Apicomplexan protozoan parasites have complex life cycles that involve phases of asexual and sexual reproduction. Some genera have intermediate insect hosts, for example, Plasmodium spp. (the cause of malaria), but related genera such as Eimeria spp. (causative agents of coccidiosis in poultry) have a direct life cycle occurring in only a single host. Mechanisms that regulate the life cycles of apicomplexan parasites are unknown, but the intracellular growth of avian Eimeria spp. is easily shortened by serial selection for the first parasites to complete the transition from asexual to sexual reproduction (to yield so-called precocious lines). To investigate the genetic basis of such an abbreviated life cycle, we have used the species E. tenella and analyzed the inheritance of 443 polymorphic DNA markers in 22 recombinant cloned progeny derived from a cross between parents that had selectable phenotypes of precocious development or resistance to an anticoccidial drug. The markers were placed in 16 linkage groups (which defined 12 chromosomes) and a further 57 unlinked groups. Two linkage groups showed an association (P = .0105) with the traits of precocious development or drug-resistance and were mapped to chromosome 2 (ca 1.2 Mbp) and chromosome 1 (ca 1.0 Mbp), respectively. The map provides a framework for further studies on the identification of genetic loci implicated in the regulation of the life cycle of an important protozoan parasite and a representative of a major taxonomic group.[A table with the segregation data is available as an online supplement at http://www.genome.org.]

Two genetically defined parents of Eimeria tenella, each characterized by a selectable phenotype, were mated to provide "recombinant' oocysts for an analysis of the inheritance of DNA markers into the sporocyst stage of the life-cycle. A total of 23 populations was established from infection with a putative single sporocyst and their inheritance of DNA markers provided compelling evidence that the 2 sporozoites within each sporocyst are genetically identical. EAch of the 5 polymorphic DNA markers used segregated independently within the "recombinant' parasites and only 1 of the parental forms was inherited in 22 of the populations aimed to have been established from a single sporocyst.