image: Figure 1. Genome synteny and genetic variations in testes of hybrids.
Credit: ©Science China Press
A research team led by Professor Shaojun Liu at Hunan Normal University published a study in Science China Life Sciences that elucidates the molecular mechanisms enabling fertility in hybrid fish despite significant genetic divergence. The study focuses on F1 hybrids of Megalobrama amblycephala (blunt snout bream) and Culter alburnus (topmouth culter), two cyprinid species that diverged approximately 12.74 million years ago yet produce fertile offspring, offering a unique model for exploring the impact of hybridization on fertility.
To investigate this phenomenon, the team utilized long-read sequencing and Hi-C technology to construct high-quality genomes for the reciprocal hybrids, BT and TB, achieving contig N50 lengths of 13.26 Mb and 12.40 Mb, respectively. Over 95% of the genome sequences were anchored to 48 pseudochromosomes, with 97.35% for BT and 95.79% for TB. Analysis revealed 193,978 structural variations (SVs) across eight hybrid individuals, predominantly deletions (131,204), which highlight rapid changes in the parental subgenomes post-hybridization.
Genes were classified into three categories: allelic genes (shared and conserved across species), orphan genes (species-specific and unique), and testis-specific genes (TSGs, closely linked to reproduction). The results showed that allelic genes and TSGs exhibited fewer structural variations, indicating strong selective pressure to maintain critical functions such as testicular development and reproduction. In contrast, orphan genes displayed higher variability, suggesting their role in aiding hybrids to adapt to new genetic environments.
Gene expression analysis across six tissues—muscle, testis, intestine, liver, kidney, and brain—indicated that the muscle tissue expression in hybrids more closely resembled that of blunt snout bream. The topmouth culter-derived subgenome (subgenome T) exhibited more differentially expressed genes, revealing an imbalance in parental genome contributions. ATAC-seq analysis demonstrated that chromatin accessibility, which regulates gene expression, remained largely stable, with only 2.82–4.90% of regions showing differences, primarily in subgenome T. Hi-C analysis further confirmed that chromatin regions associated with TSGs were stable, supporting their role in sustaining fertility.
Professor Shaojun Liu noted, “This study reveals how hybrid fish balance genomic stability and plasticity. Allelic genes and testis-specific genes ensure reproductive success, while orphan genes facilitate adaptation to new genetic landscapes, providing insights into the evolutionary role of hybridization in cyprinid fish and beyond.”
See the article:
Genomic and chromosomal architecture underlying fertility maintenance in the testes of intergeneric homoploid hybrids
Journal
Science China Life Sciences