Extensive splicing deficiency in a degenerating mating-type chromosome.

Splicing deficiency may represent a critical yet underexplored form of genomic erosion in non-recombining regions. Across four phytoplankton species diverged ~333-639 million years ago, genes within U (female) and V (male) "UV" mating-type regions-non-recombining chromosomal regions that determine mating compatibility-show strikingly elevated intron retention relative to genes in other genomic regions. Long-read data reveal abundant truncated, likely non-functional mRNA isoforms despite preserved coding potential. This suggests that splicing defects arose early in UV evolution and have persisted over deep time. We propose that these defects arise from evolutionary changes in sequence composition and chromatin organization that accompany recombination suppression, such as reduced GC content, altered nucleosome occupancy, and disrupted methylation, that collectively compromise splicing fidelity. Unlike sex chromosomes, which often degenerate through gene loss, splicing-deficient UV regions in green algae retain hundreds of genes, indicating that transcript-level dysfunction provides an alternative route to functional decay. Our results identify chromatin-mediated splicing deficiency as a novel axis of genomic erosion and position algal UV systems as models for studying how recombination suppression reshapes RNA processing fidelity in essential, non-recombining genomes.