|Friday, July 09|
Evolution of sexual systems in the brown algae
* Susana Coelho, Max Planck Institute for Developmental Biology, Germany
Over the last few years, the brown algae have emerged as a highly valuable group to study the evolution of sex chromosomes and reproductive systems because they exhibit a remarkable diversity of sexual traits and because there have been multiple transitions between sexual systems over a relatively short evolutionary time period. The maintenance of a high level of diversity of life cycle and sexual features in a single, evolutionarily young group is outstanding among the eukaryotes, and points to a complex evolutionary history of the underlying regulatory systems. I will describe how we are using the brown algae to gain novel insights into the mechanisms and evolutionary trajectories of sex determination systems and to reveal the functional and evolutionary interactions between the sex chromosomes and key reproductive and life cycle traits.
A Conserved Genetic Switch Determines Volvocine Algal Sex and Mating-Types
* James Umen, Donald Danforth Plant Science Ctr, United States
Sa Geng, Donald Danforth Plant Science Ctr
Takashi Hamaji, Chuo University
Volvocine green algae have well-studied haplontic sexual cycles that are typical for chlorophytes. In heterothallic volvocines the MID (minus dominance) gene governs sexual differentiation and is found exclusively in the MT- haplotype or male sex-determining region. In isogamous unicellular Chlamydomonas reinhardtii (Chlamydomonas) and oogamous multicellular Volvox carteri (Volvox) the expression of MID is necessary and sufficient to induce minus or male gametogenesis, respectively, while in its absence plus or female differentiation occurs as a default program. However, it has remained unknown how the default programs of female or plus are specified and are modified by expression of Mid protein. Using a comparative transcriptomics approach, we identified a conserved gene, VSR1, whose expression correlates with gametogenesis in both Volvox and Chlamydomonas. Using genome editing we generated a Volvox vsr1 null mutant whose novel phenotype was blockage of spermatogenesis and the conversion of sperm cell precursors to vegetative stem cells. Female vsr1 mutants made sterile egg-like cells that also reverted to vegetative stem cells. Similarly, a Chlamydomonas vsr1 null mutant was sterile in both mating types. We tested and validated a simple model for mating type or sexual differentiation where Vsr1 is required for female or plus gametogenesis when expressed on its own, but becomes a male or minus differentiation factor when co-expressed with Mid. Our data fill a major gap in understanding green algal sex determination and provide a paradigm for understanding the evolution of sexual differentiation in other green lineage systems through interactions between conserved transcription factors.
Ancient, gene-rich sex chromosomes in Ceratodon harbor conserved regulators of sexual development
Sarah B. Carey, University of Florida, United States
* Stuart F. McDaniel, University of Florida
Non-recombining sex chromosomes, like the mammalian Y, often lose genes and accumulate transposable elements, a process termed degeneration. The correlation between suppressed recombination and degeneration is clear in animal XY systems, but the absence of recombination is confounded with other asymmetries between the X and Y. In contrast, UV sex chromosomes, like those found in bryophytes, experience symmetrical population genetic conditions. Here we generate and use nearly gapless female and male chromosome-scale reference genomes of the moss Ceratodon purpureus to test for degeneration in the bryophyte UV sex chromosome system. We show the moss sex chromosomes evolved over 300 million years ago and expanded via two chromosomal fusions. Although the sex chromosomes show signs of weaker purifying selection than autosomes, we find suppressed recombination alone is insufficient to drive gene loss on sex-specific chromosomes. Instead, the U and V sex chromosomes harbor thousands of broadly-expressed genes, including numerous key regulators of sexual development across land plants.
Sex determination system in the liverwort Marchantia polymorpha
* Takayuki Kohchi, Kyoto University, Japan
Yukiko Yasui, Kyoto University
Katsuyuki Yamato, Kindai University
Sexual reproduction is a key process to increase genetic diversity. Land plants have a life cycle with alternation of generations and show sexual differentiation in either (or both) gametophyte (n) and sporophyte (2n) generations. The regulation of sex-specific differentiation has been studied extensively on sporophyte organ development in flowering plants, but our knowledge on sex determination and sex-dependent differentiation in the gametophyte generation is limited. The liverwort Marchantia polymorpha is a haploid dioicous species with sex chromosomes, U for female or V for male. Amongst bryophytes, the liverwort, M. polymorpha is a model liverwort for which genome sequences and molecular genetic tools are available (Yamato et al., 2007; Bowman et al., 2017; Montgomery et al., 2020; Ishizaki et al., 2016), and provides unique opportunities for research on fundamental aspects of reproduction. Spontaneous M. polymorpha segregants having both male and female sex chromosomes develop the female morphology, suggesting that M. polymorpha has a dominant sex-determining “feminizer” gene on the U (Haupt 1932), but its identity was not known. Previously we reported that a molecular toggle switch for sexual differentiation in M. polymorpha is composed of FEMALE GAMETOPHYTE MYB (MpFGMYB), a transcriptional regulator for female differentiation, and SUPRESSIOR OF FEMINIZATION (MpSUF), an antisense long non-coding RNA, at a single autosomal locus (Hisanaga et al., 2019). The alternative expression of MpFGMYB or MpSUF is dependent on the presence or absence of the U chromosome, respectively. In this symposium, we would like to report current progress in the identification of the feminizing factor encoded by the U chromosome in the liverwort M. polymorpha, and genetic relationship between “feminizer” lined to U-chromosome and autosomal MpFGMYB-MpSUF.
Whither the sex chromosome during the evolution of monoicy from ancestral dioicy
* John Bowman, Monash University, Australia
Shilpi Singh, Monash University
Liverworts comprise one of the six primary lineages of land plants, with the predicted origin of the extant diversity of liverworts dating to the Silurian. The ancestral condition of liverworts is dioicy, with monoicy evolving multiple times independently. In cases where it has been investigated, in dioicious liverworts sex is determined chromosomally, with the female possessing a U chromosome and males having a V chromosome, with the discovery of plant sex chromosomes being made by Charles Allen in Sphaerocarpos donnellii more than a century ago. Subsequently, chromosomes presumed to be sex chromosomes have been described in members of all three lineages of extant liverworts, suggesting that the ancestral dioicious liverwort also harboured sex chromosomes. We have been investigating the fate of the sex chromosomes during the evolution of monoicy from dioicy, and we will present data primarily based on our analysis of the Ricciocarpos natans genome.