Abrupt deceleration of molecular evolution linked to the origin of arborescence in ferns

Publication Type:Journal Article
Year of Publication:2010
Authors:P. Korall, Schuettpelz, E., Pryer, K. M.
Start Page:2786
Type of Article:Brief communication
Keywords:Generation time, leptosporangiates, life history, molecular rate heterogeneity, tree ferns.
Full Text

Molecular DNA analysis of the three genome compartments (nuclear, plastid and mitochrondrial) of ferns, has revealed a link between the origin of arborescence in and the rate of molecular evolution. Reference to the phylogenetic tree shows that branch lengths within the ‘tree fern’ clade are substantially shorter than those of neighbouring closely related lineages; heterosporous ferns and the polypods. Approximately 190 mya saw the beginnings of the ‘tree’(arborescent) ferns, coinciding with which was an abrupt deceleration of their molecular evolutionary rate (19% the rate of heterosporous ferns, and 15% that of polypods). It is known that the tree ferns are long-lived, and consequent to this, have an inherently slow rate of molecular evolution. This implies that over any given time period, a smaller number of DNA mutations occur when compared to more short-lived species. The short phylogenetic branch lengths of the tree ferns therefore signify less evolutionary change.
Studies have shown that the molecular evolutionary rates of arborescent species are markedly lower than their herbaceous counterparts. Results of this study strongly suggest that there is an inverse relationship between molecular evolutionary rate and generation time. As one goes up, the other goes down. It is perplexing why this should be so, and further work will hopefully highlight contributing metabolic processes.
One hypothesis is related to the alternations of generations seen in land plants. Arborescent species are known to have an exceptionally long sporophytic stage, whereas the gametophytic stage is comparable to herbaceous plants. It is thought that perhaps these two developmental stages are differentially susceptible to mutational change. If, for example, the gametophyte was more prone to mutation than the sporophyte, then arborescent taxa such as tree ferns, could conceivably display slowed rates of molecular evolution. (Korall et al, 2010)

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Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith