The allotetraploid invasive weed Bromus hordeaceus L. (Poaceae):: Genetic diversity, origin and molecular evolution

Bromus hordeaceus (section Bromus, Poaceae), a predominantly self-fertilizing tetraploid (2n=28), is an annual weed native to the Mediterranean Basin, which now has a world-wide distribution. High morphological variation led to the recognition of four subspecies, three of which correlated with habitat-type. We examined genetic diversity at enzyme loci in 15 populations from the Mediterranean and the Atlantic region. Although sampled over a larger range of ecological and geographical conditions, the North-African populations appeared less genetically differentiated than populations from Brittany, suggesting higher levels of gene flow among the first ones (Nm = 3.756 and 1.066 respectively). No genetic differentiation was encountered among the four subspecies. The populations were homozygous at homologous loci, suggesting high rates of selfing, but they frequently exhibited fixed intergenomic heterozygosity. The meiotic chromosome behaviour and disomic inheritance encountered are in accordance with the previously proposed allopolyploid origin of the species. The diploids B. arvensis and B. scoparius have been previously implicated in the parentage of B. hordeaceus on the basis of morphology and serology. We compared B. hordeaceus with related diploid species belonging to the same section (section Bromus) using different sources of data (flow cytometry, karyotypes, RAPD and DNA sequences). Molecular phylogeny based on internal transcribed spacer sequences of nuclear ribosomal genes provided the first clear scheme of relationships among monogenomic species of the section. A new hypothesis is proposed concerning the origin of B. hordeaceus: We found that it diverged earlier than all other species of section Bromus excluding the diploid B. caroli-henrici which is basal in this group. The 13 autapomorphies accumulated by B. hordeaceus, and the absence of intra-individual sequence heterogeneity are also consistent with the relatively ancient origin of the species within the section.