Which came first?

Modern-day angiosperms occupy just about every corner of our planet. But things were not always this way; early flowering plants are thought to have evolved as woody understorey species growing in warm, wet environments. So how did angiosperms come to be where they are today? More specifically, how did they adapt to living in areas which are prone to freezing?

This is the question that Amy Zanne and colleagues set out to answer in their latest paper:

Zanne et al. (2013) Three keys to the radiation of angiosperms into freezing environments. Nature, doi: 10.1038/nature12872.

Before telling you what I thought of the paper, I’m first going to briefly summarize it (hopefully by then I will have come up with something clever to say…).

Freezing poses a problem to vascular plants because during freeze-thaw cycles air bubbles are formed inside the xylem and these can jam hydraulic pathways. This process, referred to as freezing induced embolism, occurs much more frequently in species that have large conduits. The authors suggest that there are three main ways through which plants can solve the freezing problem: they can (1) develop smaller diameter vessels, (2) interrupt hydraulic transport during freezing periods by becoming deciduous, and (3) become herbaceous. The question is, did plants develop these adaptations in response to the new environmental conditions into which they were venturing, or had these traits evolved previously? Which came first, the trait or the evolutionary pressure?

To answer this question, Zanne and colleagues put together a massive dataset in which they characterize thousands of flowering plants species according to whether or not they are exposed to freezing in their range, deciduous vs. evergreen, large vs. small conduits and on the basis of their growth form (woody vs. herbaceous). They combine this with the newest, most complete molecular phylogeny of flowering plants (you have to check it out!), and set out to test how plants have shifted among these various trait states as they expose themselves to the cold.

As you might expect, angiosperms that are exposed to freezing in their range are either herbaceous, or have remained woody by developing small conduits and/or dropping their leaves. Plants lineages shift relatively frequently between freezing and non-freezing environments. When they do so, they tend to shift from evergreen to deciduous (i.e., deciduousness evolves as a response to temperatures dipping below zero). In contrast, although plants that have moved into colder regions tend to be either herbaceous or have small vessels, both of these traits seem to be pre-requisites to successful colonization (i.e., plants had already evolved as herbaceous or to have small conduits before encountering freezing temperatures). All of this is summarized beautifully in figures 2 and 3 of the paper.

I’ve been waiting to read this paper for the last couple of months (since I saw Amy present the work at INTECOL in London). It’s a really neat study; a smart way of taking full advantage of a global dataset to answer fundamental questions that bridge the gap between ecology and evolution. Most importantly, it makes you think. Here a few questions to which I don’t know the answer:

1)      If small conduits and herbaceous life style did not evolve as a way of coping with freezing, what evolutionary pressure is behind these traits? Drought seems a likely candidate to me (see this paper by Choat et al. which Howard brought up in one of previous our meetings). Many of the adaptations needed for plants to resist freezing (small vessels, herbaceous and even deciduous) are the same that allow plants to cope with high soil water deficits. In order for angiosperms to colonize higher latitudes, did they first have to contend with arid environments?

2)      I was surprised to see how few extant woody species have large conduits, regardless of whether or not they are exposed to freezing (see figure 2 in the paper). The authors define large vessels as anything above 0.044mm in diameter, as this is the threshold “above which freezing-induced embolisms are believed to become frequent at modest tensions”. But of the 860 species for which vessel diameter data was available, only 2% fall within the large conduit category. Plants are much more likely to transition from large to small vessels than vice versa (regardless of freezing/non-freezing). Why is this? Would changing the vessel diameter threshold affect these results?

What do you think?

Tommaso

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