Since their discovery, minor introns, which make up around 0.5% of the introns in the human genome, have been a mystery. The minor spliceosome, a unique splicing complex, eliminates these introns. Both have a long evolutionary history that dates back to the Last Eukaryotic Common Ancestor (LECA), as seen by minor intron enrichment in particular gene families such as the voltage-gated sodium and calcium ion channels, mitogen-activated protein kinases, and E2F transcription factors. In genes with a preponderance of major introns, the majority of minor introns are found as single introns. This arrangement increases the likelihood of missplicing since Minor Intron-Containing Gene (MIG) expression necessitates the coordinated action of two spliceosomes. Therefore, one would anticipate modest intron loss through purifying selection. At least nine eukaryotic lineages have experienced complete minor intron loss as a result of this. The importance of minor introns is highlighted by the embryonic lethality that results from the inactivation of the minor spliceosome in land plants and metazoans, where they are highly conserved. Rapidly proliferating progenitor cells are extremely vulnerable to minor spliceosome loss, as demonstrated by conditional inactivation of the minor spliceosome. In fact, we discovered that MIGs were considerably enriched in a 341 cycling cell line screen for genes necessary for survival. Here, we suggest that minor introns were randomly inserted into genes in LECA or earlier and were later conserved in genes essential for cycling cell viability. We propose that minor introns survived the unicellularity of early eukaryotic evolution because of the essentiality of MIGs. Further supporting our essentiality paradigm for MIG conservation, we found 59 MIGs that appeared after LECA, several of which are crucial for cycling cell survival. This shows that the development of minor introns is dynamic throughout the evolution of eukaryotes and that minor introns should not be thought of as molecular fossils. We further suggest that minor intron splicing was used as a regulatory switch for en masse control of MIG expression and the biological processes it regulates during multicellular evolution. According to domestication syndrome, which shows that MIGs are enriched in common candidate genes for animal domestication, this mode of regulation could specifically control cell proliferation and consequently body size.
