The little brown bat nuclear genome contains an entire mitochondrial genome: Real or artifact?

Nuclear mitochondrial DNA sequences (NUMTs) have been documented in almost all eukaryotic genomes studied. Recently, with the number of sequenced genomes increasing, extremely large NUMTs, even a nearly entire mitochondrial genome, have been reported in some plants and animals. However, few such studies provided strong experimental evidences for these important discoveries. In this study using a computer-based search method an entire mitochondrial genome (NUMT-1) was found in the nuclear genome of a bat species (Myotis lucifugus). This super-large NUMT shared a same scaffold with a 754 bp nuclear genomic sequence and a second NUMT (NUMT-2, 3292 bp). If NUMT-1 was real, it will be the largest NUMT found in animals and this finding will provide valuable insights into the mode of generation of NUMTs in the nuclear genome. Unfortunately, although the initial sequencing technology of the published M. lucifugus genome makes the possibility of artifact less likely, our results from both the PCR amplification followed by Sanger sequencing and mapping method based on the whole-genome resequencing datasets suggested that the scaffold containing the entire mitochondrial genome was artifact possibly due to a misassembly of mitochondrial and the nuclear DNA sequences. Our current study highlights the necessity to validate the authenticity of extremely large NUMTs identified in previous searches on whole-genome sequence assemblies.