Woltering et al. 2020 study genes to elucidate finger origins

Woltering et al. 2020
attempted to elucidate the transition from fins to fingers by studying the genes of extant lungfish, which don’t have fingers and their ancestors never had fingers.

From the abstract
“How the hand and digits originated from fish fins during the Devonian fin-to-limb transition remains unsolved.

No. The large reptile tree (LRT; subset Fig. 1) solved that problem in 2019 following the work of Boisvert, Mark-Kurik and Ahlberg 2008. These authors found four finger buds on Panderichthys. Thereafter four fingers appear on all basalmost tetrapods in the LRT, like Trypanognathus (Fig. 2), a taxon found in Carboniferous strata with Middle Devonian origins. Taxon exclusion is once again the problem here.

Late Devonian taxa with supernumerary digits, like Acanthostega and Ichthyostega, are the traditional ‘go-to’ taxa for the fin-to-finger transition. That was supplanted in 2019 by phylogenetic analysis in the LRT (subset Fig. 1). Simply adding taxa recovers Acanthostega and Ichythyostega as terminal taxa. They have more derived skulls and bodies sporting larger limbs and more digits.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Woltering et al. 2020 report,
“Controversy in this conundrum stems from the scarcity of ontogenetic data from extant lobe-finned fishes. We report the patterning of an autopod-like domain by hoxa13 during fin development of the Australian lungfish, the most closely related extant fish relative of tetrapods.”

In other words, Woltering et al. looked at genes in lungfish that never had digits.

Figure 6. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Figure 2. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs at the fin-to-finger transition. Acanthostega and Ichythyostega have more derived bodies with larger limbs and more digits.

Why study lungfish
when we have fossil taxa (Fig. 3) in the lineage of tetrapods? Why study genes when genomic studies produce false positives in deep time? Taken together the Woltering et al. study seems like a waste of effort on both fronts, but they didn’t realize this at the time. Paleontologists love genomics like Isaac Newton loved alchemy.

Figure 3. Forelimb of several basal tetrapods rearranged to more closely fit the LRT. Four fingers turns out to be the primitive number. Five is a recent mutation. Six was a short-lived experiment in Tulerpeton.

Figure 3. Forelimb of several basal tetrapods rearranged to more closely fit the LRT. Four fingers turns out to be the primitive number. Five is a recent mutation. Six was a short-lived experiment in Tulerpeton.

Woltering et al. 2020 report,”
“Differences from tetrapod limbs include the absence of digit-specific expansion of hoxd13 and hand2 and distal limitation of alx4 and pax9, which potentially evolved through an enhanced response to shh signaling in limbs. These developmental patterns indicate that the digit program originated in postaxial fin radials and later expanded anteriorly inside of a preexisting autopod-like domain during the evolution of limbs. Our findings provide a genetic framework for the transition of fins into limbs that supports the significance of classical models proposing a bending of the tetrapod metapterygial axis.”

Be wary of genetic studies over deep time. They have been shown to deliver false positives way too often to be trusted, or even attempted. Fossils and phenomic studies are better in all respects because they recover cladograms in which all taxa demonstrate a gradual accumulation of derived traits.


References
Boisvert CA, Mark-Kurik E and Ahlberg PE 2008.
 The pectoral fin of Panderichthys and the origin of digits. Nature 456:636–638.
Woltering JM et al. (5-co-authors) 2020. Sarcopterygian fin ontogeny elucidates the origin of hands with digits. Science Advances 6(34): eabc3510 DOI: 10.1126/sciadv.abc3510
https://advances.sciencemag.org/content/6/34/eabc3510

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