Abstract

The genetic code is nearly universal across sexually reproducing life, yet at least sixteen variant nuclear codes have been documented in asexual organisms, and mitochondrial codes diverge from the standard in most animal lineages. We propose that sexual reproduction is the mechanism responsible for this pattern. When two organisms with different codon assignments mate, the zygote contains two competing versions of the translation machinery after zygotic genome activation. Every protein the cell produces is subject to stochastic amino acid misincorporation at every occurrence of the affected codon, producing proteome-wide misfolding. This underdominance barrier purges any codon-reassignment allele from a sexual population within one to a few generations. We survey all documented codon reassignments and show that every nuclear reassignment occurs in an asexual lineage and every mitochondrial reassignment occurs in a uniparentally inherited genome that does not recombine. No exception is known. Ciliates provide a natural experiment within a single organism: the sexually recombining micronuclear genome uses the standard code, while the non-sexually-recombining macronuclear genome in the same cell carries variant codes. The rarest sense codon in a typical eukaryotic proteome appears tens of thousands of times across thousands of genes, far too many positions for a reassignment to be selectively neutral. The prediction is specific and falsifiable: a single codon reassignment in the nuclear genome of a sexually reproducing organism with standard meiosis would refute it.