The first chicken oocyte nucleus whole transcriptomic profile defines the spectrum of maternal mRNA and non-coding RNA genes transcribed by the lampbrush chromosomes

Abstract

Lampbrush chromosomes, with their unusually high rate of nascent RNA synthesis, provide a valuable model for studying mechanisms of global transcriptome up-regulation. Here, we obtained a whole-genomic profile of transcription along the entire length of all lampbrush chromosomes in the chicken karyotype. With nuclear RNA-seq, we obtained information about a wider set of transcripts, including long non-coding RNAs retained in the nucleus and stable intronic sequence RNAs. For a number of protein-coding genes, we visualized their nascent transcripts on the lateral loops of lampbrush chromosomes by RNA-FISH. The set of genes transcribed on the lampbrush chromosomes is required for basic cellular processes and is characterized by a broad expression pattern. We also present the first high-throughput transcriptome characterization of miRNAs and piRNAs in chicken oocytes at the lampbrush chromosome stage. Major targets of predicted piRNAs include CR1 and long terminal repeat (LTR) containing retrotransposable elements. Transcription of tandem repeat arrays was demonstrated by alignment against the whole telomere-to-telomere chromosome assemblies. We show that transcription of telomere-derived RNAs is initiated at adjacent LTR elements. We conclude that hypertranscription on the lateral loops of giant lampbrush chromosomes is required for synthesizing large amounts of transferred to the embryo maternal RNA for thousands of genes.