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ECOLOGY & EVOLUTION
OF PLANT SEXUAL SYSTEMS

Welcome to the website of John Pannell's lab! Our group works on plant evolution in the Department of Ecology and Evolution of the University of Lausanne (Switzerland)

About Us

"There are several groups of plants in which all the species are diœcious, and these exhibit no rudiments in the one sex of the organs proper to the other. About the origin of such plants nothing is known."

This quote by Charles Darwin from his book "The Different Forms Of The Same Species" (1877) is surprisingly still relevant today. Why dioecy in plants is rare and why species that are dioecious are so are questions that have not yet been fully resolved.

In the wake of these old questions, the group is particularly focused on addressing questions concerning transitions between sexual systems, and their implications for mating, resource allocation, demography and the evolution of the genome and transcriptome, especially sex chromosomes. Some of these are summarised under the project descriptions below. These projects use a combination of theory, wet-lab work, bioinformatics, and fieldwork. See our recent publications for further details of results from these and other projects.

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Transitions between combined and separate sexes in Mercurialis annua

Much of our research aims to explain and understand transitions between combined and separate sexes, and the evolution of sex chromosomes, in the European annual plant Mercurialis annua, which displays dioecy (fully separate sexes), monoecy (self-fertile functional hermaphroditism) and androdioecy (the coexistence of males and hermaphrodites) and male sterility. M. annua has an assembled and annotated genome and shows many of the hallmarks of Y-chromosome degeneration. Interestingly, male-function loci regulating pollen production are not located on the Y chromosome, but the secondary sexual dimorphism influencing inflorescence architecture is Y-linked and has introgressed among related species, likely due to strong frequency-dependent selection.

Related projects are examining phenotypic changes in sex allocation, changes in gene expression, and the underlying genomic basis of these changes in experimental populations of M. annua in which mate availability has been manipulated.

Introgression & Polyploidy

Phylogenetic reconstruction of the genus Mercurialis on the basis of variation at a large number of loci at autosomal and sex-linked loci reveals a complex history of hybridization and genome duplication, transitions between sexual systems and the introgression of a Y chromosome into androdioecious populations of M. annua (in which males co-occur with hermaphrodites) from a more distantly related (perennial) species.

Image: Jörn Gerchen and Paris Veltsos.

Evolution Experiment

In a key ongoing experiment, the removal of males from dioecious populations has led to the evolution over several generations of substantial male expression by females and thus a rapid transition from dioecy to monoecy. The figure shows the rapid evolution of sex allocation in 'leaky' females of the dioecious plant Mercurialis annua after the experimental removal of males from three populations (blue lines) compared with those in populations in which males were maintained (red lines). Patterns measured in the field experimental populations were similar to those measured in a common garden in which all genotypes from all generations (G0 – G4) were grown together under the same conditions. Females evolving in the absence of males dramatically increased their male allocation.

Graphs from Cossard et al. (2020, Current Biology).

Evolution of sexual dimorphism

Although plants do not usually show morphological and other differences between males and females as strikingly as do animals, sexual dimorphism in some plants can be strong. We are interested in the ecological contexts in which different strategies are selected in males and females of dioecious plants, and the way that natural and sexual selection modify their genomes and the transcriptomes. Here, we are using as models both the striking variation among sexual systems in M. annua as well as the variation among species in terms of sexual dimorphism in the South African genus Leucadendron. We recently showed that genes with sex-biased expression among 10 species of the genus sampled widely across the phylogeny have elevated rates of expression evolution. While this might suggest a role of sexual selection in driving expression evolution, our analysis indicates that in fact the sex-biased genes had been evolving more rapidly than average before they had become sex-biased, suggesting that sex-biased genes might be drawn from a class of genes that are less constrained in their expression patterns.

Inferring paths of sexual-system evolution in the context of genome duplication

We are interested in the affects that demographic processes such as metapopulation dynamics, population-size fluctuations, and range expansions have of the evolution of the mating system, sex allocation and the adaptive potential of populations. Our research here uses both demographic and population-genetic simulations, as well as bioinformatic analysis of NGS gene-capture data. We have been using Approximate Bayesian Computation to understand the order of key that demographic events, hybridization and whole-genome duplication have played in the diversification of plant lineages.

We have also used transcriptome data to infer the parental origin of the different homeologous genomes of the allopolyploid dioecious species Mercurialis canariensis and the fate of the different genomes in terms of sex-biased gene expression.

Diagrams depicting four of the many possible scenarios underlying genome duplication via autotetraploidy and subsequent diploidisation of the genome (the cessation of gene flow between homoleogous genomes). Tube width reflects population size. Approximate Bayesian Computation is being used to discriminate among these and other scenarios.

Image: Camille Roux.

The variation in the topology of 3135 gene trees for in the inferred phylogeny linking Ricinus communis, Mercurialis annua, and the two subgenomes of M. canariensis. The graph indicates that genes in M. can1 tend to be more closely related to M. annua (and were derived from a more recent ancestor of the two lineages) than those in M. can2 (which have a more ancient common ancestor). The Y chromosome of M. canariensis is derived from M. annua.

Image: Melissa Toups.

Do not hesitate to contact us!

If you are interested in learning more about our research, or if you have any questions or comments, please don't hesitate to contact us. We look forward to hearing from you!

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