A suppressor of a mating-type limited zygotic lethal allele also suppresses uniparental chloroplast gene transmission in Chlamydomonas monoica.
Uniparental inheritance of Chlamydomonas chloroplast genes is thought to involve modification of maternal (mt+) chloroplast genomes to protect against a nuclease that is activated after gamete fusion. The mating-type limited mtl-1 mutant strain of Chlamydomonas monoica is unable to protect mt(+)-derived chloroplast DNA. Zygotes homozygous for mtl-1 lose all chloroplast DNA and fail to germinate. We have selected for suppression of this zygote-specific lethality, and have obtained 20 mutant strains that produce viable homozygotes despite the continued presence of the mtl-1 allele. Genetic analysis indicates that the suppressor mutations are all recessive alleles at a single locus (sup-1) which is unlinked to mtl-1. Crosses between sup-1 strains carrying distinctive chloroplast antibiotic resistance markers also show predominantly biparental chloroplast gene transmission. Chloroplast nucleoids of both parental origins (stained with the DNA-specific fluorochrome, DAPI) are retained in the zygotes homozygous for sup-1. The data are compatible with the idea that the sup-1 (suppressor of uniparental inheritance) locus may encode a chloroplast DNA nuclease that is expressed from both parental genomes.
VanWinkle-Swift
K
K
Department of Biological Sciences, Northern Arizona University, Flagstaff 86011.
Hoffman
R
R
Shi
L
L
Parker
S
S
D000483:Alleles; D000818:Animals; D002696:Chlamydomonas / Q000235:genetics* / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism*; D005111:Extrachromosomal Inheritance; D005804:Genes, Lethal; D017125:Genes, Protozoan; D005808:Genes, Recessive; D016153:Genes, Suppressor; D006720:Homozygote; D009154:Mutation; D015053:Zygote / Q000378:metabolism