From cryptogene to gene? ND8 editing domain reduction in insect trypanosomatids

Mitochondrial ND8 gene is pan-edited in most known trypanosomatid species and 5'-edited only in a few trypanosomatid species. In this work, the ND8 nucleotide sequences from three species of insect trypanosomatids ("Wallaceina" sp. Wsd, Leptomonas rigidus and Leptomonas collosoma) were obtained and compared with previously known ND8 cryptogene sequences and mature mRNA sequences from other trypanosomatid species. We found a new pattern of editing: only 18 U residues are inserted in the 5' region of the primary transcript in all investigated species and the potential start codon is encoded on the DNA level. This is the first case when a pre-edited ND8 sequence is found to contain a putative ORF. Previously, a 5'-edited ND8 cryptogene was known only from Strigomonas oncopelti, but it contained only about half of the ORF and no start codon. Here we present the ND8 gene sequence from Angomonas deanei which has similar cryptogene structure. We also show that according to 18S rRNA phylogenetic analysis, the species with the encoded ORF group together and so do species with the 5'-edited and pan-edited forms of the gene. This result sheds light on the evolution of cryptogene structure which involved multiple events of editing domain length reduction.