Genetic divergence in the snorkel snail,Rhiostoma housei,a species complex in Thailand (Caenogastropoda: Cyclophoridae)

Allozyme variation was examined in 292 samples of the snorkel snail, Rhiostoma housei, collected from 12 localities within four regions of Thailand. Ten allozyme loci were screened across all the samples using horizontal starch gel electrophoresis, and eight of these loci, from eight enzyme systems, were found to be polymorphic. Within these eight loci the expected heterozygosity (H_exp) was moderate to high ranging from 0.015 to 0.148 with an overall mean of 0.081 ± 0.041, whilst a high genetic heterogeneity among samples was found (F_st = 0.798). R. housei exhibited a weak pattern of isolation by distance over the entire tested species range in Thailand. The high F_st and moderate to high H_exp suggest gene flow amongst populations is partially restricted resulting in fragmentation into local gene pools. Based on allele frequencies and an allozyme dendrogram, six groups were identified, and these entities may represent separate biological species. The large values of genetic distances and fixed allelic differences obtained for each combination of allopatric samples imply the presence of cryptic species under the name R. housei.     

MATING SYSTEM AND GENETIC STRUCTURE OF THE DISTYLOUS TROPICAL TREE PSYCHOTRIA FAXLUCENS (RUBIACEAE)

The population genetics and mating system of the understory tropical rain forest tree Psychotria faxlucens were studied at two plots in Los Tuxtlas, Veracruz, México, on the Gulf of México coast. This species is distylous and is pollinated by moths, mainly small hawkmoths. The seeds are dispersed by gravity and by frugivorous birds. Controlled pollinations indicate that the trees are self-incompatible and that intramorph pollinations have lower compatibilities than intermorph pollinations. The pollen: ovule ratio is high, suggesting obligate xenogamy. Using electrophoretic allozyme methods we found that eight out of 20 loci were polymorphic (P = 0.400), the observed heterozygosity (H) was 0.198, and the mean expected heterozygosity (H) was 0.495, both relatively high values compared with that reported for tropical trees. The genetic differentiation between the two plots is low, as shown by the heterogeneity in allele frequencies and the F_st (mean F_st seedlings = 0.031, mean F_st adults = 0.026), although for some loci the plot differentiation is statistically significant. The studied populations are near Hardy-Weinberg proportions, both for seedlings (mean F = 0.128) and adults (mean F = 0.075). From the fixation rate, an indirect estimate of the outcrossing rate at equilibrium gave a mean of t = 0.898 for plot 1 and 0.685 for plot 2. Direct single loci and multiloci outcrossing rate estimates were generally not statistically different from 1.0.     

Seasonal and interannual variations of sockeye salmon (<Emphasis Type="Italic">Oncorhynchus nerka</Emphasis>) microsatellite DNA in two Kamchatka lake-river systems

Seasonal and interannual variations in the sockeye salmon populations from two lake-river systems of the East and West Kamchatka were studied. Stability of allele and genotypic frequencies of six microsatellite DNA loci in the adjacent generations and spawning populations of the sockeye salmon of the Bol’shaya River was confirmed experimentally. The pairwise intersample differentiation (F _st) of the local sockeye salmon populations from the southwestern Kamchatka coast (Ozernaya and Bol’shaya Rivers) was almost 7 times higher than the corresponding values for the spawning populations of the Bol’shaya River sockeye salmon of the adjacent years; 15 times, for the adjacent Bol’shaya River sockeye salmon generations; and four times, for the seasonal races within the Kamchatka River.     

Behavioral responses of Ceratitis capitata flies to bait spray droplets and natural food

We usedF-statistics to quantify the population structure of two sympatric species of leaf beetles,Oreina cacaliae andO. speciosissima (Chrysomelinae, Coleoptera), which share the same microhabitat since they feed on the same herbaceous host plants. We measured genetic differentiation at six allozyme loci 1) among populations separated by relatively small distances (40 to 250 kilometers), 2) within each population, and 3) between sexes within populations. For both species, the populations studied are not panmictic. For each population, heterozygosities are relatively high, but the observed heterozygosities are generally lower than the expected values. Overall, within-population differentiation is high and similar for both species (F _is=0.326 forO. cacaliae and 0.332 forO. speciosissima). Additionally, populations of both species are highly differentiated (F _st=0.234 versus 0.051 forO. speciosissima). ForO. cacaliae,F _is andF _st are greater among females than among males, while forO. speciosissima,F _st is sustantially greater among the males whileF _is is slightly greater among males. Differences in gene frequency among the sexes were statistically tested using a modifiedF _st with sex as the defining category, and the sexes differed significantly with the exception of one population inO. cacaliae. Possible explanations for this difference are discussed.     

Population differentiation in randomly amplified polymorphic DNA of red-cockaded woodpeckers Picoides borealis

Random amplified polymorphic DNA (RAPD) phenotypes generated by 13 primers were scored for 101 individuals in 14 populations of the endangered red-cockaded woodpecker Picoides borealis. Although no population-specific markers were found, the frequencies of several markers differed significantly among populations. Application of the recently developedamova method (analysis of molecular variance; Excoffier, Smouse & Quattro 1992) showed that more than 90% of phenotypic variance occurred among individuals within populations; of the remaining variance, half was attributed among groups of geographically adjacent populations and half among populations within those groups. The statistical significance of these patterns was supported by Monte Carlo sampling simulations and permutation tests. Estimation of allele frequencies from phenotypes provided somewhat weaker evidence for population structure, although among-population variance in allele frequencies was detectable (F_st= 0.19; x²₁₆₉= 509.3, P < 0.0001). Upgma cluster analyses based on Rogers' (1972) genetic distance revealed grouping of some geographically proximate populations. A Mantel test indicated a positive (r = 0.16), although not significant, correlation between geographic and genetic distances. We compared a subset of our RAPD data with data from a previous study that used allozymes (Stangel, Lennartz & Smith 1992). RAPD (n= 75) and allozyme (n= 245) results based on samples from the same ten populations showed similar patterns. Our study indicates that RAPDs can be helpful in differentiating populations at the phenotypic level even when small sample sizes, estimation bias, and inability to test for Hardy-Weinberg equilibrium complicate the genotypic interpretation. Lack of large differences among populations of red-cockaded woodpeckers may allow flexibility in overpopulation translocations, provided factors such as habitat preference, latitudinal direction of translocation, and status of donor populations are considered.     

Measures of gene flow in the Columbian ground squirrel

From analyses of published data and a review of the literature, I studied indirect and direct measures of gene flow among populations of Columbian ground squirrels, Spermophilus columbianus. New analyses were used to examine an allozyme data set (seven polymorphic loci) that had been collected by Zammuto and Millar (1985a) from six populations of ground squirrels that were spread over 183 km. G-tests indicated significant variation in allele frequencies among populations, but F-statistics revealed relatively little population differentiation (average F _ST=0.026). F _ST values were used to estimate rates of gene flow indirectly and indicated fairly high rates of gene flow (average N _e m=13.5). Recorded dispersal distances of individual ground squirrels were fairly short (most<4 km, maximum recorded distance was 8.5 km), and the minimum distance between populations used to create the allozyme data set was about 25 km. Thus, direct dispersal among the populations in the allozyme data set was highly unlikely. Small genetically effective populations may have experienced high rates of migration over short distances (about 43% of adults in local populations were immigrants), however, resulting in homogeneous allele frequencies over the geographic range. This explanation provides an alternative to invoking gene flow in the recent past to explain discrepancies between dispersal distances in the field and homogenization of allele frequencies over large ranges, Mammalian species that have virtually complete dispersal of subadult males from the natal area might be expected to exhibit relatively high rates of gene flow, regardless of actual dispersal distances. Genetically effective populations may be much smaller than more extensive ecological populations and experience higher rates of gene flow.     

Moderately and Highly Polymorphic Microsatellites Provide Discordant Estimates of Population Divergence in Sockeye Salmon, Oncorhynchus nerka

Mutation rate can vary widely among microsatellite loci. This variation may cause discordant single-locus and multi-locus estimates of F_ST, the commonly used measure of population divergence. We use 16 microsatellite and five allozyme loci from 14 sockeye salmon populations to address two questions about the affect of mutation rate on estimates of F_ST: (1) does mutation rate influence F_ST estimates from all microsatellites to a similar degree relative to allozymes?; (2) does the influence of mutation rate on F_ST estimates from microsatellites vary with geographic scale in spatially structured populations? For question one we find that discordant estimates of F_ST among microsatellites as well as between the two marker classes are correlated with mean within-population heterozygosity (H_S) and thus are likely due to differences in mutation rate. Highly polymorphic microsatellites (H_S > 0.84) provide significantly lower estimates of F_ST than moderately polymorphic microsatellites and allozymes (H_S < 0.60). Estimates of F_ST from binned allele frequency data and R_ST provide more accurate measures of population divergence for highly polymorphic but not for moderately polymorphic microsatellites. We conclude it is more important to pool loci of like H_S rather than marker class when estimating F_ST. For question two we find the F_ST values for moderately and highly polymorphic loci, while significantly different, are positively correlated for geographically proximate but not geographically distant population pairs. These results are consistent with expectations from the equilibrium approximation of Wright's infinite island model and confirm that the influence of mutation on estimates of F_ST can vary in spatially structured populations presumably because the rate of migration varies inversely with geographic scale.     

Analysis of F ST outliers at allozyme loci in Pacific salmon: implications for natural selection

Natural selection has been invoked to explain the observed geographic distribution of allozyme allele frequencies for a number of teleost species. The effects of selection on allozyme loci in three species of Pacific salmon were tested. A simulation-based approach to estimate the null distribution of population differentiation (F _ST) and test for F _ST outliers was used. This approach showed that a majority of allozyme loci conform to neutral expectations predicted by the simulation model, with relatively few F _ST outliers found. No consistent F _ST outlier loci were found across species. Analysis of population sub-groups based on geography and genetic identity reduced the number of outlier loci for some species, indicating that large geographic groups may include genetically divergent populations and/or that there is geographic heterogeneity in selection pressure upon allozyme loci. Two outlier allozyme loci found in this analysis, lactate dehydrogenase-B and malic enzyme, have been shown to be influenced by selection in other teleost species. This approach is also useful in identifying allozyme loci (or other genetic markers) that meet assumptions for population genetic study.     

Genetic Coadaptation in the Chromosomal Polymorphism of DROSOPHILA SUBOBSCURA. I. Seasonal Changes of Gametic Disequilibrium in a Natural Population

Seasonal changes in gene arrangement and allozyme frequencies have been investigated in Drosophila subobscura for several years. Some arrangements (O_st and O₃₊₄₊₇) show seasonal variation, which suggests that chromosomal polymorphism is flexible in this species. Seasonal changes in allozyme frequencies for Lap and Pept-1 loci, both located within the same inversions of chromosome O, are significant only inside the O_st arrangement, but not inside O₃₊₄ arrangement. This arrangement-dependent response of allozyme generates variation in arrangement-allozyme disequilibrium. The historical hypothesis on the maintenance of disequilibria cannot explain these seasonal changes, and some kind of natural selection must be invoked. Association between Lap and Pept-1 is also seasonal inside O_st but not inside O₃₊₄. We propose that O_st probably consists of a finite array of supergenes that are differentially favored in each season by natural selection. The present evidence on this supergene selection and other genetic, biogeographic and phylogenetic data points to O₃₊₄ as the most primitive gene order among the present arrangements.     

The spatial distribution of chloroplast DNA and allozyme polymorphisms within A population of Silene alba (Caryophyllaceae)

Genetic structure within a population of Silene alba was studied using a chloroplast DNA (cpDNA) and six allozyme polymorphisms. A 20 × 65 m area was sampled by constructing a 5 × 5 m grid and determining the genotype of the plant nearest to each grid node. Analysis of the spatial distribution of genotypes by Moran's I and join-counts showed a significant degree of association of like cpDNA haplotypes (I = 0.52, S.N.D. = 2.55) but a random or slightly repulsed distribution of allozyme genotypes. A second sample was taken by collecting all individuals from within a 1-m wide transect established along the perimeter of the grid. Genotype and allele frequencies were calculated by grouping individuals from arbitrary 3-m intervals along the transect, and analyzed by Wright's F statistics. The F_st value calculated for cpDNA (0.875) differed markedly from that based on all allozymes (0.027). Taken together, the results suggest that in this population gene flow in the maternally inherited cpDNA is limited by restricted seed dispersal, whereas gene flow in the nuclear DNA based allozymes is more pervasive owing to the added effects of pollen dispersal. The utility of cpDNA polymorphism for the study of fine-scale gene flow is discussed.     

Biochemical assessment of the taxonomic diversity of the operculate land snail, Cyclophorus fulguratus (Gastropoda: Cyclophoridae), from Thailand

Allozyme variation was examined in 223 samples of the operculate land snail Cyclophorus fulguratus from 13 localities across three regions of Thailand. Using horizontal starch gel electrophoresis, 13 allozyme presumed loci (12 polymorphic) were screened. Heterozygosity was moderate in C. fulguratus (H_exp = 0.008–0.127) with a high genetic heterogeneity among samples (F_st = 0.734). Populations showed a greater genetic differentiation in central Thailand (F_st = 0.380) than in northeastern Thailand (F_st = 0.108), suggesting frequent gene flow among populations in northeastern Thailand. C. fulguratus exhibits a strong pattern of isolation by distance over the entire tested species range in Thailand and may potentially have been involved in an extensive local fragmentation. Results of the distance analysis revealed that large genetic divergence has occurred among the central, northeastern and eastern Thailand groups [D = 0.361–0.701], strongly suggesting populations from these three geographical regions may actually represent or else be evolving into separate species.     

Minimal gene flow from introduced brown trout (Salmo trutta L.) after 30 years of stocking

Lake Tinnsjø, Norway, has been heavily stocked over three decades with two different brown trout stocks. A population genetic survey based on 13 allozyme coding loci was conducted to investigate the genetic composition of the present trout population and the genetic impact of the two donors. Contrary to expectations, highly significant differences in allelic frequencies between Lake Tinnsjø trout and the two donor stocks were found at several loci, suggesting minimal gene flow from stocked trout to the wild populations. Pairwise genetic distance values were several times higher between donor stocks and Lake Tinnsjø samples than between the various spawning inlets to Lake Tinnsjø. F_st increased from 0.024 to 0.090, when samples from the donor stocks were included in the material. There were no deviations from expected Hardy–Weinberg distribution of genotypes in the spawning inlets. It was concluded that the donor stocks contributed little to the trout gene pool in the lake. A possible exception is the trout population below the lake outlet.     

ANNUAL DIFFERENCES IN FEMALE REPRODUCTIVE SUCCESS AFFECT SPATIAL AND COHORT-SPECIFIC GENOTYPIC HETEROGENEITY IN PAINTED TURTLES

Long-term ecological data were used to evaluate the relative importance of movements, breeding structure, and reproductive ecological factors to the degree of spatial and age-specific variation in genetic characteristics of painted turtles (Chrysemys picta) on the E. S. George Reserve in southeastern Michigan. Estimates of the degree of spatial genetic structuring were based on the proportion of total genotypic variance partitioned within and between subpopulations (inferred from hierarchical F-statistics based on variation at 18 protein loci), and in terms of gene correlations (co-ancestry among individuals derived from reproductive data on full-sib families of females nesting at specific nesting areas). Little variation in allele frequency was observed among turtles from different marshes (F_mt = 0.003), though significant variation was observed among turtles from different nesting areas associated with each marsh (F_nm = 0.046). Gene correlations among individuals within nesting areas varied greatly over years (0.032-0.171; mean = 0.069) and were negatively correlated to the proportion of females that successfully nested during each year. General concordance between independent estimates of genotypic correlations (i.e., F_nm derived from protein electrophoretic variation vs. mean co-ancestry) suggests that allozyme data, when collected over spatial scales consistent with species behavioral characteristics and reproductive ecology, may accurately reflect the apportionment of gene diversity within and among subpopulations. The magnitude and patterning of allelic variation among nesting areas and individuals appears to be primarily a function of gametic correlations among members of full-sib families, irrespective of the degree of gene flow or female nesting-site fidelity. Comparisons of genetic characteristics among 11 cohorts (1974-1984) revealed that heterozygosity (H) and inbreeding coefficients (F) varied greatly. Cohort estimates of H and F were correlated to female nesting success and to estimates of co-ancestry for the same years. Results clearly reflect the concomitant importance of ecological factors (principally the proportion of the female population that successfully produce offspring during each year) in determining the magnitude and patterning of gene correlations within and among groups, and to the genotypic composition of offspring born during each year.     

How does population genetic diversity change over time? An experimental seed bank study of Atriplex tatarica (Chenopodiaceae)

Atriplex tatarica is an annual, early successional, facultative halophilic species of frequently disturbed human-made habitats in Central and Eastern Europe. We investigated to what extent the plants grown from seeds extracted from soil seed bank differed genetically to mature aboveground plants in experimental populations of A. tatarica over two successive years. At each of five plots 50 aboveground plants and 50 plants extracted from seeds stored in soil were assayed for allozyme analysis in 2003 and 2004. At the start of experiment, we introduced 1000 seeds of the study species into each of five experimental plots. While the species dominated in all of the experimental plots in the first year, the second year A. tatarica coverage decreased dramatically. Overall allele frequencies of soil seeds and mature plants showed significant differences between life history stages in both years, but not within years in soil seeds as well as mature plants stages. While mature plants showed a significantly greater amount of single and multilocus heterozygosity in both consecutive years, comparison between years did not yield any significant differences. In the same way, despite a relatively large seed bank the species population genetic parameters, i.e. allelic richness (A), observed heterozygosity (H_o), gene diversity (H_s), inbreeding coefficient (F_IS) and fixation index (F_ST), did not change over years between as well as within life history stages. The soil seeds and mature plants significantly differed in H_o, H_s and F_IS, while the A and F_ST were not significantly different between life history stages.     

Estimation and adjustment of microsatellite null alleles in nonequilibrium populations

Nonamplified (null) alleles are a common feature of microsatellite genotyping and can bias estimates of allele and genotype frequencies, thereby hindering population genetic analyses. The frequency of microsatellite null alleles in diploid populations can be estimated for populations that are in Hardy–Weinberg equilibrium. However, many microsatellite data sets are from nonequilibrium populations, often with known inbreeding coefficients (F) or fixation indices (F_IS or F_ST). Here, we propose a novel null allele estimator that can be used to estimate the null allele frequency and adjust visible allele frequencies in populations for which independent estimates of F, F_IS or F_ST are available. The algorithm is currently available as an Excel macro that can be downloaded at no cost from http://www.microchecker.hull.ac.uk/ and will be incorporated into the software micro ‐checker .     

Genome-wide analysis of across herd <Emphasis Type="Italic">F</Emphasis> <Subscript>st</Subscript> Heterogeneity in holsteinized cattle

To form a reference population necessary for genomic selection of dairy cattle, it is important to acquire information on the genetic diversity of the base population. Our report is the first among the studies on breeding of farm animals to implement Wright’s F-statistics for this purpose. Genotyping of animals was performed using BovineSNP50 chip. In total, we genotyped 499 heifers from 13 breeding farms in the Leningrad region. We calculated Weir and Cockerham’s F_st estimate for all pairwise combinations of herds from breeding farms and the values obtained were in the range of 0.016–0.115 with the mean of 0.076 ± 0.002. Theoretical F_st values for the same pairwise combinations of herds were calculated using the ADMIXTURE program. These values were significantly (P < 0.05) higher than Weir and Cockerham’s F_st estimates and fell in the range of 0.063–0.136 with the mean of 0.100 ± 0.001. We discuss the reasons for this discrepancy between the two sets of F_st data. The obtained F_st values were used to identify reliable molecular-genetical differences between the herds. The ADMIXTURE program breaks the pool of 476 heifers into 16 subpopulations, the number of which is close to the number of herds used in the experiment. Results of the comparison between F_st values obtained using SNP markers with published data obtained on microsatellites are in support of the common opinion that microsatellite analysis results in underestimation of F_st values. On the whole, the obtained across-herd F_st values are in the range Fst data reported for cattle breeds. Results of comparison of F_st values with the data on the origin of bulls imported from different countries lead to the conclusion on the expediency of the use of F_st data to assess heterogeneity of the herds. Thus, we have demonstrated that use of F_st data provides the means to assess genetic diversity of cattle herds and is a necessary step in the formation of a reference population for dairy cattle.     

Allozyme variability in common shrew Sorex araneus of Western Siberia

The allozyme variability of the common shrew Sorex araneus of Western Siberia is studied. The small level of genetic variability and lack of reliable differences in indices of polymorphism for taiga, subtaiga, and forest-steppe populations are revealed. A significant level of genetic differentiation in populations belonging to adjacent chromosomal races—Novosibirsk and Serov (F _st = 0.3) — has been shown.     

Genetic variation at enzyme loci in North Atlantic minke whales,Balaenoptera acutorostrata

Electrophoretic variation within and between North Atlantic minke whale samples(Balaenoptera acutorostrata) from West Greenland, Iceland, and Norway was investigated. In the West Greenland samples, 28 enzyme systems were examined, representing 36 loci, of which 6 were found to be polymorphic. In Icelandic and Norwegian samples, 22 enzyme systems were examined, representing 29 loci, of which 6 and 5 were found to be polymorphic, respectively. The average heterozygosity was 0.058 (SE=0.024) in samples from West Greenland, 0.074 (SE=0.028) in samples from Iceland, and 0.054 (SE=0.023) in samples from Norway. No significant deviations from the expected Hardy—Weinberg genotypic frequencies, within samples taken from the same area, were found. Significant differences in allele frequencies were observed, however, between samples from the three different areas. The average Nei's genetic distance was 0.014 and the averageF _st value was 0.126. The genetic differences between the samples from the different areas indicate that those from West Greenland, Iceland, and Norway represented different breeding populations.     

Analysis of Pharmacogenomic Variants Associated with Population Differentiation

In the present study, we systematically investigated population differentiation of drug-related (DR) genes in order to identify common genetic features underlying population-specific responses to drugs. To do so, we used the International HapMap project release 27 Data and Pharmacogenomics Knowledge Base (PharmGKB) database. First, we compared four measures for assessing population differentiation: the chi-square test, the analysis of variance (ANOVA) F-test, F_st, and Nearest Shrunken Centroid Method (NSCM). F_st showed high sensitivity with stable specificity among varying sample sizes; thus, we selected F_st for determining population differentiation. Second, we divided DR genes from PharmGKB into two groups based on the degree of population differentiation as assessed by F_st: genes with a high level of differentiation (HD gene group) and genes with a low level of differentiation (LD gene group). Last, we conducted a gene ontology (GO) analysis and pathway analysis. Using all genes in the human genome as the background, the GO analysis and pathway analysis of the HD genes identified terms related to cell communication. “Cell communication” and “cell-cell signaling” had the lowest Benjamini-Hochberg’s q-values (0.0002 and 0.0006, respectively), and “drug binding” was highly enriched (16.51) despite its relatively high q-value (0.0142). Among the 17 genes related to cell communication identified in the HD gene group, five genes (STX4, PPARD, DCK, GRIK4, and DRD3) contained single nucleotide polymorphisms with F_st values greater than 0.5. Specifically, the F_st values for rs10871454, rs6922548, rs3775289, rs1954787, and rs167771 were 0.682, 0.620, 0.573, 0.531, and 0.510, respectively. In the analysis using DR genes as the background, the HD gene group contained six significant terms. Five were related to reproduction, and one was “Wnt signaling pathway,” which has been implicated in cancer. Our analysis suggests that the HD gene group from PharmGKB is associated with cell communication and drug binding.     

Microsatellite polymorphism and genetic differentiation in three Norwegian populations of Elymus alaskanus (Poaceae)

 Variation at seven microsatellite loci was investigated in three local E. alaskanus populations from Norway and microsatellite variation was compared with allozyme variation. The percentage of polymorphic loci was 81%, the mean number of alleles per polymorphic locus was 5.7 and expected heterozygosity was 0.37. An F-statistic analysis revealed an overall 48% deficit of heterozygotes over Hardy-Weinberg expectations. Gene diversity is mainly explained by the within population component. The averaged between population differentiation coefficient, F _st , over 7 loci is only 0.13, which accounts for only 13% of the whole diversity and was contrary to allozyme analysis. The mean genetic distance between populations was 0.12. However, a χ² -test showed that allele frequencies were different (p < 0.05) among the populations at 5 of the 7 loci. In comparison with the genetic variation detected by allozymes, microsatellite loci showed higher levels of genetic variation. Microsatellite analysis revealed that population H10576 possesses the lowest genetic variation among the tested three populations, which concur with allozyme analysis. The dendrogram generated by microsatellites agreed very well with allozymic data. Our results suggest that natural selection may be an important factor in shaping the genetic diversity in these three local E. alaskanus populations. Possible explanations for deficit heterozygosity and incongruence between microsatellites and allozymes are discussed. Received November 6, 2001; accepted April 24, 2002 Published online: November 14, 2002 Addresses of the authors: Genlou Sun (e-mail: Genlou.sun@STMARYS.CA), Biology Department, Saint Mary's University, Halifax. Nova Scotia, B3H 3C3, Canada. B. Salomon, R. von Bothmer, Department of Crop Science, The Swedish University of Agricultural Sciences, P.O. Box 44, SE-230 53, Alnarp, Sweden.