Congenic Strains of the Filamentous Form of Cryptococcus neoformans for Studies of Fungal Morphogenesis and Virulence

Abstract

cCryptococcus neoformans is an unconventional dimorphic fungus that can grow either as a yeast or in a filamentous form. To facilitate investigation of genetic factors important for its morphogenesis and pathogenicity, congenic a and ␣ strains for a filamentous form were constructed. XL280 (␣) was selected as the background strain because of its robust ability to undergo the morphological transition from yeast to the filamentous form. The MATa allele from a sequenced strain JEC20 was introgressed into the XL280 background to generate the congenic a and ␣ pair strains. The resulting congenic strains were then used to test the impact of mating type on virulence. In both the inhalation and the intravenous infection models of murine cryptococcosis, the congenic a and ␣ strains displayed comparable levels of high virulence. The a-␣ coinfections displayed equivalent virulence to the individual a or ␣ infections in both animal models. Further analyses of the mating type distribution in a-␣ coinfected mice suggested no influence of a-␣ interactions on cryptococcal neurotropism, irrespective of the route of inoculation. Furthermore, deletion or overexpression of a known transcription factor, Znf2, in XL280 abolished or enhanced filamentation and biofilm formation, consistent with its established role. Overexpression of Znf2 in XL280 led to attenuation of virulence and a reduced abundance in the brain but not in other organs, suggesting that Znf2 might interfere with cryptococcal neurotropism upon extrapulmonary dissemination. In summary, the congenic strains provide a new resource for the exploration of the relationship in Cryptococcus between cellular morphology and pathogenesis.

A Family of Secretory Proteins Is Associated with Different Morphotypes in Cryptococcus neoformans

“…4A). We further obtained gene deletion mutants in the mating type a background by dissecting the meiotic progeny from a cross between the gene deletion a strain and the congenic WT a strain (27). Again, there was a lack of obvious defects in the filamentation or sporulation of the corresponding mutants in the mating type a background.…”

“…Cryptococcus can undergo filamentation and sporulation both during a-␣ bisexual reproduction and during unisexual reproduction with cells of one mating type (24)(25)(26). We chose to use strains in the wild-type XL280␣ background for such investigations, as XL280 has been widely used for morphogenesis studies (27). We first examined the effect of the deletion of these genes during unisexual development where ␣ cells alone were cultured under mating-inducing conditions (V8 juice agar medium).…”

Congenic Strains for Genetic Analysis of Virulence Traits in Cryptococcus gattii

“…The analysis of the virulence properties of the two congenic strains indicates no difference between them and the parental R265 strain, excluding a major contribution from the mitochondrial genotype and mating type in the virulence of the outbreak isolate and thereby implicating other factors. Furthermore, the strains add a new resource for future analysis of gene function and evolution in both C. gattii and C. neoformans, as discussed in the accompanying article by Zhai et al (16).…”

Sex Determination Directs Uniparental Mitochondrial Inheritance in Phycomyces

“…A similar situation likely occurs in the related species Cryptococcus gattii. Crosses between congenic strains differing only in mating type exhibit UPI, while in crosses using strains with different genetic background both parents can contribute their mitochondrial DNA to the progeny (21,22). In addition, in C. neoformans the environmental and genetic conditions alter the proportion of uniparental versus biparental inheritance (20,(23)(24)(25), and cytological analysis indicates that the mating type ␣ cell delivers its nucleus to the a cell (26).…”