Plant & zo
The science of plants and more
Caught in the moment
Evolution, a slow process, difficult to see in action. Often it is so slow, only after a long time, you notice something has changed. Then, try finding out what caused the change. Sometimes, however, it is at once clear something has changed. An organ or a limp is missing, misplaced, or multiplied. A so-called homeotic mutation occurred. This is a mutation in a gene that has a crucial role early in organ or limp development. Often these are not to stay in the population, but sometimes they do. Such as for a variant of the Colorado blue columbine (Aquilegia coerulea).
Flowers of the Colorado blue columbine have five purple sepals, each with a yellow tip at the edge. Inside these there are five white petals with long purple tubular nectar spurs. And in the middle, there are the stamens with bright yellow pollen. Remarkable flowers pollinated by hawkmoths and bumblebees. A variant of the Colorado blue columbine, the daileyae, lacks the white nectar holding petals. In its place it has five purple sepals. This variant was first spotted in 1897, and now over 100 years later it is still there and thriving.
Researchers from California decided to unravel the success of daileyae. First, they found out why this variant is thriving. As pollinators miss their nectar treat, it was expected that they would not visit daileyae. The researchers actually found the opposite was true, both bumblebees and hawkmoths visited the normal and daileyae variant just as often. Individual pollinators did not have a preference, visiting both the normal and daileyae variants.
The big difference was spotted in the number of flowers with herbivory damage. Aphids, caterpillars, but also deer like Colorado blue columbine flowers. With both aphids and deer, having a sweet tooth, preferring the normal Colorado blue columbine flowers. There are just more daileyae flowers leftover that can set seeds.
Secondly the researchers analysed which gene and which mutation caused the daileyae variant. The gene APETALA3-3 is required for sepal development. Daileyae APETALA3-3 has a loss-of function mutation that turns off the gene. This causes the sepals to develop as petals. It turned out that in the daileyae population there were four different types of loss-of-function mutations in the APETALA3-3 gene. But with each APETALA3-3 allele has only one type of loss-of-function mutation. These loss-of-function mutations did not only show up in the daileyae population, but the normal Colorado blue columbine plants could in addition to a functional also have a loss-of-function APETALA3-3 copy.
Evolution is still in action. With a clear positive selection pressure for the daileyae Colorado blue columbine. But which of the four loss-of-function mutations in APETALA3-3 will get to dominate the population is still undecided. Also, pollination between the normal and the new Colorado blue columbine variants still occur. Does the daileyae variant take over, or do both variants find their own nice? The race is not finished yet, evolution is in full swing.
Cabin, Z., Derieg, N.J., Garton, A., Ngo, T., Quezada, A., Gasseholm, C., Simon, M., Hodges, S.A. (2022) Non-pollinator selection for a floral homeotic mutant conferring loss of nectar reward in Aquilegia coerulea. Current Biology DOI: https://doi.org/10.1016/j.cub.2022.01.066