Accessible research: Snail sex, or, why I won't turn into my mother

Posted 25 April 2013 by M. Wilson Sayres

Today we're going to talk about snail sex. There was recently a hubbub about an National Science Foundation (NSF) funding a grant to study snail sex to Maurine Neiman, John Logsdon, and Jeffrey Boore. Because, y'know, snails are so slimy, and sex is gross, so that makes snail sex... icky, and what is it good for?!? The work has been justified time and again (specifically see this response). I have complete confidence in the scientific and academic merit of this work. Here, I'm going to talk about the actual research, focusing on a paper published very recently (see below). I think we should start by getting to know the snails, and learn two important features of these snails.

Snails?

Yes, snails. This research focuses on a specific type of snail called a New Zealand mud snail (Potamopyrgus antipodarum) that lives in freshwater.

New Zealand mud snail (Potamopyrgus antipodarum), Wikimedia Commons

The first picture is a close up. We should put it in perspective. These snails are actually very, very small:

Aw, look at the tiny, adorable, New Zealand mud snails, Wikimedia Commons

The first interesting bit is that New Zealand mud snails are wonderfully invasive. This means that they are very good at invading a new territory (like a pond), reproducing prolifically to reach very high numbers of individuals, often squeezing out other native snails:

Densities have reached over 300,000 (!) individuals per square meter in the Madison River. - USGS

The second, and arguably more interesting, bit to know is that there are some New Zealand mud snails who always reproduce sexually (with male snails and female snails getting cosy), and some who always reproduce asexually (a process called "parthenogenesis", which basically means that some females produce clones of themselves). Sexual reproduction results in offspring that are a genetic mix of both parents. This combination increases the variation among individuals (for example, you have some features of your mother, and some features from your father, but are not identical to either). Asexual reproduction results in offspring that are nearly identical to the parent. This would be like if instead of being a mix of your parents you were EXACTLY LIKE YOUR MOM. (Please remind me to tell my husband that it could be sooo much worse. Uh, I mean, I love you mom!)

Sexual reproduction results in new combinations
Asexual reproduction dosen't fix what isn't broken (or even what is)

The unique situation with the New Zealand mud snail, with sexual and asexual individuals of the same species, is ideal for investigating how different types of reproduction affect the genome (the set of all a species' DNA), and why, or when, one type might be advantageous over another.

To understand how sexual and asexual reproduction affect the genome, it would be useful to know what the genome looks like. Until recently, however, there were no genome-wide resources for the New Zealand mud snail. Wilton et al., present us with that new resource. They developed reference maps for four mud snail lineages (two sexual, and two asexual). These reference maps are of a subset of the whole genome (called the transcriptome), but contain most of the elements we typically think of as being useful, most notably the coding genes. Now these resources are available for the public to use (and can be accessed here: http://www.biology.uiowa.edu/neiman/transcriptome.php). These resources will greatly assist in studying what the effects of sexual and asexual reproduction are, on a genomic level.

Sexual reproduction means you won't be "exactly" like your mother.

2013 Mar;13(2):289-94. doi: 10.1111/1755-0998.12051. Epub 2012 Dec 27.

Characterization of transcriptomes from sexual and asexual lineages of a New Zealand snail(Potamopyrgus antipodarum).

Wilton PR, Sloan DB, Logsdon JM Jr, Doddapaneni H, Neiman M.

Source

Department of Biology, University of Iowa, Iowa City, IA 52242, USA.

25 Comments

apokryltaros · 25 April 2013

I remember reading about another species of freshwater snail with both sexually and asexually reproducing lineages (may have been the New Zealand mudsnail, but I don't remember exactly), and the author said that the sexually reproducing lineages are better adapted to evolving adaptations against predators and parasites, while the asexually reproducing lineages are better adapted to taking over a new environment.

https://me.yahoo.com/a/JxVN0eQFqtmgoY7wC1cZM44ET_iAanxHQmLgYgX_Zhn8#57cad · 25 April 2013

why I won’t turn into my mother

Unless she raised you ~~right~~ wrong. Nurture, that's the secret to a good near-clone! Glen Davidson

DS · 25 April 2013

There are also populations of Daphnia pulex that have both cyclic parthenogenesis and obligate asexuality. The asexuals are produced by the contagious spread of a dominant meiosis suppressor, so the reproductive system may be more a product of intrinsic genetic constraints than adaptation. This data set might make it possible to identify such a genetic mechanism operating in the snails.

DS · 25 April 2013

Here is a link for the Daphnia genetic study:

http://www.indiana.edu/~lynchlab/PDF/Lynch163.pdf

M. Wilson Sayres · 25 April 2013

You're right, Glen. I am quite similar to her (and I think that's a good thing), but I do have the advantage that I've learned from both her successes and mistakes. I'm also very happy to have the influence of both of my parents.

M. Wilson Sayres · 25 April 2013

DS, thanks for the Daphnia reference. After a cursory read, it seems like the sexual reproduction has resulted in a male-limited chromosome. I don't know is that is the case with the mud snail (or if we just can't identify it yet).

diogeneslamp0 · 25 April 2013

Melissa,

thanks for writing up a description of this (I'm the one who requested it.)

Most of you know this, but for future reference, the "hubbub" she refers to is that Louisiana anti-creationist Zack Kopplin went on Bill Maher's TV show and some anti-science fathead arguing with Zack cited the "snail sex" study as an example of money wasted on science.

Zack's response, "You're not a scientist", was mocked at pro-ID Evolution News and Views. With this info, we can give a more substantive rebuttal to the anti-scientists.

This species appears highly invasive, spread throughout the American West, and I presume is of great economic importance. We'd better know how it reproduces.

DS · 25 April 2013

diogeneslamp0 said: Melissa, thanks for writing up a description of this (I'm the one who requested it.) Most of you know this, but for future reference, the "hubbub" she refers to is that Louisiana anti-creationist Zack Kopplin went on Bill Maher's TV show and some anti-science fathead arguing with Zack cited the "snail sex" study as an example of money wasted on science. Zack's response, "You're not a scientist", was mocked at pro-ID Evolution News and Views. With this info, we can give a more substantive rebuttal to the anti-scientists. This species appears highly invasive, spread throughout the American West, and I presume is of great economic importance. We'd better know how it reproduces.

I saw that show. I thought Zack did a great job, the anti-science guy just looked stupid. At least that was my impression.

anothernick · 25 April 2013

DS said: I saw that show. I thought Zack did a great job, the anti-science guy just looked stupid. At least that was my impression.

I also saw that show. The anti-science guy was Stephen Moore, an economist on the editorial board of the Wall Street Journal, and founder of the Club for Growth. Moore has mastered the talking head debate style so popular on television these days. He waits until someone with a viewpoint slightly left of his pauses to take a breath, pounces on some minor point (like the snail study) then refuses to stop talking until he outlasts everyone. Senator Sanders was on the show and he showed little respect to the Senator by doing the same thing - it doesn't seem to matter what he says as long as he's the last one talking. I've seen others try to outlast him. But in a marvelous adaptation to the modern television environment, I believe he can actually suspend his breathing for as long as it takes his opponents to give up. And, he laughs insanely at all of Maher's jokes. Maher just isn't that funny.

matthew.s.ackerman · 25 April 2013

DS said: There are also populations of Daphnia pulex that have both cyclic parthenogenesis and obligate asexuality. The asexuals are produced by the contagious spread of a dominant meiosis suppressor, so the reproductive system may be more a product of intrinsic genetic constraints than adaptation. This data set might make it possible to identify such a genetic mechanism operating in the snails.

MWAHAHAH! While you are faithful reporting the literature, the literature is actually wrong. The meiosis suppressor is extremely difficult to pass sexually, and there is definitive evidence that the asymmetry in reproductive contribution did not help spread asexual Daphnia (I'd get into why, but I probably shouldn't until the paper is published). Also, there is no evidence that mutation accumulation contributes to the demise of the asexual Daphnia. Always make sure you are reporting the probability that samples come from populations with different means, not the probability that the means of your samples are different. A subtle distinction, but an important one.

matthew.s.ackerman · 25 April 2013

Also :

DS said: ...so the reproductive system may be more a product of intrinsic genetic constraints than adaptation.

I'm not quite sure what you are getting at here. Are you saying that cyclical parthenogenesis is a genetic constraint or that asexuality is the product of genetic constraint? The traditional challenge is to explain the persistence of sexuality in the presence of the many clear advantages of asexuality. It is even worse with Daphnia, because asexual females are still capable of making sexual males and these males have sex. This means that traditional explanations for the persistence of sexuality won't work (adaptability etc.) because the asexual Daphnia have all the benefits of sex and none of the cost. One guess is that sexual Daphnia won't persist into the distant future.

DS · 25 April 2013

matthew.s.ackerman said:
DS said: There are also populations of Daphnia pulex that have both cyclic parthenogenesis and obligate asexuality. The asexuals are produced by the contagious spread of a dominant meiosis suppressor, so the reproductive system may be more a product of intrinsic genetic constraints than adaptation. This data set might make it possible to identify such a genetic mechanism operating in the snails.
MWAHAHAH! While you are faithful reporting the literature, the literature is actually wrong. The meiosis suppressor is extremely difficult to pass sexually, and there is definitive evidence that the asymmetry in reproductive contribution did not help spread asexual Daphnia (I'd get into why, but I probably shouldn't until the paper is published). Also, there is no evidence that mutation accumulation contributes to the demise of the asexual Daphnia. Always make sure you are reporting the probability that samples come from populations with different means, not the probability that the means of your samples are different. A subtle distinction, but an important one.

Thanks for the response. I'd love to see your data.

DS · 25 April 2013

matthew.s.ackerman said: Also :
DS said: ...so the reproductive system may be more a product of intrinsic genetic constraints than adaptation.
I'm not quite sure what you are getting at here. Are you saying that cyclical parthenogenesis is a genetic constraint or that asexuality is the product of genetic constraint? The traditional challenge is to explain the persistence of sexuality in the presence of the many clear advantages of asexuality. It is even worse with Daphnia, because asexual females are still capable of making sexual males and these males have sex. This means that traditional explanations for the persistence of sexuality won't work (adaptability etc.) because the asexual Daphnia have all the benefits of sex and none of the cost. One guess is that sexual Daphnia won't persist into the distant future.

Exactly. If asexuality spreads through populations contagiously, it doesn't matter whether sex has an advantage in the long term, it might get wiped out in the short term. Just another example of the lack of foresight and planning shown in nature.

matthew.s.ackerman · 25 April 2013

DS said: Exactly. If asexuality spreads through populations contagiously, it doesn't matter whether sex has an advantage in the long term, it might get wiped out in the short term. Just another example of the lack of foresight and planning shown in nature.

The problem is that using the word contagious implies that the reproductive asymmetry itself is responsible for the advantage of asexuality in Daphnia. However, asexual Daphnia are replacing sexual Daphnia clonally (i.e. without actually having sex with the sexual Daphnia). The benefit of asexuality in Daphnia is not the reproductive asymmety, but the greater rate of clonal increase of asexual Daphnia. By reproducing asexually the Daphnia do not self at the end of a season, and they don't suffer the cost of inbreeding. Sexual Daphnia inbreed at the end of each season and thus suffer a fitness loss that the asexual Daphnia do not pay. It is a good strategy. It is an adaptive strategy. However, the asexual strategy isn't a stable strategy because no Daphnia which can asexually produce resting eggs can have sex with any individual who can produce resting eggs asexually. If magically a new species of Daphnia existed (Daphnia utopia) which could do this, but was in every other way identical to Daphnia pulex, it would replace Daphnia pulex. Thus it is not the contagious-ness of the Asexuality which is responsible for it out-competing sexuality (since Daphnia utopia would out compete Daphnia pulex, without having sex with them).

matthew.s.ackerman · 25 April 2013

DS said: Thanks for the response. I'd love to see your data.

It should be in PNAS in a month or so. The first author will be Tucker. We're figuring out where to put the Data now.

DS · 25 April 2013

Matthew, if you read the paper I linked to, you will see that meiosis suppressors can convert cyclic parthenogens to obligate parthenogens through males since the suppressor is sex-limited and dominant. There is good evidence that the suppressor arose in glacial refuge in the northeast and is spreading westward. In this sense, asexuality is indeed contagious.

diogeneslamp0 · 25 April 2013

For your entertainment, DI asshole Joshua Youngkin has posted at least two, increasing hysterical attacks on Zack Kopplin, which I'll discuss here. First Zack Attack: Youngkin mocks Zack Kopplin and defends Stephen Moore on TV. A more moderate statement from Youngkin, before he moves on to insaner fare:

Joshua Youngkin wrote: The audience laughed and clapped approvingly at Zack's argument, convinced by what now passes for science that the two million dollar snail sex study must go on for the sake of, say, national security. Or something.

So the DI, copying the falsehoods of Stephen Moore (whose name Youngkin misspells), changes the cost of the study from $877,000 to a cool two million. Note how the DI's official position is that there should be LESS science funding. But they say they're pro-science. What are they afraid might be discovered?

Joshua Youngkin wrote: And non-scientists everywhere learned a valuable lesson about asking questions in public: don't do it. Just don't.

Notice here that lawyer Youngkin, with mind-numbing repetition, pounds away at his accusations that Zack is a "tyrant" and against "free scientific inquiry." What a lie. Kopplin is asking for more science funding, and Youngkin and the DI want REDUCED science funding, and to that end, Youngkin defends the lies of people like Stephen Moore. It takes chutzpah for Youngkin to claim he is "pro-inquiry" when wants cuts in science funding, and tells outright lies to achieve that end. How can there be "free scientific inquiry" when the conservatives promote an end to funding for science? Second Zack Attack: Youngkin's second, more hysterical attack. His above attack was not lunatic enough, so now he goes totally off the deep end. Youngkin informs us that Zack is fighting for a world in which non-scientists who "ask questions" are lined up against the wall and shot, firing-squad style.

Joshua Youngkin wrote: ...the ominous idea behind Zack's words: in the world to come, only scientists and other cognoscenti will get to ask science-and-policy questions. The overly curious will be the first to find their backs against the wall.

Um, no. Moore and the DI IDiots are not "overly curious." They are not "asking questions." Moore and the DI are making factually false statements. They said the snail sex study cost two million dollars; it did not. Moore and the DI are not "asking questions" about the nature of the research, which could be easily answered by a simple internet search; or (heaven forbid) calling up the scientists and ASKING them. They are not asking questions, they are making false statements. When we catch them, they compare their being refuted by FACTS to being lined up against the wall to be shot. Youngkin endlessly repeats his charges of the 19-year-old Zack's "tyranny" and plays the dishonest "I'm just askin' a question" card.

Joshua Youngkin wrote: Peel that onion back further... for whatever reason, some people don't take kindly to even good faith questioning. Who? Well, tyrants mostly.

No you fascist fuck, YOU ARE NOT ASKING QUESTIONS, YOU ARE MAKING FALSE STATEMENTS AND WE CAUGHT YOU. When you're caught, you blather "I'm just askin' a question." No you're NOT just asking a question, you fascist fuck.

Joshua Youngkin wrote: Now, not all tyrants enrich uranium for "peaceful" purposes. Some work at the DMV.

No asshole, there are no tyrants who work at the DMV. There are people who you don't like who work at the DMV. People you don't like are not necessarily tyrants. But go on. Here Youngkin accuses Zack of having "a tyrannical temperament" and of running a "tyrannical repeal campaign." Zack, as you know, is campaigning to repeal the falsely named Louisiana Science Education Act (LSEA).

Joshua Youngkin wrote: ...not all tyrants enrich uranium for "peaceful" purposes. Some work at the DMV. In a free country nothing defeats a tyrannical temperament or a tyrannical repeal campaign like self-disclosure.

No you fascist fuck, no one who works for the Discovery Institute can ever accuse anyone else of tyranny, given their universal employment of authoritarian language and philosophy in everything they write, from the "Wedge Document", the chilling Mein Kampf of Intelligent Design, to today. That especially applies to lawyer Joshua Youngkin, who, when Louisiana Gov. Bobby Jindal frankly stated that purpose of the DI-authored LSEA law was to facilitate teaching of creationism in public schools, Youngkin replied that we are not allowed to interpret the plain meaning of the Governor's words because we are not lawyers, like Youngkin and half the assholes at the Discovery Institute. No, Youngkin said, we are not allowed to interpret the plain meaning of the Governor's words because that is not our "proper role". The DI lawyer reminds us of our "proper role" and how it does not involve interpreting politicians' words.

Like the critics of the LSEA, Governor Jindal is not a lawyer. He delivers no legal memo. However, unlike the critics of the LSEA, Governor Jindal is a Rhodes Scholar with a [undergraduate!] degree in biology, an experienced politician who knows what his role is... (Failing to grasp your role is not just a problem with critics of the LSEA. An astronomer just declared vouchers in Louisiana illegal.) [“Gotcha! Governor Jindal Avoids Lawyering on TV”, Joshua Youngkin, ENV, April 18, 2013]

"Grasp your role", says lawyer Youngkin: Shaddap and stop asking questions.

harold · 26 April 2013

…the ominous idea behind Zack’s words: in the world to come, only scientists and other cognoscenti will get to ask science-and-policy questions. The overly curious will be the first to find their backs against the wall.

Because when someone is highly paid to express their opinion on television, but another person on the same show is allowed to offer a critiquing opinion, that's the same as "censoring" the first highly paid person - and then executing them by firing squad. However, I do recommend that science supporters consider saying this... "The real problem is not that you are not a scientist, it is that what you are saying is inaccurate and seems to be ill-informed." Of course, it is also not terribly wrong to use the phrase "you are not (formally qualified in the field of expertise related to what you are opining about)" as a polite way of saying "you don't know what you are talking about".

matthew.s.ackerman · 26 April 2013

DS said: Matthew, if you read the paper I linked to, you will see that meiosis suppressors can convert cyclic parthenogens to obligate parthenogens through males since the suppressor is sex-limited and dominant. There is good evidence that the suppressor arose in glacial refuge in the northeast and is spreading westward. In this sense, asexuality is indeed contagious.

The heart of the problem is whether the term contagious merely implies that asexuality can be inherited through males, or whether the term contagious implies that the selective advantage of asexuality derives from male production. If one merely means that asexual Daphnia are apomictic with male function, then I agree, this is the case. However, contagious asexuality my proliferate not only because asexual are superior competitors, but also because asexuality is contagious: an asexual female cannot be fertilized by a sexual male, while a sexual female can be fertilized by an asexual male. This asymmetry only arises in systems where asexuality is contagious, and one of the implications of using the term contagious is that asexuality is beneficial because it is contagious. It is this strong implication, an implication that Susanne's paper adopts in her model for the spread of asexuality in the equations bottom right of page 325 and in figure 6, that is incorrect. Some asexuals do produce males, but the production of males contributes little to the immediate fitness of asexuals. This can be seen through a number of lines of evidence, only one of which I will discuss. A substantial fraction of asexuals cannot produce males (you will see the estimate of 48% in the paper), yet these asexuals are still capable of out-competing a sexual population. Finally, at least part of the story about glacial refugia is incorrect, since asexuals likely originated and definitely spread long after the end of the last ice age.

DS · 26 April 2013

Matthew,

We seem to be talking past each other. I am definitely not claiming that asexuality is beneficial, in fact quite the opposite. I am saying that it may spread despite being maladaptive in some circumstances. I do not see how "contagious" implies "beneficial". Lots of diseases are contagious without being in the least beneficial to the host.

As for glacial refugia, the issue is the origin of the meiosis suppressor, not the origin of asexuality. If the proposed hypothesis is not correct, you will have to take it up with Mike Lynch, it's his hypothesis.

matthew.s.ackerman · 26 April 2013

DS said: Matthew, We seem to be talking past each other. I am definitely not claiming that asexuality is beneficial, in fact quite the opposite. I am saying that it may spread despite being maladaptive in some circumstances. I do not see how "contagious" implies "beneficial". Lots of diseases are contagious without being in the least beneficial to the host. As for glacial refugia, the issue is the origin of the meiosis suppressor, not the origin of asexuality. If the proposed hypothesis is not correct, you will have to take it up with Mike Lynch, it's his hypothesis.

I agree. I am claiming asexuality is beneficial. Contagious implies maladaptive. Calling it contagious makes it seem as if asexuality is an unfortunate driving process, like a segregating disorder. I take issue with calling it contagious because this implies that a drive like process is going on. But there is no drive like process occurring. Asexuality isn't a maladaptive trait that is increasing because of drive, it's an adaptive trait that is increasing because it increases carrying capacity, decreases inbreeding, permits maintenance of some ecologically superior genotype or whatever. The only problem is that this idyllic state can't be maintained because Daphnia haven't invented a way to have the right amount of sex. By themselves sexuals have too much sex and asexuals have too little. When both asexuals and sexuals mix, Daphnia have just the right amount of sex. Unfortunately the asexuals begin to dominate (since they can't share the benefit of asexuality with sexuals) and then Daphnia start having too little sex. I have taken up the glacial refugia with Mike. I have convinced him that the spread of asexuality occurred recently, but we haven't talked much about the date of the origin of the meiosis suppressor relative to the last period of glaciation. We don't discuss this in the paper since the data on the time of origin of asexuality are less certain. I have a bunch of pulicaria sequence to look at that might pin down the date of origin, but that is for a later time.

matthew.s.ackerman · 26 April 2013

DS said: ...take it up with Mike Lynch, it's his hypothesis.

I hope I haven't come across too argumentative. You have no way of knowing any of this, since the paper hasn't been published yet. I am just excited and wanted to share. It will be much more fun to talk about once we can talk about the data.

matthew.s.ackerman · 26 April 2013

DS said: ...I am definitely not claiming that asexuality is beneficial, in fact quite the opposite.

Also, just to clarify, I am not claiming that asexuality can never be maladaptive. It just happens to be the case that asexuality isn't maladaptive in Daphnia pulex.

DS · 26 April 2013

matthew.s.ackerman said:
DS said: ...I am definitely not claiming that asexuality is beneficial, in fact quite the opposite.
Also, just to clarify, I am not claiming that asexuality can never be maladaptive. It just happens to be the case that asexuality isn't maladaptive in Daphnia pulex.

That may be true. SInce many genetically divergent lineages have been created by the spread of the meiosis suppressor, there is probably a large degree of genetic diversity in the asexual forms. THis may mean that for some time to come they may have a short term advantage in many different environments. It remains to be seen what the long term consequences of such a genetic structure are. Are you working in the Lynch lab? If so, I am envious. I met Mike a couple of times and was very impressed.

_Arthur · 27 April 2013

If I remember well, the common dandelion reproduce asexually.

http://en.wikipedia.org/wiki/Dandelion

http://en.wikipedia.org/wiki/Apomixis

Talk about an ill-adapted species !