# Comparative analysis of the organization and complexity of immunoglobulin light chain loci in equids

## TL;DR

This study compares the structure and gene expression of immunoglobulin light chains in donkeys and horses, revealing conserved patterns useful for antibody development.

## Contribution

The first comprehensive elucidation of the donkey immunoglobulin light-chain locus structure and expression preferences.

## Key findings

- Donkey IGλ locus has a Vλ-(Jλ-Cλ)-Vλ configuration with 7 Cλ genes and 156 upstream and 98 downstream Vλ genes.
- Expression profiling shows strong preferences for specific Vλ and Vκ genes in donkeys, similar to horses.
- Breed-specific differences in IGL expression are limited to low-frequency genes, indicating high conservation in key subgroups.

## Abstract

This study investigated three donkey breeds—Guanzhong, Jiami, and Northern Shaanxi—to characterize the structural organization and diversification mechanisms of the immunoglobulin light-chain (IgL) loci and to conduct both intra-breed and interspecies comparisons with horses. The donkey IGλ locus is located on chromosome 8 and arranged in a Vλ-(Jλ-Cλ)-Vλ configuration. It contains 7 Cλ genes, each preceded by a corresponding Jλ gene to form a Jλ-Cλ cluster. Upstream of this cluster, 156 Vλ genes were identified, including 29 potential functional genes; downstream, 98 Vλ genes were detected, of which 22 were potentially functional. The IGκ locus resides on chromosome 6 and exhibits a Vκ-Jκ-Cκ structure, comprising one Cκ gene, 5 Jκ genes, and 72 Vκ segments, including 22 potential functional Vκ genes. Expression profiling revealed clear light-chain usage preferences in donkeys. For the λ-chain, Vλ72, Vλ135, Vλ150, Vλ190, and Vλ196—mainly belonging to the IGLV1 and IGLV3 subgroups—were preferentially utilized. For the κ-chain, Vκ67 and Vκ71 were dominantly expressed, highlighting the prominent role of the IGκV4 subgroup. Horses showed a similar pattern, with strong biases toward the IGLV1, IGLV3, and IGκV4 subgroups. Although breed-specific differences were observed in certain IGL expression profiles (e.g. V-J combinations), these variations were largely restricted to the λ-chain and involved low-frequency genes. Thus, highly utilized genes and subgroups exhibit high conservation among donkey breeds. This study is the first to comprehensively elucidate the structure and expression preferences of the donkey IgL locus, laying a solid foundation for the development of donkey-derived antibody resources.

Three Chinese donkey breeds exhibit unique immunoglobulin gene diversification and similar gene expression patterns to horses, thus offering a robust cost-effective immune model for developing species-specific antibodies.