# A Loss-of-Function Mutation in bco1l Underlies Yellow Coloration in Large Yellow Croaker (Larimichthys crocea)

## TL;DR

A genetic mutation in the bco1l gene causes yellow coloration in large yellow croaker fish, which can be used to improve aquaculture breeding.

## Contribution

Discovery of a 10 bp deletion in the bco1l gene as the genetic basis for yellow body coloration in large yellow croaker.

## Key findings

- A 10 bp deletion in the bco1l gene causes loss of enzymatic activity for carotenoid cleavage.
- The deletion leads to yellow body coloration due to carotenoid accumulation in the skin.
- The mutation is rare and provides a validated marker for marker-assisted selection in aquaculture.

## Abstract

Body color represents a commercially valuable phenotypic trait in the large yellow croaker (Larimichthys crocea). Here, we identify a 10 bp deletion in the carotenoid-cleaving enzyme gene bco1l that underlies its distinctive yellow body coloration. This loss-of-function mutation abolishes enzymatic activity, thereby impairing the metabolic cleavage of dietary carotenoids and resulting in their selective accumulation in the skin. Our work establishes a causal genetic mechanism for this economically important trait and delivers a robust, functionally validated molecular marker to accelerate marker-assisted selection in aquaculture breeding programs.

Carotenoid-based coloration significantly influences the ornamental appeal and market value of aquatic species. This study identifies the genetic basis of yellow body coloration in the large yellow croaker (Larimichthys crocea), a commercially important aquaculture species in China. Through a genome-wide association study (GWAS), we identified a 10 bp deletion (bco1l-Δ10) in the bco1l gene, a critical enzyme in carotenoid metabolism. This deletion, located in exon 10, causes a frameshift mutation and premature stop codon, resulting in loss of the enzymatic function essential for carotenoid cleavage. Sanger sequencing confirmed that all yellow juveniles were homozygous for the deletion, while none of the wild-type juveniles harbored this mutation. Functional assays demonstrated that the recombinant Bco1l-Δ10 protein failed to catalyze β-carotene cleavage, validating the genetic association with the yellow phenotype. The bco1l-Δ10 mutation was detected only in a limited number of juvenile individuals, indicating it is a rare occurrence in the population. Our findings provide a valuable genetic marker for marker-assisted selection (MAS) to enhance yellow pigmentation in aquaculture breeding programs, offering a potential strategy to improve the market value of L. crocea.