Pansies undergo high-speed evolution when pollinators disappear

How will plants adapt to the decreasing number of pollinators? Pierre‐Olivier Cheptou and colleagues examined Viola arvensis or Field Pansy. They found that flowers are getting smaller and paler as they increasingly rely on self-pollination and not insect visitors. Their results, published in the American Journal of Botany, show that selfing rates have risen by over 25% over the past twenty years.

Some reporters have proclaimed the decline in insect numbers in recent years as a herald of ‘insectageddon‘. There are many indications that insect populations are falling but, for many plants, this is a matter of survival. Some plants need insects to carry pollen to partners and, if there are no couriers, they cannot reproduce. Others, like V. arvensis can get by if no insects are available through self-pollination. The process of self-pollination has different demands than insect pollination. If there are few pollinators, then a plant that makes changes to improve self-pollination could be at an advantage to its neighbours. Over time, a species can benefit from evolving more self-friendly flowers. Cheptou and colleagues wanted to see if this was what had happened with V. arvensis.

The team used an approach called ‘resurrection ecology‘. Resurrection ecology brings back a population from the past to compare with their more recent relatives. In this case, Cheptou and colleagues compared two groups of pansies. One group from 2012 and another from 1991, two decades earlier. The scientists wanted to see if they could see measurable differences between the two generations of pansies over this relatively short time.

What they found was that the more recent pansies had smaller flowers. If the flowers aren’t likely to attract pollinators, then a plant is wasting resources looking attractive to visitors who aren’t there. The flowers were also shorter-lived. Again, why stay open, if no one will come? The botanists also found that the more recent plants had more capability to self-pollinate.

Cheptou and colleagues note that this self-pollination might be a small source of bias in the project. Plants usually favour outcrossing, so develop inbreeding depression. Seeds from self-pollination could be, on average, less viable. This difference means that when you germinate seeds from a seed bank, you’re not getting a complete cross-section of past plant populations. Instead, you’re getting germinated plants with a bias towards seeds that can survive long-term storage. The botanists calculated if this bias could be a cause of their results, but seed mortality showed that this could not explain all the shift to self-pollination that they observed.

Cheptou and colleagues conclude: “The pattern of evolution found for Viola arvensis has the potential to disrupt plant–pollinator interactions. Assuming that pollinator decline has caused the evolution of a selfing syndrome, this evolutionary change may reinforce pollinator decline and lead to a positive feedback loop and ultimately to the breakdown of a major trophic relationship in ecosystems. In this context, there is an urgent need to analyze patterns of mating system evolution in floras to see whether the pattern in Viola arvensis is general or not. Resurrection ecology appears as a powerful methodology to analyze such patterns. In Europe, the large number of seed banks (Ensconet network) has the potential to reveal patterns of plant evolution in the last decades.”

READ THE ARTICLE

Cheptou, P.-O., Imbert, E. and Thomann, M. (2022) “Rapid evolution of selfing syndrome traits in Viola arvensis revealed by resurrection ecology,” American Journal of Botany. https://doi.org/10.1002/ajb2.16028

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