How individual cells develop from a syncytium: merogony in Theileria parva (Apicomplexa)

Abstract

The central problem for Theileria parva during merogony is how to form numerous individual, uninucleate merozoites from a syncytial schizont so that each merozoite contains a single nucleus and a prescribed assortment of organelles. The way T. parva packages all the requisite organelles into free merozoites is by binding these organelles to the nuclear envelope, which in turn becomes associated, both directly and through the rhoptry complex, with the schizont plasma membrane. Formation of the merozoites occurs in a synchronous manner by a budding process. The merozoites develop with the rhoptry complex at the apical end by the progressive, outward evagination of the schizont plasma membrane. This evagination of the plasma membrane is associated with, and presumably induced by, the development of an orderly array of tubules that originate from the apical end and progressively form a longitudinal basket enclosing first the rhoptry complex, then the mitochondria and ribosomes, and finally the nucleus. The process of merogony is compared to sporogony within the tick salivary gland and with the differentiation of the intra-erythrocytic piroplasm stage. Because all three processes occur by a morphologically similar mechanism, the possibility that the parasite uses a single cassette of genes to perform each of these similar processes is discussed.