MOLECULAR-GENETICS OF GROWTH AND DEVELOPMENT IN POPULUS .2. SEGREGATION DISTORTION DUE TO GENETIC LOAD

Distortion of expected Mendelian segregation ratios, commonly observed in many plant taxa, has been detected in an experimental three-genera tion inbred pedigree of Populus founded by interspecific hybridization between P. trichocarpa and P. deltoides. An RFLP linkage map was cons tructed around a single locus showing severe skewing of segregation ra tio against F-2 trees carrying the P. trichocarpa allele in homozygous form. Several hypotheses for the mechanism of segregation distortion at this locus were tested, including directional chromosome loss, segr egation of a pollen lethal allele, conflicts between genetic factors t hat isolate the parental species, and inbreeding depression as a resul t of genetic load. Breeding experiments to produce inbred and outcross ed progenies were combined with PCR-based detection of RFLPs to follow the fate of the deficient allele throughout embryo and seedling devel opment. A recessive lethal allele, lth, inherited from the P. trichoca rpa parent, was found to be tightly linked to the RFLP marker locus PO P1054 and to cause embryo and seedling mortality. Heterozygotes (lth/) appear to be phenotypically normal as embryos, seedlings, and young trees.