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The mechanisms controlling chromosome number, size, and shape, and the relationship of these traits to genome size, remain some of the least understood aspects of genome evolution. Across vascular plants, there is a striking disparity in chromosome number between homosporous and heterosporous lineages. Homosporous plants (comprising most ferns and some lycophytes) have high chromosome numbers compared to heterosporous lineages (some ferns and lycophytes and all seed plants). Many studies have investigated why homosporous plants have so many chromosomes. However, homospory is the ancestral condition from which heterospory has been derived several times. Following this phylogenetic perspective, a more appropriate question to ask is why heterosporous plants have so few chromosomes. Here, we review life history differences between heterosporous and homosporous plants, previous work on chromosome number and genome size in each lineage, known mechanisms of genome downsizing and chromosomal rearrangements, and conclude with future prospects for comparative research. 

Abstract The large size and complexity of most fern genomes have hampered efforts to elucidate fundamental aspects of fern biology and land plant evolution through genome-enabled research. Here we present a chromosomal genome assembly and associated methylome, transcriptome and metabolome analyses for the model fern species Ceratopteris richardii . The assembly reveals a history of remarkably dynamic genome evolution including rapid changes in genome content and structure following the most recent whole-genome duplication approximately 60 million years ago. These changes include massive gene loss, rampant tandem duplications and multiple horizontal gene transfers from bacteria, contributing to the diversification of defence-related gene families. The insertion of transposable elements into introns has led to the large size of the Ceratopteris genome and to exceptionally long genes relative to other plants. Gene family analyses indicate that genes directing seed development were co-opted from those controlling the development of fern sporangia, providing insights into seed plant evolution. Our findings and annotated genome assembly extend the utility of Ceratopteris as a model for investigating and teaching plant biology. 

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.

 

Visions of science teaching and learning in the newest U.S. standards documents are dramatically different than those found in most classrooms. This research addresses these differences through closely examining one professional development (PD) project that connects teacher learning and teacher practice with student learning/achievement. This study examines the effects on eighth grade science teachers and their students in the context of a PD focused on the integration of information communication technologies and reformed science teaching practices. Findings from this investigation suggest that teachers who participated in PD for two years learned more about technology, improved their practice, and their students’ achievement was significantly higher compared to teachers who participated in one year of the PD or non‐participating peers. Science educators face multiple challenges as they attempt to deliver instruction in fundamentally different ways than what they experienced as learners. The delivery of this professional learning suggest that PD for science teachers should include educative learning experiences if understandings of reforms supported by research are to be realized.

 

Spore‐bearing plants are capable of dispersing very long distances. However, it is not known if gene flow can prevent genetic divergence in widely distributed taxa. Here we address this issue, and examine systematic relationships at a global geographic scale for the fern genusPteridium.

We sampled plants from 100 localities worldwide, and generated nucleotide data from four nuclear genes and two plastid regions. We also examined 2801 single nucleotide polymorphisms detected by a restriction site‐associatedDNAapproach.

We found evidence for two distinct diploid species and two allotetraploids between them. The “northern” species (Pteridium aquilinum) has distinct groups at the continental scale (Europe, Asia, Africa, and North America). The northern European subspeciespinetorumappears to involve admixture among all of these. A sample from the Hawaiian Islands contained elements of both North American and AsianP. aquilinum. The “southern” species,P. esculentum, shows little genetic differentiation between South American and Australian samples. Components of African genotypes are detected on all continents.

We find evidence of distinct continental‐scale genetic differentiation inPteridium. However, on top of this is a clear signal of recent hybridization. Thus, spore‐bearing plants are clearly capable of extensive long‐distance gene flow; yet appear to have differentiated genetically at the continental scale. Either gene flow in the past was at a reduced level, or vicariance is possible even in the face of long‐distance gene flow.