Zygotic gene activation in the chicken occurs in two waves, the first involving only maternally derived genes.

Young Sun Hwang,Minseok Seo,Jae Yong Han,Sohyun Bang,Heebal Kim,Sang Kyung Kim

doi:10.7554/elife.39381

Young Sun Hwang, Minseok Seo + Show 4 more

Open Access

https://doi.org/10.7554/elife.39381

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Abstract

The first wave of transcriptional activation occurs after fertilisation in a species-specific pattern. Despite its importance to initial embryonic development, the characteristics of transcription following fertilisation are poorly understood in Aves. Here, we report detailed insights into the onset of genome activation in chickens. We established that two waves of transcriptional activation occurred, one shortly after fertilisation and another at Eyal-Giladi and Kochav Stage V. We found 1544 single nucleotide polymorphisms across 424 transcripts derived from parents that were expressed in offspring during the early embryonic stages. Surprisingly, only the maternal genome was activated in the zygote, and the paternal genome remained silent until the second-wave, regardless of the presence of a paternal pronucleus or supernumerary sperm in the egg. The identified maternal genes involved in cleavage that were replaced by bi-allelic expression. The results demonstrate that only maternal alleles are activated in the chicken zygote upon fertilisation, which could be essential for early embryogenesis and evolutionary outcomes in birds.

Highlights

The genetic events of early embryogenesis are initiated by zygotic genome activation (ZGA) (Lee et al, 2014; Tadros and Lipshitz, 2009)
We examined primary transcripts toassess the existence and timing of transcriptional activation accurately, using previously generated bulked embryonic whole-transcriptome sequencing (WTS) data (Hwang et al, 2018aHwang et al, 2018c) (Figure 1A)
Hierarchical clustering of precursor mRNA expression demonstrated that zygotes differed from oocytes, suggesting dynamic changes in primary transcripts after fertilisation (Figure 1B)

Summary

Introduction

The genetic events of early embryogenesis are initiated by zygotic genome activation (ZGA) (Lee et al, 2014; Tadros and Lipshitz, 2009). By examining gene expression in embryos from mixed-breed chickens, Hwang, Seo et al showed that there are two stages of zygotic genome activation in chickens. Large numbers of up- and downregulated mRNAs and long intergenic noncoding RNAs (lincRNAs) were observed during the 1st wave, while other RNAs were mostly downregulated after fertilisation (Figure 1—figure supplement 3), suggesting a potential role for long transcripts in the early cleavage stages.

Results

Conclusion