The fate of sex chromosomes during the evolution of monoicy from dioicy in liverworts

Why should we study plant sex chromosomes?

Understanding plant sex chromosomes involves studying interactions between developmental and physiological genetics, genome evolution, and evolutionary ecology. We focus on areas of overlap between these. Ideas about how species with separate sexes (dioecious species, in plant terminology) can evolve are even more relevant to plants than to most animal taxa because dioecy has evolved many times from ancestral functionally hermaphroditic populations, often recently. One aim of studying plant sex chromosomes is to discover how separate males and females evolved from ancestors with no such genetic sex-determining polymorphism, and the diversity in the genetic control of maleness vs femaleness. Different systems share some interesting features, and their differences help to understand why completely sex-linked regions may evolve. In some dioecious plants, the sex-determining genome regions are physically small. In others, regions without crossing over have evolved sometimes extensive regions with properties very similar to those of the familiar animal sex chromosomes. The differences also affect the evolutionary changes possible when the environment (or pollination environment, for angiosperms) changes, as dioecy is an ecologically risky strategy for sessile organisms. Dioecious plants have repeatedly reverted to cosexuality, and hermaphroditic strains of fruit crops such as papaya and grapes are desired by plant breeders. Sex-linked regions are predicted to become enriched in genes with sex differences in expression, especially when higher expression benefits one sex function but harms the other. Such trade-offs may be important for understanding other plant developmental and physiological processes and have direct applications in plant breeding.

Syntrichia ruralis: emerging model moss genome reveals a conserved and previously unknown regulator of desiccation in flowering plants

Water scarcity, resulting from climate change, poses a significant threat to ecosystems. Syntrichia ruralis, a dryland desiccation-tolerant moss, provides valuable insights into survival of water-limited conditions. We sequenced the genome of S. ruralis, conducted transcriptomic analyses, and performed comparative genomic and transcriptomic analyses with existing genomes and transcriptomes, including with the close relative S. caninervis. We took a genetic approach to characterize the role of an S. ruralis transcription factor, identified in transcriptomic analyses, in Arabidopsis thaliana. The genome was assembled into 12 chromosomes encompassing 21 169 protein-coding genes. Comparative analysis revealed copy number and transcript abundance differences in known desiccation-associated gene families, and highlighted genome-level variation among species that may reflect adaptation to different habitats. A significant number of abscisic acid (ABA)-responsive genes were found to be negatively regulated by a MYB transcription factor (MYB55) that was upstream of the S. ruralis ortholog of ABA-insensitive 3 (ABI3). We determined that this conserved MYB transcription factor, uncharacterized in Arabidopsis, acts as a negative regulator of an ABA-dependent stress response in Arabidopsis. The new genomic resources from this emerging model moss offer novel insights into how plants regulate their responses to water deprivation.

A subclass II bHLH transcription factor in Marchantia polymorpha gives insight into the ancestral land plant trait of spore formation

Sporopollenin is often said to be one of the toughest biopolymers known to man. The shift in dormancy cell wall deposition from around the diploid zygotes of charophycean algae to sporopollenin around the haploid spores of land plants essentially imparted onto land plants the gift of passive motility, a key acquisition that contributed to their vast and successful colonization across terrestrial habitats.1,2 A putative transcription factor controlling the land plant mode of sporopollenin deposition is the subclass II bHLHs, which are conserved and novel to land plants, with mutants of genes in angiosperms and mosses divulging roles relating to tapetum degeneration and spore development.3,4,5,6,7 We demonstrate that a subclass II bHLH gene, MpbHLH37, regulates sporopollenin biosynthesis and deposition in the model liverwort Marchantia polymorpha. Mpbhlh37 sporophytes show a striking loss of secondary wall deposits of the capsule wall, the elaters, and the spore exine, all while maintaining spore viability, identifying MpbHLH37 as a master regulator of secondary wall deposits of the sporophyte. Localization of MpbHLH37 to the capsule wall and elaters of the sporophyte directly designates these tissue types as a bona fide tapetum in liverworts, giving support to the notion that the presence of a tapetum is an ancestral land plant trait. Finally, as early land plant spore walls exhibit evidence of tapetal deposition,8,9,10,11,12 a tapetal capsule wall could have provided these plants with a developmental mechanism for sporopollenin deposition.

Immunobiodiversity: Conserved and specific immunity across land plants and beyond

Angiosperms represent most plants that humans cultivate, grow, and eat. However, angiosperms are only one of five major land plant lineages. As a whole lineage, plants also include algal groups. All these clades represent a tremendous genetic diversity that can be investigated to reveal the evolutionary history of any given mechanism. In this review, we describe the current model of the plant immune system, discuss its evolution based on the recent literature, and propose future directions for the field. In angiosperms, plant-microbe interactions have been intensively studied, revealing essential cell surface and intracellular immune receptors, as well as metabolic and hormonal defense pathways. Exploring diversity at the genomic and functional levels demonstrates the conservation of these pathways across land plants, some of which are beyond plants. On basis of the conserved mechanisms, lineage-specific variations have occurred, leading to diversified reservoirs of immune mechanisms. In rare cases, this diversity has been harnessed and successfully transferred to other species by integration of wild immune receptors or engineering of novel forms of receptors for improved resistance to pathogens. We propose that exploring further the diversity of immune mechanisms in the whole plant lineage will reveal completely novel sources of resistance to be deployed in crops.

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.

Sex chromosomes: How to make a hermaphrodite

The existence of sex chromosomes complicates the evolution of cosexuality (hermaphroditism). Four new genomic studies from haploid-dominant plants show commonalities and differences in mechanisms of the evolution of cosexuality, raising questions about the genetics of sexual dimorphism and the fate of cosexual lineages.