COMPLEX HYBRIDITY IN ISOTOMA-PETRAEA .8. VARIATION FOR SEED ABORTING LETHAL GENES IN THE RING-OF-6 PIGEON ROCK POPULATION

The highly inbreeding Pigeon Rock population of Isotoma petraea consists of about one-third primitive structural homozygotes and two-thirds derived ring-of-six (or ring-of-ten) complex heterozygotes. A majority of the structural homozygotes exhibit significant proportions of seed abortion in their selfed capsules, ranging from about 1 to 33.8 per cent, while all the structural heterozygotes exhibit significant levels of seed abortion, ranging from 8.8 to 59.9 per cent. Seed abortion ratios appear to be determined by genic interactions which are sensitive to plant growth conditions. It is suggested that seed aborting recessive lethal genes are of adaptive utility and have accumulated in this population because they prevent the allocation of resources to inbred homozygotes, which cannot contribute to future generations. The seed aborting systems may be modelled in terms of recessive seed-aborting lethal genes and independently assorting dominant modifiers of those genes. Appropriate mutations may be generated by the transposition of internal chromosome segments.