SELECTION AGAINST INBREDS IN EARLY LIFE-CYCLE PHASES IN PINUS-LEUCODERMIS ANT

Conifers are self-compatible hermaphroditic plants which usually show very low levels of selfing and severe inbreeding depression. Positive fixation indices are observed in the seed stage owing to partial selfing but not in the adult stage and selection against inbreds has been frequently invoked to explain this observation. To determine the stage of elimination of inbreds in Pinus leucodermis Ant., a narrow-ranged and relic species characterized by 18-28 per cent selfing rates, fixation indices at isozyme gene loci were estimated in nursery grown individuals at three different juvenile life-cycle phases and in adult parent trees. Two populations with different levels of selfing were studied. In both populations a deficiency of heterozygotes as a result of selfing was observed in dormant and in germinated embryos, while an excess of heterozygotes comparable with that of the adult trees was found in 5-year-old plantlets. Young plantlets of the same two populations grown in the wild also had negative fixation indices confirming that selection takes place in the first years of growth.