Hemoglobin Digestion Genes Are Conserved in Lizard-Infective Plasmodium Species With Different Host Cellular Niches

In their vertebrate hosts, malaria parasites typically inhabit erythrocytes where they harvest the cell's abundant supply of hemoglobin as a nutrient source. A byproduct of hemoglobin digestion is free heme, which the parasites detoxify by converting it to a brown crystal known as hemozoin. Hemozoin is a hallmark of Plasmodium infection, and this enzymatic pathway is well studied in mammalian Plasmodium species. Despite their evolutionary relatedness to mammalian Plasmodium, wildlife malaria parasites, particularly those that infect birds and lizards, are understudied, leaving their vast genetic diversity to be explored. Plasmodium floridense, Plasmodium azurophilum, and Plasmodium leucocytica infect Anolis lizards throughout the Caribbean islands, including the endemic anole on the island of Saba. Like other Plasmodium species, P. floridense infects red blood cells and produces hemozoin. P. azurophilum also infects red blood cells, however, its sister species, P. leucocytica, infects white blood cells. This is atypical for Plasmodium parasites and represents an expansion into a new cellular niche. Two of these three parasites (P. azurophilum and P. leucocytica) also do not produce hemozoin and have seemingly evolved alternative mechanisms of hemoglobin digestion for nutrient acquisition. To investigate this, we assembled parasite transcriptomes from infected Saban anole blood samples and analyzed them for hemoglobin digestion transcripts. The transcriptome results indicate that all three lizard parasites transcribe the genes canonically involved in the hemoglobin digestion pathway. This is the first evidence that these parasites possess genes for the same digestive enzymes as the better characterized mammalian Plasmodium, indicating conservation of this pathway across the Plasmodium tree. However, there is evidence for shifts in selective pressure on some of these proteins in all three lizard-infective species. These genes may not be as functionally important relative to other Plasmodium species. Since the genes are not yet pseudogenes, however, there remains the alternative hypothesis that these genes also play additional roles in malaria parasite biology.