Comprehensive molecular characterization of complete mitogenome assemblies of 33 Eimeria isolates infecting domestic chickens

Background: Coccidiosis caused by Eimeria is one of the most severe chicken diseases and poses a great economic threat to the poultry industry. Understanding the evolutionary biology of chicken Eimeria parasites underpins development of new interactions toward the improved prevention and control of this poultry disease.

Methods: We presented an evolutionary blueprint of chicken coccidia by genetically characterizing complete mitogenome assemblies of 33 isolates representing all seven known Eimeria species infecting chickens in China. Further genome- and gene-level phylogenies were also achieved to better understand the evolutionary relationships of these chicken Eimeria at the species level.

Results: 33 mitogenomes of chicken eimerian parasites ranged from 6148 bp to 6480 bp in size and encoded typical mitochondrial compositions of apicomplexan parasites including three protein-coding genes (PCGs), seven fragmented small subunit (SSU) and 12/13 fragmented large subunit (LSU) rRNAs. Comparative genomics provided an evolutionary scenario for the genetic diversity of PCGs-cytochrome c oxidase subunits 1 and 3 (cox1 and cox3) and cytochrome b (cytb); all were under purifying selection with cox1 and cox3 being the lowest and highest evolutionary rates, respectively. Genome-wide phylogenies classified the 33 Eimeria isolates into seven subgroups, and furthermore Eimeria tenella and Eimeria necatrix were determined to be more closely related to each other than to the other eight congenic species. Single/concatenated mitochondrial protein gene-based phylogenies supported cox1 as the genetic marker for evolutionary and phylogenetic studies for avain coccidia.

Conclusions: To our knowledge, these are the first population-level mitogenomic data on the genus Eimeria, and its comprehensive molecular characterization provides valuable resources for systematic, population genetic and evolutionary biological studies of apicomplexan parasites in poultry.