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Spliceosomal introns evolved early in eukaryogenesis, originating from self-splicing group II introns that invaded the proto-eukaryotic genome 1 . Elements of these ribozymes, now called snRNAs (U1, U2, U4, U5, U6), were co-opted to excise these invasive elements. Prior to eukaryotic diversification, the spliceosome is predicted to have accumulated hundreds of proteins 2 . This early complexification has obscured our understanding of spliceosomal evolution. Reduced systems with few introns and tiny spliceosomes give insights into the plasticity of the splicing reaction and provide an opportunity to study the evolution of the spliceosome 3 , 4 . Microsporidia are intracellular parasites possessing extremely reduced genomes that have lost many, and in some instances all, introns 5 . In the purportedly intron-lacking genome of the microsporidian Pseudoloma neurophilia 6 , we identified two introns that are spliced at high levels. Furthermore, with only 14 predicted proteins, the P. neurophilia spliceosome could be the smallest known. Intriguingly, the few proteins retained are divergent compared to canonical orthologs. Even the central spliceosomal protein Prp8, which originated from the proteinaceous component of group II introns, is extremely divergent. This is unusual given that Prp8 is highly conserved across eukaryotes, including other microsporidia. All five P. neurophilia snRNAs are present, and all but U2 have diverged extensively, likely resulting from the loss of interacting proteins. Despite this divergence, U1 and U2 are predicted to pair with intron sequences more extensively than previously described. The P. neurophilia spliceosome is retained to splice a mere two introns and, with few proteins and reliance on RNA-RNA interactions, could function in a manner more reminiscent of presumed ancestral splicing., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 Elsevier Inc. All rights reserved.)

In this Quick guide, Thomas Whelan and Naomi Fast introduce the microsporidia: obligate intracellular parasites with the most extremely reduced genomes known in eukaryotes., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Whereas most eukaryotic genes are interrupted by introns removed by the U2 (major) spliceosome, U12-type introns are extremely rare. New work uncovers a case of extensive U12-type intron gain, and an unexpectedly flexible and efficient U12 (minor) spliceosome., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Microsporidian genomes are extraordinary among eukaryotes for their extreme reduction: although they are similar in form to other eukaryotic genomes, they are typically smaller than many prokaryotic genomes. At the same time, their rates of sequence evolution are among the highest for eukaryotic organisms. To explore the effects of compaction on nuclear genome evolution, we sequenced 685,000 bp of the Antonospora locustae genome (formerly Nosema locustae) and compared its organization with the recently completed genome of the human parasite Encephalitozoon cuniculi. Despite being very distantly related, the genomes of these two microsporidian species have retained an unexpected degree of synteny: 13% of genes are in the same context, and 30% of the genes were separated by a small number of short rearrangements. Microsporidian genomes are, therefore, paradoxically composed of rapidly evolving sequences harbored within a slowly evolving genome, although these two processes are sometimes considered to be coupled. Microsporidian genomes show that eukaryotic genomes (like genes) do not evolve in a clock-like fashion, and genome stability may result from compaction in addition to a lack of recombination, as has been traditionally thought to occur in bacterial and organelle genomes.

The recent discovery of novel biochemical activities of intron-encoded endonucleases emphasizes the selfish nature of mobile genetic elements.