Isozyme variation and linkage in six conifer species

Abstract

Abstract: Isozymes of female gametophyte tissue were analyzed for allelic variation in knobcone, lodgepole, loblolly, Jeffrey, and sugar pines and in Douglas-fir. Linkage was studied in the five pines. The average number of alleles and average herozygosity per enzyme locus were estimated. Knobcone pine ranked lowest among the six species in number of alleles and average heterozygosity; loblolly pine and Douglas-fir ranked highest. Numerous linkages were identified, and one chromosome segment contained between one-fourth and one-third of all loci examined. Linked loci had a consistent gene order in the different pine species. Linkage relationships support the concept that similar isozymes represent the same loci in each pine species. The consistency with which loci map in the same order and with similar map distances in species from different subsections of the genus, further suggests that evolution of the pines is not associated with major chromosome rearrangements. The analysis of genetic variation in forest trees entered a new phase with the application of electrophoresis to detect allelic differences at isozyme loci. Gametophytes from seed of single trees provide excellent material for assessing isozyme variation. Enzymes are analyzed at the just-germinated stage of development, and diploid genotypes of parent trees are inferred from the segregation of allele phenotypes of the haploid female gametophytes. Seed from some of the heterozygous parent trees are analyzed in large numbers to determine if enzyme variants segregate according to simple Mendelian ratios. A one-toone segregation ratio of phenotypic variants is evidence for allelism. Homozygous loci within a single tree are identified without error and the samples per tree are sufficient to minimize the misclassification of heterozygous loci. This paper reports a two-stage analysis. In the first stage, the amount of genetic variation in several species of two conifer genera was estimated. Gametophytes from seed of individual trees were analyzed for segregation and data were obtained on the number of alleles and the average heterozygosity per locus. The six species compared were Douglas-fir and five North American pines. In the second stage, linkage from segregation in female gametophytes of heterozygous trees was estimated. Linkage data are provided for the five pine species.