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Baileyan trends in asterids

Are major evolutionary shifts in wood anatomy triggered by paleoclimatic events? A thorough study of the Baileyan trends in asterids

Dr. Frederic Lens, Dr. Vincent Merckx, Dr. Steven Jansen

frederic.lens@naturalis.nl, vincent.merckx@naturalis.nl, steven.janssens@br.fgov.be

Period & duration
Ca. 6 months, project can start soon

Study and level:
MSc project

Background and context
Baileyan trends in wood anatomy are arguably one of the most common textbook examples of evolutionary patterns in plant anatomy. This set of linear “primitive-to-specialised” transformation series in wood anatomical features found its origin in a very broad comparison of the size of the water conductive cells in woody land plants (Bailey & Tupper, 1918). Later, Bailey’s students hypothesised that long and slender gymnosperm tracheids evolved into long and narrow, angiosperm vessel elements with scalariform perforations including many bars (often > 20). These primitive vessel elements further developed into wider and thus hydraulically more efficient water conducting cells (Carlquist, 1975). This evolutionary scheme was later confirmed by paleobotanical evidence, showing that scalariform perforations were much more abundant in the Cretaceous than in the Tertiary (Wheeler & Baas, 1991).

Illustrations of SEM surfaces

Fig. 1 Illustrations of SEM surfaces (c-d) showing the marked difference between the scalariform vessel perforations of Viburnum (c) and the simple vessel perforations of Sambucus (d). 

Objectives and goals
Angiosperm wood with simple vessel perforations has evolved many times independently from wood with scalariform perforations, but detailed studies to understand why these Baileyan trends in wood evolution have happened are lacking. The major goal of this MSc thesis is to dig deeper into the paleoclimatic triggers that have driven major evolutionary shifts in the wood of asterids, which is one of the main groups of angiosperms. This MSc thesis follows up on a recent paper where we investigated the climatic drivers behind the striking difference in wood anatomy between two closely related asterid genera, Viburnum and Sambucus. We found that the lack of selective pressure for high conductive efficiency during early diversification of Viburnum and the potentially adaptive value of scalariform perforations in frost-prone cold temperate climates have retained the primitive wood syndrome, while higher temperatures during early diversification of Sambucus have triggered the evolution of simple vessel perforations, allowing a more efficient long distance water transport. 

Material, tasks and approach
The Viburnum-Sambucus group represents only one of the 10 major Baileyan shifts within asterids. To interpret the Baileyan trends in a wider framework, we propose a broad-scale molecular dating approach at the flowering plant level, enabling us to use credible fossil calibrations. Using this single analysis strategy, we can compare the dating estimates of all the 10 Baileyan shifts simultaneously, and link these dates to specific paleoclimatic conditions. 

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. It is expected that a small number of new sequences needs to be generated and added to the database in order to include all the major shifts.

Experience with the software package Geneious is recommended, as well as some background in performing evolutionary reconstruction analyses (Bayesian, Maximum Likelihood).

Bailey IW, Tupper WW. 1918. Size variation in tracheary cells: I. A comparison between the secondary xylems of vascular cryptogams, gymnosperms and angiosperms. Proceedings of the American Academy of Arts and Sciences USA 54: 147-204.

Carlquist S. 1975. Ecological strategies of xylem evolution. Berkeley, USA: University of California Press.

Janssens SB, van der Niet T, Merckx V, BaasP, Aguirre Gutierrez J, Jacobs B, Chacon Dória L, Smets E, Erik Smets, Lens F. Submitted. Baileyan trend in wood anatomy triggered by differences in climate during the evolution of Adoxaceae. New Phytologist

Wheeler EA, Baas P. 1991. A survey of the fossil record for dicotyledonous wood and its significance for evolutionary and ecological wood anatomy. IAWA Journal 312: 275-318.