Next, we investigated the effect of disrupting the FIS1 gene on the mitochondrial morphology in an industrial sake yeast strain. Mitochondria were observed to transform from having a tubular morphology to a dotted morphology during sake brewing in an industrial strain (K7 [RAK1536] ( 23) +pRS413GPD-mito-GFP) ( 21 and 24), similar to the transformation observed in the laboratory strain ( Fig. 1B). Since the industrial sake yeast strain is a diploid and disruption of the FIS1 gene requires two steps, a haploid strain (K7H868) having a brewing performance similar to that of the parental diploid strain was obtained by sporulation, and the FIS1 gene was disrupted in K7H868. The mitochondrial morphology of the haploid fis1Δ strain of sake yeast (K7 haploid fis1::natMX4 leu2::kanMX4+pYX142-mito-GFP) remained networked throughout sake brewing as observed in the laboratory fis1Δ strain ( Fig. 1B). These findings clearly indicate that Fis1p is also responsible for this fragmentation in the industrial strains.