Complex hybridity in Isotoma petraea. III. Lethal system in Θ₁₂ Bencubbin

Abstract

Analysis of selfed capsules of a ring of 12 (Θ₁₂) complex heterozygote confirms the presence of a balanced lethal system. One of the complexes, S' is two interchanges removed from the primitive chromosome end sequence and when homozygous, causes the abortion of seeds after differentiation of the testa has begun. The other complex, N^e, is three interchanges removed from the primitive sequence and the lethals in it operate, when homozygous, before the testa differentiates. Twin hybrids from crosses between the Θ₁₂ and an alethal structural homozygote exhibit epistatic segregation ratios in their selfed capsules, demonstrating that the lethals in the two complexes are not controlled by single loci. An hypothesis based on relatively translocated internal chromosome segments is developed as the basis for the evolutionary elaboration of the balanced lethal systems in I. Petraea complex hybrids.