Does the bowfin gas bladder represent an intermediate stage during the lung-to-gas bladder evolutionary transition?

Area of Science:

• Evolutionary developmental biology
• Comparative anatomy
• Ichthyology

Background:

• The evolution of the gas bladder in Actinopteri (ray-finned fishes) from ancestral lungs is a key area of study.
• A significant difference lies in the foregut budding direction: ventral for lungs and dorsal for gas bladders.
• The developmental morphology of the bowfin gas bladder, sister lineage to teleosts, has been debated, with historical suggestions of lateral budding.

Purpose of the Study:

• To investigate the precise budding location during bowfin gas bladder development.
• To determine if bowfin gas bladder development represents an intermediate stage between ventral lung budding and dorsal gas bladder budding.
• To verify historical studies on gas bladder developmental morphology using advanced imaging.

Main Methods:

• Utilized nano-CT scanning for high-resolution visualization of early bowfin gas bladder development.
• Examined the morphological changes and budding site during embryonic development.
• Compared observed budding patterns with known lung and gas bladder development.

Main Results:

• The bowfin gas bladder originates from a definitively dorsal budding site on the anterior foregut.
• Early development shows a temporary posterior twist to the right, creating a transient lateral appearance.
• As development progresses, this twist lessens, and the organ shifts to a mid-dorsal position.

Conclusions:

• The bowfin gas bladder develops dorsally, refuting the hypothesis of a lateral-budding intermediate stage.
• This dorsal budding pattern clarifies the evolutionary trajectory from ventral lungs to dorsal gas bladders.
• The temporary lateral twist is a developmental feature, not indicative of the primary budding direction.