Molecular ecology highlights at the American Naturalist 2020 meeting

A great blue heron keeps watch over the dunes of Asilomar State Beach.

Even-numbered years are distinguished by Olympic Games (summer or winter), U.S. Congressional elections, and the American Society of Naturalists biennial meeting at Asilomar, a retreat center embedded in a California state park near the northern tip of the Monterey Peninsula. AmNat2020, which took place over the first weekend of the decade, featured the full range of biological research represented by the American Society of Naturalists and the U.S. scientific journal with the longest continuous publication history, but genetic and genomic data were key to many of the meeting’s highlights.

The tone was beautifully set with the meeting keynote by Anurag Agrawal, this year’s winner of the society’s E.O. Wilson Naturalist Award. Agrawal discussed his extensive work on the insect herbivore community that specializes on milkweeds — most famously monarch butterflies, but also multiple other lepidoptera, beetles, bugs, and even a grasshopper and a fly. Adapting to cope with milkweed species’ array of chemical and physical defenses has created what, in one framing, looks like a remarkable set of convergent features across all these taxa. These range from specific mutations to a sodium pump subunit that increase tolerance of milkweed toxins — dissected in amazing detail via gene-edited Drosophila larvae in a recent blockbuster paper — to orange-and-black aposematic coloration. However, as Agrawal concluded, the "milkweed syndrome" is actually variable enough that it could equally well be considered a case study in how rarely evolution truly repeats itself.

A climbing milkweed, <i>Funastrum cynachoides</i>, near Palm Springs, California. (jby)
A climbing milkweed, Funastrum cynachoides, near Palm Springs, California. (jby)

Genetic and genomic examinations of mutualism filled up a lot of my other notes from the meeting, alongside some interesting practical population genetics and work on hybridization and introgression.

Liana Burghardt (who is, like me, an alumnus of Peter Tiffin’s lab) presented the next stage of her research on the legume-rhizobium mutualism using evolve and resequence experiments, which demonstrates how legumes’ selection of particular rhizobia strains shapes the diversity of rhizobia in the nearby soil community.

Rebecca Batstone, working in the same system, used qPCR to track the change in frequency of a cooperative rhizobium strain over multiple host-plant generations, starting from a 50-50 mix with an uncooperative strain. Batstone followed up with a GWAS for loci associated with rhizobial fitness, and found that variants associated with better fitness for the symbionts were also also associated with better host-plant performance, bolstering the emerging view that host and symbiont interests are generally aligned in this mutualism.

Tom Booker presented simulation results that should interest anyone using population genomic divergence "scans" for locally adapted loci: if sampled populations are all undergoing adaptation to the same environmental factor, the process of selective sweeps distributed over space can create ephemeral spikes of differentiation — that is, an apparent signal of local adaptation created by global adaptation.

Sheela Turbek presented some impressive population genomic data for two species of southern capuchino seedeaters, colorful songbirds that nest in the same wetlands. In spite of the obvious opportunity for hybridization, and a genome-wide Fst below 0.01, they’re maintaining strong behavioral isolation — Turbek found evidence for lots of extra-pair mating, but no hybrid nestlings.

Finally, Chad Brock walked through analysis-in-progress of 80 genome sequences from cichlids in three lakes within the Lake Victoria "superflock" of species — the results so far are fine-grained enough to dissect out how recombination interacts with pervasive introgression.

About Jeremy Yoder

Jeremy Yoder is an Assistant Professor of Biology at California State University, Northridge. He also blogs at Denim and Tweed, and tweets under the handle @jbyoder.
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