Sorry parents, but the birds and bees conversation just became a lot harder..
I’ve been studying visualizing evolution for the past few years and learned that evolution occurs through a combination of vertical (like reproduction, where DNA is passed to the next generation) and horizontal processes (like HGT, where DNA is passed between organisms without reproduction). I’m interested though in how reproduction could be considered both a vertical and horizontal process. Its definition is in much turmoil and I’m curious to see what a big redefinition might mean for our understanding of evolution. In Evo-Repro: Painting with Natural Selection, we’re building a simulation to visualize the role of different reproductive strategies in evolution.
But to do all that, we should know what sex is. So, what is your definition of sex?
Off the top of my head, I thought sex was:
Right? Maybe not.
I came across a great paper called “Sex reduces genetic variation – A Multidisciplinary review” (!) that has a great overview of how hard it is to define sexual reproduction (Gorelick, Heng 2010). The rest of this post is me trying to summarize/digest that section. Any mistakes are entirely mine. Please correct me or add to the discussion!
What is sex?
(1) Some people say sex = meiosis. Meiosis is the process that creates eggs and sperm, so it seems pretty fundamental to sex.
BUT, this definition creates some awkwardness: male Drosophila undergo meiosis but skip recombination so they would be called asexual. ALSO, crossing-over can actually happen during mitosis (asexual reproduction) so maybe this isn’t a good definition.
(2) Sex = “any process that mixes genes”. This definition means that processes like horizontal gene transfer, transposable elements or mitosis with recombination would also be considered sex! The authors dismiss this definition, but I’m actually very curious about it since it might allow an expansion of the concept of reproduction to encompass two types of processes: genome alteration and multiplication/growth of the phenotype.
The authors don’t mention bacterial conjugation but that would also de defined as sexual reproduction – which really makes sense to me.
(3) Sex = fertilization (egg + sperm) between genetically different individuals. This would mean that self-fertilization would not be considered sex. It would also mean that sex between cloned organisms, and so genetically identical, would also not be considered sex. In these cases though, organisms are undergoing all the steps of meiosis above and of fertilization, reminding us that fertilization is actually two steps, the fusion of the gametes and the fusion of their nucleus (syngamy):
Should reproduction between genetically-identical organisms that includes meiosis and syngamy really not be considered sex? What about when we consider that “different” organisms actually need to be very, very genetically similar to achieve sexual reproduction (ie = belong to the same species). The criteria of “genetic difference” between the parents seems not to be very strong.
(Are there organisms where the sperm binds the egg but the cells don’t fuse?)
(4) The authors and other researchers propose then that sex = meiosis + syngamy. But this description is challenged by some borderline cases where meiosis occurs (duplication, segregation, recombination) but skips one of the cellular divisions and so does not require syngamy (parthenogenesis).
These examples make clear why some people think that syngamy isn’t necessary to define sex (going back to definition 1: sex = meiosis) and why some say that syngamy might be just a “modified form of meiosis.”
But, the authors suggest sticking with this definition sex = meiosis + syngamy… or at least, hanging on to it as hard as you can.
And just to make sure it’s not simple, the authors add that the cases where meiosis is followed by endomitosis (duplication of chromosomes) so that the haploid cell becomes diploidal without fertilization or syngamy would also be a form of sex.
Here is a summary of the different definitions and examples, from what I understand:
Are there other examples to consider that have other combinations of these options?
Either way: EVO-REPRO is coming!