Abstract: Angiosperm phylogeny has been investigated extensively using organellar sequences; recent efforts using nuclear genes have also been successful in reconstructing angiosperm phylogenies at family or deeper levels. However, it is not clear whether nuclear genes are also effective in understanding relationships between species in a genus. Here we present a case study of phylogeny at generic and specific levels with nuclear genes, using Brassicaceae taxa as examples. Brassicaceae includes various crops and the model plant Arabidopsis thaliana. A recent study showed that nuclear genes can provide well-resolved relationships between tribes and larger lineages in Brassicaceae, but few species were included in any given genus. We present a phylogeny with multiple species in each of five genera within Brassicaceae for a total of 65 taxa, using three protein-coding nuclear genes, MLH1, SMC2, and MCM5, with up to approximately 10 200 base pairs (in both exons and introns). Maximum likelihood and Bayesian analyses of the separate gene regions and combined data reveal high resolution at various phylogenetic depths. The relationships between genera here were largely congruent with previous results, with further resolution at the species level. Also, we report for the first time the affinity of Cardamine rockii with tribe Camelineae instead of other Cardamine members. In addition, we report sequence divergence at three levels: across angiosperms, among Brassicaceae species, and between Arabidopsis ecotypes. Our results provide a robust species-level phylogeny for a number of Brassicaceae members and support an optimistic perspective on the phylogenetic utility of conserved nuclear data for relatively recent clades.

Key words: Brassica, Brassicaceae, Cardamine, Lepidium, Rorippa, nuclear gene, species-level phylogeny.