Given the likelihood that the ctenophore lineage is the deepest branch among metazoans, let's have a closer look at the toolkit contained by these creatures to determine if the basic theme of front-loading evolution continues to hold up.
Consider two classes of transcription factors that are known to play crucial roles in metazoan development: the T-box proteins and the SOX proteins. Both factors activate a variety of genes involved in body plan formation, organ formation and cell specification. And as we might expect from a front-loading perspective, PCR analyses have retrieved a rather complex array of both transcription factors from the comb jellies.
Consider the conclusions from two different studies:
T-box genes abruptly evolved around the birth of metazoans, then diversified into at least three of the five known families and at least six of the eight known groups before the divergence of the cnidarians and the bilaterians.
Our analyses show unexpected complexity of the T-box gene family in the diploblastic animals, which is consistent with other investigations in these animal groups.
Surprisingly complex T-box gene complement in diploblastic metazoans
Atsuko Yamada, Kevin Pang, Mark Q. Martindale, and Shin Tochinaia
EVOLUTION & DEVELOPMENT 9:3, 220"“230 (2007)
For the first time, HMG domain sequences from non-bilaterian phyla (Calcispongia, Demospongiae, Ctenophora and Cnidaria) have been included in a phylogenetic study of the SOX family. A first striking finding was the high number of SOX gene sequences recovered from both the cnidarian and the ctenophore representatives (10 and 13 from C. hemisphaerica and P. pileus, respectively). Given that most of these sequences probably represent true paralogues (see Section 3), the SOX family seems to be significantly more diverse in these two non-bilaterian species than it is in D. melanogaster (8 genes), C. elegans (5 genes) or C. intestinalis (6 genes) (Koopman et al., 2004). Among the completely sequenced bilaterian genomes, only the vertebrates have a higher number of SOX genes (e.g., 20 genes in H. sapiens and M. musculus, 24 genes in F. rubripes).
We conclude that SOX genes are likely to be part to the molecular toolkit associated with the acquisition of multicellularity.
Expansion of the SOX gene family predated the emergence of the Bilateria
Muriel Jager, Eric QuÃ©innec, Evelyn Houliston, and MichaÃ«l Manuel
Molecular Phylogenetics and Evolution 39 (2006): 468"“477.
No surprise here.
BTW, a SOX gene has been isolated from single-celled choanoflagellates, but no T-box gene was seen. I predict will we eventually find a T-box gene in some protozoan.