Past exam of the genetics course of the University of Cambridge / 2024 / Paper 4 / 6 / Solution

The selfish-invasion hypothesis concerns the origin and spread of introns, whereas current function concerns the consequences of their presence today. A sequence can begin by improving its own transmission, later become dependent on host machinery, and subsequently acquire a host-beneficial function. Conversely, present-day persistence need not mean that every intron was retained because it helps the organism.

Group II introns provide a mechanistic model for mobile, self-splicing ancestors of spliceosomal introns. Their structural and catalytic similarities to the spliceosome support a historical relationship, although the detailed route and timing of eukaryotic intron origins remain reconstructed hypotheses. In ordinary spliceosomal introns, many activities needed for RNA processing are supplied in trans by a shared cellular machinery rather than by a complete mobile apparatus encoded in each intron. This dependence limits autonomy but does not demonstrate that all selfish proliferation has ended.

Purifying selection constrains the burden. Insertions that disrupt coding information, essential regulation or reliable splicing can reduce host fitness. Intron loss can remove transcriptional, processing and mutational costs, and selection preserves splice signals needed to produce functional RNA. The balance depends on effective population size, mutation and loss rates, gene expression and the effect of a particular insertion. Weakly deleterious introns can persist when drift overwhelms selection; a large genome-wide intron count is therefore insufficient evidence for either universal adaptation or unrestricted selfish spread.

The host has also harnessed intronic sequence and the processing step. Alternative splicing can connect one gene to several regulated transcripts, while exon shuffling can rearrange protein modules. Introns can contain enhancers, microRNAs and small nucleolar RNAs; splicing can be coupled to RNA export and surveillance. Some alternative events regulate transcript abundance through nonsense-mediated decay rather than producing a stable alternative protein. These examples establish possible functions, not that every alternative isoform or every intronic base has been selected for such a function.

Specific experiments demonstrate functions that an origin story alone cannot predict. Julie Parenteau and colleagues found that deleting introns from budding-yeast genes often impaired performance during nutrient depletion and connected intron presence with control of growth-related expression. Jeffrey T. Morgan and colleagues identified excised introns that become stable linear RNAs under particular growth-inhibitory conditions and contribute to growth regulation. These are evidence for regulated intron-derived activity in defined conditions, not a license to regard all excised introns as functional.^[ref][ref]

To distinguish function from a correlated consequence, an intron deletion must be assessed for effects on expression, local DNA regulation and splicing, with rescue or appropriate sequence controls. Comparative conservation, turnover and experimentally measured fitness costs help determine whether host-level selection maintains a given feature. The extent of containment is thus substantial but incomplete and lineage dependent: shared processing and host selection constrain the elements, while retained introns span functional, tolerated and potentially deleterious cases.