COMPLEX HYBRIDITY IN ISOTOMA-PETRAEA .7. ASSEMBLY OF THE GENETIC SYSTEM IN THE O-6 PIGEON ROCK POPULATION

The Pigeon Rock population of Isotoma petraea is highly inbreeding and it is polymorphic with respect to interchange hybridity, allozyme variation and lethal system components. Two thirds of the plants in this population are O6 interchange heterozygotes and may be considered as N.S complex heterozygotes. The N complexes are two interchanges removed from the standard “axillaris” chromosome end sequence characteristic of Isotoma structural homozygotes and they carry the PGM 3 allele. Most of the N complexes are non-transmissible through the pollen and where not so, carry a recessive lethal factor. The associated S complexes in the O6 interchange heterozygotes carry the standard chromosome end sequence and either PGM 1 or PGM 2. It is suggested that in highly inbreeding species with terminally localized chiasmata the occurrence of balanced deleterious recessives within supergenes results in permanent hybridity and that the yield of fully heterozygous offspring must be very low, following selling, or very occasional, following rare cross pollination events. Deterministic models incorporating the above observations and deductions indicate that O6 hybrids should occur at a frequency of approximately 66 per cent under a regime involving approximately 98 per cent recurrent selling irrespective of whether the N complex is transmissible or not transmissible in the pollen. These values are comparable to those actually found in the Pigeon Rock population. It is concluded that complex hybridity in I. petraea had its origin in the Pigeon Rock population, and arose within highly inbreeding lineages in which mutations contributing to an increasingly efficient transmission of permanently heterozygous parental genotypes were selected. © 1990 The Genetical Society of Great Britain.