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Genetic architecture and evolution of the S locus supergene in Primula vulgaris

Nature Plants volume 2, Article number: 16188 (2016) Cite this article

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Abstract

Darwin's studies on heterostyly in Primula described two floral morphs, pin and thrum, with reciprocal anther and stigma heights that promote insect-mediated cross-pollination. This key innovation evolved independently in several angiosperm families. Subsequent studies on heterostyly in Primula contributed to the foundation of modern genetic theory and the neo-Darwinian synthesis. The established genetic model for Primula heterostyly involves a diallelic S locus comprising several genes, with rare recombination events that result in self-fertile homostyle flowers with anthers and stigma at the same height. Here we reveal the S locus supergene as a tightly linked cluster of thrum-specific genes that are absent in pins. We show that thrums are hemizygous not heterozygous for the S locus, which suggests that homostyles do not arise by recombination between S locus haplotypes as previously proposed. Duplication of a floral homeotic gene 51.7 million years (Myr) ago, followed by its neofunctionalization, created the current S locus assemblage which led to floral heteromorphy in Primula. Our findings provide new insights into the structure, function and evolution of this archetypal supergene.

Heterostyly evolved independently in at least 28 families of animal-pollinated angiosperms1. In the Primulaceae the majority of species2 produce dimorphic flowers3, a characteristic inherited as a simple Mendelian trait; alleles are defined as S (Short style) and s (long style)4. The two floral forms are known as pin and thrum; thrums behave as heterozygous S/s and pins homozygous s/s5. Classical genetic studies on mutation, linkage and recombination by Bateson and Gregory5, Bridges6, Ernst7,8, De Winton and Haldane9, Darlington10 and others established Primula as an early genetic model, and led to the definition of a co-adapted linkage group of three genes at the S locus, G (Griffel (style) length), P (Pollen size) and A (Antheren (anther) position)7, which control distinct aspects of heteromorphic flower development; this locus defined the archetypal supergene11. Studies of heterostyly in Primula contributed significantly to the foundation of modern genetic theory and the neo-Darwinian synthesis. Supergenes have subsequently been shown to control other multitrait complex phenotypes in plants, animals and fungi12.

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Figure 1: P. vulgaris floral phenotypes and genotypes.
Figure 2: Organization of S locus haplotypes.
Figure 3: Linkage of the S haplotype to the thrum phenotype.
Figure 4: Expression and genomic organization of S locus genes.
Figure 5: Phylogenetic analysis and the date of duplication of GLOT from GLO.

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Acknowledgements

We thank M. Lappage, M. Hughes and P. Wells for horticultural support; colleagues at TGAC for Illumina sequencing; A. Thanki for TGAC Browser support; O. Kent for P. elatior GLO and GLOT sequences; Norfolk Wildlife Trust, Suffolk Wildlife Trust and Norfolk County Council for permission to sample P. veris, P. elatior and P. vulgaris respectively; M. Gage, B. Davies and D. Bowles for comments on the manuscript; W. Wang for advice on k-means analysis; BBSRC for funding via grant BB/H019278/2, and prior awards G11027 and P11021; The Gatsby Foundation for early stage funding; University of Leeds, Durham University and University of East Anglia for support to P.M.G. over several years of the project. CvO was funded by the Earth & Life Systems Alliance (ELSA). P.M.G.'s laboratory is hosted at the John Innes Centre under the UEA-JIC Norwich Research Park collaboration.

Author information

Author notes

  1. Sarah Dyer & Mario Caccamo

    Present address: †Present address: National Institute for Agricultural Botany, Huntingdon Road, Cambridge CB3 0LE, UK,

  2. Jinhong Li and Jonathan M. Cocker: These authors contributed equally to this work.

Authors and Affiliations

  1. School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK

    Jinhong Li, Jonathan M. Cocker, Margaret A. Webster & Philip M. Gilmartin

  2. John Innes Centre, Norwich Research Park, Norwich, NR4 7UH, UK

    Jinhong Li, Jonathan M. Cocker, Margaret A. Webster & Philip M. Gilmartin

  3. The Earlham Institute, Norwich Research Park, Norwich, NR4 7UH, UK

    Jonathan Wright, Mark McMullan, Sarah Dyer, David Swarbreck & Mario Caccamo

  4. School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK

    Cock van Oosterhout

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Contributions

J.L. contributed to project design, performed all molecular analyses, generated the S locus assembly, manually annotated the S locus gene structures and undertook data analysis. J.M.C. carried out bioinformatic analyses, including automated annotation of the S locus region, undertook in silico gene expression and k-means clustering analyses, assembled genome sequences and library scaffolds, generated the molecular phylogeny, undertook recombination analysis of the S locus flanking regions and contributed to project design. J.W. assembled genome sequences and library scaffolds, contributed to genome annotation and generated the automated gene model predictions across the S locus, aligned sequencing reads to the S locus assembly and contributed to project design. M.A.W. contributed the inbred long homostyle line, other genetic resources and classical genetics, identified the short homostyle mutant and generated the three-point cross used to demonstrate linkage. M.M. and C.v.O. contributed to the molecular phylogeny construction, evolutionary data analysis and recombination analysis. S.A., D.S. and M.C. contributed to the genome sequencing strategy, assembly and annotation that underpins this project. P.M.G. conceived, designed and directed the project, contributed to data analysis, prepared the figures and drafted the manuscript, with revision input from C.v.O.; all authors contributed to editing the manuscript.

Corresponding author

Correspondence to Philip M. Gilmartin.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Methods, Supplementary References, Supplementary Figures 1–4, Supplementary Tables 1–6, Supplementary Sequence Analyses 1–3. (PDF 1701 kb)

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Li, J., Cocker, J., Wright, J. et al. Genetic architecture and evolution of the S locus supergene in Primula vulgaris. Nature Plants 2, 16188 (2016). https://doi.org/10.1038/nplants.2016.188

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