Genome of the tropical plant Marchantia inflexa: implications for sex chromosome evolution and dehydration tolerance

Higher order polyploids exhibit enhanced desiccation tolerance in the grass Microchloa caffra

Desiccation tolerance evolved recurrently across diverse plant lineages to enable survival in water-limited conditions. Many resurrection plants are polyploid, and several groups have hypothesized that polyploidy contributed to the evolution of desiccation tolerance. However, due to the vast phylogenetic distance between resurrection plant lineages, the rarity of desiccation tolerance, and the prevalence of polyploidy in plants, this hypothesis has been difficult to test. Here, we surveyed natural variation in morphological, reproductive, and desiccation tolerance traits across several cytotypes of a single species to test for links between polyploidy and increased resilience. We sampled multiple natural populations of the resurrection grass Microchloa caffra across an environmental gradient ranging from mesic to xeric in South Africa. We describe two distinct ecotypes of M. caffra that occupy different extremes of the environmental gradient and exhibit consistent differences in ploidy, morphological, reproductive, and desiccation tolerance traits in both field and common growth conditions. Interestingly, plants with more polyploid genomes exhibited consistently higher recovery from desiccation, were less reproductive, and were larger than plants with smaller genomes and lower ploidy. These data indicate that selective pressures in increasingly xeric sites may play a role in maintaining and increasing desiccation tolerance and are mediated by changes in ploidy.

Divergent outcomes of genetic conflict on the UV sex chromosomes of Marchantia polymorpha and Ceratodon purpureus

In species with separate sexes, the genome must produce two distinct developmental programs. Sexually dimorphic development may be controlled by either sex-limited loci or biased expression of loci transmitted through both sexes. Variation in the gene content of sex-limited chromosomes demonstrates that eukaryotic species differ markedly in the roles of these two mechanisms in governing sexual dimorphism. The bryophyte model systems Marchantia polymorpha and Ceratodon purpureus provide a particularly striking contrast. Although both species possess a haploid UV sex chromosome system, in which females carry a U chromosome and males carry a V, M. polymorpha relies on biased autosomal expression, while in C. purpureus, sex-linked genes drive dimorphism. Framing these genetic architectures as divergent outcomes of genetic conflict highlights comparative genomic analyses to better understand the evolution of sexual dimorphism.

An enhancer trap system to track developmental dynamics in Marchantia polymorpha

A combination of streamlined genetics, experimental tractability and relative morphological simplicity compared to vascular plants makes the liverwort Marchantia polymorpha an ideal model system for studying many aspects of plant biology. Here we describe a transformation vector combining a constitutive fluorescent membrane marker with a nuclear marker that is regulated by nearby enhancer elements and use this to produce a library of enhancer trap lines for Marchantia. Screening gemmae from these lines allowed the identification and characterization of novel marker lines, including markers for rhizoids and oil cells. The library allowed the identification of a margin tissue running around the thallus edge, highlighted during thallus development. The expression of this marker is correlated with auxin levels. We generated multiple markers for the meristematic apical notch region, which have different spatial expression patterns, reappear at different times during meristem regeneration following apical notch excision and have varying responses to auxin supplementation or inhibition. This reveals that there are proximodistal substructures within the apical notch that could not be observed otherwise. We employed our markers to study Marchantia sporeling development, observing meristem emergence as defining the protonema-to-prothallus stage transition, and subsequent production of margin tissue during the prothallus stage. Exogenous auxin treatment stalls meristem emergence at the protonema stage but does not inhibit cell division, resulting in callus-like sporelings with many rhizoids, whereas pharmacologically inhibiting auxin synthesis and transport does not prevent meristem emergence. This enhancer trap system presents a useful resource for the community and will contribute to future Marchantia research.

Genome Evolution in Plants: Complex Thalloid Liverworts (Marchantiopsida)

Why do some genomes stay small and simple, while others become huge, and why are some genomes more stable? In contrast to angiosperms and gymnosperms, liverworts are characterized by small genomes with low variation in size and conserved chromosome numbers. We quantified genome evolution among five Marchantiophyta (liverworts), measuring gene characteristics, transposable element (TE) landscape, collinearity, and sex chromosome evolution that might explain the small size and limited variability of liverwort genomes. No genome duplications were identified among examined liverworts and levels of duplicated genes are low. Among the liverwort species, Lunularia cruciata stands out with a genome size almost twice that of the other liverwort species investigated here, and most of this increased size is due to bursts of Ty3/Gypsy retrotransposons. Intrachromosomal rearrangements between examined liverworts are abundant but occur at a slower rate compared with angiosperms. Most genes on L. cruciata scaffolds have their orthologs on homologous Marchantia polymorpha chromosomes, indicating a low degree of rearrangements between chromosomes. Still, translocation of a fragment of the female U chromosome to an autosome was predicted from our data, which might explain the uniquely small U chromosome in L. cruciata. Low levels of gene duplication, TE activity, and chromosomal rearrangements might contribute to the apparent slow rate of morphological evolution in liverworts.

An overview of bioinformatics, genomics, and transcriptomics resources for bryophytes

Bryophytes are useful models for the study of plant evolution, development, plant-fungal symbiosis, stress responses, and gametogenesis. Additionally, their dominant haploid gametophytic phase makes them great models for functional genomics research, allowing straightforward genome editing and gene knockout via CRISPR or homologous recombination. Until 2016, however, the only bryophyte genome sequence published was that of Physcomitrium patens. Throughout recent years, several other bryophyte genomes and transcriptome datasets became available, enabling better comparative genomics in evolutionary studies. The increase in the number of bryophyte genome and transcriptome resources available has yielded a plethora of annotations, databases, and bioinformatics tools to access the new data, which covers the large diversity of this clade and whose biology comprises features such as association with arbuscular mycorrhiza fungi, sex chromosomes, low gene redundancy, or loss of RNA editing genes for organellar transcripts. Here we provide a guide to resources available for bryophytes with regards to genome and transcriptome databases and bioinformatics tools.

Desiccation-rehydration measurements in bryophytes: current status and future insights

Desiccation-rehydration experiments have been employed over the years to evaluate desiccation tolerance of bryophytes (Bryophyta, Marchantiophyta, and Anthocerotophyta). Researchers have applied a spectrum of protocols to induce desiccation and subsequent rehydration, and a wide variety of techniques have been used to study desiccation-dependent changes in bryophyte molecular, cellular, physiological, and structural traits, resulting in a multifaceted assortment of information that is challenging to synthesize. We analysed 337 desiccation-rehydration studies, providing information for 351 species, to identify the most frequent methods used, analyse the advances in desiccation studies over the years, and characterize the taxonomic representation of the species assessed. We observed certain similarities across methodologies, but the degree of convergence among the experimental protocols was surprisingly low. Out of 52 bryophyte orders, 40% have not been studied, and data are lacking for multiple remote or difficult to access locations. We conclude that for quantitative interspecific comparisons of desiccation tolerance, rigorous standardization of experimental protocols and measurement techniques, and simultaneous use of an array of experimental techniques are required for a mechanistic insight into the different traits modified in response to desiccation. New studies should also aim to fill gaps in taxonomic, ecological, and spatial coverage of bryophytes.

Diversity, phylogeny, and adaptation of bryophytes: insights from genomic and transcriptomic data

Bryophytes including mosses, liverworts, and hornworts are among the earliest land plants, and occupy a crucial phylogenetic position to aid in the understanding of plant terrestrialization. Despite their small size and simple structure, bryophytes are the second largest group of extant land plants. They live ubiquitously in various habitats and are highly diversified, with adaptive strategies to modern ecosystems on Earth. More and more genomes and transcriptomes have been assembled to address fundamental questions in plant biology. Here, we review recent advances in bryophytes associated with diversity, phylogeny, and ecological adaptation. Phylogenomic studies have provided increasing supports for the monophyly of bryophytes, with hornworts sister to the Setaphyta clade including liverworts and mosses. Further comparative genomic analyses revealed that multiple whole-genome duplications might have contributed to the species richness and morphological diversity in mosses. We highlight that the biological changes through gene gain or neofunctionalization that primarily evolved in bryophytes have facilitated the adaptation to early land environments; among the strategies to adapt to modern ecosystems in bryophytes, desiccation tolerance is the most remarkable. More genomic information for bryophytes would shed light on key mechanisms for the ecological success of these 'dwarfs' in the plant kingdom.

Plant genome sequence assembly in the era of long reads: Progress, challenges and future directions

Third-generation long-read sequencing is transforming plant genomics. Oxford Nanopore Technologies and Pacific Biosciences are offering competing long-read sequencing technologies and enable plant scientists to investigate even large and complex plant genomes. Sequencing projects can be conducted by single research groups and sequences of smaller plant genomes can be completed within days. This also resulted in an increased investigation of genomes from multiple species in large scale to address fundamental questions associated with the origin and evolution of land plants. Increased accessibility of sequencing devices and user-friendly software allows more researchers to get involved in genomics. Current challenges are accurately resolving diploid or polyploid genome sequences and better accounting for the intra-specific diversity by switching from the use of single reference genome sequences to a pangenome graph.

Charting the genomic landscape of seed-free plants

During the past few years several high-quality genomes has been published from Charophyte algae, bryophytes, lycophytes and ferns. These genomes have not only elucidated the origin and evolution of early land plants, but have also provided important insights into the biology of the seed-free lineages. However, critical gaps across the phylogeny remain and many new questions have been raised through comparing seed-free and seed plant genomes. Here, we review the reference genomes available and identify those that are missing in the seed-free lineages. We compare patterns of various levels of genome and epigenomic organization found in seed-free plants to those of seed plants. Some genomic features appear to be fundamentally different. For instance, hornworts, Selaginella and most liverworts are devoid of whole-genome duplication, in stark contrast to other land plants. In addition, the distribution of genes and repeats appear to be less structured in seed-free genomes than in other plants, and the levels of gene body methylation appear to be much lower. Finally, we highlight the currently available (or needed) model systems, which are crucial to further our understanding about how changes in genes translate into evolutionary novelties.

Plant sex chromosomes defy evolutionary models of expanding recombination suppression and genetic degeneration

Hundreds of land plant lineages have independently evolved separate sexes in either gametophytes (dioicy) or sporophytes (dioecy), but 43% of all dioecious angiosperms are found in just 34 entirely dioecious clades, suggesting that their mode of sex determination evolved a long time ago. Here, we review recent insights on the molecular mechanisms that underlie the evolutionary change from individuals that each produce male and female gametes to individuals specializing in the production of just one type of gamete. The canonical model of sex chromosome evolution in plants predicts that two sex-determining genes will become linked in a sex-determining region (SDR), followed by expanding recombination suppression, chromosome differentiation and, ultimately, degeneration. Experimental work, however, is showing that single genes function as master regulators in model systems, such as the liverwort Marchantia and the angiosperms Diospyros and Populus. In Populus, this type of regulatory function has been demonstrated by genome editing. In other systems, including Actinidia, Asparagus and Vitis, two coinherited factors appear to independently regulate female and male function, yet sex chromosome differentiation has remained low. We discuss the best-understood systems and evolutionary pathways to dioecy, and present a meta-analysis of the sizes and ages of SDRs. We propose that limited sexual conflict explains why most SDRs are small and sex chromosomes remain homomorphic. It appears that models of increasing recombination suppression with age do not apply because selection favours mechanisms in which sex determination depends on minimal differences, keeping it surgically precise.

Centromere organization and UU/V sex chromosome behavior in a liverwort

In 1917, sex chromosomes in plants were discovered in a liverwort with hetermorphic U and V chromosomes. Such heteromorphy is unexpected because, unlike the XY chromosomes in diploid-dominant plants, in haploid-dominant plants the female U and the male V chromosomes experience largely symmetrical potential recombination environments. Here we use molecular cytogenetics and super-resolution microscopy to study Frullania dilatata, a liverwort with one male and two female sex chromosomes. We applied a pipeline to Illumina sequences to detect abundant types of repetitive DNA and developed FISH probes to microscopically distinguish the sex chromosomes. We also determined the phenotypic population sex ratio because biased ratios have been reported from other liverworts with heteromorphic sex chromosomes. Populations had male-biased sex ratios. The sex chromosomes are monocentric, and of 14 probes studied (eight satellites, five transposable elements and one plastid region), four resulted in unique signals that differentiated the sex chromosomes from the autosomes and from each other. One FISH probe selectively marked the centromeres of both U chromosomes, so we could prove that during meiosis each U chromosome associates with one of the opposite telomeres of the V chromosome, resulting in a head-to-head trivalent. The similarity of the two U chromosomes to each other in size and in their centromere FISH signal positions points to their origin via a non-disjunction event (aneuploidy), which would fit with the general picture of sex chromosomes rarely crossing-over and being prone to suffer from non-disjunction.

When and how do sex-linked regions become sex chromosomes?

The attention given to heteromorphism and genetic degeneration of "classical sex chromosomes" (Y chromosomes in XY systems, and the W in ZW systems that were studied first and are best described) has perhaps created the impression that the absence of recombination between sex chromosomes is inevitable. I here argue that continued recombination is often to be expected, that absence of recombination is surprising and demands further study, and that the involvement of selection in reduced recombination is not yet well understood. Despite a long history of investigations of sex chromosome pairs, there is a need for more quantitative approaches to studying sex-linked regions. I describe a scheme to help understand the relationships between different properties of sex-linked regions. Specifically, I focus on their sizes (differentiating between small regions and extensive fully sex-linked ones), the times when they evolved, and their differentiation, and review studies using DNA sequencing in nonmodel organisms that are providing information about the processes causing these properties.

The Diversity of Plant Sex Chromosomes Highlighted through Advances in Genome Sequencing

For centuries, scientists have been intrigued by the origin of dioecy in plants, characterizing sex-specific development, uncovering cytological differences between the sexes, and developing theoretical models. Through the invention and continued improvements in genomic technologies, we have truly begun to unlock the genetic basis of dioecy in many species. Here we broadly review the advances in research on dioecy and sex chromosomes. We start by first discussing the early works that built the foundation for current studies and the advances in genome sequencing that have facilitated more-recent findings. We next discuss the analyses of sex chromosomes and sex-determination genes uncovered by genome sequencing. We synthesize these results to find some patterns are emerging, such as the role of duplications, the involvement of hormones in sex-determination, and support for the two-locus model for the origin of dioecy. Though across systems, there are also many novel insights into how sex chromosomes evolve, including different sex-determining genes and routes to suppressed recombination. We propose the future of research in plant sex chromosomes should involve interdisciplinary approaches, combining cutting-edge technologies with the classics to unravel the patterns that can be found across the hundreds of independent origins.

Expression dynamics of dehydration tolerance in the tropical plant Marchantia inflexa

Tolerance to prolonged water deficit occurs along a continuum in plants, with dehydration tolerance (DhT) and desiccation tolerance (DT) representing some of the most extreme adaptations to water scarcity. Although DhT and DT presumably vary among individuals of a single species, this variability remains largely unstudied. Here, we characterized expression dynamics throughout a dehydration-rehydration time-course in six diverse genotypes of the dioecious liverwort Marchantia inflexa. We identified classical signatures of stress response in M. inflexa, including major changes in transcripts related to metabolism, expression of LEA and ELIP genes, and evidence of cell wall remodeling. However, we detected very little temporal synchronization of these responses across different genotypes of M. inflexa, which may be related to genotypic variation among samples, constitutive expression of dehydration-associated transcripts, the sequestration of mRNAs in ribonucleoprotein partials prior to drying, or the lower tolerance of M. inflexa relative to most bryophytes studied to date. Our characterization of intraspecific variation in expression dynamics suggests that differences in the timing of transcriptional adjustments contribute to variation among genotypes, and that developmental differences impact the relative tolerance of meristematic and differentiated tissues. This work highlights the complexity and variability of water stress tolerance, and underscores the need for comparative studies that seek to characterize variation in DT and DhT.

Evolutionary Genomics of Plant Gametophytic Selection

It has long been recognized that natural selection during the haploid gametophytic phase of the plant life cycle may have widespread importance for rates of evolution and the maintenance of genetic variation. Recent theoretical advances have further highlighted the significance of gametophytic selection for diverse evolutionary processes. Genomic approaches offer exciting opportunities to address key questions about the extent and effects of gametophytic selection on plant evolution and adaptation. Here, we review the progress and prospects for integrating functional and evolutionary genomics to test theoretical predictions, and to examine the importance of gametophytic selection on genetic diversity and rates of evolution. There is growing evidence that selection during the gametophyte phase of the plant life cycle has important effects on both gene and genome evolution and is likely to have important pleiotropic effects on the sporophyte. We discuss the opportunities to integrate comparative population genomics, genome-wide association studies, and experimental approaches to further distinguish how differential selection in the two phases of the plant life cycle contributes to genetic diversity and adaptive evolution.

Comparative Mitogenome Analysis of the Genus Trifolium Reveals Independent Gene Fission of ccmFn and Intracellular Gene Transfers in Fabaceae

The genus Trifolium is the largest of the tribe Trifolieae in the subfamily Papilionoideae (Fabaceae). The paucity of mitochondrial genome (mitogenome) sequences has hindered comparative analyses among the three genomic compartments of the plant cell (nucleus, mitochondrion and plastid). We assembled four mitogenomes from the two subgenera (Chronosemium and Trifolium) of the genus. The four Trifolium mitogenomes were compact (294,911-348,724 bp in length) and contained limited repetitive (6.6-8.6%) DNA. Comparison of organelle repeat content highlighted the distinct evolutionary trajectory of plastid genomes in a subset of Trifolium species. Intracellular gene transfer (IGT) was analyzed among the three genomic compartments revealing functional transfer of mitochondrial rps1 to nuclear genome along with other IGT events. Phylogenetic analysis based on mitochondrial and nuclear rps1 sequences revealed that the functional transfer in Trifolieae was independent from the event that occurred in robinioid clade that includes genus Lotus. A novel, independent fission event of ccmFn in Trifolium was identified, caused by a 59 bp deletion. Fissions of this gene reported previously in land plants were reassessed and compared with Trifolium.