Studying natural variation leads to interesting biology

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Model organisms are models for a number of reasons: they’re relatively easy to work with in the lab, or there are a lot of experimental tools available, or maybe even simple interia. But given that they’re models and various neat mutagenesis assays are available for toying around with them, people often forget that even model organisms often show a massive amount of natural variation. There’s a reason to map this natural variation over variation due to mutagenesis: it’s been visible to selection, and so may highlight key nodes of networks that are open for adaptive (or neutral change).

A beautiful example of this has just been published in an elegant paper in Nature, in the nemotode C. elegans. In some strains of this species, after a male mates with a hemaphrodite (males are rare, and most reproduction occurs via selfing), it deposits a “copulatory plug” that decreases the fitness of those males that follow him (stained red in the figure on the right). In other strains, there’s no such plug. So a natural question is: what’s the genetic basis for this phenotype, and what is its evolutionary history?

First, the answer to the first question: the authors map the trait to a retrotransposon insertion into a previously unknown gene. This gene share homology with the mucins, and is specifically expressed in around 12 cells of the male vas deferens (in green on the right). Personally, I never cease to be amazed by the discovery of new genes in sequenced model organisms, but it’s happened so much that perhaps I should get used to it.

The authors then go on to show that strains that express the plug overlap in their ranges considerably with strains that don’t. This suggests little fitness effect of the retrotransposon insertion, and this seems to make sense–C. elegans evolved from a species with obligate male=female reproduction (where presumably the plug was advantageous) to a species where most reproduction is by selfing (where a plug is likely more neutral). Overall, a really nice story.



  1. snicker… 
    Most of the available natural isolates of the nematode Caenorhabditis elegans have been examined and compared with the standard laboratory wild type (Bristol N2). Molecular markers, in particular transposon restriction fragment length polymorphisms, were used to assign these isolates to 22 different races, for which brood size and spontaneous male frequency were determined. Several distinctive traits were observed in some of these races. One example is mab-23, in a race from Vancouver, which leads to severe distortion of male genitalia and prevents male mating. Another is gro-1, segregating in a Californian race, which is associated with slow growth, heat resistance and longevity. Many races differ from N2 in carrying a dominant allele at the plg-1 locus, causing copulatory plug formation by males. Properties and possible advantages of the plugging trait have been investigated. The dominant plg-1 allele does not lead to increased male mating efficiency, but males from a Stanford race (CB4855), in which the plugging trait was first observed, are much more virile than N2 males. Crosses between N2 and CB4855 indicate that the higher virility is due to multiple factors. Size differences between N2 and CB4855 are associated with factors mapping to LGV and LGX.

  2. or maybe even simple interia.  
    Did you use this word just to annoy the commenters on the previous thread? ;) 
    (I had to look up a Latin dictionary to confirm that this word does in fact exist.)

  3. Did you use this word just to annoy the commenters on the previous thread? ;) 
    ha. nope, just a typo.

  4. males from a Stanford race (CB4855), in which the plugging trait was first observed, are much more virile than N2 males 
    clearly not conserved in humans :)

  5. s/N2/Berkeley/g 
    There, fixed it for you!