We have seen that that last common ancestor of all eukaryotes had a genome that contained as many, if not more, introns as complex, metazoan life forms. So how did these ancient organisms process all these introns? Did they have a simple mechanism for doing so or did they rely on something like a modern-day spliceosome?
Recently, a study was published that addressed just this issue [1]. It began by listing three possible hypotheses:
Investigating the distribution of splicing mechanisms and spliceosome components among eukaryotic lineages can reveal how splicing and the spliceosome evolved within eukaryotes. In this study, we investigate three hypotheses of spliceosome evolution.
The first is that the spliceosome appeared in eukaryotes shortly after the eukaryotic ancestor, possibly by invasion by self-splicing introns. It is possible under this hypothesis that some eukaryotic lineages do not contain introns or spliceosomal components.
The second hypothesis is that the eukaryotic ancestor had a basic spliceosome that increased in complexity in multicellular eukaryotes. This complexity increase through time would be similar to intron length which appears to have increased in multicellular eukaryotes. Under this scenario, we could expect to find some, but not many, highly conserved splicing proteins present throughout extant eukaryotes.
These first two hypotheses are not mutually exclusive in that an invading self-splicing intron could lead to a spliceosome that increased in complexity over time.
The third hypothesis is that the eukaryotic ancestor contained a spliceosome that is similar in complexity to the spliceosome present in today’s eukaryotes, with the expectation that we could find many spliceosomal proteins throughout eukaryotic lineages.
So which hypothesis is best supported by the evidence?
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