Achiasmy
One-sentence definition. Achiasmy is the complete absence of crossovers (chiasmata) during meiosis in one sex, eliminating the obligate crossover that normally tethers homologous chromosomes together during their segregation.
One-sentence analogy. Achiasmy is like removing the velcro between chromosome pairs during meiosis: they must still separate correctly, but they do so without the physical connection that normally ensures the pair stays together long enough to be pulled to opposite poles.
Why it matters. Achiasmy is a downstream consequence of the fragile Y ratchet. Achiasmy-causing mutations fix ~4× faster on the Y chromosome than on the X and ~18× faster than on autosomes under sexual antagonism, because Y-linked mutations causing achiasmy specifically reduce male aneuploidy (the very problem the Y is prone to). In young sex-chromosome systems, sexual antagonism drives achiasmy; in highly diverged heteromorphic systems, aneuploidy reduction takes over as the primary driver. Achiasmatic clades like beetle Trechitae show dramatically fewer XO species than expected, confirming that achiasmy stabilizes Y retention.
Where you meet it in the wiki.
- Fragile Y hypothesis — achiasmy is an evolutionary escape from the PAR-aneuploidy ratchet.
- Sex chromosome evolution — models of when achiasmy evolves.
Primary citation.
“Y chromosomes have a higher tendency to fix these mutations across the studied parameter space and do so with faster dynamics, reaching equilibrium 4 times faster than when the mutation is on the X chromosome and 18 times faster than when it is on an autosome under the same parameters.” — Barboza & Blackmon 2025, Finding 1
Prerequisites: pseudoautosomal region, recombination suppression Next, learn about: aneuploidy, holocentric chromosome