Blastogregarines are poorly studied parasites of polychaetes
superficially resembling gregarines, but lacking syzygy and gametocyst
stages in the life cycle. Furthermore, their permanent multinuclearity
and gametogenesis by means of budding considerably distinguish them from
other parasitic Apicomplexa such as coccidians and haematozoans. The
affiliation of blastogregarines has been uncertain: different authors
considered them highly modified gregarines, an intermediate apicomplexan
lineage between gregarines and coccidians, or an isolated group of
eukaryotes altogether. Here, we report the ultrastructure of two
blastogregarine species, Siedleckia nematoides and Chattonaria mesnili
and provide the first molecular data on their phylogeny based on SSU,
5.8S, and LSU rDNA sequences. Morphological analysis reveals that
blastogregarines possess both gregarine and coccidian features. Several
traits shared with archigregarines likely represent the ancestral states
of the corresponding cell structures for parasitic apicomplexans: a
distinctive tegument structure and myzocytotic feeding with a welldeveloped
apical complex. Unlike gregarines but similar to coccidians
however, the nuclei of male blastogregarine gametes are associated with
two kinetosomes. Molecular phylogenetic analyses reveal that
blastogregarines are an independent, early diverging lineage of
apicomplexans. Overall, the morphological and molecular evidence
congruently suggests that blastogregarines represent a separate class of
Apicomplexa.