The Life-history and Chromosome Cycle of <i>Aggregata eberthi</i> [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 2 x . (Between stages G and I many nuclear changes occur, but only one—the so-called “fertilization spindle”—is figured ( H ) in the diagram.)