Plants May Switch Between Selfing and Outcrossing More Easily Than Thought

Plant populations do not always have to commit to a single way of reproducing. A new theoretical study from the Hebrew University of Jerusalem reports that self-fertilizing plants and plants that require a mating partner can coexist for very long periods in a previously undescribed evolutionary state, even as the proportions of the two groups swing dramatically up and down. The work, led by Dr. Tamar Friedlander, Dr. Amit Jangid, and Prof. Ohad Noy Feldheim, was published in the journal PRX Life and is based on computer simulations of evolving plant populations rather than direct field observations.

The researchers describe this newly identified condition as a “volatile mixed mode.” In it, a population remains genuinely mixed over very long timescales, containing both self-compatible individuals, which can fertilize themselves, and self-incompatible individuals, which reject their own pollen and depend on genetically different partners. What makes the state unusual is that it is not a stable equilibrium. The relative abundance of each reproductive type can rise and fall considerably over time, yet neither strategy disappears. Instability, in other words, does not necessarily signal that a population is heading toward one final outcome.

“What surprised us was that instability does not necessarily mean that a population is on its way to one final state,” the researchers said. “In our model, self-compatible and self-incompatible plants can coexist for a very long time while their proportions continually change, even in the absence of fluctuations in external conditions. It suggests that what may look like a temporary or unstable situation in nature could actually be a persistent evolutionary state.”

For flowering plants, the choice between self-fertilization and cross-fertilization carries significant evolutionary trade-offs. Self-fertilization can be advantageous when potential mates or pollinators are scarce, because a plant does not need pollen arriving from another individual in order to reproduce. The cost, however, is inbreeding depression: repeatedly reproducing with oneself increases the likelihood that harmful genetic mutations will affect offspring. This genetic burden is one of the central forces shaping which reproductive strategy prevails in any given population.

Many plants avoid inbreeding through a genetic mechanism called self-incompatibility, which allows a plant to recognize and reject its own pollen. The system promotes mating between genetically different individuals and helps maintain diversity within a population. Self-incompatibility is found in roughly 40% of plant families and occurs in agriculturally important groups that include wild tomatoes, potatoes, apples, pears, citrus fruits and coffee. But the mechanism is not foolproof. Various genetic changes can disrupt it, allowing some plants to become self-compatible, which is why populations in nature can contain both types simultaneously.

Scientists have long debated what happens next in such mixed populations. One influential idea held that mixed populations are largely transitional, and that eventually one reproductive strategy should take over. An alternative proposal was that mixed populations could persist in fixed, stable proportions. To test whether either assumption necessarily holds, the team built computer simulations of evolving plant populations and examined the possible long-term outcomes. They found three: a population could become entirely self-compatible, entirely self-incompatible, or remain in a mixed state containing both types. It was the third outcome, and its unexpected volatility, that surprised the researchers most.

The simulations also allowed the team to map which conditions favor each of the three states. They focused on two major forces. The first was the genetic cost of self-fertilization, meaning how severely offspring are affected by inbreeding. The second involved the tolerance of the molecular recognition system that plants use to determine which pollen they accept. By varying these conditions across the model, the researchers could identify the regions of parameter space in which full self-compatibility, full self-incompatibility, or the volatile mixed state were likely to emerge.

The study produced a counterintuitive result concerning pollen shortages. A lack of pollen, which can occur when pollinators are scarce or when plants colonize new environments, might be expected to favor self-fertilization, since a plant capable of reproducing by itself appears to hold an advantage when external pollen is hard to find. Yet the model showed that pollen limitation could actually expand the conditions under which self-incompatibility is maintained. The reason lies in the differing selection pressures that pollen limitation places on the male and female parts of the self-incompatibility system. Once those sex-specific pressures are accounted for, the outcome becomes more complex than the simple assumption that fewer pollinators should automatically push plants toward selfing.

The findings may also challenge the notion that the evolutionary shift from self-incompatibility to self-fertilization is a one-way street. Across much of the conditions tested in the model, populations could potentially move in either direction between reproductive strategies. Under certain conditions, a population that had shifted toward self-fertilization could return to a system favoring cross-fertilization.

The researchers are careful to stress that the study is theoretical. Mixed populations containing both self-compatible and self-incompatible plants have already been observed in nature, but long-term studies tracking how the proportions of the two groups change over time are still lacking. Such field studies will be necessary to determine whether the long-lasting fluctuating state predicted by the model actually occurs in real populations. Understanding these dynamics could nonetheless become increasingly important as plants face changing environmental conditions, since climate change, shifting habitats and declining pollinator populations may all alter the pressures that determine how plants reproduce. By showing that reproductive strategies do not always settle neatly into one state or another, the study offers a new way of thinking about how plant populations adapt and persist: rather than evolution always pushing a population toward a single reproductive solution, sometimes the lasting solution may be to keep changing.

Subject of Research: Evolutionary dynamics of mixed self-compatible and self-incompatible plant reproductive strategies

Article Title: Plants may switch between reproductive strategies more easily than thought

Article References: Plants may switch between reproductive strategies more easily than thought. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: plant reproduction, self-fertilization, self-incompatibility, evolutionary biology, computer simulations, inbreeding depression, pollen limitation, pollinators, population genetics, PRX Life, theoretical modeling, Hebrew University of Jerusalem