Notes From the Field: Going to where the snails are, because they are not going to come to me

Lake Gunn in the fiordlands. Lots of tetraploids and triploids in there!

Hello from the land of Kiwis (the fruit, the bird and the people)! As I mentioned in my last post, I’m a coevolution nut, and down here with all the kiwis there is also an excellent system for studying coevolutionary interactions between hosts and parasites. So during the most frigid part of the terrible winter in Washington state, I take off to the sunshine and summer of the southern hemisphere to do my field work! It’s a rough job, I know.

A little over a quarter century ago, Curt Lively, noted this adorable little New Zealand snail (Potamopyrgus antipodarum) has sexual and asexual forms that coexist at varying frequencies in lakes across New Zealand. This variation suggests that there are some environments where it is advantageous to reproduce asexually and some environments where it is better to be sexual.

From then on P. antipodarumhas become an excellent system to study the evolution and maintanence of sexual reproduction, a long standing debate in evolutionary biology (See Maynard Smith 1978, Williams 1975, Bell 1982, Kondershov 1988).

What’s more, when Curt started looking at these populations more closely, one pattern emerged almost immediately. There was a strong correlation between the number of sexuals in each population and the population of the trematode parasite Microphallus. Lots of parasites therefore equals an advantage to being genetically different, providing empirical evidence for Bell’s reconception of Van Valen’s “Red Queen Hypothesis”. Curt’s work definitively showed empirical evidence of a population overcoming the two-fold cost  of producing males!

A road sign in New Zealand

Curt has continued to do excellent work in the P. antipodarum system, demonstrating negative frequency dependent selection (Jokela et al. 2009), infection of the most common clonal genotypes (Lively and Dybdahl 2000) and generally establishing this as one of the few natural systems to study host/parasite coevolutionary interactions. He has also had some excellent students and post docs that I’ve had the pleasure of interacting with including Kayla King, Deanna Soper, Danielle Vergara, Britt Koskella, my MS advisor Maurine Neiman and my current PhD advisor Mark Dybdahl. Lots of cool people doing lots of cool work!

Which now brings me to what I’m doing here in New Zealand! My strongest interest at the moment is understanding the specific genes or proteins involved in coevolutionary interactions. In host and parasites that means genes that provide hosts stronger resistance to infection and parasites stronger ability to infect. How do these genes vary across populations (ie how selection is acting on these genes) and how that leads to local adaptation? How do the genetics of one species affect the divergence of the other (a question I’m asking in collaboration with fellow contributor to this blog and former snailer, Devin Drown)? How does gene movement between populations affect the strength of selection within populations? And my favorite snails are an excellent system for answering these questions! We know that local adapatation occurs across populations, such that Microphallus is better able to infect local hosts than foreign hosts. We also know that the snails do not appear to move between populations, but the trematode does. However, the amount of migration, and the genes involved, are entirely unknown. This leaves a wide open door for me to gracefully make my entrance into the world of the snail.

Inoculated populations in my kitchen/laboratory

Despite all these benefits there are a few drawbacks. This system is still woefully lacking in genetic sequence and molecular markers available to answer these questions. Luckily (and thankfully) I spend half my time at the University of Idaho that has an excellent sequencing facility and a core program (IBEST) that seems willing to fund this poor poor graduate student. So part of my work down here is doing sympatric and allopatric inoculations so that I can look at the differences in the transcriptomes of successful and unsuccessful parasites and infected and uninfected snails. This will allow me to zero in on potential genes that might be involved in both sides of the coevolutionary interaction and create a great jumping off point for all the other questions I mentioned above!

My final interest is a bit more specific. I mentioned above that the snails are either sexual or asexual, but what I didn’t mention is that this difference also corresponds to differences in ploidy: sexual snails are diploid, asexuals are triploid. What’s even more exciting is tetraploids have recently been found within mostly triploid populations, generating a ton exciting variation in ploidy! The big question this raises for me is what does this variation mean for the coevolutionary interaction? You’ve got an extra genome, just think of all that extra genetic material! Do you express it? Will that generate too much protein? Is it silenced? Is it differentially silenced dependent on the tissue? And what about these tetraploids, huh? Traditionally in this system ploidy has largely been used as a marker to determine if a snail is sexual or asexual, and the potential consequences of being a polyploidy have remained unstudied.

Until now. In fact this year one of my biggest experiments is inoculating snails from lakes that have been found to be triploid/tetraploid and seeing which gets infected more. This holds the question of sex vs. asex constant and allows me to ask, do tetraploids or triploids have an advantage? This question is one of those exciting questions in science because no matter what the answer is, it’s interesting.

All of the above research means I spend the month of January wearing jandles (the Kiwi word for flip-flops) driving between lakes all over the south island of New Zealand and collecting my favorite snail. Then at night I pull out a bottle of some excellent NZ cider and work on some theory, finish manuscripts and write blog posts.

I have to go to where the snails are, they are not going to come to me! It’s a rough life.

Literature Cited:
Bell G (1982) The masterpiece of nature. Croon Helm, London.
Jokela J, Dybdahl MF and CM Lively (2009) The maintanence of sex, clonal dynamics, and host-parasite coevolution in a mixed population of sexual and asexual snails. American Naturalist 174, S43-S53.
Kondrashov AS (1988) Deleterious mutations and the evolution of sexual reproduction. Nature 336: 435-440

Lively, CM and MF Dybdahl (2000) Parasite adaptation to locally common host genotypes. Nature 405, 679-681.

Maynard Smith J (1978) The evolution of sex. Cambridge University Press, London.

Williams GC (1975) Sex and Evolution. Princeton University Press, Princeton, N.J.

2 comments on “Notes From the Field: Going to where the snails are, because they are not going to come to me

  1. […] mentioned the Red Queen in my last post, so I’ll just touch on it again here. Famed paleontologist Van Valen (1973) noted that […]

  2. […] in which natural populations show signs of coping with parasites by having more sex, including C.J.’s favorite mollusks, and there is one good experimental example in which the worm Caenorhabditis elegans evolved to […]

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