Dr. Guillaume Cossard

Ph.D project

The evolution of sexual systems in plants has puzzled scientists for decades, and Darwin himself has written three major books on plant reproduction. My phD concerns this exact same topic and address the question of sexual system transitions with the help of new technologies that permit us to have a look at the causes and consequences of shifts in sexual systems at the genomic level.

Ecological factors have first been highlighted and investigated to try to understand to maintenance of the great diversity of sexual system in angiosperms. With the recent understanding of the prevalence of whole genome duplication in the evolutionary history of plants, a new area of causes has been opened to try to understand their evolution. The model species I am studying, Mercurialis annua, and in a broader sense the whole Mercurialis genus, present a diversity of sexual systems and ploidy level that are entangled. Indeed, diploid plants, that are found in most parts of Europe are dioecious (separate male and female individuals), tetraploids found on the Atlantic coast of South Morocco (below Fes) are monoecious (combined male and female flowers on the same individual) and hexaploids, presenting a coastal distribution in Spain and Northern Africa, are androdioecious (coexistence of monoecious and males). Higher ploidy levels (up to dodecaploids) are even present in Corsica and Sardinia.

The aim of my PhD is then to further unravel the ecological and genetic factors responsible for this diversity and to address to what extent each factor plays a role in shaping such a complex and amazing system.