Format: ORAL

Jrn F. Gerchen1, John R. Pannell2

1 Department of Botany, Faculty of Science, Charles University, Prague, Czech republic 2 Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland

The annual lineage of the plant genus Mercurialis (Euphorbiaceae) is found in Europe and Northern Africa and shows a remarkable diversity in sexual systems and ploidy levels. While M. annua is diploid and dioecious (separate males and females) across large parts of Europe, hexaploid M. annua, which are found on the Iberian peninsula and in Northern Africa, are monoecious (male and female inflorescences on the same individual) or androdioecious (male and monoecious individuals in the same population). Here we present results concerning the evolutionary history of these polyploid Mercurialis lineages and discuss how it may be related to the variation in sexual systems. We show that polyploid Mercurialis lineages are the result of a complex series of allopolyploidization events. In addition, the presence of common male-specific DNA sequences suggests that maleness is based on an ancestrally shared XY system of sex determination in both dioecious and androdioecious lineages. Surprisingly, phylogenetic trees based on Y-linked sequences suggest that the Y chromosome in androdioecious hexaploid lineages has been introgressed from distantly related perennial lineages instead of more closely related annual lineages, which contributed to its allopolyploid subgenomes. Using additional population genomic datsets we test for further signatures of introgression genome-wide and propose the hypothesis that the strong outcrossing advantage of the male phenotype could have favored such inter-ploidy introgression specifically for the Y chromosome.