Speciation is the process leading to the emergence of new species. While being usually progressive, it can sometimes be fast with rapid emergence of reproductive barriers leading to high level of reproductive isolation. Some reproductive barriers might leave signatures in the genome, through elevated level of genetic differentiation at specific loci. Similar signatures might also be the results of linked selection acting in low recombination regions. Nottingham catchfly (Silene nutans) is a Caryophyllaceae species composed of four genetically differentiated lineages for which strong and asymmetric levels of reproductive isolation have been identified. Using population transcriptomic data from several individuals of the four lineages, we inferred the best evo-demographic scenario leading to the current reproductive isolation of these four lineages. We also tested whether loci exhibiting high level of genetic differentiation represented barrier loci or were located in low recombination regions, evolving under strong influence of linked selection. Overall, the four lineages of S. nutans have diverged in strict isolation, likely during the different glacial period, through migration in distinct glacial refugia. Speciation between these four lineages appeared to be particularly fast, likely due to fast evolving plastid genome accelerating plastid-nuclear co-evolution and the probability of plastid-nuclear incompatibilities in inter-lineage hybrids.

Distyly, an example of convergent evolution, is governed by a supergene, the S-locus, in several species. Recent studies highlight similar genomic architectures of independently evolved S-loci, but its mode of origin and whether similar regulatory pathways underlie the convergent evolution of distyly remains unclear. We examined the evolution of supergenes and mechanisms underlying distyly in Linum species that diverged c. 33 million years ago (Ma). Using haplotype-resolved genomes and population genomics, we identified and characterized the S-loci of Linum perenne (distylous) and Linum grandiflorum (style length dimorphic), and compared them to that of Linum tenue (distylous). We then tested for a conserved hormonal mechanism regulating style length polymorphism in Linum. The S-locus supergene was consistently hemizygous in short-styled individuals across all three species, although it showed variation in size, gene content, repeat elements and extent of recombination suppression. Two S-linked candidate genes, TSS1 (style length) and WDR-44 (anther height/pollen self-incompatibility), were conserved. Consistent with a brassinosteroid-dependent role of TSS1, epibrassinolide treatment revealed a conserved, morph-specific effect on style length. S-locus structural polymorphism, candidate distyly genes and mechanisms regulating style length remain conserved > 30 Ma in Linum. In combination with findings from other systems, our results suggest that the brassinosteroid pathway frequently contributes to style length polymorphism.

Distyly is an iconic floral polymorphism governed by a supergene, which promotes efficient pollen transfer and outcrossing through reciprocal differences in the position of sexual organs in flowers, often coupled with heteromorphic self-incompatibility. Distyly has evolved convergently in multiple flowering plant lineages, but has also broken down repeatedly, often resulting in homostylous, self-compatible populations with elevated rates of self-fertilization. Here, we aimed to study the genetic causes and genomic consequences of the shift to homostyly in Linum trigynum, which is closely related to distylous Linum tenue. Building on a high-quality genome assembly, we show that L. trigynum harbors a genomic region homologous to the dominant haplotype of the distyly supergene conferring long stamens and short styles in L. tenue, suggesting that loss of distyly first occurred in a short-styled individual. In contrast to homostylous Primula and Fagopyrum, L. trigynum harbors no fixed loss-of-function mutations in coding sequences of S-linked distyly candidate genes. Instead, floral gene expression analyses and controlled crosses suggest that mutations downregulating the S-linked LtWDR-44 candidate gene for male self-incompatibility and/or anther height could underlie homostyly and self-compatibility in L. trigynum. Population genomic analyses of 224 whole-genome sequences further demonstrate that L. trigynum is highly self-fertilizing, exhibits significantly lower genetic diversity genome-wide, and is experiencing relaxed purifying selection and less frequent positive selection on nonsynonymous mutations relative to L. tenue. Our analyses shed light on the loss of distyly in L. trigynum, and advance our understanding of a common evolutionary transition in flowering plants.

• Background and Aims Organelle genomes are usually maternally inherited in angiosperms. However, biparental inheritance has been observed, especially in hybrids resulting from crosses between divergent genetic lineages. When it concerns the plastid genome, this exceptional mode of inheritance might rescue inter-lineage hybrids suffering from plastid–nuclear incompatibilities. Genetically differentiated lineages of Silene nutans exhibit strong postzygotic isolation owing to plastid–nuclear incompatibilities, highlighted by inter-lineage hybrid chlorosis and mortality. Surviving hybrids can exhibit variegated leaves, which might indicate paternal leakage of the plastid genome. We tested whether the surviving hybrids inherited the paternal plastid genome and survived thanks to paternal leakage.
• Methods We characterized the leaf phenotype (fully green, variegated or white) of 504 surviving inter-lineage hybrids obtained from a reciprocal cross experiment among populations of four genetic lineages (W1, W2, W3 and E1) of S. nutans from Western Europe and genotyped 560 leaf samples (both green and white leaves for variegated hybrids) using six lineage-specific plastid single nucleotide polymorphisms.
• Key Results A high proportion of the surviving hybrids (≤98 %) inherited the paternal plastid genome, indicating paternal leakage. The level of paternal leakage depended on cross type and cross direction. The E1 and W2 lineages as maternal lineages led to the highest hybrid mortality and to the highest paternal leakage from W1 and W3 lineages in the few surviving hybrids. This was consistent with E1 and W2 lineages, which contained the most divergent plastid genomes. When W3 was the mother, more hybrids survived, and no paternal leakage was detected.
• Conclusions By providing a plastid genome potentially more compatible with the hybrid nuclear background, paternal leakage has the potential to rescue inter-lineage hybrids from plastid–nuclear incompatibilities. This phenomenon might slow down the speciation process, provided hybrid survival and reproduction can occur in the wild.

The Plasmodium falciparum life cycle includes obligate transition between a human and mosquito host. Gametocytes are responsible for transmission from the human to the mosquito vector where gamete fusion followed by meiosis occurs. To elucidate how male and female gametocytes differentiate in the absence of sex chromosomes, we perform FACS-based cell enrichment of a P. falciparum gametocyte reporter line followed by single-cell RNA-seq. In our analyses we define the transcriptional programs and predict candidate driver genes underlying male and female development, including genes from the ApiAP2 family of transcription factors. A motif-driven, gene regulatory network analysis indicates that AP2-G5 specifically modulates male development. Additionally, genes linked to the inner membrane complex, involved in morphological changes, are uniquely expressed in the female lineage. The transcriptional programs of male and female development detailed herein allow for further exploration of the evolution of sex in eukaryotes and provide targets for future development of transmission blocking therapies.