Gene dosage effects on MDH isozyme expression in diploid, triploid, and tetraploid treefrogs of the genus Hyla

Electrophoretic staining intensities at a polymorphic malate dehydrogenase locus (MDH-1) in the tetraploid treefrog Hyla versicolor and diploid Hyla chrysoscelis were compared using intraspecific and interspecific crosses. Gene dosage levels within the three ploidies were additive for the expression of isozymes encoded by this locus. Using densitometry, phenotypes from intraspecific tetraploid progeny conformed to a tetraploid gene dosage model, while phenotypes from interspecific hybrids conformed to a triploid gene dosage model. Intra- and interspecific progeny phenotypes and ratios agree with theoretical models predicting differences in such phenotypes and ratios, depending on the ploidy of the parents involved. Heterozygous phenotypes revealed that diploid crosses produced only symmetrically heterozygous progeny while tetraploid crosses produced two classes of asymmetrically heterozygous progeny in addition to symmetrically heterozygous progeny. Interspecific crosses produced only asymmetrically heterozygous progeny. Interspecific crosses involving a homozygous diploid genotype and symmetrically heterozygous tetraploid genotype produced three progeny phenotypes, while the reciprocal cross produced only two progeny phenotypes. the value of interspecific crosses between diploids and tetraploids in determining gene silencing at duplicated loci is considered.