Interdigital webbing retention in bat wings illustrates genetic changes underlying amniote limb diversification

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Molecular Biology

Background:

• Programmed cell death (apoptosis) is crucial for sculpting embryonic structures, including limbs in vertebrates.
• Species-specific differences in interdigital tissue apoptosis lead to variations like freed digits in mice and webbed feet in ducks.
• Bone morphogenetic proteins (Bmps) typically induce apoptosis, while Bmp antagonists inhibit it.

Purpose of the Study:

• To investigate the molecular mechanisms underlying interdigital tissue maintenance in the phyllostomid bat (Carollia perspicillata).
• To understand the role of Bmp signaling and other factors in bat limb development and the evolution of webbed forelimbs.
• To identify conserved or divergent pathways in amniote limb development.

Main Methods:

• Analysis of Bmp signaling components and apoptosis in bat forelimb and hindlimb interdigital tissues.
• Investigation of fibroblast growth factor 8 (Fgf8) and Gremlin expression patterns.
• Functional assays to determine the effects of Fgf and Bmp signaling on interdigital tissue maintenance.

Main Results:

• Bat hindlimbs exhibit interdigital apoptosis, similar to other vertebrates, despite expressing Bmp signaling components.
• Bat forelimbs retain interdigital tissue, correlating with unique expression of Fgf8 and the Bmp inhibitor Gremlin.
• Functional assays confirm that high Fgf signaling combined with Bmp inhibition maintains interdigital tissue in bat wings.

Conclusions:

• The bat forelimb utilizes a distinct mechanism involving Fgf8 and Gremlin to maintain interdigital webbing, differing from conserved amniote pathways.
• These molecular changes in Gremlin and Fgf8 expression likely contributed to the evolutionary development of the unique bat wing structure.
• Limb development and webbing formation show significant evolutionary plasticity across vertebrate species.