Short, Aidan W., and Matthew A. Streisfeld. "Disentangling Complex Histories of Hybridisation: The Genomic Consequences of Ancient and Recent Introgression in Channel Island Monkeyflowers." Molecular Ecology (2025): e17778.

 What They Did

The researchers examined the possibility of gene flow between different subspecies of monkeyflower, Mimulus aurantiacus. One subspecies, parviflorus, is only found on islands off the coast of California. Another subspecies, longiflorus, grows both on the islands and the mainland. The researchers compared DNA from both island and mainland populations of longiflorus, parviflorus, and aridus the sister subspecies of parviflorus.

Examining the whole genome at once shows that the two populations of longiflorus are most closely related to each other and that parviflorus is most closely related to aridus. Comparing only parts of the genome at a time, however, reveals where gene flow may have occurred. In some sections of the genome, the island population of longiflorus is more similar to parviflorus than to the mainland longiflorus population. This suggests that some gene flow occurred between the two subspecies on the island. In other parts of the genome, parviflora is more similar to both island and mainland longiflorus than it is to aridus, suggesting that there was some gene flow between the ancestor of parviflora and the ancestor of longiflorus, before the mainland and island populations diverged.

Looking more closely at one chromosome, they found a section where many alleles matched in parviflora and in island longiflorus, but not in aridus, the sister subspecies of parviflorus. They did not, however, see many matches between parviflora and island longiflorus that differed from mainland longiflorus. This suggests that gene flow caused adaptive changes in the parviflora genome but not the other way around.

Further Exploration

Back when I was a biology lab TA, I remember learning about some other monkeyflower that underwent “instant speciation” when there was a genome replication event. It’s not all that rare for the chromosomes to fail to separate during meiosis in plants, so that one gamete has a double set of chromosomes and one has none. When, for example, a diploid egg is fertilized by a haploid sperm, the resulting offspring is triploid, with three sets of chromosomes. Plants can often survive fine with extra sets of chromosomes, but when they undergo meiosis, the chromosome numbers might not be typical for their species, which can lead to reproductive isolation and speciation.

Monkeyflowers are often used in research about evolution and speciation because there’s a lot of phenotypic variation but still enough genetic similarity to cross taxa pretty easily. The plants also grow quickly and produce a lot of seeds (see https://monkeyflower.eeb.uconn.edu/a-new-genetic-model-system-monkeyflowers/). The different flower shapes and colors attract different pollinators, and the plants grow in all sorts of habitats and can be herbs or shrubs (see https://phytozome-next.jgi.doe.gov/info/Mguttatus_v2_0).

Since monkeyflowers of different species can often crossbreed, it’s not surprising that the subspecies mentioned in today’s journal article also show evidence of gene flow. The fact that (at least on one chromosome), alleles from longiflorus became fixed in parviflorus suggests that those alleles were useful enough to be selected for. It would be interesting to compare the areas of high gene flow on the other chromosomes, but that’s a rabbit hole for another day!

Image credit: Dawn Endico

https://commons.wikimedia.org/wiki/File:Mimulus_aurantiacus_(148194242).jpg