Figure 2.

Genetic code alterations act as a barrier for sexual reproduction. The ability of S. cerevisiae cells expressing C. albicans G₃₃ tRNA_CAG^Serto reproduce sexually was determined by their sporulation and mating efficiencies. (A) The number of tetrads produced by diploid ambiguous cells decreased by 30% when compared with control cells, indicating that sporulation efficiency was lower in CUG ambiguous cells. (B) Spore viability was reduced in ambiguous cells, since most tetrads yielded only one (42%) or no viable spores (35%), while tetrads from control cells produced mainly two viable spores (92%). (C) Ambiguous cells produced fewer viable spores and spores grew slower, as shown by growth on solid selective medium. The U₃₃ tRNA spores shown had mutations in the tRNA_CAG^Ser gene. (D) Haploid control and ambiguous cells with opposite mating types were mixed and serial dilutions of the mixtures were plated onto selective media. C × C, G × G and C × G indicate the crosses between the control cells, or ambiguous G₃₃ tRNA cells or control and ambiguous G₃₃ tRNA cells, respectively. The reduced amount of diploids produced by crossing ambiguous (G₃₃ tRNA) cell lines showed that mating efficiency was also negatively affected by CUG ambiguity. In all cases, the U₃₃ tRNA_CAG was lethal in haploid backgrounds. Alternatively, its gene acquired mutations that inactivated the U₃₃ tRNA.