No, we haven’t gone all Carrie Bradshaw on all of y’all. Last evening’s enthralling discussions with this week’s fantastic @RealScientists curator @turtlesatJCU about temperature-dependent sex determination in turtles – how the temperature of the sand they are surrounded by dictates the sex of the developing turtle embryo – reminded this particular RS admin just how wonderfully bizarre sex determination is in an evolutionary sense.
It’s generally the case that if something’s important, it’s important to make sure that the instructions for making it aren’t easy to lose. Anyone who’s tried to assemble a flat pack Ikea wardwobe without the cartoon diagrams or the Allen key would attest to this. So it is with vertebrate development. The developmental pathways – the basic blueprints, floorplan, instructions, Lego pieces, however you want to think of them – which underpin key developmental structures are, in general, strongly conserved (that is, they’ve not changed much through evolution.) The genes which are expressed and the cell types which are involved in directing the making of a limb – in particular its outgrowth and its ‘patterning’ (a developmental biology term for how future adult structures are laid out in the developing embryo) – are the same between all vertebrates. All there is, really, is a little tweaking in terms of expression domains (i.e. the precise timing of when key developmental regulators are switched on and off, and in what cells of the developing limb) to produce a whale flipper from the same basic genetic ‘blueprints’ as the bat wing. This idea that if it’s important, it’s conserved, runs right through evolution (and particularly ‘evo devo’, the research field at the intersection of evolution and development). One of the most important genes in human heart development is the analogue of one which directs development of the heart in the fruit fly Drosophila, called Tinman.*
Except you can chuck all that out the window with sex determination. You’d think that since sex was such an important evolutionary construct – the ability to reshuffle the genes of your offspring to create the potential for evolutionary advantage is clearly crucial to the success of the majority of animal species, since so many have it – that the instructions for how to create different genders which can make with the bunga-bunga in order to recombine chromosomes and create new genotypes would go right back in the evolutionary lineage.
Mammals like us (assuming you are a human, which I feel is a reasonable leap of confidence) have a system based on the inheritance of the sex chromosomes X and Y, where embryos with two Xs develop as females and those who inherit an X and a Y develop as males. This was pinned down in the late 80s/early 90s to a particular gene on the Y chromosome called SRY (sex determining region on the Y chromosome – mouse and human researchers aren’t as interesting with their gene names as Drossie peeps) which acts as a ‘switch’ midway through gestation. If it’s there, its expression is switched on briefly in the embryonic precursor tissue of the gonad (which, uniquely in development, is a developmental structure that can go on to form two completely different organs from the same origin tissue) and the testis development pathway is kicked off – if it’s not, i.e. if you’re XX, SRY isn’t expressed, and the pathway ‘defaults’ to female (and yes this is a slightly patriarchal view which has been challenged but we’ll set that aside for the moment). But SRY and XX/XY is by-and-large a mammalian deal. Reptiles, as discussed on @RealScientists last night, have temperature-dependent sex determination; it’s thought a reptile with TSD may have been the evolutionary precursor of vertebrate species with chromosomal sex determination. Birds such as chickens have chromosomal sex determination, a bit like XX/XY (it’s ZZ/ZW) but there’s no analogous sex determining gene driving the development of one sex or the other. Some fish have sex determining chromosomes or genes a bit like an SRY in broad function, but nothing like it in terms of molecular or genetic similarity; other fish have TSD. Drosophila have an inheritable sex chromosome (XX/X0) but again the molecular ‘bits of Lego’ involved are nothing alike other species. Nematode worms (C. elegans, another invertebrate ‘model organism’) have an unrelated XX/XO system which directs development of males, females and hermaphrodites. What we have here then is a critical developmental process, central to the survival of the majority of successful animal species on Earth, which has bugger-all in common at the level of how that process is switched on between those species. It looks like case after case of evolutionary novelty – that is, nearly each species has evolved a completely different way of determining sex, and has run with it.
What’s interesting though is the ‘downstream’ stuff often IS conserved, even in species with very different means of reproduction, or gonad structures. Hormones like aromatase, and key regulatory genes such as DAX1, DMRT1 and SOX9 which are critical in mammalian sex determination and gonad development keep turning up in the pathways of other species, even those without ‘genetic’ sex determination like reptiles with TSD. In an evo-devo sense, pretty much the same genetic players and partners are deployed in the process of making boy and girl bits – they’ve just been co-opted into the process by completely different switch mechanisms, which are evolved and selected for with no apparent commonality.
For further reading on this, have a search for ‘evolution of sex determination’ on Google Scholar. Or look it up on Wikipedia, which is generally pretty good for SCIENTS stuff.
And to finish, BABBY TURTLES.
*Drosophila researchers have always been big on the quasi-amusing gene names, usually named for some characteristic of a ‘mutant’ strain generated with that gene ‘knocked out’. They go a bit weird cooped up in fly rooms counting and classifying wing variants on tiny fruit flies. Many of which have a habit of flying up the noses of the investigators, if not anaesthatised correctly. Apparently they taste like sesame seeds. Don’t ask.