Temporal and palaeoclimatic context of the evolution of insular woodiness in the Canary Islands.

Insular woodiness (IW), referring to the evolutionary transition from herbaceousness toward woodiness on islands, has arisen more than 30 times on the Canary Islands (Atlantic Ocean). One of the IW hypotheses suggests that drought has been a major driver of wood formation, but we do not know in which palaeoclimatic conditions the insular woody lineages originated. Therefore, we provided an updated review on the presence of IW on the Canaries, reviewed the palaeoclimate, and estimated the timing of origin of woodiness of 24 insular woody lineages that represent a large majority of the insular woody species diversity on the Canaries. Our single, broad-scale dating analysis shows that woodiness in 60%-65% of the insular woody lineages studied originated within the last 3.2 Myr, during which Mediterranean seasonality (yearly summer droughts) became established on the Canaries. Consequently, our results are consistent with palaeoclimatic aridification as a potential driver of woodiness in a considerable proportion of the insular woody Canary Island lineages. However, the observed pattern between insular woodiness and palaeodrought during the last couple of million years could potentially have emerged as a result of the typically young age of the native insular flora, characterized by a high turnover.

Plastid Phylogenomics of Dendroseris (Cichorieae; Asteraceae): Insights Into Structural Organization and Molecular Evolution of an Endemic Lineage From the Juan Fernández Islands.

Dendroseris D. Don comprises 11 species endemic to the Juan Fernández islands in Chile. They demonstrate spectacular and unusual growth forms of rosette trees with extremely variable morphology and occupy wide ecological ranges on the islands. These unique plants are now highly threatened with extinction with very small population sizes, typically consisting of 10 or fewer individuals in wild. Despite morphological and ecological divergence among species of Dendroseris, their monophyly has been supported in previous studies, but with little resolution among subgeneric groups. We assembled seven complete plastome sequences from seven species of Dendroseris, including representatives from three subgenera, and carried out comparative phylogenomic analyses. The plastomes are highly conserved in gene content and order, with size ranging from 152,199 to 152,619 bp and containing 130 genes (87 coding genes, 6 rRNA genes, and 37 tRNA genes). Plastid phylogenomic analyses based on both the complete plastome sequences and 81 concatenated coding genes only show Dendroseris nested within Sonchus sensu lato, and also that inter-subgeneric relationships are fully resolved. Subg. Phoenicoseris is resolved as sister to the remaining species of the genus and a sister relationship between the two subgenera Dendroseris and Rea. Ten mutation hotspots from LSC and SSC regions and variable SSRs are identified as potential chloroplast markers for future phylogenetic and phylogeographic studies of Sonchus and related groups.

Parallel evolution of arborescent carrots (Daucus) in Macaronesia.

PREMISE: Despite intensive research, the pathways and driving forces behind the evolution of derived woodiness on oceanic islands remain obscure. The genus Daucus comprises mostly herbs (therophytes, hemicryptophytes) with few rosette treelets (chamaephytes) endemic to various Macaronesian archipelagos, suggesting their independent evolution. To elucidate the evolutionary pathways to derived woodiness, we examined phylogenetic relationships and the habit and secondary xylem evolution in Daucus and related taxa. METHODS: Sixty taxa were surveyed for molecular markers, life history, and habit traits. Twenty-one species were considered for wood anatomical characters. A dated phylogeny was estimated using Bayesian methods. The evolution of selected traits was reconstructed using parsimony and maximum likelihood. RESULTS: Daucus dispersed independently to the Canary Islands (and subsequently to Madeira), Cape Verde, and the Azores in the late Miocene and Pleistocene. Life span, reproductive strategy, and life form were highly homoplastic; the ancestor of Daucus was probably a monocarpic, biennial hemicryptophyte. Rosette treelets evolved independently in the Canarian-Madeiran lineage and in Cape Verde, the latter within the last 0.13 Myr. Treelets and hemicryptophytes did not differ in wood anatomy. Pervasive axial parenchyma in wood occurred more often in polycarpic rather than monocarpic species. CONCLUSIONS: Life span and life form in Daucus are evolutionarily labile and may change independently of wood anatomy, which is related to plant reproductive strategy rather than to life form. Insular woodiness may evolve rapidly (as demonstrated in D. bischoffii), and in Daucus, it does not seem to be an adaptation to lower the risk of xylem embolism.

Island woodiness underpins accelerated disparification in plant radiations.

The evolution of secondary (insular) woodiness and the rapid disparification of plant growth forms associated with island radiations show intriguing parallels between oceanic islands and tropical alpine sky islands. However, the evolutionary significance of these phenomena remains poorly understood and the focus of debate. We explore the evolutionary dynamics of species diversification and trait disparification across evolutionary radiations in contrasting island systems compared with their nonisland relatives. We estimate rates of species diversification, growth form evolution and phenotypic space saturation for the classical oceanic island plant radiations - the Hawaiian silverswords and Macaronesian Echium - and the well-studied sky island radiations of Lupinus and Hypericum in the Andes. We show that secondary woodiness is associated with dispersal to islands and with accelerated rates of species diversification, accelerated disparification of plant growth forms and occupancy of greater phenotypic trait space for island clades than their nonisland relatives, on both oceanic and sky islands. We conclude that secondary woodiness is a prerequisite that could act as a key innovation, manifest as the potential to occupy greater trait space, for plant radiations on island systems in general, further emphasizing the importance of combinations of clade-specific traits and ecological opportunities in driving adaptive radiations.

Evolutionary Comparison of the Chloroplast Genome in the Woody Sonchus Alliance (Asteraceae) on the Canary Islands.

The woody Sonchus alliance consists primarily of woody species of the genus Sonchus (subgenus Dendrosonchus; family Asteraceae). Most members of the alliance are endemic to the oceanic archipelagos in the phytogeographic region of Macaronesia. They display extensive morphological, ecological, and anatomical diversity, likely caused by the diverse habitats on islands and rapid adaptive radiation. As a premier example of adaptive radiation and insular woodiness of species endemic to oceanic islands, the alliance has been the subject of intensive evolutionary studies. While phylogenetic studies suggested that it is monophyletic and its major lineages radiated rapidly early in the evolutionary history of this group, genetic mechanisms of speciation and genomic evolution within the alliance remain to be investigated. We first attempted to address chloroplast (cp) genome evolution by conducting comparative genomic analysis of three representative endemic species (Sonchus acaulis, Sonchus canariensis, and Sonchus webbii) from the Canary Islands. Despite extensive morphological, anatomical, and ecological differences among them, their cp genomes were highly conserved in gene order and content, ranging from 152,071 to 152,194 bp in total length. The number of repeat variations and six highly variable regions were identified as valuable molecular markers. Phylogenetic analysis of 32 species in the family Asteraceae revealed the phylogenetic position of the woody Sonchus alliance within the tribe Cichorieae and the sister relationship between the weedy Sonchus oleraceus and the alliance.