The Genetic Gamesmanship of a Seven-Sexed Creature

By Patrick Morgan | March 4, 2011 9:40 am

What could be better than two types of sexes? For one organism, the answer isn’t three, but seven! And to top it off, these seven sexes aren’t evenly distributed in a population, although researchers have now developed a mathematical model that can accurately estimate the probabilities in this crap-shoot game of sexual determination.

Meet Tetrahymena thermophila, which in addition to its seven different sexes—conveniently named I, II, III, IV, V, VI, and VII—has such a complex sex life that it requires an extra nucleus. This fuzzy, single-celled critter has a larger macronucleus that takes care of most cellular functions and a smaller micronucleus dedicated to genetic conjugation.

The other odd thing about this one-celled wonder is that the population of the seven sexes are skewed, leading Unversity of Houston researcher Rebecca Zufall and her colleagues to ask: What gives?

To answer that question, they created mathematical models of T. thermophila populations, and discovered that different versions of the same gene, or alleles, gave advantages to different sexes. Unlike humans, in which an individual’s sex is fully determined by its genes, the genotypes of these creatures provide only probabilities of developing certain sexes—probabilities that are influenced partly by genetics and partly by surrounding temperatures. The sex-influencing gene is called mat, and different alleles make certain sex types more likely than others: A T. thermophila born with the mat2 allele, for example, has no chance of becoming type I, a 15% chance of being type II, a 9% chance of developing into type III, and so forth. For convenience, this mat2 gene variety and the other 13 known alleles are also grouped into an A-group or a B-group: The A-group alleles lead to the I, II, III, V, and VI sexes, and the B-group alleles to the II, III, IV, V, VI, and VII sexes. In terms of evolutionary advantages, the researchers’ models back up exactly what we’d expect: The alleles that produce a variety of sexes outcompete the alleles that produce only one; variation is a nice thing to have, after all.

Epilogue: You may still be wondering why a species would have seven sexes; researchers are wondering the same thing. Some past research suggests that it’s to maximize the choice of sexual partners (pdf), since partners must be of a different sex type. The jury’s still out as to why they’re specifically at the seven-sex level now. Tiago Paixão, the study’s lead author, says that “one possible answer is that this is the number of sexes that populations of T. thermophila typically support, and further increases in number of sexes would not lead to any noticeable increase in fitness.”

That said, keep in mind that scientists are playing loose with the term “sex” when they define these protozoa; as Cleveland State University professor F. Paul Doerder and his colleagues write (pdf), “To recognize suitable partners, ciliates [such as T. thermophila] are differentiated into mating types, a kind of self-not-self discrimination system, as opposed to true sexes.”

Related Content:
80beats: What Makes a Boy Lizard? Genes, Temperature, and Egg Size
Discoblog: The Strange, Violent Sex Lives of Fruit Flies and Beetles
Discoblog: NCBI ROFL: Sex Differences in Approaching Friends with Benefits Relationships.
The Loom: Through the Sexual Looking Glass

Image: Wikimedia Commons / Ayacop

  • Josh

    That would make for an interesting bar scene.

  • Robert S-R

    Would that make these creatures… sexasexual?

  • Old Rockin’ Dave

    “I’m a Kinsey 6 VI-sexual.”
    “This ad is demeaning to IIIs”
    “The IV-ine Mystique”
    “MWII looking for a fourteen-some”
    “You don’t have a chance. That I is strictly I-sexual.”

    The possibilities are endless.

  • Andrew

    And no reference to William Tenn’s 1947 story “Venus and the Seven Sexes” ?

  • Idlewilde

    I wish it talked more about the specific properties of the different sexes. That’s what’s most interesting to me.

  • Fruma Klass

    See “Venus and the Seven Sexes” by Willim Tenn, available in the book Immodest Proposals, published by NESFA Press, 2001. There were actually only six xexes that transmitted the gamete; the seventh was a coordinator. This story, first published in 1949, drew a fan letter from Hans Bethe (later Nobel Prixe winner) in which he diagrammed the probable gamete charts.

  • Jenn

    The idea that there are only two sexes in human beings (ie the existence of the gender binary) is fast getting out of date. Gender is as much a continuous phenotype as any other – and to think otherwise would be denying the existence of genderqueer, bigender and trans people.

  • Adam

    Jenn, you’re missing the issue here. The research isn’t focusing on sexual orientation – it’s dealing with reproductive potential.

    As far as the ability to naturally reproduce is concerned, there are “only two” sexes in human beings.

    Not saying that any of the other “sexes” you listed are necessarily wrong, but it’s beyond the scope of the study.

  • aztec

    7 different types of gays would make religious lose their minds, if they didn’t already.

  • Amos Zeeberg (Discover Web Editor)

    Good work on the comments, folks. “Sexasexual” — nice.

    @Idlewilde, what are the specific properties of the different sexes?

  • JK

    Adam, that’s not entirely accurate. Somatic mosaicism, while rare, nevertheless is the exception to the rule that demonstrates that we have an incomplete vocabulary for describing and defining “sexes,” even with respect to the capacity for pairs to reproduce.

  • Auto

    At the risk of sounding homophobic –

    “Gay” is not a sex. It’s a preference.

    If an entire species were gay, it wouldn’t exist in the first place.

  • Matt`

    For biological purposes, I think I’m right in saying that the type of gametes you produce define your sex. Still doesn’t produce a perfect binary, and there are always edge cases in any such definition, but it’s still fair to say that a very sizeable percentage of the human species can be categorised two different ways on that basis.

    Gender is then a psychological construction about what you identify as, combining biological sex, social influence and personal feeling, which may (in the majority case) be tightly connected with your biological sex, but evidently isn’t constrained by that.

  • Alan Gunhouse

    So this is a real creature?

  • Caleb Johnsen

    This is very interesting. What if it turns out that the reason that the ratios of the seven sexes in this species is being skewed because of a process of elimination acting on each of these varieties that takes place among all early species when they first develop any form of sexual reproduction to begin with, thereby lead to the natural selection of the most efficient means of maximizing genetic diversity in a stable community as the probability ratios shifted in favor of specific subsets of the same species.
    Then, as this plethora of stable and readily induced genetically diverse populations come into play the subsets adapt to their more rigged gender roles and become a completely new species. This could have huge ramifications for the concept of how early sexually reproductive species developed in the first place.
    If gene swapping was constantly taking place, if early protocells were constantly intermixed from complex biological communities into refined cellular machines, and if gender ratios were constantly tipping due to selective processes as I mentioned earlier in tandem with all of these, it would explain every thing perfectly. Someone, publish a paper on this possibility now.
    Oh and mention me in it too, ‘kay? Thanks, I look forward to an advanced copy. Laters.
    Signed, Apollo High school senior
    Bye. (^_^)

  • Caleb Johnsen

    Actually, if more stable reproduction of cell genetic material becomes more refined by the existence of multiple quasi-gender groups that don’t reproduce themselves perfectly then these roles become more stable overtime as the number of each gender type and their specialization to these roles becomes more and more refined, it could explain not only the arising of two partner sexual reproduction, with stable minor variations being introduced from one generation to another, but also asexual reproduction based off of the perfect or near-perfect reproduction of a cell type from generation to generation.
    That would then imply that the reproduction of cells did not start out start out asexually at all, but based of an incomprehensible number of quasi-gendered groups constantly exchange genetic information and maybe even basic cellular structures in order to better survive and more success fully reproduce themselves.
    Then it would just require a large number of imperfect and unstable biological agents to give rise to stable and semi-stable varieties, individual imperfect mechanisms that give rise to more perfect mechanisms for not merely reproductive success but more stable forms as would be a natural consequence in any natural system, to explain everything.
    It’s now so obvious.

  • Matt B.

    They would actually be septasexual. “Sexta-” has a t in it and means “six”.

    @ Caleb Johnsen, “It’s now so obvious.”
    Like putting too much air in a balloon! /Futurama


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