Haemogregarines are a group of apicomplexan parasites composed of 3 families that infect a wide range of hosts. Many species within these families have been subjected to reclassifications and reassignments, especially because the use of molecular tools to estimate their phylogenetic relationships became more widespread. The 18S rRNA gene has been the only widely used gene for studying the diversity of haemogregarines and recent phylogenetic analyses of this gene have indicated incongruences with the current taxonomy, such that a new genus Bartazoon has recently been proposed. To investigate the current taxonomic situation further, we conducted an overview of all published 18S rRNA sequence data for haemogregarines. We highlight that our understanding of the real diversity and phylogenetic relationships of haemogregarines is still limited, which undermines the proposed systematic revision. Notably all the molecular evidence comes from a single gene, and many studies have shown that single-gene trees often do not reflect species trees. Combined with doubts over the relationships of Hemolivia, the recent identification of a new lineage that could also warrant creation of a new genus, and issues with the type species for Hepatozoon, we suggest that any taxonomic changes now would be premature. In our opinion, type species need to be assessed, sampling across hosts improved, and multiple genes employed prior to taxonomic alterations. Otherwise taxonomic instability will be likely.

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