In situ and Ex situ Conservation of Commercial Tropical Trees - ITTO

215Genetic Conservation in AppliedTree Breeding ProgramsRANDY JOHNSON 1 , BRAD ST. CLAIR 1 AND SARA LIPOW 21)USDA-Forest Service, Pacific Northwest Research Station,Corvallis, OR, USA 97331-44012)Dept. of Forest Science, Oregon State University,Corvallis, OR, USA 97331-5752Randy.Johnson@orst.eduAbstract. This paper reviews how population size and structure impacts the maintenance ofgenetic variation in breeding and gene resource populations. We discuss appropriate populationsizes for low frequency alleles and point out some examples of low frequency alleles in theliterature. Development of appropriate breeding populations and gene resource populations arediscussed.IntroductionThe primary objective of a breeding program is to increase the frequency ofdesirable alleles found in the breeding population. While breeders know thetraits they wish to improve, they do not know which alleles (genes) favorablyimpact the traits or their distribution in the native population. Breeding programsmust maintain sufficient genetic variation to allow for continued genetic gainsover multiple generations. Complicating matters is the fact that traits of interestchange over time in response to new pests or changes in markets. Populationsizes needed to maintain gains in polygenic traits of current interest are muchsmaller than population sizes needed to find potentially rare traits that may bedesired in the future. This paper considers the impact of breeding populationsize and structure on the maintenance of genetic variation and on continuedgenetic gain.The breeder needs to consider both short- and long-term objectiveswhen structuring a breeding program. Short-term objectives usually includeobtaining substantial gains in current traits of interest in the first few generationsof breeding while maintaining well-adapted trees. Long-term objectives includethe maintenance of low frequency alleles and control of inbreeding. A majorconflict arises between short- and long-term objectives. Selection intensity mustbe high to obtain substantial genetic gains, yet maintaining rare alleles requireskeeping a large breeding population in subsequent generations. There are waysto structure the breeding population and make selections to reduce this conflict.