and sporadic; in contrast with genome size inferences in earlier studies, no hexaploids were found. Diploids and tetraploids show different geographic ranges: only tetraploids were found in the northernmost portion of the range (Washington, Oregon, and British Columbia) and only diploids were found in the Sierra Nevada of California. Diploid, triploid, and tetraploid cytotypes were found to co-occur in relatively few localities: in the southern (San Diego County, California) and desert Southwest (Arizona) parts of the range, and along the Pacific Coast of California.

CONCLUSIONS

Tetraploids occupy a wider bioclimatic niche than diploids both within P. triangularis and at the genus-wide scale. It is unknown whether the wider niche of tetraploids is due to their expansion upon the diploid niche, if diploids have contracted their niche due to competition or changing abiotic conditions, or if this wider niche occupancy is due to multiple origins of tetraploids.

Polyploidization: A Biological Force That Enhances Stress Resistance

Organisms with three or more complete sets of chromosomes are designated as polyploids. Polyploidy serves as a crucial pathway in biological evolution and enriches species diversity, which is demonstrated to have significant advantages in coping with both biotic stressors (such as diseases and pests) and abiotic stressors (like extreme temperatures, drought, and salinity), particularly in the context of ongoing global climate deterioration, increased agrochemical use, and industrialization. Polyploid cultivars have been developed to achieve higher yields and improved product quality. Numerous studies have shown that polyploids exhibit substantial enhancements in cell size and structure, physiological and biochemical traits, gene expression, and epigenetic modifications compared to their diploid counterparts. However, some research also suggested that increased stress tolerance might not always be associated with polyploidy. Therefore, a more comprehensive and detailed investigation is essential to complete the underlying stress tolerance mechanisms of polyploids. Thus, this review summarizes the mechanism of polyploid formation, the polyploid biochemical tolerance mechanism of abiotic and biotic stressors, and molecular regulatory networks that confer polyploidy stress tolerance, which can shed light on the theoretical foundation for future research.

Hydraulic trade-offs underlie enhanced performance of polyploid trees under soil water deficit

The relationships between aerial organ morpho-anatomy of woody polyploid plants with their functional hydraulics under water stress remain largely understudied. We evaluated growth-associated traits, aerial organ xylem anatomy, and physiological parameters of diploid, triploid, and tetraploid genotypes of atemoyas (Annona cherimola x Annona squamosa), which belong to the woody perennial genus Annona (Annonaceae), testing their performance under long-term soil water reduction. The contrasting phenotypes of vigorous triploids and dwarf tetraploids consistently showed stomatal size-density trade-off. The vessel elements in aerial organs were ∼1.5 times wider in polyploids compared with diploids, and triploids displayed the lowest vessel density. Plant hydraulic conductance was higher in well-irrigated diploids while their tolerance to drought was lower. The phenotypic disparity of atemoya polyploids associated with contrasting leaf and stem xylem porosity traits that coordinate to regulate water balances between the trees and the belowground and aboveground environments. Polyploid trees displayed better performance under soil water scarcity, presenting as more sustainable agricultural and forestry genotypes to cope with water stress.

Genetic Variability in Balkan Paleoendemic Resurrection Plants Ramonda serbica and R. nathaliae Across Their Range and in the Zone of Sympatry

The genus Ramonda includes three Paleoendemic and Tertiary relict species that survived in refugial habitats of the Balkan Peninsula (R. nathaliae and R. serbica) and the Iberian Peninsula (R. myconi). They are all "resurrection plants," a rare phenomenon among flowering plants in Europe. Ramonda myconi and R. nathaliae are diploids (2n = 2x = 48), while R. serbica is a hexaploid (2n = 6x = 144). The two Balkan species occur in sympatry in only two localities in eastern Serbia, where tetraploid potential hybrids (2n = 4x = 96) were found. This observation raised questions about the existence of gene flow between the two species and, more generally, about the evolutionary processes shaping their genetic diversity. To address this question, genetic markers (AFLP) and an estimate of genome size variation were used in a much larger sample and at a larger geographic scale than previously. The combination of AFLP markers and genome size results suggested ongoing processes of interspecific and interploidy hybridization in the two sites of sympatry. The data also showed that interspecific gene flow was strictly confined to sympatry. Elsewhere, both Ramonda species were characterized by low genetic diversity within populations and high population differentiation. This is consistent with the fact that the two species are highly fragmented into small and isolated populations, likely a consequence of their postglacial history. Within sympatry, enormous variability in cytotypes was observed, exceeding most reported cases of mixed ploidy in complex plant species (from 2x to >8x). The AFLP profiles of non-canonical ploidy levels indicated a diversity of origin pathways and that backcrosses probably occur between tetraploid interspecific hybrids and parental species. The question arises whether this diversity of cytotypes corresponds to a transient situation. If not, the question arises as to the genetic and ecological mechanisms that allow this diversity to be maintained over time.

Differences in Floral Scent and Petal Reflectance Between Diploid and Tetraploid Chamerion angustifolium

Genome duplication in plants is thought to be a route to speciation due to cytotype incompatibility. However, to reduce cross-pollination between cytotypes in animal-pollinated species, distinctive floral phenotypes, which would allow pollinator-mediated assortative mating between flowers, are also expected. Chamerion angustifolium is a Holarctic species that forms a hybrid zone between diploid and tetraploid populations in the North American Rocky Mountains. Extensive research has shown that these cytotypes differ in many ways, including some floral traits, and that pollinators can discriminate between cytotypes, leading to assortative mating. However, two signals commonly used by insect pollinators have not been measured for this species, namely petal colour and floral scent. Using greenhouse-grown diploids and tetraploids of C. angustifolium from the ploidy hybrid-zone in the North American Rocky Mountains, we show that both floral scent signals and petal reflectance differ between cytotypes. These differences, along with differences in flower size shown previously, could help explain pollinator-mediated assortative mating observed in previous studies. However, these differences in floral phenotypes may vary in importance to pollinators. While the differences in scent included common floral volatiles readily detected by bumblebees, the differences in petal reflectance may not be perceived by bees based on their visual sensitivity across the spectra. Thus, our results suggest that differences in floral volatile emissions are more likely to contribute to pollinator discrimination between cytotypes and highlight the importance of understanding the sensory systems of pollinators when examining floral signals.

Changes in ploidy affect vascular allometry and hydraulic function in Mangifera indica trees

The enucleated vascular elements of the xylem and the phloem offer an excellent system to test the effect of ploidy on plant function because variation in vascular geometry has a direct influence on transport efficiency. However, evaluations of conduit sizes in polyploid plants have remained elusive, most remarkably in woody species. We used a combination of molecular, physiological and microscopy techniques to model the hydraulic resistance between source and sinks in tetraploid and diploid mango trees. Tetraploids exhibited larger chloroplasts, mesophyll cells and stomatal guard cells, resulting in higher leaf elastic modulus and lower dehydration rates, despite the high water potentials of both ploidies in the field. Both the xylem and the phloem displayed a scaling of conduits with ploidy, revealing attenuated hydraulic resistance in tetraploids. Conspicuous wall hygroscopic moieties in the cells involved in transpiration and transport indicate a role in volumetric adjustments as a result of turgor change in both ploidies. In autotetraploids, the enlargement of organelles, cells and tissues, which are critical for water and photoassimilate transport at long distances, point to major physiological novelties associated with whole-genome duplication.

Intricate Distribution Patterns of Six Cytotypes of Allium oleraceum at a Continental Scale: Niche Expansion and Innovation Followed by Niche Contraction With Increasing Ploidy Level

The establishment and success of polyploids are thought to often be facilitated by ecological niche differentiation from diploids. Unfortunately, most studies compared diploids and polyploids, ignoring variation in ploidy level in polyploids. To fill this gap, we performed a large-scale study of 11,163 samples from 1,283 populations of the polyploid perennial geophyte Allium oleraceum with reported mixed-ploidy populations, revealed distribution ranges of cytotypes, assessed their niches and explored the pattern of niche change with increasing ploidy level. Altogether, six ploidy levels (3x-8x) were identified. The most common were pentaploids (53.6%) followed by hexaploids (22.7%) and tetraploids (21.6%). Higher cytotype diversity was found at lower latitudes than at higher latitudes (>52° N), where only tetraploids and pentaploids occurred. We detected 17.4% of mixed-ploidy populations, usually as a combination of two, rarely of three, cytotypes. The majority of mixed-ploidy populations were found in zones of sympatry of the participating cytotypes, suggesting they have arisen through migration (secondary contact zone). Using coarse-grained variables (climate, soil), we found evidence of both niche expansion and innovation in tetraploids related to triploids, whereas higher ploidy levels showed almost zero niche expansion, but a trend of increased niche unfilling of tetraploids. Niche unfilling in higher ploidy levels was caused by a contraction of niche envelopes toward lower continentality of the climate and resulted in a gradual decrease of niche breadth and a gradual shift in niche optima. Field-recorded data indicated wide habitat breadth of tetraploids and pentaploids, but also a pattern of increasing synanthropy in higher ploidy levels. Wide niche breadth of tetra- and pentaploids might be related to their multiple origins from different environmental conditions, higher "age", and retained sexuality, which likely preserve their adaptive potential. In contrast, other cytotypes with narrower niches are mostly asexual, probably originating from a limited range of contrasting environments. Persistence of local ploidy mixtures could be enabled by the perenniality of A. oleraceum and its prevalence of vegetative reproduction, facilitating the establishment and decreasing exclusion of minority cytotype due to its reproductive costs. Vegetative reproduction might also significantly accelerate colonization of new areas, including recolonization of previously glaciated areas.

Intra- and Inter-Specific Crosses among Centaurea aspera L. (Asteraceae) Polyploid Relatives-Influences on Distribution and Polyploid Establishment

How polyploids become established is a long-debated question, especially for autopolyploids that seem to have no evolutionary advantage over their progenitors. The Centaurea aspera polyploid complex includes diploid C. aspera and two related tetraploids C. seridis and C. gentilii. Our purpose was to study the mating system among these three taxa and to analyze its influence on polyploid establishment. The distribution and ploidy level of the Moroccan populations, and forced intra- and inter-specific crosses were assessed. Allotetraploid C. seridis produced more cypselae per capitulum in the intra-specific crosses. It is a bigger plant and autogamous, and previous studies indicated that selfing forces the asymmetric formation of sterile hybrids. All these characteristics help C. seridis to avoid the minority-cytotype-exclusion effect and become established. Inter-specific hybridization was possible between C. aspera and C. gentilii, and with the symmetric formation of hybrids. However, 49% of the hybrid cypselae were empty, which probably reveals postzygotic barriers. Autotetraploid C. gentilii produced the same number of cypselae per capitulum as those of the diploid parental, has an indistinguishable field phenotype, is allogamous, and symmetrically produces hybrids. Therefore, C. gentilii does not seem to have the same competitive advantages as those of C. seridis.