Recent reading: 29 April 2022

Joshua trees in profuse flower in the San Bernardinos (jby)

How is this month already almost over? Four weeks ago I was just starting to realize that an unexpected, astonishingly good flowering season for Joshua trees meant I needed to shoehorn in some fieldwork, eyeing the data analysis I needed to do for not one but two in-person conferences coming up in the first weeks of May, and hitting the hard phase of a training sequence for a marathon. Now I’m a week out from that first conference and still grasping for free days to drive into the desert, and the marathon has been … delayed a week? (This is not a thing that happens so late in the game! And yet.) Also I’ve had multiple workweeks with more days spent on campus than not, to the point that the morning commute feels dangerously routine.

Whether or not we’re ready for it there’s a whole backlog of things that weren’t properly possible for the last two years that are suddenly crashing back into place.

I have managed to do some reading, though! Here’s the highlights:

Lewontin R. 1972. The apportionment of human diversity. Evolutionary Biology. doi: 10.1007/978-1-4684-9063-3_14

Not sure how long it’s been since I last read this, but it really holds up. Just a very clear, systematic walk through the fundamental question (is human genetic diversity meaningfully reflected in racial groupings?) the data to address it (electrophoretic alleles of selected proteins) and the math necessary to quantify variation within and between populations, racial groups, and across all of humanity. And then after 16 pages of this clear, matter-of-fact analysis, the conclusion:

Human racial classifcation is of no social value and is positively destructive of social and human relations. Since such racial classification is now seen to be of virtually no genetic or taxonomic significance either, no justification can be offered for its continuance.

Spottiswoode et al. 2022. Genetic architecture facilitates then constrains adaptation in a host-parasite coevolutionary arms race. PNAS. doi: 10.1073/pnas.2121752119

Brood parasitic birds are often adapted to lay eggs that match the color and patterning of host species’ eggs, to evade recognition and rejection by the host. Many brood parasites use multiple host species with diverse egg coloration. For this reason, it has long been hypothesized that egg mimicry is sex-linked — in birds, females are heterogametic, so mimicry determined by genes on the W locus would be maternally inherited, and not disrupted by mating with males raised by different host species. This paper presents clear evidence of host-associated maternal (mitochondrial) lineages in a brood-parasitic African finch species, and shows that genome-wide RADseq data is consistent with unrestricted mating among those lineages.

Very tidy result presented in just two figures! A new one for the textbook. But the second half of the title, that maternal inheritance of egg mimicry and host association “then constrains” coevolution with hosts, is all in the Discussion — logical consequences of what is shown, but not actually supported by direct evidence presented.

AJ Weisberg et al. 2022. Pangenome evolution reconciles robustness and instability of rhizobial symbiosis. mBio. doi: 10.1128/mbio.00074-22

A classic “paradox” of the legume-rhizobium nitrogen fixation symbiosis is that legumes can (generally) successfully sanction or avoid interaction with rhizobia that don’t fix nitrogen effectively — but natural rhizobia populations vary substantially in their ability to fix nitrogen. One possible explanation is that rhizobia can relatively rapidly acquire and drop nitrogen fixation in response to host-imposed selection, and this would be greatly facilitated if the genes needed for fixation and symbiosis are packaged in mobile genetic elements like plasmids and integrative conjugative elements. To examine the genetic basis of symbiosis and fixation in Bradyrhizobium, the authors compiled genome sequences for strains isolated from a wide diversity of hosts, and assembled new genomes for 85 strains isolated from wild populations of Acmispon strigosus, characterizing each strain as able to nodulate (initiate symbiosis) or not and able to fix nitrogen or not.

  • The genome assemblies reveal Bradyrhizobium symbiosis and fixation is driven by genes on an array of mobile genetic elements — most strains have them on integrative conjugative elements, but a couple have megaplasmids.
  • Sequences of these “symICEs” are consistent with a high degree of mobility and recombination, allowing remixing of genes involved in nodulation and those involved in fixation — otherwise quite different “species” of Bradyrhizobium that share hosts have shared MGEs for nodulation, and closely related strains that vary in symbiosis effectiveness have widely differing MGEs.
  • Within the Acmispon strigosus isolates, there is evidence that symbiosis MGEs of ineffective (non-fixing) strains are derived from the MGEs of effective strains; and effective strains carry MGEs from two very different lineages. This is consistent with “modularity” of the MGEs that allows separate shuffling of symbiosis initiation functions and nitrogen fixation functions.
  • Not to plug my own thing, but this is very consistent with a model of independently co-evolving sanctions and recognition systems.

About Jeremy Yoder

Jeremy B. Yoder is an Assistant Professor of Biology at California State University Northridge, studying the evolution and coevolution of interacting species, especially mutualists. He is a collaborator with the Joshua Tree Genome Project and the Queer in STEM study of LGBTQ experiences in scientific careers. He has written for the website of Scientific American, the LA Review of Books, the Chronicle of Higher Education, The Awl, and Slate.
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