Population genetics of conifer seed orchards

Seed orchards represent the link between breeding and silvicultural activities. They were expected to act as closed, panmictic populations in Hardy-Weinberg equilibrium, meaning that desired genes drafted during previous selection stage would be effectively transmitted from parental to offspring populations; however, extensive research has indicated that this expectation is not met. Scrutinizing seed orchards’ efficiency is of vital importance as it determines the genetic quality of future forest stands. Population genetics of four tree species’ seed orchards (western larch, Douglas-fir, lodgepole pine, and western redcedar) was studied using microsatellite DNA markers. Partial (family array) and full (bulk seed) pedigree reconstruction of offspring population (seed crops) were conducted using the likelihood-based parentage inference program CERVUS to estimate parental reproductive success, selfing rate, pollen contamination, effective number of parents (Ne), and seedlot genetic worth. Several simplified methods for predicting seed crops’ genetic quality and quantity were evaluated by comparing parental reproductive success with parental fecundities. In all species, the top 20% of males contributed approximately one half of successful within-orchard pollen, substantially reducing male Ne (45 to 62% of the orchards’ census numbers). Even larger distortion was observed among females (the top 20% of females produced 77% of seed crop in Douglas-fir), reducing female Ne to as little as 13% of the census. Selfing and pollen contamination rates were in the range of previously reported studies, with the exception of high (15.2%) and low (7.3%) selfing rates in Douglas-fir and western redcedar, respectively. Pollen bud production and seed-cone volume were found to be the most reliable proxies to parental reproductive success, genetic worth, and Ne estimates. An optimization protocol was developed for creating custom seedlots with maximized genetic gain at any Ne while collectively considering parental male and female fecundities, co-ancestry among parents, inbreeding, and variation in seed germination capacity. This protocol can be utilized in any generation’s seed orchard e.g. when seed supply exceeds demand, for mixing surpluses from multiple years, or if a given seed lot fails to meet minimum Ne requirements.Forestry, Faculty ofGraduat