Very interesting summary of the results.
Pdf can be accessed here: A comparative genomic framework for the fish-tetrapod transition
Ahlberg PE. A comparative genomic framework for the fish-tetrapod transition. Sci China Life Sci. 2021 Apr;64(4):664-666. doi: 10.1007/s11427-021-1903-x. Epub 2021 Mar 1. PMID: 33660224.
Some highlights:
The evolution of tetrapods (land vertebrates) from lobe-finned fishes, nearly 400 million years ago, was a hugely important event. For one thing, if these ancient fishes had not found a way to adapt to life out of the water, we humans would not exist. To investigate this event we can look at the fossil record, but that only tells us about changes in the skeleton. The equally important changes that must have occurred in the soft anatomy and physiology would appear to be lost, undocumented, in the mists of time. Or are they?
In two linked papers published this week in Cell, Wang et al. (2021) and Bi et al. (2021) presented a tour-de-force of comparative genomic analysis that shines a light on all those aspects of the transition that cannot be observed from the fossil record. By placing whole-genome sequences from a range of carefully chosen fishes in a phylogenetic context spanning the fish-tetrapod transition (Figure 1), they are able to show in remarkable detail exactly what happened to the genome and the phenotype during that transition
On genome sizes of the lungfish:
Lungfish genomes are gigantic, the largest of any animal. The genome assembly for the African lungfish presented by Wang et al. measures 40 Gb and is the largest ever published for any organism.
On consilience of phylogenies:
So, what have they found? The first thing to note is that their whole-genome phylogenies, unlike some previous molecular analyses based on fewer genes, are compatible with phylogenies derived from morphological data. This seemingly straightforward result is really important, because it strongly suggests that the analyses have identified the true branching pattern of the tree, which in turn means that evolutionary inferences drawn from that branching pattern should be robust. From now on we can assume that we understand how these various animals are related to each other.
A bit about lungs and swim bladders:
Another important result concerns lungs and swim bladders. Morphologists have long argued that these are homologous structures, and that lungs are ancestral to swim bladders. This is confirmed by the genomic and transcriptomic comparisons, which not only find that lung and swimbladder expression profiles cluster together but that the ancestral condition for Osteichthyes as a whole is a lung-type expression profile. Interestingly, lung-related genes are present not only in Osteichthyes but in Chondrichthyes as well, even though they lack lungs, showing that these genes originated at the base of the jawed vertebrates.
And I can’t resist also posting this final statement:
This curious juxtaposition seems like a fitting summation of the findings of these landmark studies. By illuminating genomic evolution among Osteichthyes and across the fishtetrapod transition in such detail, they uncover an extraordinary range of different evolutionary steps. Some are expected, like those associated with the fin-to-limb transition; others introduce unexpected subtleties that we could not have predicted, like the enhanced surfactant function in the lungs of Sarcopterygii; and a few are just weird. Who could possibly have guessed that the common ancestor of lungfishes and tetrapods evolved the molecular basis of a more relaxed attitude to life?
My bold.
