Figure 5. A model for sex determination and paternal mtDNA inheritance in mussels.
Sex is determined by a nuclear locus S, with two alleles, S1 and S2, with different “dosage” effect (S1 = 1, S2 = 2). For maleness it is required that the dosage is 2 or higher. Only the maternal allele is expressed during the development of sex. The paternal allele is masked and contributes a dosage of zero, regardless of whether it is S1 or S2. Thus, sex is determined by the mother's genotype. The genotype of the male parent affects the sex of the progeny of his daughters. Paternal mtDNA transmission is affected by three nuclear genes. Locus W is male-expressed. It's product labels the outer surface of sperm mitochondria and differentiates them from egg mitochondria. Locus X is female-expressed. It supplies the egg with a factor that interacts with W and prevents sperm mitochondria from aggregating and co-segregating into the first germ cells. Locus Z is also female-expressed. It segregates for an active (Z) and an inactive (z) allele. It supplies the eggs with factor Z which suppresses factor X and allows sperm mitochondria to aggregate and co-segregate into the first germ cells. There is a tight linkage between Z and S2 and between z and S1 so that only the combinations ZS2 and zS1 may occur in an egg. Sperm mitochondria occur in the sperm's mid-piece, but are shown in the “head” for convenience. Egg mitochondria far out-number sperm mitochondria but we show one of each kind for convenience.