Genomic Anatomy of Homozygous XX Females and YY Males Reveals Early Evolutionary Trajectory of Sex-determining Gene and Sex Chromosomes in Silurus Fishes.

Sex chromosomes display remarkable diversity and variability among vertebrates. Compared with research on the X/Y and Z/W chromosomes, which have long evolutionary histories in mammals and birds, studies on the sex chromosomes at early evolutionary stages are limited. Here, we precisely assembled the genomes of homozygous XX female and YY male Lanzhou catfish (Silurus lanzhouensis) derived from an artificial gynogenetic family and a self-fertilized family, respectively. Chromosome 24 (Chr24) was identified as the sex chromosome based on resequencing data. Comparative analysis of the X and Y chromosomes showed an approximate 320 kb Y-specific region with a Y-specific duplicate of anti-Mullerian hormone type II receptor (amhr2y), which is consistent with findings in 2 other Silurus species but on different chromosomes (Chr24 of Silurus meridionalis and Chr5 of Silurus asotus). Deficiency of amhr2y resulted in male-to-female sex reversal, indicating that amhr2y plays a male-determining role in S. lanzhouensis. Phylogenetic analysis and comparative genomics revealed that the common sex-determining gene amhr2y was initially translocated to Chr24 of the Silurus ancestor along with the expansion of transposable elements. Chr24 was maintained as the sex chromosome in S. meridionalis and S. lanzhouensis, whereas a sex-determining region transition triggered sex chromosome turnover from Chr24 to Chr5 in S. asotus. Additionally, gene duplication, translocation, and degeneration were observed in the Y-specific regions of Silurus species. These findings present a clear case for the early evolutionary trajectory of sex chromosomes, including sex-determining gene origin, repeat sequence expansion, gene gathering and degeneration in sex-determining region, and sex chromosome turnover.

Complete sequence and characterization of the Silurus lanzhouensis (Siluriformes: Siluridae) mitochondrial genome.

The complete mitochondrial DNA (mtDNA) sequence of Silurus lanzhouensis was constructed from whole-genome Illumina sequencing data. The 16 523 bp circular genome comprises typical mtDNA components. All 13 protein-coding genes (PCGs) are initiated by an ATG except for COX1, which uses GTG. Some PCGs harbor TAG (ND2 and ND3) or an incomplete stop codon T (COX2, ND4, and CYTB), while others use TAA as their stop codon. 12S rRNA and 16S rRNA secondary structures are composed of four domains with 45 helices and six domains with 54 helices, respectively. All tRNAs are predicted to fold into the expected typical cloverleaf secondary structure except tRNA-Ser((AGN)). The largest intergenic spacer sequence was predicted to be the origin of light-strand replication. Eight conserved sequences were identified in the control region (CR). This complete S. lanzhouensis mitogenome provides useful data for further studies on molecular systematics, taxonomic status, stock evaluation, and conservation genetics.