Data from: where the minor things are: a pan-eukaryotic survey suggests neutral processes may dominate minor spliceosomal intron evolution

Spliceosomal introns are gene segments removed ("spliced") from
RNA transcripts by large ribonucleoprotein machineries called
spliceosomes. In some eukaryotes a second spliceosome (the minor/
U12-type) is responsible for processing a tiny minority of introns.
Despite its seemingly modest role, minor splicing has persisted for
roughly 1.5 billion years of eukaryotic evolution. Identifying and
cataloging minor introns in > 3000 eukaryotic genomes, we report
diverse evolutionary histories including surprisingly high numbers of
minor introns in some fungi and green algae, repeated massive loss, as
well as several general biases in the positional and genic distributions
of minor introns. We estimate that ancestral minor intron densities were
comparable to those of the most minor intron-rich species, suggesting a
trend of long-term stasis. Finally, three findings suggest a major role
for neutral processes in minor intron evolution. First, we find highly
similar patterns of minor and major intron evolution, in contrast to the
predictions of both functionalist and deleterious models. Second, we find
that observed functional biases among minor intron-containing genes are
largely explained by these genes' greater ages. Third, we find no
association of intron splicing with cell proliferation in a minor
intron-rich fungus, suggesting that regulatory roles are lineage-specific
and thus cannot offer a general explanation for minor splicing's
persistence. These data constitute the most comprehensive view to date of
modern minor introns, their evolutionary history, and the forces shaping
minor splicing, and provide a foundation for future studies of these
remarkable genomic elements.