#Evol2017 catch-up — Effects of range expansions on mating system

Two weeks (more about that in a post I’ve written for Wednesday!) after the closing day of the 2017 Evolution Meetings, the Molecular Ecologists have all dispersed from Portland, though some may have left things behind! Still, the conference was so big that there’s a lot we missed the first time around — many great talks were scheduled against each other. Fortunately, hundreds of talks were recorded on video and posted online, so it’s possible to go back and catch up with them all. Over the next few days, we’ll highlight some recommendations from the conference’s well-organized YouTube channel
Matthew Koski’s talk on mating system evolution (The effects of historic range expansion on drift load, inbreeding depression, and autonomous selfing) is one I somehow missed.

Patterns of historical migration can influence geographic structuring of the magnitude of inbreeding depression and genetic drift, both of which are correlated with the evolution of mating systems. For example, low costs of inbreeding can facilitate the evolution of selfing, and small populations that experience strong drift are more likely to evolve elevated self-fertilization. Here, we identify clinal patterns of autonomous self-fertilization across 24 populations of the short-lived herb, Campanula americana. Specifically, autonomous fruit set is elevated at the leading range edges, but is low in populations near glacial refugia. We predict that the populations further from glacial refugia that display elevated autonomy also express higher drift load, and have reduced inbreeding depression. In each population we estimated the amount of inbreeding depression and heterosis (drift load) by conducting controlled hand-pollinations, and evaluating the fitness of progeny. We explore geographic patterns of inbreeding depression and drift load, and associate each parameter with the observed spatial variation in autonomous selfing. Our study highlights how genetic structuring following range expansion can affect the evolution of mating systems.

Koski presented work done with Jeremiah Busch and Laura Galloway on when we might expect selfing to evolve. The benefits include the automatic transmission of genes, but the costs include inbreeding depression.
By studying migration from glacial refugia, we can explore how migration may structure genetic loads and mating system variation. At the leading edges, we should expect increased selfing as a form of reproductive assurance (aka Baker’s Law). Thus, we may expect a cline in mating system patterns. As we get further away from the refugium, we should expect to see increasing frequency of selfing.
They studied 24 populations of Campanula americana, the American bellflower, across the North American range.

This species spread from the southeastern part of the continent following the last glacial maximum.

Screen grab of migration schematic for C. americana      © Matthew Koski

They posited three hypotheses:

  1. The leading edge should have an increased capability of selfing
  • They found the northwestern populations exhibited more autonomous fruit set in a common garden with no pollinators. There was, therefore, latitudinal and longitudinal clines in selfing, occurring farther away from the glacial refugium.
  1. The leading edge should have increased drift load
  • Looking at the variation in flower production, the northwestern populations showed evidence of increased drift load, consistent with migration route and selfing capability. However, when they looked at the cumulative drift load and not focused solely on germination, bolting or flower production, the longitudinal pattern dissolved. So, for this hypothesis, Koski gave it a “luke warm” check mark!
  1. The leading edge should have decreased inbreeding depression
  • There was no evidence for inbreeding depression for those leading edge populations.

Thus, the genetic underpinnings of inbreeding depression and drift load may differ. Importantly, the evolution of mating systems will depend on the magnitude of selection for reproductive assurance and inbreeding depression. We can forget to asses the historical processes and their effects on mating system evolution.
Here’s the video with some pretty cool heat maps of selfing!

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