Inbreeding depression plays a key role in the evolution of plant mating systems. Conifers have high inbreeding depression, high genetic diversity and low self-fertilization rates compared to other plant groups. A conspicuous exception in conifers is the genus Thuja (Cupressaceae) which has shown low inbreeding depression, low genetic diversity and high selfing rates. In western redcedar (Thuja plicata) genetic diversity measured both at neutral molecular markers and phenotypic traits is among the lowest in conifers. Using one polymorphic isozyme marker we estimated outcrossing rates in six natural populations of T. plicata in southwestern British Columbia. Population outcrossing rates ranged from 17 to 100% (mean: 71%), indicating a labile mating system with marked among population differences. Early inbreeding depression in redcedar is lower than in other conifers and can allow for ecological differences among populations and trees to influence the outcrossing rate. Selection against inbreeding could be occurring at the embryo developmental stage. In conifers, several fertilized embryos can co-occur in a female gametophyte but only one eventually survives. In species with fewer embryonic lethals, polyembryony can increase outcrossing when both self and outcross pollen are present, but also allow for reproductive assurance through selfing when outcross pollen availability is low. We conducted controlled pollinations in a seed orchard to first assess the levels of inbreeding depression in T. plicata. Pollinations with self and outcross pollen mixtures were also conducted to assess selection for outcross pollen at the embryo stage. Isozymes were used to assay the seedlings to assess the relative success of each pollen type.

Key words: conifer,evolution,plant mating system,polyembryony,Thuja plicata