Defining Populations and Predicting Future Suitable Niche Space in the Geographically Disjunct, Narrowly Endemic Leafy Prairie-Clover (Dalea foliosa; Fabaceae)

Conservation actions for rare species are often based on estimates of population size and number, which are challenging to capture in natural systems. Instead, many definitions of populations rely on arbitrarily defined distances between occurrences, which is not necessarily biologically meaningful despite having utility from a conservation management perspective. Here, we introduce a case study using the narrowly endemic and highly geographically disjunct leafy prairie-clover (Dalea foliosa), for which we use nuclear microsatellite loci to assess the current delimitations of populations and management units across its entire known range. We model future potential suitable niche space for the species to assess how currently defined populations could fare under predicted changes in climate over the next 50 years. Our results indicate that genetic variation within the species is extremely limited, particularly so in the distal portions of its range (Illinois and Alabama). Within the core of its range (Tennessee), genetic structure is not consistent with populations as currently defined. Our models indicate that predicted suitable niche space may only marginally overlap with the geology associated with this species (limestone glades and dolomite prairies) by 2070. Additional studies are needed to evaluate the extent to which populations are ecologically adapted to local environments and what role this could play in future translocation efforts.

Divergent gene expression levels between diploid and autotetraploid Tolmiea relative to the total transcriptome, the cell, and biomass

PREMISE OF THE STUDY

Studies of gene expression and polyploidy are typically restricted to characterizing differences in transcript concentration. Using diploid and autotetraploid Tolmiea, we present an integrated approach for cross-ploidy comparisons that account for differences in transcriptome size and cell density and make multiple comparisons of transcript abundance.

METHODS

We use RNA spike-in standards in concert with cell size and density to identify and correct for differences in transcriptome size and compare levels of gene expression across multiple scales: per transcriptome, per cell, and per biomass.

KEY RESULTS

In total, ~17% of all loci were identified as differentially expressed (DEGs) between the diploid and autopolyploid species. The per-transcriptome normalization, the method researchers typically use, captured the fewest DEGs (58% of total DEGs) and failed to detect any DEGs not found by the alternative normalizations. When transcript abundance was normalized per biomass and per cell, ~66% and ~82% of the total DEGs were recovered, respectively. The discrepancy between per-transcriptome and per-cell recovery of DEGs occurs because per-transcriptome normalizations are concentration-based and therefore blind to differences in transcriptome size.

CONCLUSIONS

While each normalization enables valid comparisons at biologically relevant scales, a holistic comparison of multiple normalizations provides additional explanatory power not available from any single approach. Notably, autotetraploid loci tend to conserve diploid-like transcript abundance per biomass through increased gene expression per cell, and these loci are enriched for photosynthesis-related functions.

Niche divergence between diploid and autotetraploid Tolmiea

PREMISE OF STUDY

Polyploidy is common in eukaryotes and is of major evolutionary importance over both short and long time-scales. Compared to allopolyploids, autopolyploids remain understudied; they are often morphologically cryptic and frequently remain taxonomically unrecognized, although there is increasing recognition of the high frequency of autopolyploidy in angiosperms. While autopolyploidy can serve as an instant speciation mechanism, little is known about the ecological consequences of this process. We describe the ecological divergence of a diploid-autotetraploid species pair in Tolmiea.

METHODS

We investigated whether abiotic niche divergence has shaped the current allopatric distribution of diploid T. diplomenziesii and its autotetraploid derivative, T. menziesii, in the Pacific Northwest of North America. We employed field measures of light availability, as well as niche modeling and a principal component analysis of environmental space. Within a common garden, we also investigated physiological responses to changes in soil moisture.

KEY RESULTS

Diploid and autotetraploid Tolmiea inhabit significantly different climatic niche spaces. The climatic niche divergence between these two species is best explained by a shift in precipitation availability, and we found evidence of differing physiological response to water availability between these species.

CONCLUSIONS

We found that spatial segregation of T. diplomenziesii and T. menziesii was accompanied by adaptation to changes in climatic regime. Tolmiea menziesii is not a nascent autotetraploid, having persisted long enough to be established throughout the Pacific Northwest, and therefore both polyploidization and subsequent evolution have contributed to the observed differences between T. menziesii and T. diplomenziesii.

Polyploidy: Pitfalls and paths to a paradigm

Investigators have long searched for a polyploidy paradigm-rules or principles that might be common following polyploidization (whole-genome duplication, WGD). Here we attempt to integrate what is known across the more thoroughly investigated polyploid systems on topics ranging from genetics to ecology. We found that while certain rules may govern gene retention and loss, systems vary in the prevalence of gene silencing vs. homeolog loss, chromosomal change, the presence of a dominant genome (in allopolyploids), and the relative importance of hybridization vs. genome doubling per se. In some lineages, aspects of polyploidization are repeated across multiple origins, but in other species multiple origins behave more stochastically in terms of genetic and phenotypic change. Our investigation also reveals that the path to synthesis is hindered by numerous gaps in our knowledge of even the best-known systems. Particularly concerning is the absence of linkage between genotype and phenotype. Moreover, most recent studies have focused on the genetic and genomic attributes of polyploidy, but rarely is there an ecological or physiological context. To promote a path to a polyploidy paradigm (or paradigms), we propose a major community goal over the next 10-20 yr to fill the gaps in our knowledge of well-studied polyploids. Before a meaningful synthesis is possible, more complete data sets are needed for comparison-systems that include comparable genetic, genomic, chromosomal, proteomic, as well as morphological, physiological, and ecological data. Also needed are more natural evolutionary model systems, as most of what we know about polyploidy continues to come from a few crop and genetic models, systems that often lack the ecological context inherent in natural systems and necessary for understanding the drivers of biodiversity.

A global perspective on Campanulaceae: Biogeographic, genomic, and floral evolution

PREMISE OF THE STUDY

The Campanulaceae are a diverse clade of flowering plants encompassing more than 2300 species in myriad habitats from tropical rainforests to arctic tundra. A robust, multigene phylogeny, including all major lineages, is presented to provide a broad, evolutionary perspective of this cosmopolitan clade.

METHODS

We used a phylogenetic framework, in combination with divergence dating, ancestral range estimation, chromosome modeling, and morphological character reconstruction analyses to infer phylogenetic placement and timing of major biogeographic, genomic, and morphological changes in the history of the group and provide insights into the diversification of this clade across six continents.

KEY RESULTS

Ancestral range estimation supports an out-of-Africa diversification following the Cretaceous-Tertiary extinction event. Chromosomal modeling, with corroboration from the distribution of synonymous substitutions among gene duplicates, provides evidence for as many as 20 genome-wide duplication events before large radiations. Morphological reconstructions support the hypothesis that switches in floral symmetry and anther dehiscence were important in the evolution of secondary pollen presentation mechanisms.

CONCLUSIONS

This study provides a broad, phylogenetic perspective on the evolution of the Campanulaceae clade. The remarkable habitat diversity and cosmopolitan distribution of this lineage appears to be the result of a complex history of genome duplications and numerous long-distance dispersal events. We failed to find evidence for an ancestral polyploidy event for this clade, and our analyses indicate an ancestral base number of nine for the group. This study will serve as a framework for future studies in diverse areas of research in Campanulaceae.

Evolution and biogeography of the endemic Roucela complex (Campanulaceae: Campanula) in the Eastern Mediterranean

At the intersection of geological activity, climatic fluctuations, and human pressure, the Mediterranean Basin – a hotspot of biodiversity – provides an ideal setting for studying endemism, evolution, and biogeography. Here, we focus on the Roucela complex (Campanula subgenus Roucela), a group of 13 bellflower species found primarily in the eastern Mediterranean Basin. Plastid and low‐copy nuclear markers were employed to reconstruct evolutionary relationships and estimate divergence times within the Roucela complex using both concatenation and species tree analyses. Niche modeling, ancestral range estimation, and diversification analyses were conducted to provide further insights into patterns of endemism and diversification through time. Diversification of the Roucela clade appears to have been primarily the result of vicariance driven by the breakup of an ancient landmass. We found geologic events such as the formation of the mid‐Aegean trench and the Messinian Salinity Crisis to be historically important in the evolutionary history of this group. Contrary to numerous past studies, the onset of the Mediterranean climate has not promoted diversification in the Roucela complex and, in fact, may be negatively affecting these species. This study highlights the diversity and complexity of historical processes driving plant evolution in the Mediterranean Basin.

Origins and diversity of a cosmopolitan fern genus on an island archipelago

Isolated oceanic islands are characterized by patterns of biological diversity different from that on nearby continental mainlands. Isolation can provide the opportunity for evolutionary divergence, but also set the stage for hybridization between related taxa arriving from different sources. Ferns disperse by haploid spores, which are produced in large numbers and can travel long distances in air currents, enabling these plants to become established on most oceanic islands. Here, we examine the origins and patterns of diversity of the cosmopolitan fern genus Pteridium (Dennstaedtiaceae; bracken) on the Galapagos Islands. We use nucleotide sequences from two plastid genes, and two nuclear gene markers, to examine phylogeography of Pteridium on the Galapagos Islands. We incorporate data from a previous study to provide a worldwide context. We also sampled new specimens from South and Central America. We used flow cytometry to estimate genome size of some accessions. We found that both plastid and nuclear haplotypes fall into two distinct clades, consistent with a two-diploid-species taxonomy of P. aquilinum and P. esculentum. As predicted, the allotetraploid P. caudatum possesses nuclear haplotypes from both diploid species. Samples from the Galapagos include P. esculentum subsp. arachnoideum, P. caudatum and possible hybrids between them. Multiple Pteridium taxa were also observed growing together at some sites. We find evidence for multiple origins of Pteridium on the Galapagos Islands and multiple origins of tetraploid P. caudatum throughout its range in Central and South America. We also posit that P. caudatum may include recent diploid hybrids, backcrosses to P. esculentum, as well as allotetraploid plants. The Galapagos Islands are positioned close to the equator where they can receive dispersing propagules from both hemispheres. This may partly explain the high levels of diversity found for this cosmopolitan fern on these islands.

An Exploration into Fern Genome Space

Ferns are one of the few remaining major clades of land plants for which a complete genome sequence is lacking. Knowledge of genome space in ferns will enable broad-scale comparative analyses of land plant genes and genomes, provide insights into genome evolution across green plants, and shed light on genetic and genomic features that characterize ferns, such as their high chromosome numbers and large genome sizes. As part of an initial exploration into fern genome space, we used a whole genome shotgun sequencing approach to obtain low-density coverage (∼0.4X to 2X) for six fern species from the Polypodiales (Ceratopteris, Pteridium, Polypodium, Cystopteris), Cyatheales (Plagiogyria), and Gleicheniales (Dipteris). We explore these data to characterize the proportion of the nuclear genome represented by repetitive sequences (including DNA transposons, retrotransposons, ribosomal DNA, and simple repeats) and protein-coding genes, and to extract chloroplast and mitochondrial genome sequences. Such initial sweeps of fern genomes can provide information useful for selecting a promising candidate fern species for whole genome sequencing. We also describe variation of genomic traits across our sample and highlight some differences and similarities in repeat structure between ferns and seed plants.

Population genetic variation, geographic structure, and multiple origins of autopolyploidy in Galax urceolata

PREMISE OF THE STUDY

Whereas population genetic studies have examined allopolyploids, comparable studies of naturally occurring autopolyploids remain rare. To address fundamental questions regarding autopolyploidy, we undertook a detailed population genetic study of one of the classic examples of autopolyploidy, Galax urceolata (Diapensiaceae), which comprises diploid, triploid, and autotetraploid cytotypes. Galax is endemic to the Appalachian Mountains, the adjacent piedmont, sandhills, and coastal plain and represents perhaps the most widely known example of autopolyploidy in nature.

METHODS

Flow cytometry was used to diagnose ploidal level of ∼1000 individuals across 71 populations. We used 10 microsatellite markers to examine genetic variation across the geographic range of Galax and assessed multiple origins though comparisons of diploid, triploid, and tetraploid accessions using multiple analytical approaches.

KEY RESULTS

Tetraploids had higher levels of heterozygosity than diploids did. Genetic variation in diploid and tetraploid Galax is geographically structured among the ecoregions of the southeastern United States. Autotetraploidy in Galax urceolata has occurred independently at least 46 times, with triploidy having occurred a minimum of 31 times.

CONCLUSIONS

Genetic differentiation among ecoregions suggests historical patterns of local adaptation. The numerous independent origins of tetraploid Galax reported here are among the highest frequencies of independent polyploidizations ever reported for any polyploid (auto- or allopolyploid).

Multiple origins and chromosomal novelty in the allotetraploid Tragopogon castellanus (Asteraceae)

Tragopogon includes two classic examples of recently formed allopolyploid species in North America: T. mirus and T. miscellus. Older Tragopogon allotetraploids from Eurasia offer ideal taxa for comparing the longer term outcomes of allopolyploidy. To help resolve the ancestry of one of these older polyploids, phylogenetic analyses of multiple populations of the allotetraploid T. castellanus (2n = 24) and its putative diploid parents, T. crocifolius and T. lamottei, were conducted using sequences from nuclear (internal transcribed spacer, ITS; and alcohol dehydrogenase 1A, Adh) and plastid (trnT-trnL spacer, trnL intron, trnL-trnF spacer and rpl16 intron) loci. Fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH) were used to investigate the chromosomal constitution of T. castellanus. Our data confirm that the widely distributed T. crocifolius and the Iberian endemic, T. lamottei, are the diploid parents of T. castellanus, and that this polyploid formed at least three times. One group of populations of T. castellanus is distinct in exhibiting two pairs of rearranged chromosomes. These data suggest that some of the chromosomal variants that originate in young polyploids (here, an intergenomic translocation) may become fixed in populations, contributing to novelty in older polyploid lineages. The geographical distributions of the allopolyploids and parents are also complex, with allotetraploid populations being disjunct from one or both of the most closely related diploid parental populations.

Polyploidy and novelty: Gottlieb's legacy

Nearly four decades ago, Roose & Gottlieb (Roose & Gottlieb 1976 Evolution 30, 818-830. (doi:10.2307/2407821)) showed that the recently derived allotetraploids Tragopogon mirus and T. miscellus combined the allozyme profiles of their diploid parents (T. dubius and T. porrifolius, and T. dubius and T. pratensis, respectively). This classic paper addressed the link between genotype and biochemical phenotype and documented enzyme additivity in allopolyploids. Perhaps more important than their model of additivity, however, was their demonstration of novelty at the biochemical level. Enzyme multiplicity-the production of novel enzyme forms in the allopolyploids-can provide an extensive array of polymorphism for a polyploid individual and may explain, for example, the expanded ranges of polyploids relative to their diploid progenitors. In this paper, we extend the concept of evolutionary novelty in allopolyploids to a range of genetic and ecological features. We observe that the dynamic nature of polyploid genomes-with alterations in gene content, gene number, gene arrangement, gene expression and transposon activity-may generate sufficient novelty that every individual in a polyploid population or species may be unique. Whereas certain combinations of these features will undoubtedly be maladaptive, some unique combinations of newly generated variation may provide tremendous evolutionary potential and adaptive capabilities.

The polyploidy revolution then…and now: Stebbins revisited

Polyploidy has long been considered a major force in plant evolution. G. Ledyard Stebbins, Jr., an architect of the Modern Synthesis, elegantly addressed a broad range of topics, from genes to chromosomes to deep phylogeny, but some of his most lasting insights came in the study of polyploidy. Here, we review the immense impact of his work on polyploidy over more than 60 years, from his entrance into this fledgling field in the 1920s until the end of his career. Stebbins and his contemporaries developed a model of polyploid evolution that persisted for nearly half a century. As new perspectives emerged in the 1980s and new genetic tools for addressing key aspects of polyploidy have become available, a new paradigm of polyploidy has replaced much of the Stebbinsian framework. We review that paradigm shift and emphasize those areas in which the ideas of Stebbins continue to propel the field forward, as well as those areas in which the field was held back; we also note new directions that plant geneticists and evolutionists are now exploring in polyploidy research. Perhaps the most important conclusion from recent and ongoing studies of polyploidy is that, following Levin and others, polyploidy may propel a population into a new adaptive sphere given the myriad changes that accompany genome doubling.