The Life-history and Chromosome Cycle of Aggregata eberthi [Protozoa: Sporozoa: Coccidia]

Abstract

The life-history of Aggregata, eberthi is summarized in the accompanying diagram (Text-fig. 3). The stages shown above the dotted line occur in Sepia officinalis, those below in Portunus.Merozoites (R), when swallowed by the cuttlefish, pass through the epithelium and enter the submucous tissue of the caecum or intestine, where they develop into young (undifferentiated) parasites (A) which grow into males (B) or females (E). The former, after undergoing repeated nuclear divisions (C), produce broods of microgametes (D)—each of which (D*) has a complicated structure. Females (E), on the other hand, are bodily converted into macrogametes (F), which undergo a complex series of nuclear changes during maturation. No reduction occurs during gametogenesis.Fertilization (G) is effected by the union of one microgamete with one macrogamete, and is followed by the formation of a fertilization membrane which, in part, gives rise to the oocyst. The zygote (H) becomes a sporont, which, after repeated division of its nucleus (I, J), ultimately segments—leaving no residuum—into a large number of spherical uninucleate sporoblasts (K). Bach of these is then converted into a spore (L) containing three sporozoites and a small sporocystic residue.The ripe spores pass out of the intestine with the cuttlefish's excrement; and when swallowed by a Portunus they open in its intestine and liberate their contained sporozoites (M). These penetrate the epithelium of the crab's midgut, and enter the subepithelial connective tissue, where they grow into schizonts (N, O). Finally, the schizonts undergo schizogony, producing large broods of merozoites (P, Q). No further development takes place unless the schizogonic cysts, in the crab's coelome, are eaten by a cuttlefish: in which case they liberate their contained merozoites (R) and the development just described then begins anew.A. eberthi is, at every stage but one in the whole life-cycle, a haploid organism. Every nuclear division is mitotic: and every nucleus (except the zygote nucleus) contains a single set of six differentiated and unpaired chromosomes. In the zygote nucleus alone there are 12 chromosomes—six homologous pairs—derived from the two gamete nuclei: but at the first division of this nucleus (first division in sporont, Text-fig. 3, I) the chromosomes are again halved to six. Reduction (meiosis) thus occurs immediately after fertilization. No halving of the chromosome number occurs during gametogenesis or schizogony; and males, females, and asexual parasites all possess an identical chiomosomic constitution. Every nucleus in Text-fig. 3 is therefore an x nucleus, with the exception of stage H, which is 2x. (Between stages G and I many nuclear changes occur, but only one—the so-called “fertilization spindle”—is figured (H) in the diagram.)