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Red tilapia are favored by consumers, but the molecular genetic basis for this color pattern is unknown. Here we report on the genetic and physical mapping of the red locus in two strains of tilapia. We raised ~3000 hybrid individuals to map the red locus to a single bacterial artificial chromosome clone on linkage group 3. Long-read sequencing allowed us to assemble contigs spanning both the black and red haplotypes. The red haplotype contains additional repetitive sequence totaling almost one megabase that includes no obvious candidate genes. We suggest that the red phenotype may arise from substitutions in a protein in the primary cilia (Ccdc149), or changes in the expression of a nearby gene (nckx2). Red mutations in several unlinked loci have now been identified, creating an opportunity to identify the best allelic combinations for aquacultural production. 

ABSTRACT Cichlid fishes have the highest rates of evolutionary turnover of sex chromosomes among vertebrates. Many large structural polymorphisms in the radiation of cichlids in Lake Malawi are associated with sex chromosomes and may also carry adaptive variation. Here, we investigate the structure and evolutionary history of an inversion polymorphism that includes both a ZW sex locus and an orange‐blotch colour polymorphism in the rock‐dwelling cichlid fishes of Lake Malawi. We use long‐read sequencing to characterise the sequence and breakpoints of the inversion. We quantify allele frequency differences across the inversion in population samples of the generaMetriaclimaandLabeotropheus. We also examine expression differences of genes in the inversion. The simple inversion spans 7 Mb and is flanked by CACTA transposons that may have catalysed the rearrangement. The region includes ~600 genes, several of which show large differences in expression. Some of these genes are candidates for the sex and colour phenotypes. This inversion is an accessible model system for studying the role of structural polymorphisms and sex chromosome turnover in the adaptive radiation of cichlids in the lakes of East Africa.