Besnoitia darlingi (Apicomplexa,Sarcocystidae, Toxoplasmatinae): Transmission between Opossums and Cats1

ABSTRACT. Opossums (Didelphis marsupialis), act as intermediate hosts for Besnoitia darlingi and could be infected orally with sporozoites (oocysts) and bradyzoites (tissue cysts), or intraperitoneally (i.p.) with tachyzoites. Infections could presumably be transmitted through cannibalism. Cats (Felis catus), the definitive host, could be infected only with bradyzoites but not sporozoites. Oocysts shed by cats measure about 12 × 12 μm, resemble similarly sized oocysts of Toxoplasma gondii and Hammondia hammondi, and must be differentiated by the appearance of tissue cysts after experimental infection of intermediate hosts. Cats did not form tissue cysts of B. darlingi. Tachyzoites from the related B. jellisoni could be used in the Sabin-Feldman dye test to determine the development of antibody to B. darlingi in opossums after infection.     

Heterothallism in Saccharomyces cerevisiae isolates from nature: effect of HO locus on the mode of reproduction

Understanding the evolution of sex and recombination, key factors in the evolution of life, is a major challenge in biology. Studies of reproduction strategies of natural populations are important to complement the theoretical and experimental models. Fungi with both sexual and asexual life cycles are an interesting system for understanding the evolution of sex. In a study of natural populations of yeast Saccharomyces cerevisiae , we found that the isolates are heterothallic, meaning their mating type is stable, while the general belief is that natural S. cerevisiae strains are homothallic (can undergo mating-type switching). Mating-type switching is a gene-conversion process initiated by a site-specific endonuclease HO; this process can be followed by mother–daughter mating. Heterothallic yeast can mate with unrelated haploids (amphimixis), or undergo mating between spores from the same tetrad (intratetrad mating, or automixis), but cannot undergo mother–daughter mating as homothallic yeasts can. Sequence analysis of HO gene in a panel of natural S. cerevisiae isolates revealed multiple mutations. Good correspondence was found in the comparison of population structure characterized using 19 microsatellite markers spread over eight chromosomes and the HO sequence. Experiments that tested whether the mating-type switching pathway upstream and downstream of HO is functional, together with the detected HO mutations, strongly suggest that loss of function of HO is the cause of heterothallism. Furthermore, our results support the hypothesis that clonal reproduction and intratetrad mating may predominate in natural yeast populations, while mother–daughter mating might not be as significant as was considered.     

Microsatellite markers for assessing DNA polymorphism of Emericella nidulans in nature

Based on genome sequence of Emericella nidulans, primers for 25 microsatellites were designed, of which, seven were chosen as polymorphic markers (five to 12 alleles). Simple sequence repeat (SSR) polymorphism was tested in 44 isolates from extremely arid soils of the southern Negev desert, Israel and some related species of Aspergillus. The SSR marker set appears suitable for population studies in diverse ecological conditions.     

DIFEKTITA AUTOTROFO DE MUTACIITA CHLAMYDOMONAS1

A mutant of Chlamydomonas moewusii is described which, thoughcapable of CO₂-fixation and O₂-evolution in light, was unableto grow at appreciable rates in the absence of an organic substrate.Growth of the mutant approached that of the wild type in citratemedia; fumarate, succinate or malate stimulated growth lesseffectively. (Received August 8, 1960; )     

Tissue-Dwelling Intracellular Parasites: Infection and Immune Responses in the Mammalian Host to Toxoplasma, Sarcocystis and Trichinella

The cycles, pathogenicity and immune processes observed in 3intracellular infections, with the protozoa Toxoplasma gondii,several Sarcocystis species, and the nematode Trichinella spirahsare described. All three organisms have cycles that have co-existedwith their hosts for an evolutionarily long period. Naturalhosts of the protozoa generally are able to develop immunityafter primary infection in a timely manner. However infectionis often fatal in certain unusual hosts not naturally exposedto Toxoplasma and Sarcocystis. It is suggested that whereashosts may have selected strains of intermediate pathogenicity,the two protozoa may have selected for hosts that can developtimely immune responses and survive with chronic persistentinfection. Stability of cycle is maintained by co-evolutionin the definitive host through which the cycle must pass almostregularly. Toxoplasmosis and sarcosporidiosis join malaria andAfrican trypanosomiasis as infections that co-evolved with theirhosts, selecting for immunocompetency and intermediate to lowpathogenicity (Allison, 1982). The plea is made to considerthe probable evolutionary experience of experimental hosts whenstudying their specific immune mechanisms and before possiblygeneralizing from such studies, because the immune responseis dependent on evolutionary experience. In respect to infectionwith Trichinella spiralis one might consider carnivores to havebeen selected and pure herbivores unselected. However, infectionoccurs in a wide variety of hosts, with carnivorous, scavengingand accidental transmission, some of which are of recent modificationby man. Because infection is of low prevalence, and involvesa wide variety of hosts, and pathogenicity is more related tothe degree of hypersensitivity, than to frequency of transmissionand worm load, no uniform effects of selection are apparent.Trichinella infected athymic mice have a high worm load withlittle pathogenic effect. Immunologic processes, while restrictingthe number of worms, are much more important in making the hostsick. In all three intracellular infections both protectiveimmunity and delayed type hypersensitivity are operative. Inbalance, these immune responses are protective in Toxoplasmaand Sarcocystis infection, but are pathogenic in trichinellosis,often causing the death of the infected host.