Minifish mtDNA has abundance of repeat sequences and inefficient replication in vitro.

Paedocypris is a newly described minifish genus endemic to Southeast Asia. Besides a tiny adult size of ~8 mm in length, minifish feature fragmentary habitats of acidic peat blackwater swamps, an unusual reproduction mode, truncated development and one of the smallest known genomes. A complete sequence is absent for the minifish mitochondrial DNA (mtDNA). Here we report the complete mtDNA sequence and its unusual feature in the minifish P. progenetica (Pp). We show that the Pp mtDNA is a circular molecule of 17,382 bp in length and has the same number of similarly oriented genes as in other vertebrates. Specifically, it comprises 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes and 1 D-loop. Surprisingly, the D-loop is elusive for amplification by standard PCR conditions. The D-loop possesses a 28-bp dinucleotide TA repeat and more intriguingly, up to 25 copies of a 34-bp tandem repeat sequence. These tandem repeats predict the formation of paired regions. Hence, besides a generally conserved mtDNA with other vertebrates, the Pp mtDNA features an unusual D-loop and compromised DNA replication in vitro.