Mitochondrial genome of palpitomonas bilix: Derived genome structure and ancestral system for cytochrome c maturation

Abstract

Wehere reported themitochondrial (MT)genomeofoneof theheterotrophicmicroeukaryotes relatedto cryptophytes, Palpitomonas bilix. The P. bilix mt genome was found to be a linear molecule composed of "single copy region" (16 kb) and repeat regions (~30 kb) arranged in an inverse manner at both ends of the genome. Linear mt genomes with large inverted repeats are known for three distantly related eukaryotes (including P. bilix), suggesting that this particularmt genome structure has emerged at least three times in the eukaryotic tree of life. The P. bilix mt genome contains 47 protein-coding genes including ccmA, ccmB, ccmC, andccmF, which encode protein subunits involved in the system for cytochrome c maturation inherited from a bacterium (System I).We present data indicating that the phylogenetic relatives of P. bilix, namely, cryptophytes, goniomonads, and kathablepharids, utilize an alternative systemfor cytochrome c maturation,which hasmost likely emerged during the evolution of eukaryotes (System III). To explain the distribution of Systems I and III in P. bilix and its phylogenetic relatives, two scenarios are possible: (i) System I was replaced by System III on the branch leading to the common ancestor of cryptophytes, goniomonads, and kathablepharids, and (ii) the two systems co-existed in their common ancestor, and lost differentially among the four descendants.