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Hornworts

Cellular structures evolved several times

Munich, 10/31/2012

The hornworts are among the oldest extant land plants and possess striking cellular structures (pyrenoids) that have been regarded as a legacy from algae. Instead, pyrenoids evolved and were lost many times.

Of the three moss lineages, hornworts are the one most closely related to vascular plants. Their plastids (the intracellular organelles in which the light reactions of photosynthesis take place) contain characteristic structures called pyrenoids, which are mainly composed of the enzyme Rubisco. Rubisco plays a key role in photosynthesis, as it is required for the conversion of CO2 into sugars. Given the enzyme’s crucial function, botanists have assumed that pyrenoids must provide some evolutionary advantage, perhaps by enabling carbon fixation when ambient carbon dioxide concentrations are low.

 “Apart from their presence in hornworts, pyrenoids are found only in some algae, and are absent from all other land plants. For this reason, they have been taken to represent an evolutionary holdover from the time when the first terrestrial plants appeared,” says Professor Susanne Renner, a biologist at the LMU and Director of the Botanic Garden in Munich. Together with Juan Carlos Villarreal, a postdoctoral fellow from Panama, she has now taken a closer look at the diversification of the hornworts and the evolutionary history of the pyrenoids.

Lack of correlation with low CO2 levels
The analysis revealed that pyrenoids have actually arisen independently six times in different lineages of hornworts and have also been lost at least as often. In other words, hornwort pyrenoids do not represent an ancient trait that was inherited from alga-like ancestors and retained since the inception of the hornwort phylum.  

The new study, which is based on extensive DNA sequence comparisons, also finds no support for the hypothesis that pyrenoids confer an adaptive advantage when the level of CO2 in the atmosphere is low. “We dated the branching points in the hornwort family tree using the so-called molecular-clock technique and compared the ages of groups with or without pyrenoids with a plot depicting the changes in levels of CO2 over the past 100 million years. There was no correlation between the presence or absence of pyrenoids and the rise and fall in atmospheric CO2 concentration.”

On the basis of these results, Villarreal and Renner conclude that pyrenoids must serve some other function. Moreover, Renner suspects that they must impose an evolutionary cost on the mosses that have them, “otherwise,” she says, “it is hard to understand why they should have been lost so often.” (PNAS 30.10.2012) göd

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