Here’s another concept that is difficult to get your head around: the single-celled organism Tetrahymena thermophila has not two distinct genders — our male and female — but SEVEN.
So let's talk about sex.
The further you delve into what sex is all about, what the definitions of terms such as ‘gender’ are and the biological roles each ‘gender’ play the more muddled they become. Our classical understanding is undermined by exceptions at each level.
In nature, you might presume, it is the female who gives birth and rears the young. But not so for the pygmy marmoset, where it is the male who diligently rears the young as soon as they are born, only giving them back to their disinterested mother for feeding. The female seahorse lays the fertilised eggs into a pouch on the male, who then carries them throughout the entire pregnancy and gives birth to the baby seahorse proper. Traditional parenting roles do not, therefore, define gender.
|X and Y chromosomes|
At a genetic level, there is also no standard. Each organism contains two copies of each chromosome, a self contained package of DNA, one of many in the organism like a chapter in a book. The number of chromosome pairs differs in each species, but typically there is one pair devoted to defining the gender of the individual; in humans and mammals these are the X and Y chromosomes — XX for female, XY for male. You receive one copy of each chromosome from each parent.
But this is not the case across the animal kingdom. In some insects, XX denotes female whereas X0 denotes male, an individual with only one chromosome. In birds, the system is reversed: females are ZW, whereas males are ZZ. In social insects, such as bees and ants, fertile males have half of the number of chromosomes as females, who can reproduce without males. This produces a system in which progeny do not inherit ½ of their genes from mum and ½ from dad but instead can be statistically ¾ alike. Then, of course, there are asexual, parthenogenetic or hermaphroditic organisms which, through a variety of genetic means, can reproduce without a mate at all and therefore do not strictly have male and female genders.
It gets stranger. The black howler monkey has not one but four sex chromosome pairs and female and male platypuses are not XX and XY, respectively, but... wait for it... XXXXXXXXXX and XYXYXYXYXY.
But if we ignore the myriad elaborations on the genetic model of gender, can we at least conclude that male and female are defined according to different genetic contributions to their offspring?
In turtles and other reptiles, sex determination is not controlled by genetics but by temperature. Turtle eggs are buried together underground, creating a warm nest. Typically, the eggs in the centre, which are therefore the warmest, produce female turtles whereas those on the edge, and therefore the coolest, produce males. Exceptions, as ever, apply.
In some species of fish, too, gender is not controlled by genetics but by social hierarchy. When the sole female bluehead wrasse dies in her school, she is replaced by a male who becomes female.
The definition of male and female is not therefore related to parental contribution, physical birth or genetic determinants. It’s something altogether simpler. The female is the one who makes the egg cell (ovum, or equivalent), which contains both genetic information and means to sustain the early embryo, and the male is the one who produces a mobile cell (be it sperm, pollen or another cell type) that contributes DNA to the female ovum, rather than the other way round*. Males cannot mate with males; females cannot mate with females.
But all of these definitions and distinctions go out of the window for the weird and wonderful model organism Tetrahymena thermophila, which exists in any one of seven numbered genders, I to VII. Such a system is beneficial, for the genders are self-incompatible, as in the male–female system, allowing a greater possibility of finding a genetically compatible mate. No longer are half of the population off limits, only one-seventh. And Tetrahymena thermophila has another quirk: the seven sexes are not equally common, nor are they determined by temperature, ecology or genetics.
For them, a third mechanism of sex determination exists: probability.
In this species the gene mat, short for mating locus, has fourteen different versions, called alleles. The presence of each allele offers a different probability of being a particular gender. For example, an organism with the mat-2 allele has a probability of 0, 0.15, 0.09, 0.47, 0.05, 0.14 and 0.1 of being gender I–VII, respectively. This vague method of sexual determination produces skewed sex ratios, where mating types are not equally likely, but the fact that one individual can mate with six-sevenths of the population, not only half, gives it a competitive edge for recovery in times of stress and population loss.
But how, you may be asking, can a single-celled organism use sexual reproduction at all? An organism of only one cell surely has to put up with the basic asexual method, dividing into two without varying its genes?
It’s a wonderful and always bemusing world out there. Nature has a habit of challenging our perceptions about the world, often in the strangest of places. But if you’ve learnt nothing else from this piece, I leave you with one final strange thought. Conjugation is also the method adopted by bacteria. Yep, even bacteria have sex.
Full (main) references:
Paixão, T., Phadke, S. S., Azevedo, R. B. R. & Zufall, R. A. Sex ratio evolution under probabilistic sex determination. Evolution 65-7: 2050–2060 (2011)
Arslanyolu, M. & Doerder, F. P. Genetic and environmental factors affecting mating type frequency in natural isolates of Tetrahymena thermophila. J. Eukaryot. Microbiol. 47, 412–418 (2000)
*No doubt people will find exceptions to this also.