Thursday, 5 April 2012

Is biology zoonormative?

A geneticist colleague once told me a story about hiking with another biologist.  The forest was quiet and still, from the emergent podocarps and the tawa canopy right down to the ferns and mosses on the forest floor (sadly often the case in New Zealand forest since introduced mammals ate most of our native birds).  The other biologist's reaction to this was to say, "It's so quiet; there's nothing alive here!" To what extent is our thinking and teaching in biology zoonormative?
Nothing alive here?

Zoonormative is a word I use to describe how biologists think that what animals, and in particular humans, do is normal in biology.  Botanists, mycologists, phycologists, and bacteriologists are constantly trying to explain to students that their understanding of how animals do things isn't really a good picture of how life as a whole, in all its glory and diversity, really works.  And if students can understand the true diversity of ways to make a living in this world, well, they should see animals in a more realistic light.  Nothing demonstrates zoonormativity more than sex.

Most people think sex is all about mating, or copulation, between a male and a female.  We talk about "having sex".  That's a very zoonormative point of view.  Sex doesn't need males and females.  It doesn't even need copulation.  All sex needs is meiosis and fertilization.  Meiosis is a special kind of cell division that halves the number of chromosomes in a cell.  Strictly, it doesn't just halve the number, but because chromosomes in a diploid cell occur in pairs, meiosis separates the two members of each pair of chromosomes into daughter cells.  It also mixes up the different copies of the genes on the two chromosomes of a pair.  Fertilization is the reverse of halving the chromosomes: two such haploid cells (gametes) unite to form a new diploid cell, the zygote, that has two sets of chromosomes, one set from each gamete.

In familiar animals, there are two types of gamete, eggs (large, resource-rich, and produced in relatively low numbers) and sperms (small, resource-poor, and produced in relatively large numbers).  By convention we call eggs female and sperms male.  But the gametes can be all the same.  Many fungi and some algae produce gametes that don't specialize as either male or female, so here we have a simple sort of sex that doesn't have males and females. (Strictly speaking there are usually plus and minus mating strains, such that a plus and another plus can't unite, but the point is they're not physically and behaviorally different.)

In the unicellular green alga Chlamydomonas, for example, the cells are generally haploid, so when mating occurs, two of these cells (gametes) simply (well, it's not simple really) join together to make a single diploid zygote, which is a functioning diploid Chlamydomonas.  When such a cell divides by meiosis, the haploid stage is restored.  Yeasts (unicellular fungi) behave in much the same way.  Such organisms are isogamous, meaning their gametes are the same.

In other algae, usually multicellular ones, there's specialization of the gametes as male (small, copiously produced, resource-poor) or female (large, few, resource-rich), and usually specialization of the structures they're produced in as well.  This is anisogamy, and it has evolved many times in separate lineages of animals and plants.  In Chara, cells of the algal body are haploid (one set of chromosomes in each cell), so it's easy for some cells to become gametes: they are already haploid and they simply specialize.  Specializing seems to help the gametes to find each other.  If one (egg) stays put and sends out a signal and the other one (sperm) can move and follow that signal, it's more efficient than both constantly moving in the hope of a random collision.  (Think of it like two friends downtown on a Saturday night; one texts the other and says, "I'm at Molly Malone's pub"; the other goes there and they meet).

When the gametes fuse and make a zygote, in many simple plants the zygote divides straight away, or immediately after a resting period, by meiosis to make haploid cells again.  Thus algae like Chara don't have a multicellular diploid stage; their only diploid cell is a zygote.  Back in the third paragraph (above), I introduced the term diploid.  I was tempted to add a couple of words to explain it, and those words included saying normal cells are diploid.  That's zoonormative thinking.  I hope you can see that being diploid certainly isn't the normal state for a Chlamydomonas or a yeast.

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