Unexpected absence of exo-erythrocytic merogony during high gametocytaemia in two species of (Haemosporida: Haemoproteidae), including description of n. sp. (lineage hCWT7) and a report of previously unknown residual bodies during in vitro gametogenesis.

Neglected avian blood parasites of the genus (Haemoproteidae) have recently attracted attention due to the application of molecular diagnostic tools, which unravelled remarkable diversity of their exo-erythrocytic (or tissue) stages both regarding morphology and organ tropism levels. The development of haemoproteids might result in pathologies of internal organs, however the exo-erythrocytic development (EED) of most species remains unknown. Seven individual birds - (1) and (6) - with high gametocytaemia (between 1% and 24%) of n. sp. (hCWT7) and (lineage hWW1) were sampled in Lithuania, and their internal organs were examined extensively by parallel application of histology and chromogenic in situ hybridization methods. Tissue stages were apparently absent, suggesting that the parasitaemia was not accompanied by detectable tissue merogony. n. sp. was described and characterized morphologically and molecularly. Sexual process and ookinete development of the new species readily occurred in vitro, and a unique character for parasites was discovered - the obligatory development of several tiny residual bodies, which were associated with intracellular transformation of both macrogametocytes and microgametocytes before their escape from the host cells and formation of gametes. A DNA haplotype network was constructed with lineages that cluster in one clade with the lineage hCWT7. This clade consists of lineages mostly found in birds, indicating specificity for birds of this genus. The lineage hCWT7 is mainly a parasite of Most reports of this lineage came from Turkey, with only a few records in Europe, mostly in birds wintering in Africa where transmission probably occurs. This study highlights unexpected difficulties in the research of EED even when using sensitive molecular diagnostic tools and extends information about transformation in early stages of gametogenesis in haemosporidian parasites.