Format: ORAL

Armel Salmon1, Lin-Feng Li2, Morgane Milin1, Yan Hao2, Armand Cav-Radet1, Delphine Giraud1, Oscar Lima1,Virginie Huteau3, Olivier Coriton3, JM Aury4, P. Wincker4, Bo Li2, Malika Ainouche1

1 Université de Rennes, UMR CNRS 6553, Ecosystèmes-Biodiversité-Evolution, OSUR, Campus de Beaulieu, Bâtiment 14A, 35042 Rennes cedex, France 2 Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Center for Evolutionary Biology, Fudan University, Shanghai, China 3 Plateforme de cytogénétique moléculaire végétale, Institut de Génétique, Environnement et Protection des Plantes (IGEPP), INRA Domaine de la Motte, BP 35327, 35653 Le Rheu cedex 4 Genoscope, Institut de biologie François-Jacob, Commissariat à l’Energie Atomique CEA, Université Paris-Saclay, Evry, France

Polyploidy and genome evolution are particularly well documented in the grass family (Poaceae), although the accumulated knowledge is concentrated in subfamilies containing important crop species such as rice (Oryzoideae), maize, Sorghum (Panicoideae) or wheat (Pooideae) and related species. In contrast, one of the largest, and highly variable subfamily (notably with regard to base chromosome number and ploidy levels), Chloridoideae, is still underinvestigated. We focused on the large Sporobolus genus where various basic chromosome numbers are recorded (n=7, 8, 9, 10). In this genus, S. sect. Spartina (Schreb.) P.M.Peterson Saarela is a monophyletic clade that represents an iconic model system in evolutionary ecology. It contains several species that are considered as ecosystem engineers on coastal saltmarshes, and some of them are highly invasive, having expanded on different continents. Of particular interest is the textbook example of neopolyploidy involving hybridization in Europe between the native Sporobolus maritimus (presumed hexaploid with 2n=60) and introduced American S. alterniflorus (2n=62) during the 19th century. Genome duplication of the sterile hybrid S. x townsendii (2n=62) resulted in the successful allopolyploid species S. anglicus (2n=124). We conducted comparative analyses involving newly Sporobolus-Spartinaassembled genomes and related grass genomes. We show how two successive whole genome duplication events (WGD1 WGD2) have shaped the mesopolyploid parental genomes (chromosomal repatterning, repetitive sequence dynamics) following the diversification of the Sporobolus- Spartina clade in the last 20 MYs. Descending dysploidy appears to have resulted in a new base chromosome number following WGD1, which allowed us to revisit ploidy levels of the derived species. We also show how chromosomal restructuration accompanied divergence between S. maritimus and S. alterniflorus (2-5 MYA) during the speciation process and resulted in an additional chromosome pair (2n=60+2) in S. alterniflorus, and discuss consequences of these dramatic genome changes on species distribution and ecology.