Multilocus structure in the Pinus contorta – Pinus banksiana complex

Natural hybridization between lodgepole pine ( Pinus contorta Dougl. ex Loud.) and jack pine ( Pinus banksiana Lamb.) in western Canada provides a distinct opportunity to infer evolutionary and demographic determinants of nonrandom associations of the alleles at different loci occurring in this Pinus contorta – Pinus banksiana complex (PCBC). Here, we investigated multilocus associations among and within 40 PCBC populations sampled from central and northwestern Alberta, using 39 random amplified polymorphic DNA (RAPD) markers. For each of the 40 PCBC populations, we examined distributions of 741 pairs of linkage (gametic) disequilibria (LD) between the 39 RAPDs and measured the “average” multilocus associations using the summary statistics that allow for packaging of individual LD in each population. We then partitioned the variance of LD in the total population to assess the causes of multilocus population structure. The results showed that (i) LD were more prominent in hybrid populations than in parental populations; (ii) multilocus Wahlund effect was a much more important determinant of population structure than its single-locus counterpart, particularly in hybrid populations; and (iii) considerable mutlilocus associations across the populations within each taxa group was due to the presence of different multilocus haplotypes in different populations. Such results are best explained by the fact that PCBC populations are geographically and ecologically marginal and are produced at the balance between mixing of two distinct gene pools creating new recombinants and selection in favor of parental gametes, but against the recombinants.

Random amplified polymorphic DNA diversity of marginal and central populations in Pinus contorta subsp. latifolia

Fifteen populations of lodgepole pine (Pinus contorta subsp. latifolia) were surveyed for diversity across 52 random amplified polymorphic DNAs (RAPDs). The objective was to compare single-locus and multilocus structures in four marginal, three intermediate, and eight central populations. Single-locus estimates indicated average observed and expected heterozygosity to be 0.19 and 0.17, respectively. When these estimates were split into population categories, a clear trend of increasing diversity was detected in the direction of marginal to central populations. F-statistics indicated an excess of heterozygotes, with FIS ranging from -0.08 for marginal populations to -0.15 for central populations and averaging -0.12 over 15 populations. The estimates of FST decreased towards the margins of the species range, indicating increased population differentiation. Forty-nine of 52 RAPDs tested neutral in the Ewens-Watterson analysis. Multilocus analysis showed significant two-locus and high-order gametic disequilibria in all 15 populations. The most prominent components of the two-locus analysis were the variance of disequilibrium (VD, 46.2%) and the multilocus Wahlund effect (31.9%). This high value for VD indicated that founder effects could explain much of the observed multilocus associations. When analyzed by population categories, the VD showed a decreasing trend indicating that variation due to founder effects was more prominent in marginal populations. The two-locus Wahlund effect (WC) that is characteristic of strong population subdivision was highest in the central populations. This indicated significant levels of gene flow between populations with different allelic combinations.Key words: multilocus genetic structure, central and marginal populations, RAPD, Pinus contorta subsp. latifolia.

Zygotic Associations and Multilocus Statistics in a Nonequilibrium Diploid Population

The usual approach to characterizing and estimating multilocus associations in a diploid population assumes that the population is in Hardy-Weinberg equilibrium. The purpose of this study is to develop a set of summary statistics that can be used to characterize and estimate the multilocus associations in a nonequilibrium population. The concept of “zygotic associations” is first expanded to facilitate the development. The summary statistics are calculated using the distribution of a random variable, the number of heterozygous loci (K) found in diploid individuals in the population. In particular, the variance of K consists of single-locus and multilocus components with the latter being the sum of zygotic associations between pairs of loci. Simulation results show that the multilocus associations in the variance of K are detectable in a sample of moderate size (≥30) when the sum of all pairwise zygotic associations is greater than zero and when gene frequency is intermediate. The method presented here is a generalization of the well-known development for the Hardy-Weinberg equilibrium population and thus may be of more general use in elucidating the multilocus organizations in nonequilibrium and equilibrium populations.

The effect of an oak wilt epidemic on the genetic structure of a Texas live oak population

Oak wilt is a fungal tree disease that has killed millions of live oaks (Quercus fusiformis Small) in the oak woodlands of central Texas. Allozymes were used to characterize the genetic structure of live oak populations prior to infection (pre-epidemic) and following passage of an epidemic wave (post-epidemic). Pre-epidemic trees (N = 112) were sampled along transects in front of an expanding disease front. Post-epidemic trees (N = 109) were survivors of an epidemic that swept through an area of approximately 28 ha over a period of 20 years. Significant differences in allele and genotype frequencies existed between pre- and post-epidemic populations. Gene diversity was lower for two of the four allozyme loci in the post-epidemic population. Departures from Hardy-Weinberg equilibria occurred for two loci and multilocus associations developed in the post-epidemic population. These results demonstrate that disease can have a significant impact on the genetic structure of a natural host population. We hypothesize that selection for increasing disease resistance was the dominant evolutionary force leading to genetic change in this plant pathosystem.Key words: host-pathogen interactions, population genetics, Ceratocystis fagacearum, coevolution.