Sibling rivalry in plants
This week I will discuss two papers, both dealing with plants and competition, in the context of genetic relatedness that might be expected to moderate competition:
"Growing with siblings: a common ground for cooperation or for fiercer competition among plants?" by Ruben Milla and colleagues (Proceedings of the Royal Society), and
"Do plant parts compete for resources? An evolutionary viewpoint" by Victor Sadras and me (New Phytologist).
Earlier I discussed a paper by Susan Dudley and Amanda File showing that some plants grow less root when interacting with related than with unrelated neighbors. Spending less resources on roots could have freed resources for more seed production, but they didn't measure that. Now Milla and colleagues have.
They grow three lupine plants per pot, using either three seeds from the same plant, three seeds from different plants in the same area, or three seeds from different parts of Spain, and measured various aspects of plant growth and reproduction. In contrast to what I might have expected from Dudley and File's work, plants surrounded by siblings produced no more seeds than plants surrounded by strangers. In fact, one of their measures showed significantly more seed production from plants growing with plants from other regions.
They suggest two possible explanations. First, there was some tendency for plants to grow taller when growing with close kin, perhaps because they all germinated at the same time and thereby triggered an "arms race" to get above each other. The resulting over-investment in stem could leave less resources for seed production. Their other explanation is almost the opposite. What if closely related plants invest less in root, as Dudley and File found, and (under the conditions of Milla's experiment) this resulted in too little root for optimal uptake of water and nutrients?
When wild plants are grown in pots in a greenhouse, they may not allocate resources optimally, nor respond normally to environmental cues, including cues about the relatedness of their neighbors. But if hypothetical cooperation among closely related plants is weak enough to be undermined (even reversed) by growth conditions, the tendency to cooperate can't be very strong.
I discussed a paper by Victor Sadras in one of my first posts in This Week in Evolution, so I was intrigued when he invited me to collaborate on a paper reviewing the idea of "competition" among parts of the same plant. We argue that mechanisms that look like within-plant competition often act to maximize overall plant reproduction. A branch shaded by another branch may die, but this is more like suicide than murder. We know this because the same degree of shading isn't lethal when the whole tree is shaded equally. When only one branch is shaded, however, it can increase the frequency of its genes in the next generation by sending its nitrogen to better-lit branches, where the photosynthesis rate per unit nitrogen is greater. Seeds produced on those branches carry the same genes as those that the shaded branch could have produced itself. Selfish genes lead to unselfish branches.
Competition among seeds on the same plant is a different story. These seeds may have different fathers, whose pollen contained competing versions of various genes. Gene variants that help a seed take more than its share of resources from the mother plant will tend to increase over generations, unless countered. But mother plants have various counter-measures that tend to equalize resources among seeds. (This contrasts with birds that can only bring enough food to feed one chick. They may lay two eggs, but then let the stronger chick kill the weaker.)
We suggested that natural selection for equalizing resources among seeds has often set limits on how much seeds can grow, even when conditions turn out to be unusually favorable during seed-fill. This tradeoff may have been worth it for genetically diverse wild plants. In modern agriculture, however, whole fields may be almost identical, genetically. We might therefore be able to eliminate some of these ancestral seed-balancing mechanisms, letting seeds grow more when conditions are good.
Such tradeoffs between past natural selection and present human goals are a major theme of my forthcoming book, "Darwinian Agriculture: where does Nature's wisdom lie?"