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1. Life in the canopy: community trait assessments reveal substantial functional diversity among fern epiphytes

   https://doi.org/10.1111/nph.16607  

   Nitta, Joel H. ; Watkins, Jr, James E. ; Davis, Charles C. ( May 2020 , New Phytologist)

   The expansion of angiosperm‐dominated forests in the Cretaceous and early Cenozoic had a profound effect on terrestrial biota by creating novel ecological niches. The majority of modern fern lineages are hypothesized to have arisen in response to this expansion, particularly fern epiphytes that radiated into the canopy. Recent evidence, however, suggests that epiphytism does not correlate with increased diversification rates in ferns, calling into question the role of the canopy habitat in fern evolution.

   To understand the role of the canopy in structuring fern community diversity, we investigated functional traits of fern sporophytes and gametophytes across a broad phylogenetic sampling on the island of Moorea, French Polynesia, including > 120 species and representatives of multiple epiphytic radiations.

   While epiphytes showed convergence in small size and a higher frequency of noncordate gametophytes, they showed greater functional diversity at the community level relative to terrestrial ferns.

   These results suggest previously overlooked functional diversity among fern epiphytes, and raise the hypothesis that while the angiosperm canopy acted as a complex filter that restricted plant size, it also facilitated diversification into finely partitioned niches. Characterizing these niche axes and adaptations of epiphytic ferns occupying them should be a priority for future pteridological research.

    

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2. The C-Fern (Ceratopteris richardii) genome: insights into plant genome evolution with the first partial homosporous fern genome assembly

   https://doi.org/10.1038/s41598-019-53968-8  

   Marchant, D. Blaine ; Sessa, Emily B. ; Wolf, Paul G. ; Heo, Kweon ; Barbazuk, W. Brad ; Soltis, Pamela S. ; Soltis, Douglas E. ( December 2019 , Scientific Reports)

   Ferns are notorious for possessing large genomes and numerous chromosomes. Despite decades of speculation, the processes underlying the expansive genomes of ferns are unclear, largely due to the absence of a sequenced homosporous fern genome. The lack of this crucial resource has not only hindered investigations of evolutionary processes responsible for the unusual genome characteristics of homosporous ferns, but also impeded synthesis of genome evolution across land plants. Here, we used the model fern speciesCeratopteris richardiito address the processes (e.g., polyploidy, spread of repeat elements) by which the large genomes and high chromosome numbers typical of homosporous ferns may have evolved and have been maintained. We directly compared repeat compositions in species spanning the green plant tree of life and a diversity of genome sizes, as well as both short- and long-read-based assemblies ofCeratopteris. We found evidence consistent with a single ancient polyploidy event in the evolutionary history ofCeratopterisbased on both genomic and cytogenetic data, and on repeat proportions similar to those found in large flowering plant genomes. This study provides a major stepping-stone in the understanding of land plant evolutionary genomics by providing the first homosporous fern reference genome, as well as insights into the processes underlying the formation of these massive genomes.

    

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3. Primary tissues may affect estimates of cavitation resistance in ferns

   https://doi.org/10.1111/nph.17374  

   Pittermann, Jarmila ; Baer, Alex ; Sang, Ying ( April 2021 , New Phytologist)

   Different methods of measuring cavitation resistance in fern petioles lead to variable results, particularly with respect to the P₅₀metric. We hypothesised that the fern dictyostele structure affects air entry into the xylem, and therefore impacts the shape of the vulnerability curve.

   Our study examined this variation by comparing vulnerability curves constructed on petioles collected from evergreen and deciduous ferns in the field, with curves generated using the standard centrifuge, air‐injection and bench‐top dehydration methods. Additional experiments complemented the vulnerability curves to better understand how anatomy shapes estimates of cavitation resistance.

   Centrifugation and radial air injection generated acceptable vulnerability curves for the deciduous species, but overestimated drought resistance in the two evergreen ferns. In these hardy plants, axial air injection and bench‐top dehydration produced results that most closely aligned with observations in nature. Additional experiments revealed that the dictyostele anatomy impedes air entry into the xylem during spinning and radial air injection.

   Each method produced acceptable vulnerability curves, depending on the species being tested. Therefore, we stress the importance of validating the curves within situmeasures of water potential and, if possible, hydraulic data to generate realistic results with any of the methods currently available.

    

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4. Polyploidy: its consequences and enabling role in plant diversification and evolution

   https://doi.org/10.1093/aob/mcac132  

   Heslop-Harrison, J. S. ; Schwarzacher, Trude ; Liu, Qing ( October 2022 , Annals of Botany)

   Most, if not all, green plant (Virdiplantae) species including angiosperms and ferns are polyploids themselves or have ancient polyploid or whole genome duplication signatures in their genomes. Polyploids are not only restricted to our major crop species such as wheat, maize, potato and the brassicas, but also occur frequently in wild species and natural habitats. Polyploidy has thus been viewed as a major driver in evolution, and its influence on genome and chromosome evolution has been at the centre of many investigations. Mechanistic models of the newly structured genomes are being developed that incorporate aspects of sequence evolution or turnover (low-copy genes and regulatory sequences, as well as repetitive DNAs), modification of gene functions, the re-establishment of control of genes with multiple copies, and often meiotic chromosome pairing, recombination and restoration of fertility.

   World-wide interest in how green plants have evolved under different conditions – whether in small, isolated populations, or globally – suggests that gaining further insight into the contribution of polyploidy to plant speciation and adaptation to environmental changes is greatly needed. Forward-looking research and modelling, based on cytogenetics, expression studies, and genomics or genome sequencing analyses, discussed in this Special Issue of the Annals of Botany, consider how new polyploids behave and the pathways available for genome evolution. They address fundamental questions about the advantages and disadvantages of polyploidy, the consequences for evolution and speciation, and applied questions regarding the spread of polyploids in the environment and challenges in breeding and exploitation of wild relatives through introgression or resynthesis of polyploids.

   Chromosome number, genome size, repetitive DNA sequences, genes and regulatory sequences and their expression evolve following polyploidy – generating diversity and possible novel traits and enabling species diversification. There is the potential for ever more polyploids in natural, managed and disturbed environments under changing climates and new stresses.

    

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5. Chemical defense over decadal scales: Ontogenetic allocation trajectories and consequences for fitness in a foundation tree species

   https://doi.org/10.1111/1365-2435.13425  

   Cope, Olivia L. ; Kruger, Eric L. ; Rubert‐Nason, Kennedy F. ; Lindroth, Richard L. ; Barton, ed., Kasey ( August 2019 , Functional Ecology)

   Expression of herbivore defense traits can change dramatically during the course of plant development. Little is known, however, about the degree of genetic or sexual variation in these ontogenetic defense trajectories or whether the trajectories themselves are adaptive, especially in long‐lived species.

   We used a 13‐year dataset of chemical defense traits, growth and survivorship from a common garden of trembling aspen (Populus tremuloides) genotypes to document long‐term defense trajectories and their relationship to tree fitness during juvenile and early mature stages.

   Overall, concentrations of the two principal classes of aspen defense compounds (salicinoid phenolic glycosides [SPGs] and condensed tannins [CTs]) decreased to differing degrees in foliage of juvenile trees and then remained relatively constant in maturity. Initial values, juvenile rates of change and average mature values all exhibited significant genetic variation for both SPGs and CTs.

   Relationships between defense trajectory parameters and metrics of tree fitness (growth and survivorship) depended on compound type and tree sex. Females with higher‐allocation SPG trajectories (high initial juvenile concentrations, slow juvenile declines, high mature concentrations) grew more slowly relative to females with lower‐allocation trajectories. In males, higher‐allocation SPG trajectories had a lesser effect on growth but were associated with reduced mortality. Juvenile CT trajectories were not correlated with tree fitness, but average CT concentration in maturity was positively related to growth in females.

   These results suggest that ontogenetic defense trajectories are adaptive and subject to natural selection. Genotypic variation and ontogeny shape tree defensive chemistry, both independently and interactively. These patterns of defense expression have the potential to structure trophic interactions and the genetic composition of forests in both space and time.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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