Comparative transcriptomic insights into the evolutionary origin of the tetrapod double cone

Area of Science:

• Evolutionary biology
• Molecular biology
• Vision science

Background:

• The tetrapod double cone, comprising principal and accessory members, is present in diverse vertebrates but absent in fish and eutherian mammals.
• Understanding the evolutionary trajectory of the double cone can illuminate photoreceptor evolution in vertebrates.

Purpose of the Study:

• To investigate the evolutionary origins of the tetrapod double cone.
• To identify molecular signatures distinguishing double cone members and their evolutionary relationship to other photoreceptors.

Main Methods:

• Single-cell and single-nucleus transcriptomic analyses were performed on photoreceptors from six vertebrate species.
• Computational methods were used to separate and analyze cone subtypes and identify molecular distinctions.

Main Results:

• Principal and accessory double cone members were computationally separated in chicken and lizard, revealing distinct molecular signatures.
• Comparative transcriptomics indicate that both double cone members originated from ancestral red cones.
• Gene expression patterns suggest a conserved evolutionary order for cone subtypes (red → green → blue → UV) and that rods predated cone spectral diversification.

Conclusions:

• The tetrapod double cone evolved from ancestral red cones, with a conserved gene expression order influencing cone subtype emergence.
• Rods likely evolved before the spectral diversification of cones, indicating an early divergence in photoreceptor evolution.