Evolution of Sex, Part 2: The Build Up

We left off on the last post with a question and a tantalizing hint at an answer. Do you remember?

The problem was a bit thorny. We looked at asexual reproduction, compared it to sexual reproduction, and saw a clear-as-day numerical advantage to being an asexual reproducer, a clone. That left us scratching our heads— how did sexual reproduction get to be so common across the multicellular world if it’s such a dud prospect?

The thing is, last time was also a bit of a one-sided story. I didn’t give any of the rationale, the suspicions for why sexual reproduction has succeeded, in spite of its numerical disadvantage. There are plenty of theories about this— too many to describe here— but the leading theories rely on the advantage of variation. Asexual reproduction results in offspring that are clones of the parent, carbon copies with identical genetic information. Sexual reproduction results in unique offspring, with genetic information cobbled together from the contributions of mom and dad. The benefit of sexual reproduction, theory argues, lies in the genetic variation carried by these offspring. All that variation provides insurance against an unpredictable future. If the environment of the offspring changes drastically, there may be enough variation in a sexual organism’s offspring to allow some survivors. Clones, armed with fewer trait variations, face an increased risk of dying out if the environment changes too much. And so sexual reproduction wins the day.

There are problems with this theory, just as there are problems with theories for the success of asexual reproduction. For sex to edge out cloning, remember, it can’t just be slightly better. Because of the two-fold cost of males, it has to be twice as good. That requires a strong selective force, and no one could find an actual situation where environmental change was drastic enough to push the population that far.

So we’re back to where we finished up last time. We have our contenders: asexual reproduction is bold and brassy, confident that the current genetic make-up is good enough for the future; sexual reproduction is cautious, hedges its bets, pays the insurance premiums on the off-chance that something terribly different might happen in the next generation. We have theories arguing for the benefit of both sides. All we need now is a place to actually bring these theories to test. The promised evolutionary theory testing ground, I hinted, lies in New Zealand.

More specifically, it lies in this lake.


The beautiful Lake Alexandrina (Photo courtesy of Neil Yazma)

This is Lake Alexandrina, which marks a point right about smack-dab in the center of the South Island. This freshwater lake is filled and surrounded by bustling life— waterfowl dabble in shallow vegetation, fishes dart along the muddy bottom, trees sink their roots into the ready water source, humans park their trailers and stay for a holiday.

And then there’s the snail.

There it is! (Photo courtesy of Wikipedia)

Meet Potamopyrgus antipodarum, the New Zealand mud snail. You may already be familiar with this snail— it’s a problematic invader in waterways across much of Asia, Australia, Europe, and North America. Originally, though, it comes from the freshwater lakes of New Zealand, including Lake Alexandrina.

It carries out a typical snail existence in the lake: it crawls, it feeds, it reproduces. Oh, and that reproduction? It can be sexual or asexual.

That’s right— after all our talk about the conflict between sexual or asexual mating strategies, we’ve stumbled right on to an animal that exhibits both. (How modern, huh? I think the mud snail should be the mascot for the millennial generation, but I’d love to hear if you all have other suggestions.) The mud snail’s catholic approach to reproductive strategies operates in an interesting way: all males are sexual reproducers, but females can be either sexual or asexual (also called “parthenogenic”). The strategy a female will follow is determined before her birth, written into her genome. Sexual females have a double set of DNA (called “diploid”), while parthenogenic females have a triple set (called “triploid”.). Because of the way DNA copies divide and recombine in the process of reproduction, the offspring have the same number of DNA sets as their parent, and will pass the same on to their offspring. If your parents were sexual reproducers, you will be too. And if they were asexual, that will be the case for you and your offspring. You can’t argue with (genomic) fate.*

Snails being snails…

All of these snails appear to be very similar. They all eat the same detritus, crawl over the same mud, avoid the same predators. At a glance, it’s an undifferentiated, homogeneous group. But in fact, the snail population is actually a mosaic of different lineages. There are sexual families that interbreed and mix up their DNA with other sexuals, but there are also distinct asexual lines (basically: Clone A and her many identical progeny are different from Clone B and her progeny). All of these lineages are competing to use resources, to reproduce, to come out ahead in the ongoing contest that is natural selection. That’s what makes Lake Alexandrina such a prime ground to test predictions and outcomes of the sexual-asexual skirmish.

It’s time to introduce Curt Lively, another important character in the Lake Alexandrina scene. Curt is an evolutionary biologist who stumbled across the NZ mud snail in the 1980s and has been studying them ever since. He hasn’t been alone in this pursuit, and his research findings can’t be described without acknowledging the significant help of contributors like Lynda Delph, Jukka Jokela, Mark Dybdahl, Mandy Gibson, and many more. Each year, Lively and his co-conspirators (together, called the Snail Team) return to the freshwater lakes of New Zealand, analyzing tiny snails to unravel huge evolutionary questions.

Lynda, Mandy, me, and Alexandrina (Photo courtesy of Curt Lively)

In January, I had the opportunity to join this year’s incarnation of the Snail Team. I first met Curt, Lynda, and Mandy at the baggage claim of the Christchurch Airport. I was freshly new to all things New Zealand; they were, in a sense, returning home after a long absence. A few days later, I crouched on the shores of Lake Alex, watching this year’s rendition of the sampling process that has taken place every summer since the 1980s. Curt, bedecked in snorkle, mask, and wetsuit, trawled deep lake waters, while Mandy used a long-handled net to comb the waters closer to shore. Lynda oversaw operations from land, separating snails from lake detritus and carefully storing them. The snails, many thousands of them, would be carried back to the lab, where they’d be analyzed to determine reproductive strategy and clonal lineage.

The data from this year, 2016, would be combined with similar data on Lake Alexandrina Potamopyrgus populations stretching back three decades. That leaves the Snail Team with a hearty dataset, rich with the possibility of uncovering the answers to some of our questions. What have they found in those data?

First of all, they found consistent coexistence between sexual and asexual populations. This was anticipated— after all, they already knew that Lake Alexandrina featured a mixed population. The frequency of sexual snails varied from year to year— some years, they made up  60% of the population, some years just 30%— but even at their lowest years, sexual snails weren’t rare enough to be in danger of extinction.

It was in the asexual population that the real drama emerged. The total population of asexuals varied inversely to the sexuals (makes sense, right?). But within that general variation, different clone lineages underwent rocky rides. Certain clones would boom in population, only to go bust a few years later. Take Clone 49, for example. In 1994, she was the most common asexual snail in the lot. She had more than twice as many little clonal snails crawling around in the lake shallows than any of the other clones detected. Clone 49 was taking over! Nothing could stand in her way! She was headed full tilt towards total lake domination!

Clone 49, on her way (Courtesy of Bibliothèque Sainte-Geneviève, ms 143, fol179vs.)

…Until she wasn’t. By 2003, Clone 49 had disappeared. In fact, all of the common clones of 1994 were undetectable in the lake in 2003.

It’s not that 2003 was a year with no asexual snails— nope, it was full of happy parthenogens pumping out offspring in double-time. But the asexual snails of the 2003 lake all belonged to lineages that had been relatively rare in 1994. These rare clones, by all rights, should have slowly disappeared, losing their foothold (<- that’s a snail pun) under the competitive pressure of the highly-prevalent common clones. And the common clones, rather than disappearing, should have continued to do well until they became the only clonal types. Instead, they were…gone.

And throughout all of this clonal drama, sexual snails were simply existing. It’s like they got left out of the fight, and merely remained as a successful bystander throughout it all.

What could the Snail Team determine from these patterns? First of all, they found clear support that asexual reproduction is a risky business. When you’re good, you’re really good— but ten years later, you might be completely gone. Sexual reproduction is steadier in comparison. It might not rocket up the way certain asexual lineages do, but it’s sustainable over the long term. As the evolutionary theorists suspected, sexual reproduction is a safe bet, a hedged bet, ultimately a successful bet.

But there’s still the question of what drives those differential success rates. At last, we’ve found numerical evidence for the long-term success of sex, but we still don’t know the why behind it all. Leading theories would suggest that environmental changes are wiping out certain clone families and causing these fluctuations— but at Lake Alexandrina, Curt has never found environmental changes that are drastic enough to support that hypothesis. Plus, there’s the mysterious pattern: the most common clones disappear, and most rare clones thrive. The physical environment doesn’t cherry-pick based on frequency like that.

But there is another factor that varies from year to year. And this one does cherry-pick. Or perhaps it’s more nefarious than “cherry-picking”– this factor targets and destroys the most common clones. Remember Clone 49? With this factor around, her demise was really just a matter of time.

Find out why (at last!) in the compelling conclusion of this evolutionary mystery—
Next time,

*Except for when you can. Standing variation in ploidy has to come from somewhere, right? The current understanding is that the ancestral state for these snail populations is diploidy. Periodically, triploid offspring result from “mistakes” in sexual reproduction and join the snail population as new clonal lines. Is it really a mistake, or is it an evolutionary advantage to mix things up on occasion?

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