Now that we have seen that VOLCADRAFT_41528 (from Volvox carteri ) is likely to be homologous to human beta catenin, it becomes reasonable to propose that something very similar to beta-catenin existed in the last common ancestor of all eukaryotes.
However, if we are going to make this rather radical claim, it would help if we could find homologs in other lineages. So let me begin by pointing out that homologs of human beta catenin exist in all three basal metazoan lineages: Trichoplax, sponges, and comb jellies. Among metazoans, the beta catenin sequence is strongly conserved. For example, 340/592 (58%) of the positions have identical amino acids when sponge sequence is aligned with human sequence.
So beta-catenins are universally present among metazoans and found in the multicellular algae, Volvox. But what about true unicellular organisms?
If you go back to the table I posted a few days ago, we do have one such example – Trypanosoma brucei, a protozoan that causes African sleeping sickness, has a protein that has similar amino acids in 90/204 (45%) positions. But the E value for this match is only 2.00E-06. Smaller than 1e-04, but not that much. The bunnah wants more crunch in his carrot.
To better resolve this issue, I simply took the next logical step – I used the Volvox homolog to BLAST other green algae and various protozoan lineages. The results are shown in the below table:
| Unicellular species | % identity | % positives | E value | # Arm repeats | Tree color |
| Chlorella variabilis | 177/546 (33%) | 257/546 (48%) | 5.00E-36 | 10 | Green |
| Micromonas sp. RCC299 | 120/394 (31%) | 185/394 (47%) | 9.00E-29 | 9 | Green |
| Polysphondylium pallidum | 101/317 (32%) | 156/317 (50%) | 6.00E-22 | 8 | Purple |
| Chlamydomonas reinhardtii | 122/406 (31%) | 190/406 (47%) | 1.00E-20 | 7 | Green |
| Theileria parva | 98/373 (27%) | 172/373 (47%) | 1.00E-19 | 9 | Pink |
| Ostreococcus lucimarinus | 92/296 (32%) | 138/296 (47%) | 2.00E-19 | 5 | Green |
| Dictyostelium discoideum | 93/303 (31%) | 151/303 (50%) | 3.00E-20 | 8 | Purple |
| Perkinsus marinus | 113/367 (31%) | 165/367 (45%) | 4.00E-18 | 9 | Pink |
| Monosiga brevicollis | 100/327 (31%) | 152/327 (47%) | 2.00E-17 | 6 | Purple |
| Paramecium tetraurelia | 79/310 (26%) | 136/310 (44%) | 7.00E-12 | 8 | Pink |
| Tetrahymena thermophila | 76/285 (27%) | 121/285 (43%) | 1.00E-10 | 9 | Pink |
| Trypanosoma brucei gambiense | 134/520 (26%) | 228/520 (44%) | 1.00E-26 | 5 | Yellow |
| Leishmania major | 140/538 (27%) | 227/538 (43%) | 3.00E-17 | 5 | Yellow |
Jackpot!
As you can see, there are many homologs of the Volvox protein that was, in turn, homologous to metazoan beta-catenin. And most of them appear to have 8 or more Arm repeats. Also, in case you are not familiar with all those species, the column on the far right indicates the colored region of the phylogenetic tree from the figure below:
As you can see, 4/5 major clades are represented, vastly strengthening the case that a beta-catenin-like protein was present in the last common ancestor of all eukaryotes.
Anyway, most of those matches are listed as hypothetical proteins. In the next posting, we’ll talk about one that is not.
