Paper of the Day: Akiyama & Agren 2013, “Conflicting selection on the timing of germination in a natural population of Arabidopsis thaliana”

Read the abstract of this paper yourself here: Conflicting selection on the timing of germination in a natural population of Arabidopsis thaliana

One-sentence summary:

In this paper, the authors explore how seeds sprouting early versus sprouting late in the season can be good or bad for the plant’s overall fitness (its ability to survive and reproduce) depending on the effect that sprouting time has on the plant’s fitness at different stages in its life cycle.

Background info:

To understand what they’re studying, it is important to know what “conflicting selection” is. First, plain old selection is the process by which more adapted individuals (i.e., those with better fitness) survive and pass on their genetic information to the next generation. Many many instances of natural selection over time cause evolution to occur.

Conflicting selection is a slightly more complicated version of regular selection. Jose Gomez (2004) describes a basic example in which producing bigger acorns can be advantageous for an oak plant to make because they contain more nutrients for the sprouting plant to use, but they can also be disadvantageous because animals that eat acorns, like squirrels, prefer to collect larger acorns. In cases like this, you might expect that because the smallest acorns die without enough nutrients and the largest acorns are eaten by squirrels, the only acorns left that actually survive long enough to make more oak trees are the medium-sized ones. That’s conflicting selection at work!

Paper summary:

In Akiyama and Agren’s paper, the researchers study conflicting selection on the timing of germination, or sprouting, of a small flowering plant in the mustard family called Arabidopsis thaliana.

In order for plants to contribute genetic material to the next generation, they need to survive, and they need to reproduce. In this study, plants that sprouted early experienced lower survival as seedlings, but if they did survive to become adult plants, the adults had better survival and produced more offspring. On the other hand, plants that sprouted later experienced higher survival as seedlings, but out of the survivors, the adults had lower survival and produced fewer offspring than the plants that sprouted early.

In a situation like this where there are complicated pros and cons associated with sprouting early or late, which plants win the fitness game?

To find out, the authors measured overall fitness of the plants that sprouted at different times and found that the advantages of sprouting early outweighed the disadvantages: plants that sprouted earlier had higher overall fitness.

A word of caution from the authors: this study only took place over one year, which is not enough time to generalize these results as a consistently winning strategy for this population of plants. For example, during the year of this study, the fall and winter were fairly cool. This means that the plants that sprouted earlier may have had two advantages related to that year’s weather: 1) even though the early-sprouting seedlings were small and delicate in the fall, they may have survived drought conditions better than they usually would at that size because the fall was cooler than normal, and 2) because the early-sprouting plants had more time to grow big before winter came, they may have had an advantage over the smaller, later-sprouting plants because winter was cold and larger plants have a better chance of surviving the winter.

This is why it’s important to replicate studies – in this case, repeat it with different plants, in different places, over multiple years, etc. – before assuming all plants of the same species or even the same population employ the same early-sprouting strategy all the time. If the fall and winter had been warmer, the researchers may have found completely different results!

Why should you care?

This paper illustrates an important point about the complexity of nature – often, what’s really going on isn’t what seems initially obvious, or even what seems obvious after the second or third or fiftieth time an experiment has been done on it! Science is a slow process, and each paper like this one is a small brick added to a wall of knowledge that will always have gaps in it. Only with much rigorous science can we make the gaps smaller and smaller and the wall sturdier over time.

References:

Akiyama, R. & Agren, J. 2013. Conflicting selection on the timing of germination in a natural population of Arabidopsis thalianaJournal of Evolutionary Biology 27:193-199.

Gomez, Jose M. 2004. Bigger is not always better: Conflicting selective pressures on seed size in Quercus ilex. Evolution 58(1):71-80.

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