POPULATION STUDIES IN PREDOMINANTLY SELF‐POLLINATED SPECIES. II. ANALYSIS OF QUANTITATIVE GENETIC CHANGES IN A BULK‐HYBRID POPULATION OF BARLEY

Genetic variation among mainland and island populations of a native perennial grass used in restoration

Genetic marker studies can assist restoration practice through selection of seed sources that conserve historical levels of gene diversity and population genetic differentiation. We examined genetic variation and structure within and among mainland and island populations of Elymus glaucus, a perennial bunchgrass species native to western North American grasslands that is targeted for grassland restoration. Island populations of E. glaucus represent sensitive sites and potentially distinctive seed sources for reintroduction, and little is known of their genetic composition. Genetic diversity and structure were estimated using amplified fragment length polymorphism markers for 21 populations and 416 individuals distributed across two coastal California mainland locations and three California Channel Islands. Eight primer combinations resulted in 166 markers, of which 165 (99.4 %) were polymorphic. The number of polymorphic bands was significantly greater among mainland populations relative to island sites, and locally common alleles were present for each sampled island and mainland location. Population structure was high (62.9 %), with most variation (55.8 %) distributed among populations, 7.1 % between mainland and island locations, and the remainder (37.1 %) within populations. Isolation by distance was only apparent among islands. Using marker data to recommend appropriate seed sources for restoration, E. glaucus seeds are best derived within islands with collections representing a large number of individuals from matching environments. Given the limited gene flow and prior evidence of adaptive divergence among populations of this species, regional collections are recommended in all cases to maintain diversity and to avoid long-distance introductions of highly differentiated plant material.

Loss of genetic diversity from heterogeneous self-pollinating genebank accessions

Genebank seed accessions of predominantly self-pollinating species may be stored either as bulked (mixed) seed lines or as pure line cultivars. If seed lines are bulked in storage then when considered over several regeneration cycles, loss of genetic diversity within heterogeneous self pollinating genebank accessions is shown to be severe. This within-accession loss of diversity represents opportunities foregone through the random loss of individual genotypes. Amongst working collections, the utility and repeatability of genebank accessions is paramount in the justification of the germ plasm resource. Therefore, the only practical solution to the management of predominantly self-pollinating species is to preserve individual accessions as pure lines.

The Croonian Lecture, 1991. Genostasis and the limits to evolution

The Darwinian explanation for evolution is that it is the outcome of the interaction between genetic variation and natural selection. There is now good evidence for both the existence of genetic variation and the occurrence of natural selection, the latter potentially at high intensities. The outcome should be rapid evolutionary change; yet in practice very little change is found. Most species are very stable, and in situations where evolution is observed in one species often none is found in others despite equivalent opportunity. Evolutionary failure is commonplace. Despite the occurrence of high levels of protein polymorphism, there is good evidence that the supply of variation making a major contribution to fitness is very limited. As a result it is argued that lack of evolution in most species may be due more to lack of appropriate variability than to other causes: a condition for which the term 'genostasis' is proposed. In those situations where appropriate genetic variation is available for one reason or another, evolution is found to be very rapid. There are good theoretical and practical reasons for more attention being paid to the mechanisms of supply of new variation and to those situations where evolution appears not be taking place.

Evolution of resistance to scald, powdery mildew, and net blotch in barley composite cross II populations

Progenies of barley plants taken at random from generations F8, F13, F23, and F45 of Composite Cross II were tested for reaction to three barley pathogens, Helminthosporium teres, Erysiphe graminis, and Rhynchosporium secalis (four races). The frequency of families resistant to each of the three pathogens (excepting one race of R. secalis) increased from early to later generations. Many families carrying combinations of multiple resistance not observed among the parents were found in the later generations; one-half of the parents were susceptible to all three diseases but only 4% of the families in generation F45 were of this phenotype. The frequency of multiply resistant families also increased sharply over generations. Among the parents the greatest concentration of resistant reactions observed was triple resistance: 14% were triply resistant; none were quadruply resistant; nor were any resistant to all three pathogens. In generation F43 36% of the families were triply resistant, 32% were quadruply resistant and 6% were resistant in five of the six disease reactions. The value of later generations of Composite Cross II for breeding against pathogens thus appears to have been increased by recombination and natural selection. Significant positive correlations were found for resistance to races 40, 61, and 74 of R. secalis whereas correlations between resistance to R. secalis, E. graminis, and H. teres were generally nonsignificant. Use of these multiply resistant genotypes in breeding is thus unlikely to be hindered by negative associations between resistance to the three diseases.

The maintenance of genetic variability by mutation in a polygenic character with linked loci

It is assumed that a character under stabilizing selection is determined genetically bynlinked, mutable loci with additive effects and a range of many possible allelic effects at each locus. A general qualitative feature of such systems is that the genetic variance for the character is independent of the linkage map of the loci, provided linkage is not very tight. A particular detailed model shows that certain aspects of the genetic system are moulded by stabilizing selection while others are selectively neutral. With reference to experimental data on characters ofDrosophilaflies, maize, and mice, it is concluded that large amounts of genetic variation can be maintained by mutation in polygenic characters even when there is strong stabilizing selection. The properties of the model are compared with those of heterotic models with linked loci.

NATURAL SELECTION IN A COMPOSITE CROSS OF BARLEY

A composite of 105 crosses involving 15 parental barley varieties was grown for 5 years at nine stations of widely different latitudes in Canada, U.S.A. and Norway. A resulting sample of seed was recovered from each station and grown with original seed in comparative studies at Edmonton. Significant differences were observed among plants from different stations for heading date, maturity period, yield, spike type, and awn type. Little difference was observed for plant height and seed size. There was little or no effect on collar type, neck shape, neck length, leaf width or shape of basal rachis internode. Natural selection acted strongly against black kernel, the elimination of which was probably due to genetic linkage with a character of low competitive capacity. In general, the shift in the gene pool was consistent with plant breeding objectives.

Simulation of models involving mixed selfing and random mating : II. Effects of selection and linkage in finite populations

The joint effects of linkage, inbreeding, and drift due to finite population size were investigated in terms of population changes under selection involving gene interaction. Six-locus models with the same amount of recombination between adjacent pairs of loci, mixed selfing and random mating, and selection of basically three forms (heterotic, optimizing and mixed optimum-heterotic) were used for Monte Carlo simulation. The results were primarily described in terms of certain measures of gene dispersion, genetic variability, gametic unbalance (linkage disequilibrium) and the approach to stable gene frequency equilibria. Under both cumulative and diminutive heterosis models, a steady state with polymorphisms could be attained with random gene dispersion being small and different replicate populations evolved high degrees of gametic unbalance in the direction of excess of either coupling or repulsion phase linkages depending on the random drift in gene frequencies. Under optimum models, on the other hand, all populations approached steady decay toward fixation at all loci although gene dispersion was governed by rather complex interactions between the parameters of selfing, linkage and selection intensity. Gene dispersion was not necessarily proportionately greater with the higher levels of inbreeding. An excess of repulsion linkages with mean population fitness approaching unity was noted in all runs with the optimum models, more so with tight linkage and heavy inbreeding. Any asymmetry in the sense of selection favoring one or the other allele tends to reinforce gene fixation particularly under inbreeding. Heterozygote advantage, on the other hand, seemed to play a relatively greater role under inbreeding in terms of retaining heterozygosity. Mixed optimum-heterotic models provide a favorable compromise between these conflicting attributes of multilocus systems in terms of the maintenance of polymorphisms and the maximization of fitness in relation to certain optimal linked gene complexes. In general, for moderate to large population size these results are, as expected, in line with those reported previously for two-locus deterministic models.