The new apicomplexan species Monocystis cantonensis from the earthworm Amynthas aspergillum retains a micropore but lacks an apicoplast

Monocystis spp., early-branching members of Apicomplexa, parasitize earthworms and may negatively affect their health. Given the important role of earthworm in agricultural systems, such infections have the potential to influence soil ecosystem functioning. However, despite their ecological and evolutionary relevance, Monocystis have received limited attention, particularly in China, constraining our knowledge of their biology and parasitism in the Apicomplexa. In this study, we collected 121 earthworms ( Amynthas aspergillum ) from the Guangdong province, in southern China. A large number of oocysts were found in the seminal vesicles and enclosed in the brown body of the earthworm. Based on morphological characteristics and 18S rDNA sequence, a new species, Monocystis cantonensis sp. nov, was identified as a parasite of A. aspergillum . This represents the first newly described Monocystis species recorded in China. We further characterized the life cycle and ultrastructure of trophozoite, oocyst and gametocyte stages, and revealing that the parasite retains the nutrient uptake structure, micropore, but lacks the plastid-derived organelle, apicoplast. In addition, the genome of M. cantonensis was sequenced and analyzed, provide genomic support for these structural observations. These findings not only expand our knowledge of Monocystis diversity and host associations in China, but also offer important insights into the evolution of parasitism and organelle reduction within Apicomplexa.