THE IMPACT OF ELECTROPHORETIC GENOTYPE ON LIFE HISTORY TRAITS IN PINUS TAEDA

Abstract

Abstract.-Reports of positive associations between allozymic heterozygosity and measures of fitness are routine, but it has not been possible to distinguish between the two preeminent explanations of the phenomenon, dominance and overdominance. We tested several ofthe assumptions of these hypotheses in our study of the relationship between electrophoretic genotype and three life history traits in loblolly pines (Pinus taeda L.). Traits examined included the survival and growth of selfed and outcrossed progeny of 45 maternal trees, and maternal fecundity, measured as the number of surviving progeny per mother tree. Inbreeding depression was severe; the relative fitness ofthe selfed progeny was only 8% that of the outcrossed progeny. We found a heterozygote fecundity advantage, which should have resulted in an excess ofrare alleles in the progeny. Instead, there was evidence of severe survival selection against rare alleles in both heterozygous and homozygous forms. The deficit of rare alleles averaged 69 and 50% in the selfed and outcrossed progeny, respectively.The one allele in the sample that we should have suspected ofbeing maintained by overdominance (a PGI2 mid-frequency allele) appeared to be overdominant for outcrossed height growth and probably for fecundity as well. Multiple-locus genotype explained very little of the variation in growth, however, and rather than seeing evidence for overdominance as a force in maintaining most of the observed polymorphism, we were left to explain, in the face of the severe survival selection, why the rare alleles were present at all. Projection of the stand into the future through computer simulation showed how balancing selection acting on differential growth, fecundity, and mortality among genotypes could, over the life of the stand, account for the maintenance of the rare alleles in the population.

Overdominant lethals as part of the conifer embryo lethal system

“…Another explanation is that overdominant lethal factors are rarely detected due to transient expression specific to early embryo development (Williams et al, 2001). This is consistent with the concept that overdominant lethal factors persist as a consequence of antagonistic or biphasic selection at different life history stages (eg, Bush and Smouse, 1991;Charlesworth and Charlesworth, 1999).…”

Evaluation of Major Genetic Loci Contributing to Inbreeding Depression for Survival and Early Growth in a Selfed Family of Pinus Taeda

“…These estimates would have required inclusion of outcross families of 7-56 in the study in a design that allows estimation of breeding value. Two separate field studies in loblolly pine each show an average height reduction of approximately 20% with selfing after 19 and 9 years, respectively (Bush and Smouse 1991;S. E. McKeand, pers.…”

“…Dominant trees also have larger crowns that provide more sites for seed production, so they may have higher fecundity as well. Bush and Smouse (1991) treated growth potential as a direct measure of fitness. However, increased annual growth tends to be correlated with increased susceptibility to cold and drought among provenances, which may reduce observed growth rates and lead to higher rates of mortality (Ma 1987;Rehfeldt 1992a,b;Schmidtling 1994).…”

“…Evidence for alleles deleterious to early viability but with favorable heterozygous growth effects in P. taeda has been reported by Bush and Smouse (1991).…”

Genetic diversity and recruitment of the tropical palm, Euterpe edulis Mart., in a natural population from the Brazilian Atlantic Forest

“…Using allozyme markers, such studies have demonstrated a heterozygote increase towards the adult stages. Furthermore, positive associations between allozyme heterozygosity and fitness have been reported (Bush and Smouse, 1991;Aravanopoulos and Zsuffa, 1998).…”