Genetic basis of a key character in Helianthemum nummularium

Abstract

Two subspecies of Helianthemum nummularium (L.) Mill. occur in southern Scandinavia. The key character distinguishing the two subspecies is a dense cover of stellate hairs on the abaxial surface of the leaves in subsp. nummularium that are not found in subsp. obscurum. Stellate hairs on the abaxial surface of the leaves are not a discrete character, but occur in a more or less continuous range from zero stellate hairs to a thick carpet of white intertwisted stellate hairs. A template involving seven classes (scores) was used to describe the variation in stellate hairs. Forty-one natural populations were studied in the field, and their offsprings raised in a common garden were also studied. Distribution of hair scores showed little overlap or few intermediates between the two taxa. In crosses between individuals assumed to be homozygous for the key character of subsp. obscurum and subsp. nummularium, respectively, all progeny in F1 showed hair scores in the range of that found in subsp. obscurum. Consequently, heterozygous individuals could not be distinguished from homozygous individuals of subsp. obscurum. Segregation in F2 did not deviate from the expected 3:1 ratio (obscurum:nummularium), indicating one major gene with a dominant allele for the key character of subsp. obscurum and a recessive allele for the key character of subsp. nummularium. Besides the Mendelian inheritance of the presence/absence of a whitish felt of stellate hairs, the density of hairs appeared to be further modified by minor (quantitative) genes and environment. The frequency of the recessive allele for the subsp. nummularium phenotype in mixed populations showed a strong correlation between field-sampled plants and their offspring in a common garden, supporting the conclusion that the key character was controlled by Mendelian inheritance.