Understanding the genomic basis of diversification is a central goal in evolutionary biology. In recent years, the development and use of pangenomes, a genomic representation of multiple individuals within a lineage (a set of related populations, subspecies, ecotypes, or species), has enabled researchers to differentiate between DNA sequences shared by all individuals of a given lineage (core regions) from those present only in some individuals (accessory or variable regions). Differentiating between core and accessory regions has highlighted a key limitation of relying on a single reference genome: It captures the genetic code of only one individual and this biases genomic analyses and our understanding of diversification. Here, we propose that by identifying genes associated with both core and accessory regions, we can deepen our understanding of the processes underlying diversification. We suggest that analyzing pangenomes and accessory regions will provide deeper insights into diversification, hybridization, and the genetic basis of adaptation and speciation.
The mustard genus Boechera (Brassicaceae) is notorious for rampant intra- and interclade hybridization, which has produced a complex reticulate network of apomictic diploid, triploid, and tetraploid hybrids. A recent hypothesis suggested the possibility of intergeneric hybridization between Boechera and other closely related genera. Here we explored the origin of Boechera tiehmii, a putative hybrid between Boechera and Nevada.
To test the hybrid hypothesis, we leveraged a unique methodological approach that integrates results from 15 microsatellite loci from the Boechera Microsatellite Website (BMW), 1114 phased Hyb-Seq loci from a recently published phylogeny of tribe Boechereae, and a detailed investigation of morphological variation.
Our study supported the hypothesis that B. tiemhii is an intergeneric hybrid between Boechera lemmonii and Nevada holmgrenii. The placement of phased hybrid tips within the sexual diploid phylogeny was unequivocal, the microsatellite loci exhibited near perfect additivity, and B. tiehmii was shown to be morphologically intermediate between the proposed parents.
By affirming the hybrid nature of B. tiehmii, this work contributes to a deeper understanding of the evolutionary processes that shape biodiversity within the tribe Boechereae. It also highlights the importance of applying a broad range of analytical tools when addressing taxonomic ambiguities in groups where hybridization is rampant.
Multispecies invasions raise questions about the mechanisms enabling coexistence. Differences in regenerative niche requirements may reduce competition and facilitate coinvasion in related species with similar life forms, phenology, and habitat ranges. We expected closely related woody invaders to differ in germination phenology and seedling stress tolerance during early life stages.
We compared the regenerative niche requirements of four confamilial non-native shrubs (Pyracantha angustifolia, Pyracantha atalantoides, Cotoneaster franchetii, and Cotoneaster glaucophyllus), using seeds from a single invaded site where they co-occur in the Córdoba Mountains, Argentina. We evaluated germination and seedling establishment using germination chambers and greenhouse conditions. We assessed germination responses to alternating temperatures, light sensitivity, seed mass, and dormancy, and seedling performance under varying light and water levels.
While all species shared the same optimal germination temperature under an alternating regime of 25°C during the day and 15°C at night, they differed in seed mass, final germination percentage, germination temperature range and speed, and dormancy-breaking treatments. High germination of P. angustifolia contrasted with seed dormancy of the other species. Seedlings varied in performance and functional traits related to resource acquisition and use. In stressful conditions, Cotoneaster species outperformed Pyracantha, especially under low light.
Differences in germination phenology and seedling tolerance to water stress and low light suggest potential niche differentiation across irradiance and water gradients. These interspecific differences among coinvasive species may influence establishment sites, leading to spatial segregation and facilitating coexistence in multispecies-invaded ecosystems.
What maintains trait divergence in the face of gene flow? Two varieties of wild snapdragon (Antirrhinum majus) characterized by divergent flower color hybridize in their native range. Selection on flower color genes is indicated by sharp clines, but the selective agents have not been demonstrated. Although previous work has focused on pollinators, pigmentation genes can also contribute to abiotic stress tolerance. We hypothesized that pigmentation in A. majus mediates stress tolerance, which could contribute to hybrid zone maintenance through parental niche divergence or hybrid maladaptation. Specifically, we tested whether morphotype mediates drought tolerance in an experiment comparing magenta-flowered var. pseudomajus, yellow-flowered var. striatum, and their pink-flowered hybrid cross.
We experimentally compared drought tolerance of each morphotype from allopatric crosses within and between varieties using three greenhouse treatments. Control plants were watered as needed, while drought-treated plants were watered half as often, either from the transplant stage (“early” drought), or from flowering onset (“late” drought).
Parental morphotypes responded identically to drought in fitness and most phenotypic traits. However, hybrids had lower survival (14%) under late drought stress than parental morphotypes (70%). All hybrids that flowered in the late drought treatment died, compared to ~20% of flowering parental morphotypes.
Hybrid maladaptation to abiotic stress could potentially contribute to flower color divergence in the face of gene flow in A. majus. Further research should test the relevance of our results to field conditions and explicitly probe the role of flower color genes in drought tolerance.
Ecological niche differentiation of spore germination and sporophyte habitat promote the coexistence of fern species in the cloud forest, but the role of the gametophyte is unknown. Because fern sporophytes only establish where the inconspicuous gametophytes grow and reproduce, the environmental tolerance of the gametophyte has been hypothesized to be associated with the light preference of the sporophyte.
We measured environmental variables of the sporophyte and gametophyte of nine coexisting fern cloud forest species and manipulated water availability in the air and soil to quantify the desiccation tolerance of gametophytes.
Gametophytes of all species dried quickly but maintained at least half of their photosynthetic efficiency (ΦPSII) after losing 80% of their relative water content (RWC). However, pretreatment Fv/Fm values were not recovered after rehydration of gametophytes with 80%, 50%, or 20% of RWC. Passive water loss through the gametophyte cuticle was slower in species from exposed areas than in shaded areas. Gametophytes of sun species dried slowly and had high ΦPSII at low RWC (20%), whereas those of species from shaded, humid sites were less desiccation-tolerant. The gametophytes of all species, especially those from shaded habitats, were more sensitive to low air humidity than to low soil moisture.
The physiology of the gametophyte was positively linked to the preferred light habitat of the sporophyte. Low physiological resilience to low to moderate desiccation events may compromise gametophyte survival and fern diversity in the face of habitat transformation and climate change.
Diverse mechanisms of reproductive isolation can limit gene flow between species. The Orchidaceae, one of the most species-rich and morphologically diverse plant families, often exhibits strong pollinator-mediated isolation due to highly specialized pollination systems. Autonomous self-pollination is also common in orchids and contributes to both speciation and the maintenance of reproductive barriers among closely related taxa. Although agamospermy (clonal seed production without fertilization) occurs in several orchid species, its role in reproductive isolation remains unclear.
We tested whether agamospermy serves as a reproductive barrier between the agamospermous Goodyera crassifolia and the pollinator-dependent G. schlechtendaliana by assessing pollen germination, fruit, and seed production after interspecific and intraspecific pollination in conjunction with high-throughput genomic analysis.
Goodyera crassifolia pollinia failed to germinate on conspecific and heterospecific stigmas. In contrast, G. schlechtendaliana pollinia germinated readily on conspecific stigmas, with pollen tubes reaching the column base, but did not germinate on G. crassifolia stigmas. No fruit set occurred after G. crassifolia pollinia were placed on G. schlechtendaliana stigmas, whereas G. schlechtendaliana pollinia on G. crassifolia stigmas resulted in seeds that lacked paternal genetic contribution. No evidence of hybridization between G. crassifolia and G. schlechtendaliana was detected in natural populations.
The agamospermous reproductive mode of G. crassifolia likely serves as an effective barrier to interspecific gene flow from sympatric G. schlechtendaliana at the study site. To our knowledge, this is the first detailed investigation of how agamospermy functions as a mechanism of reproductive isolation in orchids.
The Glossopteridales are an extinct group of seed plants that dominated mid to high latitude floras of the supercontinent Gondwana during the Permian (298–251 million years ago). Reconstructing their functional ecology is thus of particular importance for understanding the forests of that time period. Previous studies have shown evidence for diverse strategies favoring individual persistence and regeneration in glosssopterids. This evidence includes the production of epicormic and basal shoots, but there is no evidence of root suckering (i.e., new shoots from the root system).
In previously made serial acetate peels on sections of silicified peat blocks from the late Permian of Skaar Ridge, Central Transantarctic Mountains, Antarctica, we identified two young glossopterid roots emitting a lateral axis with a non-root anatomy and studied their anatomy.
The two specimens show evidence of young Vertebraria roots producing stem-like laterals, which we interpret as evidence of root suckering.
The specimens provide new evidence in favor of high ecological resilience among glossopterids and suggest that some may have had the potential for clonal growth. This example is the oldest to date of root suckering in a fossil plant.
Flower morphology influences the architecture of plant–pollinator interaction networks. Flowers with deep corolla tubes and bilateral symmetry have a narrower pollinator range, hence are considered more specialized than shallow radial flowers. Past interspecific comparisons revealed positive correlations between flower depth and nectar production rates in a few plant communities, suggesting that specialized flowers may allocate more resources into food rewards for pollinators.
We compiled a global data set of flower morphology vs. nectar sugar production rates (N = 494 plant species) and per-flower pollen counts (N = 164 species). We applied phylogenetically controlled mixed models to examine the effects of symmetry and tube length on floral rewards.
Corolla tube lengths, symmetry type, and their interaction significantly predicted nectar production rates, with the larger effect attributed to tube length. Neither tube length nor symmetry predicted pollen number. Both nectar and pollen production were affected by phylogeny in a larger data set of 854 species that produce both pollen and nectar and 1040 species that produce only pollen as rewards for visitors. Genus explained more of the variation in nectar production and less of the variation in pollen production than family.
Our findings suggest that visual signals associated with specialized flowers indicate nectar production, but not pollen rewards. Other visual and chemical family-specific floral displays potentially advertise pollen availability to pollinators. We propose experiments to test whether nectar foragers indeed use flower depth and symmetry as combined signals that guide their choices of nectar sources.
The proper classification of taxa is often debated, particularly when organisms lack qualitative diagnostic traits. Highbush cranberry taxa (Viburnum spp.) have been the subject of such disputes since their characterization by 18th- and 19th-century botanists. Despite their allopatric distributions—V. trilobum in North America, V. opulus in Europe, and V. sargentii in Asia—these taxa have received numerous taxonomic treatments as species, subspecies, and varieties due to their morphological similarities. Genetic evidence has shown these taxa to be distinct; however, the human-mediated introduction of V. opulus and V. sargentii into North America may remove their geographic and genetic isolation, with implications for the conservation of V. trilobum.
We used single-nucleotide polymorphisms generated via reduced representation sequencing (DArTseq) to assess the genetic structure and diversity of these taxa, focusing on the impact of V. opulus and V. sargentii introduction into North America.
Consistent with prior studies, V. trilobum, V. opulus, and V. sargentii were found to be genetically distinct species. European V. opulus—and, to a lesser extent, V. opulus × V. sargentii hybrids—were found to be widely naturalizing in North America. However, interspecific V. trilobum hybrids were notably rare. All three taxa exhibited low genetic diversity and evidence of clonality. A cryptic subgroup of V. sargentii, originating in Japan, was identified.
American V. trilobum shows evidence of continued genetic isolation despite the introduction of Eurasian taxa into North America, suggesting the presence of an unknown reproductive barrier.
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