ABSTRACT
After the identification of the sexual state, a study was undertaken to determine what role, if any, mating type plays in virulence [18]. Three hundred thirty-eight isolates of C. neoformans were tested for mating type using four tester strains. Of these isolates, 105 were natural isolates and 233 were clinical isolates. The data shown in Table 2 revealed that there was a severe bias in both environmental isolates as well as clinical isolates for the a-mating type over the amating type. The ratio of MATa:MATa for environmental isolates was -40: I and for clinical isolates, -30: I. A genetic explanation for this bias was not apparent since laboratory strains yield the expected la: la ratio of offspring when crossed. The severe bias of MATa to MATa in clinical isolates at first glance appeared to suggest that MATa cells are more infectious than MATa cells. However, the MATa to MATa bias is conserved in environmental isolates as well. Therefore, it is possible that the predominance of the a-mating type in clinical isolates is due to a difference in exposure frequencies rather than an innate difference in virulence. To test this
Figure 2 Basidium and basidiospores. (Photo courtesy of R. Samson.)
possibility, Kwon-Chung and Hill tested a number of MATex and MATa isolates in the mouse model [19]. Based on the data reported in their paper, the authors could not conclude that one mating type was more virulent than the other; however, under certain conditions, ex-cells killed more mice than a-cells. The most important factor which led to increased virulence of ex-cells was inoculum size (Table 3). Continued research on C. neoformans at the molecular and genetic level has shown that this fungus is extremely heterogeneous. Therefore, it was quite possible that the variation observed in the mouse study could have been due to differences in strain
Table 2 Mating Type Bias in Cryptococcus neoformans Mating type
Source a a aaB NMb Totals Ratio a:a
Environmental 87 2 0 16 105 43.5: I Clinical 199 7 2 25 233 28.4: I Totals 286 9 2 41 338 31.8: I
Table 3 Comparison of MAra vs. MAra Cells for Virulence in Mice8
Inoculum (cells)
1 X 10" IX lOS IX lot' 1 X 107
Parental isolates NIHI2 (a) 58 8 9 10 NIH433 (8) 0 2 9 10
F 1 isolates (12 X 433) 8-3501 (a) 4 4 10 10 8-3552 (a) 0 4 7 10 8-3502 (8) 0 3 4 10 8-3556 (8) 0 0 0 I
Total a deaths 9 16 35 40 Total a deaths 0 3 4 II
background, which operated independently of mating type. In order to eliminate this possibility, a congenic pair of strains was constructed which differed only at the mating type locus [20]. These two strains, JEC21 (a) and JEC20 (a), were constructed using a series of backcrosses such that the final pair represented the FlO generation and were derived from sibling spores of different mating type. The two isolates were tested for biochemical characteristics and growth rate in order to insure that they were identical. The strains were then crossed and the progeny were scored for mating type. Five MATa and five MATa isolates for a total of 10 strains were recovered and tested for virulence in the mouse model. The data were analyzed statistically and showed that MATa cells killed more mice at a faster rate than MATa cells [20]. MATa cells were, however, virulent although the virulence was clearly less than MATa cells. These results clearly demonstrated that a-cells are indeed more virulent than a-cells. The molecular basis of this difference in virulence is being investigated in a number of laboratories. While mating type is known to play a role in virulence in other fungi, the ability of one mating type to be more virulent than the other is unexpected.
