Origin of Symphyotrichum anticostense (Asteraceae: Astereae), an endemic species of the Gulf of St. Lawrence

This thesis discusses the origin of Symphyotrichum anticostense, a decaploid species restricted to the Gulf of St. Lawrence region (Quebec, New Brunswick, and Maine). Two closely related species, S. novi-belgii, a hexaploid and S. boreale, a putatively tetraploid species have been proposed to be putative parents of S. anticostense. Symphyotrichum novi-belgii is distributed in the eastern coastal region of North America and S. boreale is widespread throughout the northern part of North America from west to east. These three species belong to the genus Symphyotrichum, which comprises ca. 91 species of which more than 50% are polyploids. The evolutionary history of the genus is poorly known even at the diploidy level. Before investigating the origin of S. anticostense, it seems necessary in the first step to elucidate phylogenetic relationships within the genus. The objectives of this thesis are therefore to delimit species boundaries excluding polyploid species and to investigate the origin of S. anticostense. The ITS data set nearly supports previous classifications based on morphological and cytological approaches, while the GAPDH nuclear gene is in poor agreement with the ITS results. Incomplete lineage sorting within Symphyotrichum is shown by stochastic sorting of ancestral polymorphisms among the GAPDH alleles. This evidence, in combination with the insufficient resolution among ribotypes, suggests recent evolutionary radiation of the genus. Univariate and multivariate morphological analyses as well as the ITS results roughly support S. anticostense to be a hybrid derivative of S. novi-belgii and S. boreale. The ITS-based network analysis suggests three independent geographical origins for S. anticostense : (1) Lake St. John; (2) Gaspe Peninsula-New Brunswick-Maine; (3) Anticosti Island. The results of two nuclear genes (GAPDH and MIPS) demonstrate relative incongruence between the phylogenetic trees. Although the results obtained from the latter markers support those obtained from the morphological and ITS data sets, incomplete lineage sorting strongly affected the phylogenetic relationships among species.;Keywords. Asteraceae, Astereae, Symphyotrichum, Symphyotrichum anticostense, polyploidy, hybridization, phylogeny, incongruence, incomplete lineage sorting, radiation.