Direct male development in chromosomally ZZ zebrafish

Area of Science:

• Developmental Biology
• Genetics
• Comparative Genomics

Background:

• Sex determination mechanisms exhibit significant variation across animal taxa, even within species.
• Domesticated zebrafish (Danio rerio) strains like AB and TU lack a strong genetic sex-determining locus.
• Wild-derived Nadia (NA) zebrafish possess a ZZ male/ZW female chromosomal sex-determination system.

Purpose of the Study:

• To elucidate the mechanisms of gonad development in the Nadia (NA) zebrafish strain.
• To compare gonad development in ZZ and ZW NA zebrafish using histology and single-cell transcriptomics.

Main Methods:

• Histological analysis of developing gonads in ZZ and ZW NA zebrafish.
• Single-cell RNA sequencing (scRNA-seq) of gonads at 19 and 30 days post-fertilization (dpf).
• Comparative transcriptomic analysis to identify cell types and gene expression patterns.

Main Results:

• ZW NA zebrafish developed oocytes by 22 dpf, while ZZ NA zebrafish directly formed testes.
• scRNA-seq at 19 dpf revealed similar bipotential gonad cell types in both ZZ and ZW embryos.
• scRNA-seq at 30 dpf showed distinct testicular (Sertoli, Leydig) and ovarian (granulosa, theca) cell transcriptomes in ZZ and ZW gonads, respectively.

Conclusions:

• Juvenile NA zebrafish initially develop bipotential gonads.
• A factor on the NA W chromosome, or the absence of two Z chromosomes, is crucial for initiating oocyte development.
• The AB and TU zebrafish strains may have lost their Z chromosome during domestication, mimicking WW zebrafish and explaining their altered sex determination.