AI Insight
Researchers created a near-complete genome assembly of the mountain swordtail fish Xiphophorus nezahualcoyotl to investigate the genetic basis of two melanin-based color patterns called "spotted side" and "marmoratus." Using genome-wide association studies and long-read sequencing, they discovered both pigmentation patterns are linked to a region near the oncogene xmrk on the sex chromosomes, with dramatic structural variation between different chromosomal versions (haplotypes). The study resolves a century-old debate about which genes control these traits and demonstrates how complex, repetitive regions of sex chromosomes can harbor genes controlling diverse physical characteristics.
Why it matters
This work demonstrates how advanced sequencing technologies can reveal the genetic architecture of traits linked to difficult-to-study genomic regions. Understanding how pigmentation genes and cancer-related genes (oncogenes) interact on sex chromosomes may provide insights into both evolutionary processes and disease mechanisms, as xmrk is associated with melanoma development in these fish models.
⚠️ Preprint – Noch nicht peer-reviewed
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Sex-linked traits are widespread, but their genetic architecture has been challenging to characterize, due in part to the repetitive and structurally complex nature of sex chromosomes. In swordtails and platyfish of the genus Xiphophorus, diverse melanic pigmentation patterns are thought to be controlled by a region on the sex chromosomes classically referred to as the "macromelanophore determining locus". Despite nearly a century of study, the identity of the causal gene remains controversial, partially due to previous inability to fully sequence the sex chromosomes. Here, we characterize and investigate two melanin-based pigmentation phenotypes in the species X. nezahualcoyotl: "spotted side" and "marmoratus". We generate a gapless near-telomere-to-telomere X. nezahualcoyotl assembly and perform GWAS to identify regions associated with pigmentation pattern variation and sex-determination. We find both patterns map near the sex-determining region and to a narrow interval near the oncogene xmrk. By generating additional long-read assemblies of sex chromosomes derived from individuals with distinct phenotypes, we find haplotypes containing xmrk can be both X- and Y-linked, and vary dramatically in gene content, structure, and accumulation of repetitive elements including a newly described composite satellite. This variability may impact the region’s stability and affect recombination between haplotypes associated with each pattern. Our results shed light on a longstanding debate surrounding the genetic architecture of sex-linked phenotypes. More generally, we showcase how long-read sequencing can reveal phenotypic variation linked to complex and dynamic genomic regions, which may contribute to the evolution of diverse sex-linked traits.