Heterochronic transcription factor expression drives cone-dominant retina development in 13-lined ground squirrels

Area of Science:

• Evolutionary biology
• Developmental neuroscience
• Genomics

Background:

• Mammalian retinas are typically rod-dominant, suited for low-light vision.
• Ground squirrels exhibit cone-dominant retinas, an adaptation for diurnal vision.
• The molecular underpinnings of this retinal shift are not well understood.

Purpose of the Study:

• To investigate the molecular mechanisms driving the evolution of cone-dominant retinas in ground squirrels.
• To uncover the regulatory basis for altered photoreceptor development during retinal neurogenesis.

Main Methods:

• Single-cell RNA sequencing (scRNA-Seq) to profile gene expression.
• Single-cell assay for transposase-accessible chromatin using sequencing (scATAC-Seq) to map regulatory elements.
• Analysis of retinal neurogenesis in the 13-lined ground squirrel (13LGS).

Main Results:

• Cone photoreceptors in 13LGS originate from both early and late-stage neurogenic progenitors, unlike rod-dominant species.
• A heterochronic shift in transcription factor expression, including sustained activity of cone-promoting factors (e.g., Onecut2, Pou2f1, Zic3) and precocious expression of differentiation factors (e.g., Thrb, Rxrg, Mef2c).
• Zic3 and Mef2c were found to be sufficient to promote cone specification and inhibit rod specification via species-specific regulatory elements.

Conclusions:

• Evolutionary changes in gene regulatory networks are responsible for the development of cone-dominant retinas.
• Understanding these mechanisms offers insights into sensory adaptation.
• This research may inform strategies for cone photoreceptor regeneration in vision disorders.