Jump to content Jump to navigation

Molecular dating of insular woody lineages

A molecular dating approach of insular woody Canary Island lineages to assess a potential link between wood development and drought stress.

Supervisor and contact

Period and duration

Ca. 6 months, project can start soon

Study and level

MSc project

Background and context

Insular floras often include species with a remarkable degree of woodiness. This so-called insular woodiness was described by Darwin who assumed that at least some woody island species have continental herbaceous ancestors (Fig. 1), an hypothesis that has been supported by more recent molecular analyses. In a recent review paper focusing on insular woodiness in the Canary Islands, at least 220 insular woody species from 38 independent lineages were found that mainly grow in the dry coastal regions [1]. Consequently, there seems to be a correlation between increased woodiness and increased drought at least in some plant groups, suggesting a new hypothesis stating that wood formation and drought stress are linked.

Macintosh HD:Users:fredericlens:My Documents:Leiden:Naturalis:UE:MSc stages:Echium.jpg

Fig. 1: Based on molecular phylogenies, we know that herbaceous Echium species of the European continent have reached the Canary Islands about 3 million years ago, followed by a rapid speciation into ca. 26 woody species. The time of colonization can be linked with past drought events.

Objectives and goals

The Canary Island flora is the only region in Europe with a high diversity of insular woody species [1], and is therefore selected as a case study. At this point, we found evidence for the drought stress hypothesis based on distribution data and based on water transport measures in stems. To provide more compelling evidence for drought stress hypothesis, this MSc proposal wants to find out whether we can link the time of origin of several insular woody groups with past drought events using a single molecular dated phylogeny.

Dating estimates of insular woody groups on the Canary Islands are only known for few genera [2-4]. These age estimates indicate that at least some of the largest insular woody clades have originated 3 million years ago or less (e.g., Convolvulus, Echium (Fig. 1), Sideritis), which makes it possible to accurately correlate wood development in these lineages with major palaeoclimatic drought events, such as the formation of the Sahara (starting 5-6 million years ago), and the onset of cooler and drier periods during the glaciation cycles co-occurring with lower sea levels (during last 2-3 million years).

Material, tasks and approach

Fossils are scarce and often unreliable on the Canaries, which sometimes force authors to use the geological age of specific islands to calibrate nodes in the molecular phylogeny of endemic Canary Island groups [2]. This may lead to inaccurate divergence times, because plant groups can arrive long after islands have originated. To overcome this problem, we propose a broad-scale molecular dating approach at the flowering plant level, enabling us to use credible fossil calibrations outside the Canary Island clades. Using this single analysis strategy, we can also compare the dating estimates of most of the 38 insular woody Canary Island groups simultaneously.

The backbone for this MSc thesis is represented by an available huge aligned dataset using rbcL and matK sequences that are retrieved from GenBank, based on over 30000 angiosperm species and over fifty fossil calibration points [Janssens et al., unpublished]. Due to the size of the dataset, treePL will be used to run the dating analyses in this project. Sequences from DNA samples and leaf material for about 100 species of insular woody subclades and their herbaceous relatives from 31 groups have already been generated. These sequences should be aligned and additional sequences should be generated for groups that are underpresented or missing in the dataset.

Requirements

Experience with evolutionary techniques and sequencing is recommended.

References

1 Lens F, Davin N, Smets E, del Arco M. 2013. Insular woodiness on the Canary Islands: a remarkable case study of parallel evolution. Int J Plant Sci 174: 992-1013.

2 Kim SC, M.R. McGowen, P. Lubinsky, J.C. Barber, M.E. Mort, A. Santos-Guerra. 2008. Timing and tempo of early and successive adaptive radiations in Macaronesia. Plos One 3: e2139.

3 Garcia-Maroto F, Manas-Fernandez A, Garrido-Cardenas, Lopez Alonso D, Guil-Guerrero JL, Guzman B, Vargas P. 2009. Δ6-Desaturase qequene evidence for explosive Pliocene radiations within the adaptive radiation of Macaronesian Echium (Boraginaceae). Mol. Phyl. Evol. 52: 563-574.

4 Carine MA 2005 Spatio-temporal relationships of the Macaronesian endemic flora: a relictual series or window of opportunity? Taxon 54: 895-903.