Why not Y naught?
I want to be careful about how I pose this question, because it is easy to pose it wrong. I am not asking why the X does not degenerate. We know why that does not happen. The X recombines in females and purifying selection holds it together. The question I actually find strange is a different one. New genetic material walks into genomes all the time. B chromosomes, aneuploids, chromosomal fragments from fissions and translocations. Any of those pieces could, in principle, capture a sex-determining factor and take over the role of the existing X. Once a new chromosome is doing the sex-determining work, there is nothing keeping the old X differentiated. Selection for linkage between sex determination and sexually antagonistic alleles would now be building up on the newcomer, not on the old X. The old X should fix as diploid in both sexes and revert to an autosome. The system left behind looks like Y0 or W0, a single small univalent segregating in the heterogametic sex with the former X or Z now sitting quietly among the autosomes. Evolution has had countless opportunities to do this. It almost never does. Michelle Jonika led a short perspective piece that our group published in Heredity in 2022 that asks why. This page walks through that argument at the level of a senior undergraduate who has had one solid evolution course.
The setup
Quick terminology. In species with male heterogamety, females carry two X chromosomes and males carry one X and one Y, written XX/XY. Heterogamety just names the sex with the mismatched pair. In species with female heterogamety, the letters change to Z and W, with ZW females and ZZ males. Sometimes the Y is absent and males are X0 (read "X zero"), carrying a single X and no partner. On the female-heterogametic side the corresponding reduction is Z0. These X0 and Z0 configurations are common. Many insect orders use them as their default. The mirror outcome, Y0 or W0, where the X or Z is the missing partner and the Y or W segregates alone, is what the paper is about.
Synthesizing karyotype data across 10,754 species with documented genetic sex determination (after excluding species with multiple sex chromosomes, which usually reflect later autosome fusions), roughly 67 percent are XX/XY and roughly 28 percent are XX/X0. Zero are confirmed Y0. The asymmetry shows up on the female-heterogametic side as well. The only confirmed W0 case in the literature is the New Zealand frog Leiopelma hochstetteri, where most populations have females carrying a single univalent W on top of 11 autosomal bivalents and a variable number of B chromosomes. A univalent is a chromosome with no pairing partner at meiosis. One case in ten thousand is the kind of number that asks for an explanation.
Why I expect Y0 to keep arising
The premise of the paper is not that Y0 is hard to build. It is that Y0 should be easy to build, and the puzzle is why it does not stick. The relevant biology is that chromosomes are not a fixed inventory. New bits of genome show up all the time. I want to list the routes that should, in principle, let a new chromosome grab sex determination away from the incumbent X.
First, univalent segregation is a solved problem. A lone chromosome with no partner at meiosis is a potential mechanical headache, and if it were a real barrier we would expect X0 and Z0 to be rare. They are not. Roughly a third of karyotyped species have univalent sex chromosomes, including large ancestrally-X0 clades like Orthoptera and Odonata. Organisms handle univalent segregation with two families of spindle behavior, and the details differ across taxa. The upshot is the same. Univalent sex chromosomes segregate reliably. Y0 would not fail at this hurdle.
Second, sex chromosome systems are visibly labile. Sex determination moves from one chromosome to another fairly often on evolutionary timescales. The ancestor of Drosophila melanogaster had its sex-determining role on the dot chromosome, which is now an autosome, and the current X and Y in Drosophila started out as autosomes. Polyneoptera, the insect clade that contains grasshoppers and cockroaches and their relatives, is ancestrally X0 and has undergone dozens of independent transitions to XY through X-to-autosome fusions. Y chromosomes have been lost many times independently, including in Lepidoptera, Nematoda, Orthoptera, Odonata, and repeatedly in Coleoptera. Neo-sex chromosomes, meaning new sex chromosomes formed when an autosome fuses to an existing sex chromosome, are everywhere you look. If the machinery can repeatedly reassign sex determination from one chromosome to another, the opportunity for a new univalent to take over is there.
Third, B chromosomes are common and already have the right shape. A B chromosome, sometimes called a supernumerary, is a small, often heterochromatic, gene-poor extra chromosome carried by some individuals in a population but not others. They turn up in 2,087 plant species, 736 animal species, and 14 fungi species in current databases, so the raw material is widespread. A B chromosome can acquire a sex-determining factor either by transposition of an existing master regulator from the Y or by a new sex-determining mutation arising directly on the B. Both routes have been documented. Three lineages of Lepidoptera appear to have W chromosomes that are captured B chromosomes, including the butterfly Dryas iulia. In fish, the cavefish Astyanax mexicanus carries a B chromosome that acts as a male-determining univalent Y, with leaky inheritance. Its congener Astyanax scabripinnis has a B chromosome that behaves like a W. The cichlid Lithochromis rubripinnis has a female-biased B chromosome that skews clutch sex ratios toward daughters. Several lineages look like they are walking this path right now.
So the ingredients are there. Segregation works, turnover is documented, and supernumerary material is common and can carry sex determination. We should see Y0 systems dotting the tree of life. We do not.
Why they do not persist
The paper lays out two main reasons, and a corollary that falls out of the second one. Read together, they predict that Y0 and W0 configurations are unstable and transitory rather than absent.
The first reason is sexual antagonism. Sexually antagonistic alleles are polymorphisms that benefit one sex and harm the other. A coloration allele that helps males attract mates but makes females easier for predators to spot is the textbook version. Theoretical and empirical work over the last four decades has established that SA polymorphism is pervasive, not a rare corner case. Whenever an SA locus sits on an autosome, any rearrangement that links the male-beneficial allele to the male-inherited sex chromosome is favored by selection. In a lineage sitting in Y0, that looks like a fusion between the univalent Y and the autosome that carries the SA locus. Once the fusion happens, the old autosome is now part of the Y and its unfused homologue, present in two copies in females and one in males, is now acting like an X. The system has just reinvented XY. The empirical fingerprint is already visible. Fusions joining the Y to an autosome are documented in the Japan Sea stickleback, and the recurrent X-to-autosome fusions driving X0-to-XY transitions in Polyneoptera are the same process running from the other direction. The same argument applies on the W0 side and predicts collapse back to ZW.
The second reason is recombination failure. A chromosome that sits alone in the heterogametic sex does not recombine, and over time that is a fitness problem. Transposable elements expand inside non-recombining regions, and weakly deleterious mutations accumulate because selection on them is inefficient. Essential genes can sometimes be preserved by strong purifying selection, as with mammalian SRY across roughly 150 million years, but most loci cannot. A univalent Y that carries a meaningful amount of gene content is on a decay clock. This is the same process that drives ordinary Y degeneration, but in the Y0 case there is no recombining X in the background and no recombination in either sex, so the decay has nothing to push against.
The corollary, which the paper makes explicit, is a size argument. The rate at which a non-recombining chromosome loses fitness scales with how many sites on it are under selection. Bigger chromosomes decay faster. For a new univalent sex chromosome to sweep and stick, it has to be small. This is a separate reason to expect B chromosomes to be the most likely raw material for Y0 or W0 origins, because B chromosomes are already small and gene-poor. It also predicts that if Y0 ever does fix, it will be a stripped-down object, nearly empty, carrying a sex-determining factor and not much else.
What this says about sex chromosome evolution
Two small reflections to close. The two sides of the Y0 question are not in tension. The "should see it" side and the "do not see it" side are describing the same system at different points in its life cycle. Y0 and W0 may arise repeatedly, each time a B chromosome or a fragment grabs sex determination, and each time the combination of sexual antagonism and mutational decay ends the run. The reason we almost never catch one in a karyotype is that the window during which they exist is short.
More generally, it is tempting to treat XY and X0 as symmetric collapse products of the same two-chromosome system. They are not. X0 works because the surviving chromosome still recombines in the homogametic sex. Y0 fails because the surviving chromosome would never recombine anywhere, and because sexual antagonism pulls the system back into a bivalent configuration before it can fix. Sex chromosome systems have more inertia than naive turnover models assume, and the inertia is asymmetric. The honest test is going to be more transitional systems like Astyanax, better models of how long a univalent sex chromosome can persist, and direct measurements of sexual antagonism in candidate lineages.
One last boundary. There is a separate puzzle about why Y chromosomes are prone to being lost at all, sometimes called the fragile-Y hypothesis. That question asks when an XY system collapses to X0. The Y-naught question is different. It takes Y loss as given and asks why some new chromosome does not step in and take over sex determination, flipping the system into something Y0-like. Different question. Different paper. Worth not blurring together.
This essay walks through a perspective piece, not an empirical study. Jonika and colleagues did not discover a new Y0 species or run a mutation accumulation experiment. They pulled together the comparative data, pointed at how extreme the asymmetry is, and proposed a mechanism. It is testable, which is the standard I care about.
Reference: Jonika MM, Alfieri JM, Sylvester T, Buhrow AR, Blackmon H. 2022. Why not Y naught. Heredity 129, 75-78. doi:10.1038/s41437-022-00543-z