Observations of the Fine Structural Changes Associated with Merogony and Gametogony in Eimeria necatrix and Localization of Two Gametocyte Proteins

Coccidian parasites possess complex life cycles involving asexual proliferation followed by sexual development, producing oocysts that are transmitted from host to host through feces, guaranteeing disease transmission. <i>Eimeria necatrix</i> is a highly pathogenic coccidian causing high mortality in birds. This study examined ultrastructural changes occurring during the third merogony, microgametogenesis, and macrogametogenesis of <i>E. necatrix</i>. The third-generation meront contained eight merozoites, each with coccidian-specific features like conoid, rhoptries, micronemes, and dense granules. Microgametes had a nucleus, mitochondrion, two flagella, and a basal apparatus. Macrogametes surrounded by two membranes (M1 and M2), contained organelles like WFB1, WFB2, endoplasmic reticulum, mitochondria, and tubular structures. Oocyst wall formation began with M2 separating from M1 and forming a loose veil around the organism. The WFB1 fused together to form the outer layer of the oocyst wall between M1 and M2, while M4 formed beneath M1. The WFB2 fused with the M4 to discharge its contents external to M4, which fused together to form the inner layer of the oocyst wall. Immunogold electron microscopy co-localization result showed that EnGAM22 localized to WFB1 and the outer wall, while EnGAM59 localized to WFB2 and the inner wall, suggesting they are key structural components of the oocyst wall.