Genomic and transcriptomic perspectives on the origin and evolution of NUMTs in Orthoptera

Nuclear mitochondrial pseudogenes (NUMTs) result from the transfer of mitochondrial DNA (mtDNA) to the nuclear genome. NUMTs, as "frozen" snapshots of mitochondria, can provide insights into diversification patterns. In this study, we analyzed the origins and insertion frequency of NUMTs using genome assembly data from ten species in Orthoptera. We found divergences between NUMTs and contemporary mtDNA in Orthoptera ranging from 0 % to 23.78 %. The results showed that the number of NUMT insertions was significantly positively correlated with the content of transposable elements in the genome. We found that 39.09 %-68.65 % of the NUMTs flanking regions (2,000 bp) contained retrotransposons, and more NUMTs originated from mitochondrial rDNA regions. Based on the analysis of the mitochondrial transcriptome, we found a potential mechanism of NUMT integration: mitochondrial transcripts are reverse transcribed into double-stranded DNA and then integrated into the genome. The probability of this mechanism occurring accounts for 0.30 %-1.02 % of total mitochondrial nuclear transfer events. Finally, based on the phylogenetic tree constructed using NUMTs and contemporary mtDNA, we provide insights into ancient evolutionary events such as species-specific "autaponumts" and "synaponumts" shared among different species, as well as post-integration duplication events.