A multilocus estimator of mating system parameters in plant populations

A multilocus method of estimating mating system parameters in populations is presented that recovers information from classification over multiple loci that single-locus estimators do not detect. It is shown that the multilocus estimator provides a standard of reference (null hypothesis) that can be useful in analyzing the effects of factors such as population heterogeneity and post-mating zygotic selection on the transmission of genetic information at the population level.

Evolutionary Changes in the Mating System of an Experimental Population of Barley (Hordeum vulgare L.)

An analysis is presented of the reproductive cycle in an experimental population of barley. The experimental design included growing three different generations of the population in the same year and environment, thus permitting an assessment of the mating system at three stages in the evolutionary history of the population, unconfounded by environmental differences. Gene frequencies sometimes differed significantly in adults and in the effective pollen pool. It was also found that out-crossing rate more than doubled during the twenty generations spanned by the study. These results provide evidence for selection during reproductive phases of the life cycle. They also demonstrate that evolution in this predominantly self-pollinating population was in the direction of increased recombinational potential.

Patterns of Genetic Differentiation in the Slender Wild Oat Species Avena barbata

Allozyme frequencies at five enzyme loci were determined for 14 California populations of Avena barbata, a species introduced to California from the Mediterranean Basin during the colonization of North America. Allelic frequencies at these loci were also determined in Mediterranean collections of this species. The pattern of divergence of the California populations from the ancestral gene pool was not random and was strongly correlated with environment; thus, the pattern is not in accord with the hypothesis that most electrophoretically detectable variants are adaptively neutral. Rates of gene substitution in California were also not in accord with the neutrality hypothesis. The observations are, however, compatible with predictions of Neo-Darwinian evolutionary theory. We interpret these observations to indicate that natural selection plays a major role in determining the unique patterns of distribution of genetic variability in the slender wild oat in California.

Microgeographical Variation in Allozyme Frequencies in Avena barbata

Microgeographical distribution of alleles of five enzyme loci and one morphological variant has been determined within populations of A. barbata occupying sites that are transitional between different vegetational zones. The results show that the spatial distribution of alleles is nonrandom and highly correlated with habitat in patterns that parallel the distribution of the same alleles in major climatic zones in California. It is concluded that the observed patterns of variation are best explained by Neo-Darwinian evolutionary models, in which selection plays a predominant role.

Further analysis of complex allozyme polymorphisms in a barley population

New theory has recently been developed for two-locus models. In the light of this theory, an earlier analysis of esterase allozyme data from an experimental barley population has been modified to take proper account of initial gametic phase (linkage) disequilibria. The results show that the directions in which two-locus genotypic frequencies deviated from products of one-locus frequencies in this population followed those predicted by neutral descent theory. The observed departures were, however, much larger in size than predicted by the new descent measure theory, indicating that selection is operating in the population.

History of plant population genetics

This review of plant population genetics focuses on the genetic foundations of the processes that have led to documentable improvements in cultivated plants since the earliest domestications took place perhaps 13,000 years ago. Nearly all human civilizations have depended heavily on inbreeding plants (particularly wheat, barley, soybeans and other inbreeding legumes), as well as outbreeding vegetatively propagated species (white potatoes, yams) as their dietary standbys. The principal exception is maize (corn), an annual seed-produced outbreeder in nature. It is noteworthy that maize joined wheat, rice, and barley as a truly major crop worldwide only after its conversion to self-pollination combined with hybridization between favorably interacting inbred lines increased yield of maize several-fold in the twentieth century.

Evolution of ribosomal DNA (rDNA) genetic structure in colonial Californian populations of Avena barbata

DNA samples from 980 plants of Avena barbata from 48 ecologically diverse sites in California and Oregon were assayed to determine their genotype for two duplicated loci governing rDNA variants. More than 40 different rDNA genotypes were observed among which 5 made up 96% of our sample in environmentally homogeneous sites; predominant genotypes were less frequent and recombinant genotypes were more frequent in environmentally heterogeneous sites. The spatial distribution of each predominant rDNA genotype was nearly an exact overlay on both macro- and microgeographical scales of a distinctive habitat and also of the distribution of an eight-locus morphological-allozyme variant genotype. In all, seven different habitat-genotype combinations (ecotypes) were distinguishable on the basis of their morphological-allozyme-rDNA genotypes. None of these seven genotypes has been found in ancestral Spanish populations; thus the above predominant multilocus genotypes (ecotypes) of the colonial populations evidently evolved subsequent to the recent introduction (within 150-200 generations) of A. barbata to California. The precise associations of specific alleles and genotypes of the morphological allozyme and rDNA loci with different specifiable habitats leads us to the conclusion that natural selection favoring particular multilocus combinations of alleles in different habitats was the main guiding force in shaping the internal genetic structure of local populations as well as the overall adaptive landscape of A. barbata over California and Oregon.

Extraordinarily polymorphic microsatellite DNA in barley: species diversity, chromosomal locations, and population dynamics

This study was undertaken to assess the extent of genetic variation in barley simple sequence repeats (SSRs) and to study the evolutionary dynamics of SSR alleles. SSR polymorphisms were resolved by the polymerase chain reaction with four pairs of primers. In total, 71 variants were observed in a sample of 207 accessions of wild and cultivated barley. Analyses of wheat-barley addition lines and barley doubled haploids identified these variants (alleles) with four loci, each located on a different chromosome. The numbers of alleles detected at a locus corresponded to the number of nucleotide repeats in the microsatellite sequences. The numbers of alleles at two loci were 28 and 37; to our knowledge these are the largest numbers of alleles for single Mendelian loci reported in plants. Three alleles were resolved by each of the other two loci. Allelic diversity was greater in wild than in cultivated barley and surveys of two generations (F8 and F53) of Composite Cross II, an experimental population of cultivated barley, showed that few of the alleles present in the 28 parents survived into generation F53, whereas some infrequent alleles reached high frequencies. Such changes in frequency indicate that the chromosomal segments marked by the SSR alleles are under the influence of natural selection. The SSR variants allow specific DNA sequences to be followed through generations. Thus, the great resolving power of SSR assays may provide clues regarding the precise targets of natural and man-directed selection.

Ecogeographical distribution and differential adaptedness of multilocus allelic associations in Spanish Avena sativa L

We determined the nine-locus isozyme genotype of 267 landrace accessions of Avena sativa from 31 provinces of Spain. Our results establish that level of genetic variability is usually high both within and among accessions of this heavily self-fertilizing hexaploid grass and that multilocus genetic structure differs in various ecogeographical regions of Spain. We concluded that selection favoring different multilocus genotypes in different environments was the main integrating force that shaped the internal genetic structure of local populations as well as the overall adaptive landscape of A. sativa in Spain. Implications in genetic resource conservation and utilization are discussed.

Evolution of multilocus genetic structure in Avena hirtula and Avena barbata

Avena barbata, an autotetraploid grass, is much more widely adapted than Avena hirtula, its diploid ancestor. We have determined the 14-locus genotype of 754 diploid and 4751 tetraploid plants from 10 and 50 Spanish sites, respectively. Allelic diversity is much greater in the tetraploid (52 alleles) than in the diploid (38 alleles): the extra alleles of the tetraploid were present in nonsegregating heteroallelic quadriplexes. Seven loci were monomorphic for the same allele (genotypically 11) in all populations of the diploid: five of these loci were also monomorphic for the same allele (genotypically 1111) in all populations of the tetraploid whereas two loci each formed a heteroallelic quadriplex (1122) that was monomorphic or predominant in the tetraploid. Seven of the 14 loci formed one or more highly successful homoallelic and/or heteroallelic quadriplexes in the tetraploid. We attribute much of the greater heterosis and wider adaptedness of the tetraploid to favorable within-locus interactions and interlocus (epistatic) interactions among alleles of the loci that form heteroallelic quadriplexes. It is difficult to account for the observed patterns in which genotypes are distributed ecogeographically except in terms of natural selection favoring particular alleles and genotypes in specific habitats. We conclude that natural selection was the predominant integrating force in shaping the specific genetic structure of different local populations as well as the adaptive landscape of both the diploid and tetraploid.

Evolution of multilocus genetic structure in an experimental barley population

Data from 311 selfed families isolated from four generations (F8, F13, F23, F45) of an experimental barley population were analyzed to determine patterns of change in character expression for seven quantitative traits, and in single-locus allelic frequencies, and multilocus genetic structure, for 16 Mendelian loci that code for discretely recognizable variants. The analyses showed that large changes in single-locus allelic frequencies and major reorganizations in multilocus genetic structure occurred in each of the generation-to-generation transitions examined. Although associations among a few traits persisted over generations, dynamic dissociations and reassociations occurred among several traits in each generation-transition period. Overall, the restructuring that occurred was characterized by gradual decreases in the number of clusters of associated traits and increases in the number of traits within each cluster. The observed changes in single-locus frequencies and in multilocus genetic structure were attributed to interplay among various evolutionary factors among which natural selection acting in a temporally heterogeneous environment was the guiding force.

Associations between nuclear loci and chloroplast DNA genotypes in wild barley

Associations among alleles at nine nuclear loci and three chloroplast DNA (cpDNA) genotypes were assessed in a sample of 247 accessions of the wild barley, Hordeum vulgare ssp. spontaneum. Alleles at two of the nine nuclear loci are marked by length variations in the intergenic spacer region of ribosomal DNA (rDNA), and those of the other seven loci are well characterized allozymes. The three chloroplast DNA (cpDNA) genotypes are marked by restriction fragment length polymorphisms resulting from three polymorphic restriction sites detected by Southern blot hybridization. The analyses were performed by dividing the nine nuclear loci into a series of two-locus subsets and constructing log-linear models to characterize associations between the subsets of two nuclear loci and the cpDNA genotypes. Statistically significant associations were detected between six of the nine nuclear loci and the cpDNA genotypes, either individually as pairwise correlations, or through interaction with another nuclear locus to form three-variate complexes. Although the sample size of the present study was inadequate for statistical evaluation of higher order interactions, the results suggest the existence of interactions in which more than two nuclear loci are involved in associations with cpDNA genotypes. The observed cytonuclear associations appear to result from interplay among a number of evolutionary forces including a mating system of predominant selfing, differentiation among gene pools of local populations, and adaptation of barley genotypes to specific environmental conditions.

Pollen migration in predominantly self-fertilizing plants: barley

In barley, a heavily self-fertilizing species (approximately 99%), most outcrosses occur between plants that grow closely adjacent to each other. Outcrosses have been detected only rarely between plants that are separated by a meter or more. In this article we present evidence that outcrosses can occur at distances up to 60 m and we discuss the implications of this longer-distance pollen migration on the maintenance of the genetic integrity of pedigreed stocks and experimental populations.

Genetic and molecular organization of ribosomal DNA (rDNA) variants in wild and cultivated barley

Twenty rDNA spacer-length variants (slvs) have been identified in barley. These slvs form a ladder in which each variant (with one exception) differs from its immediate neighbors by a 115-bp subrepeat. The 20 slvs are organized in two families, one forming an eight-step ladder (slvs 100-107) in the nucleolus organizer region (NOR) of chromosome 7 and the other a 12-step ladder (slvs 108a-118) in the NOR of chromosome 6. The eight shorter slvs (100-107) segregate and serve as markers of eight alleles of Mendelian locus Rrn2 and the 12 longer slvs (108a-118) segregate and serve as markers of 12 alleles of Mendelian locus Rrn1. Most barley plants (90%) are homozygous for two alleles, including one from each the 100-107 and the 108a-118 series. Two types of departures from this typical pattern of molecular and genetic organization were identified, one featuring compound alleles marked by two slvs of Rrn1 or of Rrn2, and the other featuring presence in Rrn1 of alleles normally found in Rrn2, and vice versa. The individual and joint effects on adaptedness of the rDNA alleles are discussed. It was concluded that selection acting on specific genotypes plays a major role in molding the strikingly different allelic and genotypic frequency distributions seen in populations of wild and cultivated barley from different ecogeographical regions.

Genetic diversity and ecogeographical differentiation among ribosomal DNA alleles in wild and cultivated barley

DNA from 267 accessions of wild barley from ecologically diverse habitats in Israel and Iran and from 92 accessions of cultivated barley from throughout the world were assayed for the 20 ribosomal DNA (rDNA) spacer-length variants that have been identified in the barley species. These 20 spacer-length variants, which are detectable by Southern blot hybridization, serve as markers of rDNA alleles of two Mendelian loci, Rrn1 and Rrn2. All of the populations of wild barley studied were polymorphic for both loci. In wild barley allele 112 (Rrn1) and allele 107 (Rrn2) behaved as widely adapted wild-type alleles; in our sample of cultivated barley allele 112 also behaved as a wild-type allele but allele 104 was somewhat more frequent than allele 107 in Rrn2. A few other alleles were locally frequent in wild barley. However, most of the 20 alleles were infrequent or rare and such alleles were often associated as "hitchhikers" with one of the wild-type alleles in compound two-component alleles. Allelic and genotypic frequencies differed widely in different habitats in correlation with eight of nine factors of the physical environment. Discrete log-linear multivariate analyses revealed statistically significant associations among alleles of Rrn1 and Rrn2. It was concluded that natural selection acting differentially on various rDNA alleles plays a major role in the development and maintenance of observed patterns of molecular and genetic organization of rDNA variability.

Effects on adaptedness of variations in ribosomal DNA copy number in populations of wild barley (Hordeum vulgare ssp. spontaneum)

Twenty alleles, 12 at Mendelian locus Rrn1 and 8 at locus Rrn2, control rRNA genes [ribosomal DNA (rDNA)] variability in barley. These alleles differ strikingly in their effects on adaptedness. In the present study, we determined variation in the copy number of 101 accessions of wild barley plants from 10 ecologically diverse sites in Israel and examined relationships between rDNA copy number and adaptedness. The average multiplicity of rDNA per haploid genome was 1881 copies and the average numbers of copies for Rrn1 and Rrn2 were 962 and 917, respectively. The total number of copies as well as the number of copies for Rrn1 and Rrn2 varied widely from plant to plant within sites and also from site to site. The predominant allele of Rrn2 had somewhat more copies on the average than the other alleles of this locus but differences between the predominant allele and other alleles of Rrn1 were not statistically significant. Overall, the results indicated that differing amounts of rDNA resulting from variations in copy number and/or number of subrepeats in the intergenic spacer region were not closely associated with adaptedness. This suggests that the high adaptedness of a few specific alleles results in large part from adaptatively favorable nucleotide sequences in the transcription units and/or the intergenic spacer regions of the favored alleles--i.e., that adaptedness in barley depends on the quality more than on the quantity of rDNA present.

Worldwide pattern of multilocus structure in barley determined by discrete log-linear multivariate analyses

Data from the electrophoretic assay for seven enzyme loci of 1,032 accessions of cultivated barley, Hordeum vulgare L., from the USDA world barley collection were analyzed for multilocus structure using discrete log-linear multivariate techniques. Three major steps were involved in the analysis: (i) identification and elimination of terms that have inconsequential effects in multilocus association; (ii) construction of a log-linear model that best describes the complete multilocus structure of the genetic system; and (iii) evaluation of each of the association terms included in the model. The results of analyses of two subsets of loci show that the multilocus genetic system of cultivated barley, including loci located on different chromosomes, is organized into hierarchically structured complexes of loci. Multilocus structure differs in various geographical regions of the world. The structure of barleys from Southwest Asia, the putative center of origin for cultivated barley, is intermediate for both subsets of loci. Differences increased progressively across the Eurasian-African landmasses in each direction with increasing distance from Southwest Asia, with the consequence that the barleys from West Europe, East Asia, and Ethiopia are maximally different from those of Southwest Asia and Middle South Asia.

Superstructure of the Drosophila ribosomal gene family

Determining the spatial organization of middle repetitive DNA has proven difficult for several reasons. Repeated arrays are often so large that molecular methods alone cannot resolve their organization, and the lack of phenotypic markers within arrays limits the value of classical genetic analysis. We have characterized the superstructure of one repeated gene family, the ribosomal gene family of Drosophila melanogaster, by a combination of recombinational and molecular analyses of spacer-length variants. The resulting genetic maps demonstrate that some spacer variants are widely dispersed, while others are limited in their distribution. Moreover, exchange among ribosomal DNA (DNA encoding rRNA) arrays was often unequal, leading to a prediction of little or no relationship between physical location in an array and relatedness of gene family members. Extensions of our procedure may be generally useful for mapping the superstructure of repetitive DNA.

The genetics of host-pathogen coevolution: implications for genetic resource conservation

The results of long-term studies of coevolution in the Hordeum vulgare-Rhynchosporium secalis pathosystem are summarized. The genetic systems of barley (host) and R. secalis (pathogen) are complementary: Gene-for-gene interactions among loci affect many traits, leading to self-regulating adjustments over generations between host and pathogen populations. Different pathotypes differ widely in their ability to damage the host, and different host-resistance alleles differ widely in their ability to protect the host from the pathogen. Among 29 resistance loci in the specific host population studied, several played major roles in providing stable resistance, but many had net detrimental effects on the yield and reproductive ability of the host. Resistance alleles that protected against the most damaging pathotypes increased sharply in frequency in the host populations. It is concluded that the evolutionary processes that take place in genetically variable populations propagated under conditions of cultivation can be highly effective in increasing the frequency of desirable alleles and useful multilocus genotypes. This enhances the value of the evolving populations as sources of genetic variability in breeding for disease resistance and other characters that affect adaptedness.

Genetic variability for pathogenicity, isozyme, ribosomal DNA and colony color variants in populations of Rhynchosporium secalis

Samples of Rhynchosporium secalis were collected from two experimental barley populations known to carry a diverse array of alleles for resistance to this fungal pathogen. Classification of 163 isolates for four putative isozyme systems, a colony color dimorphism and 20 ribosomal DNA restriction fragment length variants revealed 49 different multilocus phenotypes (haplotypes). The six most common haplotypes differed significantly in pathogenicity. Genetic analyses of the data indicated that effective population sizes of the fungus were very large, that the effects of genetic drift were small, and that negligible recombination occurred in the populations studied. Frequency dependent selection was suggested as an explanation for the maintenance of variation in pathogenicity in the fungus.

Allelic and genotypic composition of ancestral Spanish and colonial Californian gene pools of Avena barbata: evolutionary implications

Spanish explorers and colonists inadvertently started a massive experiment in evolutionary genetics when they accidentally introduced Avena barbata to California from Spain during the seventeenth and eighteenth centuries. Assays of the Spanish and Californian gene pools of this species for 15 loci show that the present day Spanish gene pool, particularly that of Southwestern Spain, is identical or virtually identical to that of California for five loci and closely similar for nine loci. Despite their similar allelic and single-locus genotypic compositions, the present-day Spanish and Californian gene pools are differently structured on a multilocus genetic basis. Evolutionary implications of these results are discussed.

Coevolution of host and pathogen populations in the Hordeum vulgare-Rhynchosporium secalis pathosystem

Isolates of Rhynchosporium secalis collected from two experimental barley populations were scored for putative isozyme, colony color, and virulence loci. Allelic frequencies, multilocus haplotype frequencies, and multilocus genetic structure differed in the two populations of R. secalis; haplotypes also differed widely from each other in virulence. The average virulence of isolates collected from the more resistant host population was greater than the average virulence of the isolates collected from the less resistant host population; also the least virulent haplotype, which made up 19% of the pathogen population collected from the less resistant host population, accounted for only 0.3% of the isolates collected from the more resistant host population. It was concluded that the genetic systems of the barley host and fungal pathogen interacted in a complementary fashion and that the genetic structures of both the host and pathogen populations were shaped by coevolutionary processes featuring interactions among loci affecting many different traits, including interactions among host resistance genes and pathogen virulence genes.

Spatial autocorrelation analysis of the distribution of genotypes within populations of lodgepole pine

Spatial autocorrelation analyses of point samples within two populations of lodgepole pine (Pinus contorta ssp. latifolia) indicate that single-locus mature tree and pollen genotypes are distributed in a nearly random fashion for most of the allozyme loci assayed. This lack of structure in the distributions of most genotypes is consistent with outcrossing rates that are very nearly 1.0 and with estimates indicating that both pollen and seed are dispersed over long distances in lodgepole pine. However, spatial autocorrelation of genotypes for a few loci suggests that genotypes at these loci may be under natural selection.

Chloroplast DNA diversity in populations of wild and cultivated barley

Chloroplast DNA (cpDNA) diversity was found within and among populations (245 accessions total) of wild barley, Hordeum vulgare L. ssp. spontaneum Koch from Israel and Iran. Three polymorphic restriction sites (HindIII, EcoRI, BclI) which define three distinct cpDNA lineages were detected. One lineage is common to populations in the Hule Valley and Kinneret of northern Israel, and in Iran. The second lineage is found predominantly in the Lower Jordan Valley and Negev. The distribution of the third lineage is scattered but widespread throughout Israel. Sixty two accessions of cultivated barleys, H. vulgare L., were found, with two exceptions, to belong to just one cpDNA lineage of wild barley, indicating that the cpDNA of cultivated barley is less variable than its wild ancestor. These results demonstrate the need for assessing intraspecific cpDNA variability prior to choosing single accessions for phylogenetic constructions at the species level and higher.

The Wilhelmine E. Key 1987 invitational lecture. Genetic changes associated with the evolution of adaptedness in cultivated plants and their wild progenitors

The results of long-term studies of changes in adaptedness in a number of experimental populations of annual plants are summarized. Measurements made of quantitative traits showed that cumulative increases in reproductive capacity continued in these experimental populations for more than 50 generations. Highly significant allelic frequency changes also occurred for marker loci governing morphological variants, disease resistance, allozymes, and rDNA restriction fragments. Individual effects of the marker loci on quantitative traits were determined by extensive progeny testing of selfed families descended from single plants isolated from various generations of the experimental populations. Comparisons between homozygotes and heterozygotes of marker loci for quantitative trait expression revealed that all the marker loci studied had statistically significant additive effects on several to many quantitative traits; thus, each Mendelian locus, in addition to being a locus for its discrete descriptive effect, was also a locus for several quantitative traits. Consistent associations were found between superior reproductive capacity (e.g., larger numbers of kernels per plant) and the alleles of marker loci that increased in frequency over generations; no other quantitative traits measured were clearly and consistently associated with alleles that increased in frequency. Multilocus analyses based on canonical correlation, log linear, and cluster analysis procedures showed that highly significant associations developed in early generations among alleles of different loci in all the predominantly selfing populations studied. Dynamic changes featuring amalgamations of alleles into fewer clusters involving larger numbers of loci continued into the late generations. Patterns of ecogenetic differentiation that developed under predominant selfing were found to be fine-scaled overlays of environmental heterogeneity. The picture of evolutionary change that emerges is one in which the incorporation of increasing numbers of favorably interacting alleles into large synergistic complexes was accompanied in inbreeding populations by increases in adaptedness to the local environment and also by striking ecogenetic differentiation among local populations that occupy unlike habitats, including differentiation between cultivated plants and their wild progenitors. Selfing appears to promote the development and maintenance of adaptedness within populations and at the same time to facilitate the development of spatial differentiation by retarding gene flow between populations.(ABSTRACT TRUNCATED AT 400 WORDS)

Inheritance of organelle DNA in barley and Hordeum × Secale intergeneric hybrids

Four reciprocal hybrids of Hordeum vulgare (deficiens type) and seven nonreciprocal hybrids resulting from controlled pollination between two Hordeum and three Secale species were assayed for variation in their organelle DNA. The chloroplast and mitochondrial DNAs of these hybrids were distinct. Genetic variability exists within species of the genus Hordeum with organelle transmission being strictly maternal. On the other hand, biparental inheritance was obtained in the intergeneric hybrids involving Hordeum and Secale. There was selective transmission of Secale bands and the number of gene copies transmitted through the pollen was under genetic control. No recombination occurred between the two parental genomes.

Key words: DNA, organelle, intergeneric hybrids, Hordeum Secale.

Chloroplast DNA polymorphisms in lodgepole and jack pines and their hybrids

Samples taken from throughout the ranges of distribution of lodgepole pine (Pinus contorta Dougl. ex. Loud.) and jack pine (Pinus banksiana Lamb.) were assayed for Sal I and Sst I chloroplast DNA restriction fragment variation. Although the chloroplast genome is often regarded as highly conserved, at least 2 distinct Sal I and 13 distinct Sst I restriction fragment banding patterns occur in these closely related species. None of the chloroplast DNA restriction fragment banding patterns observed in allopatric lodgepole pine was observed in allopatric populations of jack pine, and vice versa, even though the two species share an extensive zone of sympatry, and gene flow between the species has been reported for nuclear genes. However, several atypical Sst I restriction fragment banding patterns occur only in or near the zone of sympatry. Chloroplasts have been reported to be inherited maternally in the great majority of species studied; however, restriction fragment analyses indicated that chloroplasts are inherited paternally in controlled matings between lodgepole pine (female) and jack pine (male).

Linkage Disequilibrium Between Allozymes in Natural Populations of Lodgepole Pine

Pairwise linkage disequilibrium values (D) were estimated for 14 allozyme loci in two natural populations of lodgepole pine (Pinus contorta ssp. latifolia). Maternal multilocus genotypes were inferred from samples of (haploid) megagametophytic seed-endosperms. Coupling/repulsion double heterozygotes were distinguished for closely linked pairs of loci. Assays of seven of the loci in seed embryos allowed estimates of D for these loci in the outcross pollen pool (estimates of outcrossing rates indicate no significant departures from random mating in either population). No disequilibrium was observed between unlinked loci in either maternal genotypes or outcross pollen. However, significant disequilibrium was observed within and between gametes for some allelic combinations of four tightly linked loci; the assumption of random association of gamete types within individuals is thus invalid for some loci in lodgepole pine. Possible causes of the observed D were examined using the noncentrality parameter of the general noncentral chi square distribution. We concluded, from estimates of population size, linkage and measurements of population substructure, that neither drift nor population subdivision was responsible for the significant values of D which were observed and that epistatic selection was the most likely cause of the disequilibrium observed.

Correlations between development rates, enzyme activities, ribosomal DNA spacer-length phenotypes, and adaptation in Drosophila melanogaster

Selection for "fast" preadult development rate among the progeny of flies collected in a natural population of Drosophila melanogaster produced a line that developed more rapidly than a line selected for "slow" preadult development rate. Assays for enzyme activity levels showed that the activities of alpha-glycerophosphate dehydrogenase, alcohol dehydrogenase, and malic enzyme were higher in the fast than in the slow line, but that the activity of superoxide dismutase was lower in the fast line. Differences in the frequencies of spacer-length phenotypes of X chromosome-linked rRNA genes (rDNA), which developed between the lines during the selection process, are larger than can be explained on the basis of genetic drift alone. Long rDNA spacers had high frequency in the fast line; short spacers, in the slow line. We conclude that enzyme levels affected adaptation under the selective regimes imposed and that the different X-linked rDNA spacer-length phenotypes are either adaptive in themselves or that they mark chromosomal segments carrying genes relevant to adaptation.

NONRANDOM MATING IN AN OPEN-POLLINATED MAIZE POPULATION

Randomness of fertilization was studied in an open-pollinated population of maize (Zea mays L.) through allozyme assays of seedlings from open-pollinated seeds produced on both tasseled and detasseled plants. Mixed-mating-model estimates of the amount of outcrossing (t) were not significantly different from t = 1.00 for four enzyme loci (Adh1, Idh2, Got1 and Acp1), indicating that fertilizations were at random in the population. However, for loci Prx1 and Est4, estimates of t were significantly smaller than unity-0.80 and 0.70 for tasseled plants and 0.81 and 0.80 for detasseled plants. The excesses of homogametic fertilizations detected on the detasseled plants could not have been due to selffertilization, s = 1 - t, because the detasseled plants shed no pollen. Analyses of allelic frequencies in the pollen that produced seed on the detasseled plants established that different maternal plants sampled genetically different populations of pollen from the outcross pollen pool. It was suggested that the causes of the differential sampling were temporal variation in the pollen pool, and/or gametophytic selection, correlated with marker-locus genotype. Two-, three- and four-locus interactions among the marker loci were often statistically significant, indicating that the factors responsible for the nonrandom gametic unions observed in the maize population studied were complexly interactive.

Ribosomal DNA spacer-length polymorphisms in barley: mendelian inheritance, chromosomal location, and population dynamics

Spacer-length (sl) variation in ribosomal RNA gene clusters (rDNA) was surveyed in 502 individual barley plants, including samples from 50 accessions of cultivated barley, 25 accessions of its wild ancestor, and five generations of composite cross II (CCII), an experimental population of barley. In total, 17 rDNA sl phenotypes, made up of 15 different rDNA sl variants, were observed. The 15 rDNA sl variants comprise a complete ladder in which each variant differs in length from adjacent variants by approximately equal to 115 nucleotide pairs. Studies of four rDNA sl variants in an F2 population showed that these variants are located at two unlinked loci, Rrn1 and Rrn2, each with two codominant alleles. Using wheat-barley addition lines, we determined that Rrn1 and Rrn2 are located on chromosomes 6 and 7, respectively. The nonrandom distribution of sl variants between loci suggests that genetic exchange occurs much less frequently between than within the two loci, which demonstrates that Rrn1 and Rrn2 are useful as new genetic markers. Frequencies of rDNA sl phenotypes and variants were monitored over 54 generations in CCII. A phenotype that was originally infrequent in CCII ultimately became predominant, whereas the originally most frequent phenotype decreased drastically in frequency, and all other phenotypes originally present disappeared from the population. We conclude that the sl variants and/or associated loci are under selection in CCII.

Inheritance of nitrite reductase and regulation of nitrate reductase, nitrite reductase, and glutamine synthetase isozymes

Banding patterns of nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS) from leaves of diploid barley (Hordeum vulgare), tetraploid wheat (Triticum durum), hexaploid wheat (Triticum aestivum), and tetraploid wild oats (Avena barbata) were compared following starch gel electrophoresis. Two NR isozymes, which appeared to be under different regulatory control, were observed in each of the three species. The activity of the more slowly migrating nitrate reductase isozyme (NR1) was induced by NO3- in green seedlings and cycloheximide inhibited induction. However, the activity of the faster NR isozyme (NR2) was unaffected by addition of KNO3, and it was not affected by treatments of cycloheximide or chloramphenicol. Only a single isozyme of nitrite reductase was detected in surveys of three tetraploid and 18 hexaploid wheat, and 48 barley accessions; however, three isozymes associated with different ecotypes were detected in the wild oats. Inheritance patterns showed that two of the wild oat isozymes were governed by a single Mendelian locus with two codominant alleles; however, no variation was detected for the third isozyme. Treatment of excised barely and wild oat seedlings with cycloheximide and chloramphenicol showed that induction of NiR activity was greatly inhibited by cycloheximide, but only slightly by chloramphenicol. Only a single GS isozyme was detected in extracts of green leaves of wheat, barley, and wild oat seedlings. No electrophoretic variation was observed within or among any of these three species. Thus, this enzyme appears to be the most structurally conserved of the three enzymes.

Mating system and genetic polymorphism in populations of Secale cereale and S. vavilovii

Electrophoretic banding patterns were determined for nine enzyme systems (IPO, PGM, PGI, LAP, GOT, EST, PHOS, MDH, CPX) in four populations of Secale cereale L. from widely different geographical areas, and in one population of S. vavilovii Grossh. Secale cereale was found to be extensively variable and S. vavilovii invariant for these enzyme systems. Formal genetic studies of nine polymorphic banding zones in S. cereale revealed that each zone was under single locus control. Mating system studies based on these loci indicated that 8% of self-fertilization occurred under field conditions in a population of S. cereale, a species with a highly developed self-incompatibility system. Each population was characterized by fewer heterozygotes than expected in random mating populations. Genotypic and allelic frequencies were nearly identical in four populations of S. cereale, despite their diverse origins and different cytological characteristics.Key words: mating system, Secale, rye, isozyme polymorphism.