This relationship (lungfish-tetrapods) was firmly established recently by comparing the genome of the Australian lungfish (Neoceratodus forsteri) with that of tetrapods (Meyer et al., 2021). The other possibility had been ceolacanth-tetrapods. Coelacanths and lungfish are related—they form the class Sarcopterygii (lobe-finned fish).
The size of the Australian lungfish genome (43 × 109 bp, 43 Gb) had precluded genome sequencing using the old technology because of the amount of repetitive DNA. The advent of long- and ultra-long sequence reads has enabled sequencing of huge genomes.
Recently the genomes of two other lungfish species have been sequenced (Schartl et al., 2024). The genome of the African lungfish (Protopterus annectens) is about 47Gb and the genome of the South American lungish (Lepidosiren paradoxa)is about 90Gb making it about 30 times larger than the human genome. All three species have about the same number of protein-coding genes (~20,000) but they differ in the amount of repetitive DNA. Most of the repetitive DNA consists of transposon-related sequences (transposable elements, TEs) and Schartl et al. were able to distinguish between old TEs that were no longer active and more recent ones that were still active and spreading in the genome.
Genome expansion in lungfish appears to have occurred over the past 300 My and it accelerated in South America lungfish since it diverged from the others about 100 million years ago. The authors estimated that the South American lungfish genome expanded at a rate equal to 3.71 Gb every 10 Mya. This is equivalent to adding an amount of DNA equivalent to the entire human genome every 10 million years. Most of this expansion seems to be due to more than 75,000 active long retrotransposons (LINEs).
The data is consistent with the standard explanation for the accumulation of junk DNA. The idea is that it is due to the multiplication of TEs that eventually accumulate mutations rendering them inactive.
Meyer, A. et al. (2021) Giant lungfish genome elucidates the conquest of land by vertebrates. Nature 590:284-289. [doi: 10.1038/s41586-021-03198-8]
Schartl, M., et al. (2024) The genomes of all lungfish inform on genome expansion and tetrapod evolution. Nature:1-8. [doi: 10.1038/s41586-024-07830-1]
6 comments :
Small quibble: if taxa must be monophyletic, then the statement Coelacanths and lungfish are related—they form the class Saropterygil (lobe-finned fish) should be modified to read "Coelacanths, lungfish, and tetrapods are related ..."
Also, you misspelled "Sarcopterygii".
Yeah, I noticed that too, but Sandwalk doesn't make it possible to go back and correct a typo. Thanks for joining me in quibbling about the Sarcopterygii.
Larry also misses an opportunity to beat the strangely still-living horse of genomic dark matter. Here we sit between fugu and lungfish. If we can all agree (one hopes) that the lungfish genome is mostly junk, what evidence is there that our genomes are any different? What evidence that we need more than a fugu has?
@Joe We are lungfish. :-)
"The tetrapods, a mostly terrestrial superclass of vertebrates, are now recognized as having evolved from sarcopterygian ancestors and are most closely related to lungfishes. Their paired pectoral and pelvic fins evolved into limbs, and their foregut diverticulum eventually evolved into air-breathing lungs. Cladistically, this would make the tetrapods a subgroup within Sarcopterygii and thus sarcopterygians themselves. As a result, the phrase "lobe-finned fish" normally refers to not the entire clade but only aquatic members that are not tetrapods."
Wikipedia: Sarcopterygii
Larry: No, we aren't lungfish. Lungfish are a clade within Sarcopterygii that's the living sister group of Tetrapoda. Saying we're lungfish is like saying we're marsupials.
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