O'CONNELL, LISA M.* AND KERMIT RITLAND. Department of Forest Sciences, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z4. - Early inbreeding depression and the mating system of western redcedar.
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