Unusual dynamics of extinction in a simple ecological model.

Studies on natural populations and harvesting biological resources have led to the view, commonly held, that (i) populations exhibiting chaotic oscillations run a high risk of extinction; and (ii) a decrease in emigration/exploitation may reduce the risk of extinction. Here we describe a simple ecological model with emigration/depletion that shows behavior in contrast to this. This model displays unusual dynamics of extinction and survival, where populations growing beyond a critical rate can persist within a band of high depletion rates, whereas extinction occurs for lower depletion rates. Though prior to extinction at lower depletion rates the population exhibits chaotic dynamics with large amplitudes of variation and very low minima, at higher depletion rates the population persists at chaos but with reduced variation and increased minima. For still higher values, within the band of persistence, the dynamics show period reversal leading to stability. These results illustrate that chaos does not necessarily lead to population extinction. In addition, the persistence of populations at high depletion rates has important implications in the considerations of strategies for the management of biological resources.

Use of yeast artificial chromosomes (YACs) in studies of mammalian development: production of beta-globin locus YAC mice carrying human globin developmental mutants.

To test whether yeast artificial chromosomes (YACs) can be used in the investigation of mammalian development, we analyzed the phenotypes of transgenic mice carrying two types of beta-globin locus YAC developmental mutants: (i) mice carrying a G-->A transition at position -117 of the A gamma gene, which is responsible for the Greek A gamma form of hereditary persistence of fetal hemoglobin (HPFH), and (ii) beta-globin locus YAC transgenic lines carrying delta- and beta-globin gene deletions with 5' breakpoints similar to those of deletional HPFH and delta beta-thalassemia syndromes. The mice carrying the -117 A gamma G-->A mutation displayed a delayed gamma- to beta-globin gene switch and continued to express A gamma-globin chains in the adult stage of development as expected for carriers of Greek HPFH, indicating that the YAC/transgenic mouse system allows the analysis of the developmental role of cis-acting motifs. The analysis of mice carrying 3' deletions first provided evidence in support of the hypothesis that imported enhancers are responsible for the phenotypes of deletional HPFH and second indicated that autonomous silencing is the primary mechanism for turning off the gamma-globin genes in the adult. Collectively, our results suggest that transgenic mice carrying YAC mutations provide a useful model for the analysis of the control of gene expression during development.

Maintenance of protective immunity against malaria by persistent hepatic parasites derived from irradiated sporozoites.

Immunization of rodents and humans with irradiation-attenuated malaria sporozoites confers preerythrocytic stage-specific protective immunity to challenge infection. This immunity is directed against intrahepatic parasites and involves T cells and interferon gamma, which prevent development of exoerythrocytic stages and subsequent blood infection. The present study was undertaken to determine how protective immunity is achieved after immunization of rodent hosts with irradiated Plasmodium berghei sporozoites. We present evidence that irradiated parasites persist in hepatocytes of rats and mice for up to 6 months after immunization. A relationship between the persistence of parasites and the maintenance of protective immunity was observed. Protective immunity was abrogated in irradiated-sporozoite-immunized rats following the application of chemotherapy to remove preexisting liver parasites. Additionally, protective immunity against sporozoite challenge was established in rats vaccinated with early and late hepatic stages of irradiated parasites. These results show that irradiation-attenuated sporozoites produce persistent intrahepatic stages in vivo necessary for the induction and maintenance of protective immunity.