# Transcription of the Extensively Fragmented Mitochondrial Genomes of Human Lice

## TL;DR

This study explores how human lice transcribe their highly fragmented mitochondrial genomes, revealing unique and shared transcription patterns compared to humans.

## Contribution

The first detailed analysis of transcription in extensively fragmented mitochondrial genomes of human lice.

## Key findings

- Mitochondrial minichromosomes are transcribed entirely, with genes transcribed at higher levels than non-coding regions.
- Transcription events often terminate near a conserved GC-rich motif, but some pass it to transcribe non-coding regions.
- Human lice share some transcriptional features with humans but have unique traits due to genome fragmentation.

## Abstract

Genome fragmentation is a drastic, large-scale mutation. How genome fragmentation affects gene and chromosome transcription is still poorly understood. In this study, we analysed the RNA-seq data of human lice to understand how their extensively fragmented mitochondrial genomes are transcribed. We find their mitochondrial minichromosomes are transcribed entirely, with genes transcribed at much higher levels than non-coding regions. The 37 mitochondrial genes of human lice are transcribed unevenly—several genes are transcribed at significantly higher levels than other genes. Many transcription events terminate near a highly conserved GC-rich motif in the non-coding regions; however, some transcription events can pass this motif, leading to the transcription of entire non-coding regions. Despite the drastic difference in mitochondrial genome organisation, the human lice share several transcriptional features with humans, but also have unique features related to their fragmented mitochondrial genome organisation. The current study represents the first major effort in the transcription of fragmented genomes and will serve as a stepping stone for further studies on other animals and eukaryotes with fragmented genomes.

The mitochondrial (mt) genomes of animals, including humans, are typically a single circular chromosome containing all mt genes. In several animal lineages, however, mt genomes have become fragmented, with genes distributed on multiple minichromosomes. How fragmented mt genomes are transcribed is still poorly understood. In this study, we investigated the transcription of the extensively fragmented mt genomes of the human head louse (Pediculus humanus capitis) and the human body louse (Pediculus humanus corporis). RNA-seq reads of both subspecies were retrieved from the NCBI Sequence Read Archive database and mapped to their mt genomes. The transcription level of each mt gene, minichromosome, motif, coding region and non-coding region, measured by RPKM (Reads Per Kilobase of transcript per Million mapped reads), TPM (Transcripts Per Million) or read coverage, was analysed statistically. In both subspecies, mt minichromosomes were transcribed entirely, with coding regions transcribed at much higher levels than non-coding regions. The 37 mt genes are transcribed unevenly, with rrnL, cox1, cox2, cox3 and atp6 transcribed at significantly higher levels than several other genes. Many transcription events terminate near a GC-rich motif in the non-coding regions; however, some transcription events pass this motif, leading to the transcription of entire non-coding regions. Despite the drastic difference in mt genome organisation, the human lice share several transcriptional features with humans, but also have unique features related to their fragmented mt genome organisation. The current study represents the first effort into the transcription of fragmented mt genomes. As more RNA-seq data become available, further studies on other animals with fragmented mt genomes are necessary to fully understand how genome fragmentation affects transcription.