The General Rule

As I noted earlier, one of the clues to support the hypothesis that introns facilitated the evolution of multicellular life was that as a general rule, introns are far more common in multicellular genomes than single-celled genomes.  I then pointed to a few specific examples to illustrate this point.  Let me help you better visualize this by considering intron counts in 29 different eukaryotic species, including metazoans, plants, fungi, and protozoans. If you go here, you will find a table from a research paper that provides the count.

Let’s now score them as being multicellular or unicellular:

Multicellular

Anopheles gambiae 37 901

Apis mellifera 145 454

Arabidopsis thaliana 91 222

Aspergillus fumigatus 18 293

Aspergillus nidulans 24 772

Caenorhabditis elegans 137 752

Ciona intestinalis 196 139

Dictyostelium discoideum 17 468

Drosophila melanogaster 19 390

Fugu rubripes 171 912

Homo sapiens 307 019

Oryza sativa 100 262

Phanerochaete crysosporium 44,855

Phytophthora sojae 34 525

Phytophtora ramorum 24 896

Ustilago maydis 4900

Unicellular

Bigelowiella natans 861

Cryptococcus neoformans 35 032

Cyanidoischyzon merolae 27

Encephalitozoon cuniculi 15

Entamoeba histolytica 3125

Ostreococcus tauri 6450

Paramecium tetraurelia 1082

Plasmodium falciparum 7426

Plasmodium yoelii 8143

Saccharomyces cerevisiae 266

Schizosaccharomyces pombe 4730

Thalassiosira pseudonana 15 636

Toxoplasma gondii 27 495

Trichomonas vaginalis 58

Yarrowia lipolytica 829

Let’s now plot the data.

Thar she blows.  As you can see, if there are more than 40,000 introns, 100% (8/8) are multicellular.  If there are less than 10,000 introns, 92% (12/13) are unicellular (if there are less than 1000, then 100% are unicellular).  In the range of 10,000 to less than 40,000 introns, there is an overlap, where 70% (7/10) are multicellular.   The overlap tells us our general rule is not an absolute rule (but we already knew that from this exception), but clearly multicellular life is pushed toward the left (intron abundance) while unicellular life is pushed toward the right (intron scarcity).

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3 responses to “The General Rule

  1. Easter Bunny on a rampage?

  2. Generally, organisms with longer genomes have a larger number of introns. I find it interesting also that Archaea can lose introns once acquired, which speaks against the possibility that the ancestral archaea had a larger amount of introns.

    I think another thing that is overlooked in this debate is that the ancestor that preceded plants and animals may not have been a single celled organism, they could have been mutlicellular “sheets”, a la Bhat’s proposal.

  3. Yes, that is an intriguing proposal.

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