Sit Up and Blink (V) – Sex determination

SUB I – Inheritance
SUB II – Cosmic vertigo
SUB III – Microbiota
SUB IV – Mass extinction

“How can a woman be expected to be happy with a man who insists on treating her as if she were a perfectly normal human being?”

– Oscar Wilde

In this SUB series we’re less concerned with philosophical stances and more about interesting scientific facts. So here’s a staggeringly obvious one to start us off: physical differences between males and females appear throughout the animal kingdom. Wow. That shouldn’t blow too many minds. A ‘sexually dimorphic’ species is one where boys and girls look visibly distinct – humans are a textbook example – and is among the most common kinds of diversity found in nature.

To continue stating the obvious, the difference an individual creature’s sex makes to its life trajectory can be enormous. I covered some of the inescapable consequences of this in an earlier blog, but to summarise; Being born male or female influences not just how you look, but how you act, your internal anatomy, longevity, the sorts of diseases you can catch, pretty much everything! And all these things put a cap on the number of offspring you can realistically expect to produce. As always, we’ll discover more as we dive deeper.

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It’s been said that a chicken is just an egg’s way of producing another egg. This is an amusing, yet curiously insightful way of thinking about biological reproduction and development. As an adult member of any species, the conflicting priorities of gathering enough food to sustain yourself while producing as many offspring as possible cannot be escaped. Resources must always be divided between those two projects, which is why fertility is one of the first things to be abandoned during times of stress or illness where an animal needs to fight for its life! Extreme cases of ‘male-female symbiosis’ illustrate just how minimal an organism can be, without shirking its reproductive inclinations.

Lil baby barnacles, from a group of crustaceans called the rhizocephala, are released from their mother and swim about in the ocean water for a few days. They don’t eat as larva as they have no mouth or digestive plumbing. Eventually, all going well, females will settle into the exposed limb joints of unlucky crabs. Having securely attached itself to a host, the young barnacle injects a mass of cells into the crab’s body through its syringe-like head. These cells grow into threads right through the crab’s soft tissues, tapping into its digestive system for nourishment. Being a female, the diffuse, formless, but singular parasite then develops an external ovary-lump emerging from the crab’s abdomen. The ordeal is an understandable burden on the crab, sterilising it without any physical impairments.

What about the boys? Well, attracted by pheromones from an established female barnacle, male rhizocephalans inject their cluster of cells directly into that sexy exposed ovary-lump! These male cells also grow like roots but toward the female’s ovary. Here they set up shop as sperm-producing germ cells, spending the rest of their existence directly fertilising new barnacle eggs. At this stage the male rhizocephalan can be considered “winning at life”, though, as over ten males can tunnel their way into each female gonad, their paternity isn’t completely assured. Still, there’s only so much one can do.

Being nothing more than parasitic sperm-forming cells within a parasitic egg-building female (herself within the body of a still-living crab), any respectable adult rhizocephalans might just represent the most basic form of male or female that can exist in the animal kingdom! But what casts the deciding vote on what sex developing offspring become? Well, it depends! It’s fascinating how different groups of animals use different genetic and environmental triggers to determine if fertilised eggs become male or female. There’s a whole range of natural diversity here too!

“All of man’s troubles have arisen from the fact that we do not know what we are, and do not agree on what we want to be”

– Jean ‘Vercors’ Bruller

To break it down, chromosomes are large chunks of DNA that carry lots and lots of genes. In humans, other than the X and Y chromosomes (aka ‘sex chromosomes’), all our other chromosomes (collectively called ‘autosomes’) come in matching pairs. When making sperm or eggs, autosome pairs splice together and exchange chunks of genetic material in a process called… wait for it… ‘recombination’. Generally speaking, recombination keeps each autosome pair looking pretty much identical over the generations – both chromosome number threes look very similar to each other, but different to both chromosome number nines, etc.

For unknown reasons, at some point in mammalian evolution, the sex chromosomes stopped recombining. Eons of divergence have resulted in the mismatched X and Y chromosomes we see today. This gradual change can be mapped in the relationship history of all mammals. From monotremes, to marsupials and us placentals, recombination became outlawed on a larger and larger portions of the sex chromosomes. But its not just mammals where chromosomes determine sex. In case you were wondering, many reptiles, some fish, a few arachnids, most flies, and all beetles use the same system! 

In a species where the presence (or the dose) of X and Y chromosomes determine the sex of the offspring, XX creates a girl, XY spits out a boy. A mismatch results in a male. But there are always more ways to skin a cat in the natural world. For whatever reason, sex-specific chromosomes are the most common way to determine the sex of an offspring, though lots of reptiles and fish use environmental triggers! In many of these cases, the temperature during embryo development will make all the difference. Higher temperatures usually result in more females for a temperature-dependent species, but it only takes a very slight increase. This raises troubling conservation issues; global warming may skew sex ratios in future generations of these creatures, if not stop the production of males entirely.

Another natural environmental trigger may be the absence of one sex within a community. Sex-changing ‘hermaphroditism’ is no big deal for hundreds of species of fish, reptiles, birds, amphibians, and invertebrates. One way to improve your chances of being a parent lies in being able to assess your social surroundings and swapping sex to meet demand! Species with this genetic superpower usually start life in an all-male or all-female brood. As a general rule, there’s an advantage to being a large female, as greater size means you can gather more resources and lay more eggs. Unless there are physical contests going on between competing males, larger size doesn’t generally increase male reproductive success. In either case, the death of a dominant – often the largest – individual of the opposite sex may present an opportunity. Events like this can bring about a sex-change in the next largest individual, often in a very short amount of time! 

In bees, wasps, and ants, there’s no chromosome mis-matching going on. Females will develop when an egg is fertilised and there are two copies of every chromosome present. Unfertilised eggs, with only one set of each chromosome will develop into males! Here, where there’s no pairing-up of chromosomes, males receive no genetic material from any biological dad. They are all mummy’s boys. Because of this genetic quirk, sister bees, wasps, or ants will be more genetically related to each other than they would to their own offspring! Having a more invested interest in the success of the colony than in making your own babies, inspires many altruistic, even self-sacrificing behaviours in these sisterhoods.

For us people-fish, it’s all about X and Y. What is it that makes our sex chromosomes so important? The human X chromosome carries about 800 different genes and is really no different to any other autosome. The Y chromosome, in stark contrast, only has about 70 genes, and less than half of those are unique! Despite the surprising lack of additional information needed to produce a boy, there is one specific gene that seems essential. Y chromosomes contain a bit of code called ‘SRY’, and this alone decides whether you are born male or female. People born with a full Y chromosome but a faulty or missing SRY gene will develop into females!

Once an organism’s sex has been determined, by any mechanism, a domino effect begins, with different genes turning each other off and on right across the genome. Downstream effects like the look of our bodies, our internal anatomy, our behaviours and our goals, can all be controlled in a smooth continuum depending on the activity of genes involved. In many animals, we picture a trade-off between beautiful but expensive features – colours, size, vocalisations, extravagant appendages, or ritualistic dance moves – and investment in gametes (eggs and sperm). Unless you’re a parasitic rhizocephalan barnacle, making babies is costly, leading to a variety of life-choices for the different sexes of different species. Depending on the genetic abilities available to you, your social environment, and what position you hold in the pecking order, it might pay to secure a single mate for life, or collect a harem of loyal partners. You do you. 

Lots of genes show a male or female bias – expressed more in either males or females – no matter what chromosome they’re on. As you might expect, we find intermediate levels of these genes expressed in species where males battle for female approval. Dominant males tend to be bigger, louder, and more colourful than subordinate males. The “lesser males” of various species may get less chances to mate, or even forgo reproduce altogether to serve the alpha, but are nonetheless bona fide males according to their sex chromosomes. Differences lie only in how strongly ‘demasculinised’ or ‘feminised’ the rest of their sex-biased genes are.

Suddenly we see that sex determination is a profoundly complex business! In humans, the SRY gene (on the Y chromosome) will determine whether an embryo’s gonads try to develop into testes. These testes will then start producing hormones that are typical for a human male. But it’s more complicated than that! Like having central heating and air conditioning continually work against each other, female-bias genes on other chromosomes keep pushing gonadal development towards ovaries. Sure, SRY puts testes on the cards, but genes like DAX1 (on the X chromosome) will attempt to build ovaries at the same time! Any chance duplication or up-regulation of the DAX1 gene can overpower SRY’s influence. Such individuals could go on to develop ovaries, even though they possess a Y chromosome.

Another gene, called DMRT1, also plays an important role in the development of testis. This stays active after gonad development is complete. If the effect of this gene is removed at any point in a male’s life, reproductive cells will revert back into ovary cells, effectively changing the sex of the individual. In females, outspoken male-bias genes like DMRT1 must be continually silenced! Sex isn’t a question of yes or no, off or on, its an ongoing cacophony of voices. Why would our bodies operate in such a chaotic way, maintaining the potential to take on characteristics of the opposite sex long after maturity? The mechanism is surely a hangover from our fishy sex-swapping origins, but there’s no concrete answer to why it’s been preserved. 

More layers of complexity arise when we realise that every hormone needs a corresponding receptor for its effect to be felt. For example, androgen receptors allow the “male hormone” testosterone to bestow its effects on development. Yet all the genes for building these receptors are located on the X chromosome. The “female chromosome” contains the receptor for testosterone! Concepts of specific “male and female behaviours and characteristics” become rather watery with so many sex-bias genes interacting with each other. Adding the complexities of human cultural and sociobiological evolution to the pot gives us plenty of incentive to retire old-fashioned male/female stereotypes. There simply are no robust reasons for assigning gender-appropriate behaviours or aesthetics.

“Culture tends to argue that it forbids only that which is unnatural. But from a biological perspective, nothing is unnatural. Whatever is possible is by definition also natural. A truly unnatural behaviour, one that goes against the laws of nature, simply cannot exist, so it would need no prohibition.”

– Yuval Noah Harari, Sapiens

While people love to assign tidy labels to each other, there’s a huge middle ground between ‘male’ and ‘female’ that relies on fluctuating hormone levels, the shape of someone’s genitalia, and the arbitrary nature of genetic traits. Sex and gender are not simple concepts – there’s a world of variation between the two piles. Whether a particular individual or culture has come to appreciate this seems irrelevant. Just because we’ve recently invented an assortment of modern labels, if LGBTQ+ individuals exist today, they’ve always existed, at every point in human history. Such diversity is a natural part of any species with a healthy, dynamic, complex, haphazard, and utterly beautiful system for determining ‘sex’.