# A maternal-to-zygotic-transition gene block on the zebrafish sex chromosome

## TL;DR

The study identifies a unique region on zebrafish sex chromosomes that plays a role in the maternal-to-zygotic transition during early embryonic development.

## Contribution

The discovery of a gene block on zebrafish sex chromosomes that is silenced in ovaries but expressed in testes and embryos during zygotic genome activation.

## Key findings

- Region-2 on chromosome 4R has testis-biased gene expression and is silent in ovaries.
- Region-2 contains maternal-specific genes and miR-430, which targets maternal RNA for degradation.
- The region is associated with the maternal-to-zygotic transition in zebrafish embryos.

## Abstract

Wild zebrafish (Danio rerio) have a ZZ/ZW chromosomal sex-determination system with the major sex locus on the right arm of chromosome-4 (Chr4R) near the largest heterochromatic block in the genome, suggesting that Chr4R transcriptomics might differ from the rest of the genome. To test this hypothesis, we conducted an RNA-seq analysis of adult ZW ovaries and ZZ testes in the Nadia strain and identified 4 regions of Chr4 with different gene expression profiles. Unique in the genome, protein-coding genes in a 41.7 Mb section (Region-2) were expressed in testis but silent in ovary. The AB lab strain, which lacks sex chromosomes, verified this result, showing that testis-biased gene expression in Region-2 depends on gonad biology, not on sex-determining mechanism. RNA-seq analyses in female and male brains and livers validated reduced transcripts from Region-2 in somatic cells, but without sex specificity. Region-2 corresponds to the heterochromatic portion of Chr4R and its content of genes and repetitive elements distinguishes it from the rest of the genome. Region-2 lacks protein-coding genes with human orthologs; has zinc finger genes expressed early in zygotic genome activation; has maternal 5S rRNA genes, maternal spliceosome genes, a concentration of tRNA genes, and a distinct set of repetitive elements. The colocalization of (1) genes silenced in ovaries but not in testes that are (2) expressed in embryos briefly at the onset of zygotic genome activation; (3) maternal-specific genes for translation machinery; (4) maternal-specific spliceosome components; and (5) adjacent genes encoding miR-430, which mediates maternal transcript degradation, suggest that this is a maternal-to-zygotic-transition gene regulatory block.

Here, Wilson and Postlethwait identify a unique region in zebrafish sex chromosomes that contains protein-coding genes silenced in ovaries but expressed in testes which are then transiently expressed in embryos as they begin to express their own genes. They report that the region also encodes maternal-specific spliceosome components and microRNAs that target maternally-stored RNA for degradation. The authors conclude that this region defines a distinct maternal-to-zygotic-transition block of genes.