Evolution of recombination suppression and sex determination on Y chromosomes of the plant genus Mercurialis

By analyzing high-quality genome assemblies and population data across the *Mercurialis* genus, this study reveals that the *M. annua* Y chromosome evolved through two distinct stages of recombination suppression involving a recent inversion and an older degenerated stratum, while identifying *APRR7* as a conserved candidate master sex-determination gene.

The Gist

The Big Picture: A Plant Family's "Secret Code"

Imagine the plant genus Mercurialis (mercury plants) as a large, extended family. Most of this family used to be "hermaphrodites," meaning every single plant had both male and female parts (like a person having both a left and right hand). But somewhere along the line, some branches of the family evolved to have separate sexes: some plants became strictly male, and others strictly female.

To do this, they developed sex chromosomes (like the X and Y in humans). This paper is a detective story about how the "Y chromosome" (the male version) in one specific plant, Mercurialis annua, changed over time. The scientists wanted to know: How did the male chromosome stop sharing information with the female one, and what happened to the genes inside it?

The Main Discovery: Two Layers of History

The researchers found that the Y chromosome isn't just one big block of secrets. Instead, it's like a Russian nesting doll or a layered cake with two distinct evolutionary "strata" (layers) that happened at different times.

Layer 1: The "Old Basement" (The Older Stratum)

Deep inside the Y chromosome, there is an ancient region that stopped swapping genetic information with the X chromosome a long time ago.

• What happened here? It's like an abandoned attic. Because it stopped communicating with the rest of the genome, it started to fall apart.
• The Evidence: The scientists found that many useful genes here have been lost or broken (like furniture rotting away). It's also filled with "junk" (transposable elements), which are like weeds taking over a garden that no one is weeding anymore.
• The Result: This area is very different between males and females. It's old, messy, and full of genetic debris.

Layer 2: The "New Attic" (The Younger Stratum)

Surrounding that old, messy basement is a brand new layer. This layer was created recently by a massive chromosomal inversion.

• The Analogy: Imagine you have a bookshelf with books in order. Suddenly, someone grabs a huge chunk of the shelf, flips it upside down, and puts it back. That's an inversion.
• What happened here? Because this chunk was flipped, the male and female chromosomes can no longer "read" each other in this section. They can't swap pages anymore.
• The Evidence: Unlike the old basement, this new area is still very clean. The genes are mostly intact, and there isn't much "junk" yet. It's like a newly locked room that hasn't had time to get messy.
• The Conclusion: The plant didn't lose its ability to swap genes all at once. First, the old basement stopped swapping. Then, much later, a giant flip (inversion) locked up a huge new section around it.

The "Master Switch": Who Decides the Sex?

Every sex chromosome system needs a "Master Switch"—a specific gene that tells the plant, "You are a boy."

The scientists hunted through the genome to find this switch. They found a gene called APRR7.

• Why is it special? In every single species of Mercurialis they looked at (from the diploid ones to the complex six-set ones), this gene was always present in males and missing (or different) in females.
• The Metaphor: Think of APRR7 as the "On/Off" button for the male factory. If the plant has this specific version of the button, it builds male flowers. If it doesn't, it builds female flowers.
• The Twist: This gene usually helps plants tell time (it's part of their internal clock). The scientists think that at some point, a copy of this clock gene got duplicated, moved to the Y chromosome, and accidentally became the boss of sex determination.

Why This Matters: A Tale of Different Families

The researchers didn't just look at one plant; they looked at the whole family tree. They found that different species of Mercurialis are at different stages of this evolution:

• Diploid M. annua: Has the clear "two-layer" system we just described.
• Other species: Some have expanded the "locked" region even further (like adding more rooms to the secret basement), while others haven't changed much at all.

This shows that evolution isn't a straight line. Different species take different paths to solve the same problem: how to keep male and female traits separate.

Summary in a Nutshell

1. The Problem: Sex chromosomes need to stop swapping genes to maintain separate sexes, but we didn't know exactly how this happens in plants.
2. The Discovery: In Mercurialis annua, the male chromosome evolved in two steps. First, an ancient core stopped swapping genes and started to rot (the old stratum). Later, a giant flip (inversion) locked up a fresh, clean area around it (the new stratum).
3. The Hero: They identified APRR7 as the likely "Master Switch" gene that determines maleness across the entire plant genus.
4. The Takeaway: Sex chromosomes are like evolving cities. Some parts are ancient, crumbling ruins, while other parts are brand new construction sites. And in this plant family, a gene originally used for telling time accidentally became the boss of gender.

Technical Summary

1. Problem Statement

The evolution of sex chromosomes is characterized by the suppression of recombination, subsequent genetic degeneration (gene loss), and the accumulation of repetitive elements. While these patterns are well-documented in animals, the mechanisms driving the step-wise expansion of non-recombining regions (evolutionary strata) and the specific causes of recombination suppression remain unclear in plants.

• Specific Gap: Previous studies on the dioecious annual plant Mercurialis annua estimated the size of the male-specific non-recombining region (NRR) using linkage maps and BAC sequences, suggesting a size of 14.5–19 Mb. However, these methods lacked the resolution of chromosome-level assemblies, potentially overestimating the region's size and gene content.
• Key Questions: How many evolutionary strata exist on the M. annua Y chromosome? What is the precise physical structure and gene content of the sex-determining region (SDR)? What is the mechanism of recombination suppression, and is there a conserved master sex-determination gene across the genus?

2. Methodology

The authors employed a multi-faceted genomic approach combining de novo assembly, comparative genomics, and population genetics.

• Genome Assembly:
  • Generated two new high-quality, long-read genome assemblies using Oxford Nanopore technology: one for an XX female and one for a YY male (derived from a self-fertilized XY male treated with cytokinin).
  • Integrated Hi-C scaffolding data to achieve chromosome-level resolution.
  • Compared these against a previously published PacBio HiFi assembly of an unrelated female (XXdtol).
  • Used a rigorous pipeline involving Canu (assembly), Racon/HyPo (polishing), purge_dups (haplotype removal), and Braker3 (annotation).
• Synteny and Structural Analysis:
  • Used GENESPACE and OrthoFinder to assess synteny and orthology across the three assemblies and 12 other angiosperm reference genomes.
  • Identified large structural variations, specifically inversions, by comparing gene order and orientation.
• Population Genomics:
  • Analyzed exon-capture data from 20 male and 20 female diploid M. annua individuals to calculate $F_{ST}$ (differentiation) between sexes.
  • Extended analysis to other Mercurialis species (diploid M. huetii, tetraploid M. canariensis, hexaploid M. annua, and perennial species) using $F_{ST}$ or $F'_{ST}$ (for allopolyploids) to assess the conservation of the SDR.
• Linkage Mapping:
  • Constructed new male and female linkage maps using RNA-seq data and a new ddRAD-seq dataset from an interspecific cross (M. annua male × M. huetii female) to validate recombination suppression patterns.
• Gene Curation:
  • Manually curated gene models in the SDR using Apollo, integrating RNA-seq evidence and checking for transposable elements (TEs) misannotated as genes.
  • Calculated synonymous ($d_S$) and non-synonymous ($d_N$) substitution rates to estimate the age of strata.

3. Key Contributions

• High-Resolution Assembly: Provided the first chromosome-level assemblies of both XX and YY M. annua individuals, allowing for precise physical mapping of the SDR.
• Strata Identification: Definitively identified two discrete, nested evolutionary strata on the Y chromosome, resolving previous ambiguities about the region's structure.
• Mechanism Elucidation: Demonstrated that the expansion of the non-recombining region occurred in two distinct steps: an ancient event creating an "old stratum," followed by a recent large inversion creating a "young stratum."
• Candidate Gene Discovery: Identified APRR7 as the only gene showing consistent male-specific inheritance across the entire genus, proposing it as the master sex-determination gene.

4. Key Results

A. Structure of the Sex-Determining Region (SDR)

• Two Evolutionary Strata: The SDR on Chromosome 1 consists of:
  1. Young Stratum: Generated by a massive ~10.9 Mb inversion on the Y chromosome (compared to ~4.8 Mb on the X). This region shows little genetic degeneration, low synonymous divergence ($d_S \approx 0.01$), and minimal TE accumulation. It contains 387 genes present on both X and Y.
  2. Old Stratum: Nested within the inverted region. It exhibits substantial degeneration, including the loss of 22 genes (present on X, absent on Y), massive accumulation of transposable elements (increasing Y size by ~6 Mb), and high synonymous divergence ($d_S \approx 0.147$ for retained genes).
• Recombination Suppression: Linkage maps confirmed that recombination is suppressed across the entire inverted region in males. The "old stratum" represents the ancestral non-recombining core, while the inversion expanded this region recently.

B. Gene Content and Degeneration

• Gene Loss: The Y chromosome lost 22 genes in the old stratum. Notably, the gene DTM1 (Defective Tapetum and Meiocytes 1), essential for male fertility in rice, is missing from the Y but present on the X. The authors hypothesize that the loss of DTM1 on the Y contributes to the sterility observed in YY males.
• Transposable Elements: The old stratum is enriched with TEs, suggesting a feedback loop where TE accumulation promotes further recombination suppression and structural variation.
• Sexually Antagonistic Candidates: The new stratum contains tandem duplications of REM16 (Reproductive Meristem 16), a gene involved in flowering time and female-biased expression in other species. The authors suggest sexual antagonism may have driven the expansion of this region via the inversion.

C. Comparative Genomics Across Mercurialis

• Species Variation: The extent of sex-linked differentiation varies significantly:
  • M. huetii (diploid): Shows a much larger region of differentiation than M. annua, suggesting further expansion of the SDR.
  • M. canariensis (tetraploid): Shows differentiation in both old and new strata, consistent with inheritance of the M. annua-like Y chromosome via allopolyploidy.
  • Perennial species and hexaploid M. annua: Show only moderate differentiation, suggesting no substantial recent expansion of the SDR in these lineages.

D. Master Sex-Determination Gene

• APRR7: The gene Arabidopsis Pseudo Response Regulator 7 (APRR7) was identified as the sole candidate with consistent male-specific inheritance across all studied species (diploid, tetraploid, and hexaploid).
• Function: APRR7 is part of the circadian clock and flowering time regulation. The authors propose that an ancestral duplication of an autosomal APRR7 copy (on Chromosome 2) neofunctionalized to become the dominant sex-determining locus, driving the evolution of dioecy in the genus.

5. Significance

• Model for Plant Sex Chromosome Evolution: This study provides a rare, high-resolution view of "young" sex chromosomes in plants, capturing the transition from a recombining region to a degenerated stratum. It confirms the "step-wise" expansion model (Lahn & Page) in a plant system.
• Decoupling Cause and Effect: By identifying a recent inversion that created a young, non-degenerated stratum, the study suggests that recombination suppression can evolve rapidly via structural variation (inversions) before significant genetic degeneration occurs.
• Single-Gene Sex Determination: The identification of APRR7 supports the hypothesis that plant sex determination can evolve via a single dominant gene (rather than the two-locus model of male/female sterility mutations), challenging the canonical view of plant sex evolution.
• Methodological Benchmark: The integration of long-read assemblies with population genomics and manual curation sets a new standard for characterizing complex, repetitive sex-determining regions in non-model organisms.