Ancient Hox code reveals the developmental roots of vertebrate hind fins and limbs
Paired appendages—the pectoral and pelvic fins of fishes and the forelimbs and hindlimbs of tetrapods—were a major innovation in vertebrate evolution. Their emergence greatly expanded the ways vertebrates could move through their environments and ultimately provided the developmental foundation for the evolution of limbs. Yet how the genetic system responsible for building paired appendages was…
The ancient "Hox code" has provided insights into the developmental origins of vertebrate hind fins and limbs, a crucial aspect of vertebrate evolution. Researchers led by Akinori Kawamura from Saitama University examined the role of Hox genes, specifically HoxA and HoxD paralog groups 9-13, in controlling the later patterning of fish fins and tetrapod limbs.
While the connection between the Hox code and the developmental program of paired appendages was suspected, direct evidence of the necessity of posterior Hox genes during the earliest stages of hind appendage formation remained limited. To address this gap, the research team utilized zebrafish with Hox gene mutations, fluorescent reporter lines for visualizing developing pelvic fin progenitors, skeletal imaging, and CRISPR-Cas9-mediated Hox gene disruption in the Iberian ribbed newt.
The experiments revealed that the simultaneous disruption of five HoxA/D paralog group 9-10 genes in zebrafish led to the complete loss of pelvic fins and the absence of detectable fin progenitor cells. This suggests that HoxA/D PG9-10 genes act prior to the formation of morphologically recognizable pelvic fin buds. Furthermore, the researchers found that these genes are also essential in the Iberian ribbed newt, a lobe-finned vertebrate lineage, as the simultaneous disruption of all seven Hox9-10 paralogs resulted in hindlimb loss and reduced expression of Prdm16 in prospective hindlimb-forming regions.
The findings indicate that the requirement for Hox9-10 genes in establishing hind appendage progenitors predates the evolutionary divergence of ray-finned fishes and lobe-finned vertebrates. This research establishes a developmental connection between the posterior Hox code of the body axis and the Hox system responsible for building paired appendages, providing functional support for the idea that an ancestral posterior Hox program contributed to the evolutionary assembly of this system.
The study also aligns with a stepwise evolutionary model where the ancient Hox9-10 program involved in early hind appendage development was later expanded to include additional posterior Hox genes and redeployed in anterior paired appendages.
Written by urgent.news from Phys.org's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
