Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae).

Numerous island-inhabiting species of predominantly herbaceous angiosperm genera are woody shrubs or trees. Such "insular woodiness" is strongly manifested in the genus Echium, in which the continental species of circummediterranean distribution are herbaceous, whereas endemic species of islands along the Atlantic coast of north Africa are woody perennial shrubs. The history of 37 Echium species was traced with 70 kb of noncoding DNA determined from both chloroplast and nuclear genomes. In all, 239 polymorphic positions with 137 informative sites, in addition to 27 informative indels, were found. Island-dwelling Echium species are shown to descend from herbaceous continental ancestors via a single island colonization event that occurred < 20 million years ago. Founding colonization appears to have taken place on the Canary Islands, from which the Madeira and Cape Verde archipelagos were invaded. Colonization of island habitats correlates with a recent origin of perennial woodiness from herbaceous habit and was furthermore accompanied by intense speciation, which brought forth remarkable diversity of forms among contemporary island endemics. We argue that the origin of insular woodiness involved response to counter-selection of inbreeding depression in founding island colonies.

Non-coding chloroplast DNA for plant molecular systematics at the infrageneric level.

With primers constructed against highly conserved regions of tRNA genes (trnTUGU, trnLUAA and trnFGAA) in chloroplast DNA, we have amplified two different non-coding spacers and one intron from four species within the genus Echium L. (Boraginaceae) and from two confamilial outgroups. The trnTUGU-trnLUAA intergenic spacer contains a greater number of polymorphic sites than the trnLUAA intron or the trnLUAA-trnFGAA intergenic spacer. We analyzed a total of 11 kb of sequence data from this non-coding DNA. Total nucleotide divergence between Echium species is on the order of 1% for these regions, all of which possess infrageneric length polymorphisms. The latter two regions contain indels which occur only in the 14 Macaronesian Island endemic species of Echium studied and suggest that these may form a monophyletic group.