By Vidya Rajan, Columnist, The Times
Charles Darwin concluded his seminal “Origin of Species by Means of Natural Selection” with this immortal paragraph: “Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
In the previous article, I discussed the purpose of sex being to generate diversity – those endless forms most beautiful. But what sort of diversity are we discussing? Along with individual diversity in the generational span, there is an accruing diversity over evolutionary time, which leads to speciation. Darwin was unable, because he knew nothing of genes or the process of inheritance, to explain how the diversity emerged. What he did say was that different traits allowed individuals to variously adapt to their specific environments. Some traits were better for survival than others, and the best adapted produced offspring that contained those better traits. As I discussed in the article about sex generating diversity, genes for traits are mixed by sexual reproduction such that every individual has a set of traits that is unique.
There is another feature of sex that is a little more obvious than the scrambling of traits, and that is its external manifestation, gender. Gardeners know that both male and female parts are present in flowers on the same, or monoecious plants, but there are also plants where male and female flowers are separated on different individuals, the so-called dioecious plants. In these plants, such as holly and winterberry, both male and female plants have to be present for berries to be produced by the female plant.
Most people think that all animals are dioecious and that two genders, male and female, exist across the board. Not so fast! There is a huge variation in how gender is determined in animals, and I want to review here some of the diversity of gender determination. I will stick to animals to avoid getting too much into the weeds. There’s some pretty crazy stuff that goes on in bacteria, for example, where the bacterium’s gender is changed by mating or by being infected by a virus, but let’s leave that for another day.
In animals invertebrates such as earthworms and the occasional vertebrate, such as the hamlet fish, both male and female body parts are present in the same individual. So the same individual has ovaries that produce eggs, and testes that produce sperm. Some animals, such as hamlet fish and sea slugs, take turns playing male or female. But strange as it may seem, some of these animals can actually mate with themselves! Since the gametes are all a little bit different because meiosis scrambles the genes in eggs and sperm, so the offspring are not clonal. This is useful, as you can imagine, when the population of the species is so small or spread out that it’s hard to come across another individual. When the opportunity presents itself, then cross-fertilization occurs. Compare it to plants which can set seed with pollen from the same flower. Now it doesn’t seem so strange, does it?
On to more strange things about gender.
Right now, as the climate warms, turtles are producing more females than males. This is because their sex is determined by the temperature at which the egg is incubated [1]. Some single-sex invertebrates and vertebrates can transition or switch between genders in an eye-popping example of gender fluidity. Even more extraordinarily, this is the norm for many organisms – they start off as one gender and change to another gender later in life. Turns out these animals don’t have a “sex chromosome” per se – their sex is due to the production of sex hormones which are regulated by external conditions. Fish exhibit gender fluidity. In sheepshead wrasses, the presence of an enormous male in the population keeps all the others as diminutive females. Astonishingly, however, a females can transform into a male and produce sperm instead of eggs if the existing male dies or even if the female reaches a critical size (see https://youtu.be/8S79UN856EE for the amazing transformation, courtesy of the inimitable David Attenborough.) Clownfish do the reverse – the largest male changes to female if the female dies. A whole slew of animals are able to switch genders, allowing population of only a few same-sex individuals to give rise to a whole population. Fecundity unleashed. Genius move, evolutionarily.
In animals that are dioecious, that is have only one fixed gender per individual, strange things can happen to for females to produce offspring by a process called parthenogenesis, where eggs grow into individuals without fertilization by sperm. For example, a microscopic invertebrate called a rotifer has not had a male in the ranks for 300 million years. Only females. Come to that, there is a lizard in the Southwest desert called the New Mexico whiptail, which also has only females in the population. No males. It reproduces just by scrambling its own chromosomes and then producing four cells, any of which can become an egg, but only one does. Then, the surprising thing happens, one of those three discarded cells that does not become an egg actually fuses with the egg, and this restores the chromosome number [2]. Lest you think that this process is unusual, there are about 80 species of vertebrates who have jettisoned the male sex as being unnecessary.
Some parasites accelerate the process of single-sex reproduction. In insects, a bacterium called Wolbachia is transmitted from the female parent to offspring through the egg. This bacterium helps the egg to be self-fertilized to make a normal offspring by parthenogenesis [3]. Imagine you’re a woman who sick with an infection, and find yourself unexpectedly pregnant as a result. That might set some tongues wagging, don’t you think?
References:
- Balaraman, K., Is Climate Change Producing Too Many Female Sea Turtles?, in Scientific American. April 3, 2017.
- Harmon, K., No Sex Needed: All-Female Lizard Species Cross Their Chromosomes to Make Babies, in Scientific American. Feb 20, 2010.
- Huigens, M.E., et al., Infectious parthenogenesis. Nature, 2000. 405(6783): p. 178-179.
