Systematic and genomic studies in the genus Aubrieta (Brassicaceae)

The study focuses on the herbaceous perennial plant genus Aubrieta (Brassicaceae). Distributed from Armenia through the Levant and Anatolia to Greece, the Balkans and Italy, the species have proved difficult to distinguish. About 12 species are currently recognised, although estimates range from only one up to about 20 or more. Furthermore, their evolutionary relationships are unknown. In order to remedy this situation molecular and cytogenetical studies were conducted. Next Generation Sequencing (NGS) data were produced to generate a complete chloroplast genome for four species of Aubrieta in order to confirm the phylogenetic position of the genus in the family. Earlier suggestions that it belongs in tribe Arabdieae were confirmed. Details of plastome structure were analysed in A. gracilis, which was shown to have 88 protein-encoding, 37 transfer-RNA and eight ribosomal-RNA genes. A phylogenetic analysis of the genus was conducted based on chloroplast and nuclear sequences as well as mitochondrial RFLPs. Five chloroplast regions (matK, trnD-trnT, ycf6-psbM, rps11-rpl36 and trnH-psbA) and two nuclear genes (duo1 and rbp2) were amplified and sequenced successfully. Six mitochondrial gene regions (Orf114, Nad9-ccmFN2, Orf25, matR, ccm FC, trnK-rps3) were studied by means of restriction enzymes. Data analyses show that Aubrieta comprises the annual, pan-Mediterranean Arabis verna plus perennial taxa that fall into one of five geographically delimited gene pools: i) Near East (Levant, Iraq and Iran); ii) Anatolia; iii) Aegean Basin; iv) Greece, Albania, Bulgaria (Pindus Mts and associated ranges); and v) Trans-Adriatic Sea and Sicily. There was some disagreement between the plastid and nuclear trees, which was attributed to hybridisation, chiefly affecting the taxa occupying the Aegean Basin. Evolution in the genus appears to have proceeded largely at the diploid level (2n=2x=16). In order to see what changes at the chromosome level have accompanied speciation, fluorescence in situ hybridization (FISH) studies were conducted. The probes pTa71 and pTa794 were used to locate the position of 45S and 5S rDNA sites on the chromosomes. The number of 45S rDNA sites are 2, 4, 5, or 6, localized on short-arms, the centromere and on long-arms. The number of 5S rDNA sites is a constant two, located either on short-arms or long-arms. These rDNA sites (45S and 5S) are either located on different chromosomes or shared by one or two chromosomes. Speciation is accompanied (promoted?) by translocations and duplications. Hybridisation was confirmed in the genus. The timing of the various bifurcations in the evolutionary tree were estimated from a study of the concatenated chloroplast sequences, but the major split into an annual lineage (Arabis verna) and a perennial lineage appears to date from 1.4 Mya. The hybridisation events involving the Aegean Basin taxa appear to date from the early to mid-Pleistocene (ca 600-800 Kya), a time when considerable parts of the Aegean were above sea level. The taxonomy of the genus is still problematic, it being impossible to diagnose the five geographical genepools by means of morphological characters. Instead, a splitting approach is adopted whereby regional or local phenotypes are recognised as species. This can be unsatisfactory in some cases where there is considerable morphological, but not geographical, overlap. A total of 21 species, including Arabis verna which is recombined into Aubrieta, is recognised.