Genetic relationships and evolution in <Emphasis Type="Italic">Cucurbita pepo</Emphasis> (pumpkin,squash, gourd) as revealed by simple sequence repeat polymorphisms

Genetic relationships among 104 accessions of Cucurbita pepo were assessed from polymorphisms in 134 SSR (microsatellite) and four SCAR loci, yielding a total of 418 alleles, distributed among all 20 linkage groups. Genetic distance values were calculated, a dendrogram constructed, and principal coordinate analyses conducted. The results showed 100 of the accessions as distributed among three clusters representing each of the recognized subspecies, pepo, texana, and fraterna. The remaining four accessions, all having very small, round, striped fruits, assumed central positions between the two cultivated subspecies, pepo and texana, suggesting that they are relicts of undescribed wild ancestors of the two domesticated subspecies. In both, subsp. texana and subsp. pepo, accessions belonging to the same cultivar-group (fruit shape) associated with one another. Within subsp. pepo, accessions grown for their seeds or that are generalists, used for both seed and fruit consumption, assumed central positions. Specialized accessions, grown exclusively for consumption of their young fruits, or their mature fruit flesh, or seed oil extraction, tended to assume outlying positions, and the different specializations radiated outward from the center in different directions. Accessions of the longest-fruited cultivar-group, Cocozelle, radiated bidirectionally, indicating independent selection events for long fruits in subsp. pepo probably driven by a common desire to consume the young fruits. Among the accessions tested, there was no evidence for crossing between subspecies after domestication.     

Numerical analysis of allozyme variation inCucurbita pepo

Cucurbita pepo, represented by cultivated forms of squash, pumpkins, and ornamental gourds, is a morphologically diverse species. Although several classifications of the cultivars have been proposed, none has been fully accepted. In an attempt to clarify the systematics of theC. pepo cultivars, allozyme variation was assayed. Twelve loci representing 6 enzyme systems (GOT, IDH, MDH, PER, PGI, and PGM) were screened. Seven of the loci were polymorphic. Electrophoretic data were collected for 50 accessions representing 14 commercial cultivars and a landrace from Mexico. Statistical analyses of the allozyme data revealed a biochemical basis for characterizing cultivars that agrees with morphology. Principal component analysis and cluster analysis revealed a major subdivision within the species above the level of cultivars. Genetic identities among groups of cultivars were much lower than those usually found among conspecific populations for outcrossing plants, possibly reflecting the influences of populational subdivision, drift, and selection particular toC. pepo.     

Parallel Evolution Under Domestication and Phenotypic Differentiation of the Cultivated Subspecies of <Emphasis Type="Italic">Cucurbita pepo</Emphasis> (Cucurbitaceae)

Parallel Evolution under Domestication and Phenotypic Differentiation of the Cultivated Subspecies of Cucurbita pepo (Cucurbitaceae). Cucurbita pepo (pumpkin, squash, gourd, Cucurbitaceae) is an ancient North American domesticate of considerable economic importance. Based on molecular genetic polymorphisms, two cultivated lineages of this species, each consisting of very many edible–fruited cultigens, have been recognized, C. pepo subsp. pepo and C. pepo subsp. texana. However, the phenotypic commonalities and differences between these two subspecies have not as yet been systematically collected and organized. Among the evolutionary developments common to the two subspecies are the increased size of the plant parts, less plant branching, and premature loss of chlorophyll in the exocarp of the fruits. In both subspecies, bush growth habit, conferred by allele Bu, is common to the cultigens grown for consumption of the immature fruits, as is the deviation from the 1:1 ratio of fruit length to fruit width. A major characteristic differentiating between the edible–fruited cultigens of the respective subspecies are the longitudinal protrusions, in subsp. pepo, versus depressions, in subsp. texana, of the fruit surface corresponding with the subsurface primary carpellary vein tracts. Subsp. pepo also has larger fruits and larger and longer seeds. In addition, some alleles affecting stem color, leaf mottling, multiple flower bud production, and fruit characteristics are frequently occurring to nearly fixed in one subspecies but are rare to less common in the other.     

Tetrahydrocannabinol levels in hemp (cannabis sativa) germplasm resources

In most of the western world where industrial hemp, Cannabis sativa, is licensed for cultivation, the plants must not exceed a level of 0.3% tetrahydrocannabinol (THC), the principal intoxicating constituent of the species. Because there are no publicly available germplasm hemp collections in North America and only a very few, recent North American cultivars have been bred, the future breeding of cultivars suitable for North America is heavily dependent on European cultivars and European germplasm collections. Based mostly on material from Europe, this study surveyed THC levels of 167 accessions grown in southern Ontario, making this the largest survey to date of germplasm intended for breeding in North America. Forty-three percent of these had THC levels ≥0.3% and, therefore, are unsuitable for hemp development in North America. Discrepancies were found between THC levels reported for some germplasm holdings in Europe when they were grown in Canada and, accordingly, verification of THC levels developed in North America is necessary.     

INTERSPECIFIC GENE FLOW IN CUCURBITA: C. TEXANA VS. C. PEPO

The potential for interspecific genetic exchange was examined by monitoring flowering patterns, pollinator movement, and gene flow among experimental populations of the Texas gourd (Cucurbita texana) and cultivars of Cucurbita pepo. While flowering patterns and pollinator movement tended to maximize self-pollination and local gene exchange, movement of effective pollen exceeded 1,300 m. This movement, mediated by the solitary bee Xenoglossa strenua and monitored by tracking allozyme variants, produced interspecific hybrids in 5% of the progeny from experimental plants. Interspecific gene exchange occurred in either direction with either species serving as staminate or pistillate parent. No obvious constraints to gene flow among plants representing C. texana and distinctive cultivars (vars. ovifera, medullosa, melopepo) of C. pepo were detected. Genetic exchange among different species and cultivars is enhanced by the foraging behavior of Xenoglossa. Multiple visits to either staminate (pollen carryover) or pistillate (multiple pollinations) flowers often result in the deposition of mixed pollen on receptive stigmas. The wild type (C. texana) can donate and receive effective pollen when growing under both weedy and natural conditions. The observed lack of interspecific reproductive isolation supports treatment of cultivars and wild types as a single species and, in conjunction with available data concerning temporal/geographical relationships among bees, squash, gourds, and humans in eastern North America, suggests the possibility of long-term genetic interaction between wild types and domesticates.     

Examining the genetic diversity of the orange blossom wheat midge,Sitodiplosis mosellana (Géhin), across North America

The orange blossom wheat midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), is a significant pest of wheat (Triticum spp.) grown in the Northern Hemisphere. It was accidently introduced to North America over 200 years ago and has subsequently spread throughout the northern Great Plains. Since 2010, several Canadian spring wheat varieties containing the resistance gene Sm1 have been released. Due to the potential of wheat midge populations to evolve virulent biotypes to Sm1, cultivars containing Sm1 are grown with a susceptible cultivar in an interspersed refuge. An understanding of the genetic diversity of wheat midge populations could provide important information on the potential development of resistance to Sm1. In the current study, we used two mitochondrial genes (CO1 and ND4) from wheat midge collected across the northern Great Plains and Québec in North America to assess population structure and genetic diversity. We found limited genetic diversity and population structure across the sampled North American populations. We also assessed North American haplotype similarity to wheat midge collected from Europe and China and found high similarity between North American and European populations, although sampling in Europe was limited. This supports the hypothesis that North American populations originated from Europe.     

Quality of a stress-sensitive Cucurbita pepo L. pollen

The quality of Cucurbita pepo L. pollen was studied using field pollinations and the fluorochromatic-reaction test. The extreme sensitivity of this pollen to dehydration and ageing is demonstrated. Controlled stress applied to mature pollen leads to the development of seedless fruits. Molecular signals seem to be involved in the induction of this parthenocarpy. These results indicate the existence of distinct sequences involved in the completion of the fertilization program of pollen. With pollen altered by stress, the fertilization process may be stopped at different stages of its completion. We bring evidence that Cucurbita pepo plants have developed special adaptations in order to compensate for the poor viability of their pollen.Abbreviation FCR fluorochromatic reaction     

Microbial Diversity Inside Pumpkins: Microhabitat-Specific Communities Display a High Antagonistic Potential Against Phytopathogens

Recent and substantial yield losses of Styrian oil pumpkin (Cucurbita pepo L. subsp. pepo var. styriaca Greb.) are primarily caused by the ascomycetous fungus Didymella bryoniae but bacterial pathogens are frequently involved as well. The diversity of endophytic microbial communities from seeds (spermosphere), roots (endorhiza), flowers (anthosphere), and fruits (carposphere) of three different pumpkin cultivars was studied to develop a biocontrol strategy. A multiphasic approach combining molecular, microscopic, and cultivation techniques was applied to select a consortium of endophytes for biocontrol. Specific community structures for Pseudomonas and Bacillus, two important plant-associated genera, were found for each microenvironment by fingerprinting of 16S ribosomal RNA genes. All microenvironments were dominated by bacteria; fungi were less abundant. Of the 2,320 microbial isolates analyzed in dual culture assays, 165 (7%) were tested positively for in vitro antagonism against D. bryoniae. Out of these, 43 isolates inhibited the growth of bacterial pumpkin pathogens (Pectobacterium carotovorum, Pseudomonas viridiflava, Xanthomonas cucurbitae); here only bacteria were selected. Microenvironment-specific antagonists were found, and the spermosphere and anthosphere were revealed as underexplored reservoirs for antagonists. In the latter, a potential role of pollen grains as bacterial vectors between flowers was recognized. Six broad spectrum antagonists selected according to their activity, genotypic diversity, and occurrence were evaluated under greenhouse conditions. Disease severity on pumpkins of D. bryoniae was significantly reduced by Pseudomonas chlororaphis treatment and by a combined treatment of strains (Lysobacter gummosus, P. chlororaphis, Paenibacillus polymyxa, and Serratia plymuthica). This result provides a promising prospect to biologically control pumpkin diseases.     

Cytokinins antagonize the jasmonates action on the regulation of potato (Solanum tuberosum) tuber formation in vitro

Cucurbita pepo L. (squash, pumpkin) is a highly polymorphic vegetable species of major importance. Our study characterized a spectrum of C. pepo germplasm for the ability to regenerate in vitro by direct organogenesis from cotyledon explants. Cultivars tested included both cultivated subspecies, texana and pepo, and nearly all of their respective cultivar-groups. Direct shoot regeneration occurred in all accessions, and was generally high (56–94%), with a single exception of 22% (‘Bolognese’). There was no significant difference between the percentage regeneration of the two subspecies. Shoot regeneration per responding explant was uniform (1.2–1.6 shoots per explant). Only ‘True French’ produced statistically more shoots (3.9 per explant) than other accessions. The morphology of regeneration varied. Most cultivars produced long shoots, often fasciated, amid a few small buds. Some subspecies pepo cultivars (Beirut, Yugoslavia 7, Ma’yan and True French) produced short, massive, hollow shoots, sometimes accompanied by shoots that were more normal. Two subspecies texana cultivars (Creamy Straightneck and Small Bicolor) produced single (sometimes double) shoots without other buds. The production of chimeric (mixoploid) regenerants varied and there was a tendency to regenerate chimeric plants from the widest-fruited accessions (i.e. lowest length-to-width ratio) in each subspecies. Subspecies pepo Pumpkin Group ‘Tondo di Nizza’ showed significantly greater production of chimeric regenerants. In comparison with the great range of variation observed in fruit shape, the variation of in vitro responses (mostly less than 2-fold in regeneration and shoot production) was less than expected.     

Comparative analysis of quality and nutritional traits from Lonicera caerulea (Honeyberry) cultivars and other berries grown in Scotland

Lonicera caeruleabrix is a perennial shrub native to North America, Europe and Asia. It produces dark blue berries known as honeyberries or haskap berries which are produced commercially in several territories including Canada, Japan, Russia and Poland. Plants are suited to UK environments, but it is yet to be widely commercially developed in the UK. In the present work, quality and nutritional traits of six honeyberry cultivars grown in Scotland were compared with other commonly grown berry fruits (strawberry, raspberry, blueberry, blackberry, blackcurrant) to aid the identification of environmentally stable, high-quality honeyberry cultivars suitable for UK cultivation. Differences were observed in fruit quality variables (soluble solids, titratable acidity and Brix/acid ratios) between honeyberry cultivars. Three of six cultivars examined exhibited notable variation in soluble solids dependent on harvest year with ‘Aurora’ and ‘Strawberry Sensation’ having consistently high ^oBrix values. Titratable acidity exhibited cultivar differences and there was limited variation over harvest years. ‘Aurora’ exhibited consistently high ^oBrix/titratable acidity ratio reflected by high glucose and fructose content. Honeyberry fruit had good nutritional profile relative to other soft fruits with higher polyphenol and anthocyanin content than strawberry, blueberry, blackberry and raspberry, manifested in greater antioxidant capacity. The major anthocyanins in aqueous honeyberry fruit extracts were cyanidin, pelargonidin and peonidin glycosides. These findings indicate that L. caerulea represents a crop suitable for UK cultivation capable of producing high quality fruit with a valuable nutritional profile relative to other soft fruits. Cultivars exhibit significant differences in fruit quality and nutritional profile as well as harvest consistency and growers should consider this when establishing new plantations.     

Biological and molecular characterization of <Emphasis Type="Italic">Prunus necrotic ringspot virus</Emphasis> isolates from three rose cultivars

Prunus necrotic ringspot virus (PNRSV) is a rose and stone fruit tree pathogen. Three different PNRSV isolates, originating from three rose cultivars were studied. These PNRSV isolates were characterized using molecular techniques. Nearly the complete nucleotide sequence (1,630 nucleotides) of RNA3 of the isolate PNRSV-R1 has been determined (GenBank Acc. No. DQ003584). The sequence of the MP gene of the PNRSV-R1 isolate was determined, the first such results for a rose-derived PNRSV isolate. The reaction of PNRSV infection on test plants was also investigated. Cucumis sativus cv. Wisconsin, Cucurbita maxima cv. Buttercup and Cucurbita pepo cv. Melonowa Żółta appeared to be the most useful test plants for the differentiation of isolate-specific pathogenicity.     

Resistance to Tomato leaf curl New Delhi virus in Cucurbita spp.

Tomato leaf curl New Delhi virus (ToLCNDV) is a bipartite begomovirus (family Geminiviridae) first reported in India and its neighbouring countries. ToLCNDV severely affects zucchini crop (Cucurbita pepo) in the main production areas of Southern Spain since 2012. This emerging begomovirus is a serious threat to this and other cucurbit crops. Breeding resistant cultivars is the most promising method for disease control, but requires the identification of sources of resistance in the Cucurbita genus. In this work, we screened for ToLCNDV resistance a large collection of Cucurbita spp. accessions, including landraces and commercial cultivars of the main cultivated species, C. pepo, Cucurbita moschata and Cucurbita maxima and wild species. The screening was performed using mechanical and whitefly inoculation. The level of resistance was assessed by scoring symptom severity and by measuring the virus content with quantitative polymerase chain reaction in selected genotypes. Diversity in the response was observed within and among species. Severe symptoms and high viral amounts were found at 30 days after mechanical and whitefly inoculation in C. pepo, in all accessions belonging to the Zucchini morphotype and to other morphotypes of both subspecies, pepo and ovifera, and even in the wild relative Cucurbita fraterna. C. maxima was also highly susceptible. This species showed characteristic symptoms of leaf decay and intense yellowing, different from those of mosaic, curling and internode shortening found in C. pepo. The only species showing resistance was C. moschata. Four accessions were symptomless or had some plants with only mild symptoms after three independent rounds of mechanical inoculation with different inoculum sources. Two of them also remained symptomless after virus inoculation with viruliferous whiteflies. ToLCNDV was detected in these asymptomatic accessions at 15 and 30 days post inoculation, but viral amounts were much lower than those found in susceptible genotypes, suggesting a high level of resistance. The symptoms in the susceptible accessions of this species were also different, with a characteristic leaf mottling, evolving to a severe mosaic. The newly identified C. moschata resistant accessions are good candidates for breeding programmes to avoid the damage caused by ToLCNDV.     

Inheritance Profile of Weathered Chlordane and P,P′-DDTs Accumulation by cucurbita pepo hybrids

Cucurbita pepo ssp pepo (zucchini) accumulates significant levels of persistent organic pollutants in its roots, followed by unexpectedly high contaminant translocation to the stems. Most other plant species, including the closely related C. pepo ssp ovifera (squash), do not have this ability. To investigate the mechanism of contaminant accumulation, two cultivars each of parental zucchini and squash, as well as previously created first filial (F1) hybrids and F1 backcrosses (BC) of those parental cultivars, were grown under field conditions in a soil contaminated with weathered chlordane (2.29 μg/g) and DDX residues (0.30 μg/g; sum of DDT, DDE, DDD). The parental zucchini had stem-to-soil bioconcentration factors (BCF, contaminant ratio of stem to soil) for chlordane and DDX of 6.23 and 3.10; these values were 2.2 and 3.7 times greater than the squash, respectively. Chlordane and DDX translocation factors, the ratio of contaminant content in the stems to that in the root, were 2.1 and 3.2 times greater for zucchini than for squash. The parental zucchini and squash also differed significantly in chlordane component ratios (relative amounts of trans-nonachlor [TN], cis-chlordane [CC], trans-chlordane [TC]) and enantiomer fractions for the chiral CC and TC. Hybridization of the parental squash and zucchini resulted in significant differences in contaminant uptake. For both the three separate component ratios (CR) and two sets of enantiomer fraction (EF) values, subspecies specific differences in the parental generation became statistically equivalent in the F1 hybrid zucchini and squash. When backcrossed (BC) with the original parental plants, the zucchini and squash F1 BC cultivars reverted to the statistically distinct CR and EF patterns. This pattern of trait segregation upon hybridization suggests either single gene or single locus control for persistent organic pollutant (POP) uptake ability by C. pepo ssp pepo.     

Abnormal chromosomes and DNA content in micropropagated rhubarb (<Emphasis Type="Italic">Rheum rhaponticum </Emphasis>L<Emphasis Type="Italic">.</Emphasis>) PC49

Rhubarb (Rheum rhaponticum L.) is a tetraploid (2n = 44) vegetable crop growing in Europe and North America. The results observed 44 chromosomes for both cultivars Timperly Early and PC49, but showing significant differences in DNA contents between them. Abnormal chromosome numbers (2n = 22 and 32) and chromosomal fragments were observed only in micropropagated plants of rhubarb PC49, suggesting somaclonal variation. The results provide further information on understanding morphological variation and increased disease susceptibility of micropropagated rhubarb PC49 in our previous investigation.     

A Comparison of Stem Peroxidases in Bush and Vine Forms of Squash (Cucurbita maxima Duch.and C. pepo L.)

Total peroxidase activities and peroxidase isozymes were comparedamong 10 bush- and vine-squash cultivars of Cucurbita maximaDuch. and C. pepo (L.) Several qualitative variations of isozymeswere detected between C. maxima and C. pepo, and some differenceswere evident among varieties of C. pepo. There were no qualitativedifferences between genetically similar bush and vine strains.Quantitative differences were difficult to resolve, althoughit did appear that in varieties of C. maxima exhibiting lowlevels of peroxidase activity, isozymes C₃, C₄, and C₆ wereless intense. Depending upon the cultivars compared, total peroxidase activityin bush forms was either higher or lower than, or the same astheir vine counterparts. Since the major bush genes in the varietiesstudied are believed to be allelic, the results indicate thatthe over-all genotype of squash varieties can influence therelative expression of peroxidase activity between bush andvine forms.     

Summer Squash Identification by High-Resolution-Melting (HRM) Analysis Using Gene-Based EST–SSR Molecular Markers

Cucurbita pepo (squash, pumpkin, gourd), a worldwide cultivated vegetable of American origin, is extremely variable in fruit characteristics. Most of its widely grown commercial types are known as summer squashes and belong to the elongated forms of C. pepo ssp. pepo (Cocozelle, Vegetable marrow and Zucchini groups). Here, we have integrated the high-resolution-melting (HRM) analysis method with expressed sequence tags–simple sequence repeat (EST–SSR) marker genotyping, in order to facilitate the identification of 36 summer squash landraces originated from Greece. The six EST–SSR loci used were informative and generated a unique melting curve profile of EST-derived microsatellites for each accession allowing their comparison and classification. Moreover, HRM was highly informative, as by using only four microsatellite markers we were able to discriminate 36 summer squash landraces and by using six EST–SSRs. We were able to construct a highly informative and discriminative dendrogram where the 36 genotypes were classified in six distinct clusters. Furthermore, we acquired information about the genes containing the EST–SSRs using bioinformatics tools. We found that the EST–SSRs used in this study were hybridizing to genes involved in stress response to heavy metals and biotic stresses or the production of flavonoids or symporters of important nitrogen sources, like xanthine and uric acid amongst others. The results presented here suggest that the panel of EST–SSR markers used in combination with HRM analysis could be useful in a variety of applications, like squash biodiversity assessment but most importantly in managing squash germplasm to improve breeding programs.     

Effects of soil phosphorus on pollen production,pollen size,pollen phosphorus content,and the ability to sire seeds in Cucurbita pepo (Cucurbitaceae)

To determine the effects of soil phosphorus on pollen production, pollen grain size, phosphate concentration per pollen grain, and the siring ability of pollen, two cultivars of the common zucchini (Cucurbita pepo) were grown under two soil phosphorus conditions in an experimental garden. Overall, soil phosphorus availability had a significant effect on reproductive output through the female function and on traits affecting the male function of plants (staminate flower production, pollen production per flower, and pollen grain size). In addition, pollen produced by plants in the high phosphorus soils had a higher phosphate concentration than pollen produced by plants in the low phosphorus soils. A pollen mixture experiment revealed that pollen produced by plants in the high phosphorus treatment sired significantly more seeds than pollen produced by plants in the low phosphorus treatment. This study showed that growing conditions such as soil phosphorus can influence the size of a pollen grain and its chemical composition, which, in turn, can affect its ability to sire mature seeds.     

Behavioral constancy for interspecies dependency enables Nearctic Chymomyza amoena (Loew) (Diptera: Drosophilidae) to spread in orchards and forests in Central and Southern Europe

Nearctic Chymomyza amoena, an eastern US forest drosophilid, was initially known only to breed in damaged or parasitized nuts and had been little studied. It has been spreading in Europe since its discovery in the former Czechoslovakia in 1975. By the time it arrived in Switzerlands Canton Ticino in 1988, research in the United States revealed it had long been in domestic habitats, overwinters in the third instar larval stage in endemic substrates (black walnut husks Juglans nigra, native crabapples Malus coronaria), domestic [imported] apples Malus domestica and ornamental fruits (crabapples) and uses these plus other substrates for breeding from spring and summer through autumn. Female oviposition in firm substrates in Michigan and the mid-South (North Carolina, Virginia) as fallen unripe and ripe frassy apples, acorns, black walnut husks, native and ornamental crabapples is mediated by prior insect attack. Although large numbers of C. amoena coming to banana bait in Canton Ticino suggested that founder effects involving attraction to fermenting substrates might have occurred, experimental studies with European flies in Michigan and continuing research in Zürich and Canton Ticino revealed that behavioral constancy had been maintained. This enabled prediction that C. amoena would spread into apple orchards in northern Switzerland and into Italy. Research in July 2000 established that it is in apple orchards on the German border and in the chestnut forests and in old orchard apples in Italys Valtellina region, Lombardy Province. Other European drosophilids have not exploited parasitized fruits and nuts, indicating C. amoena entered a vacant niche. Facilitation provided by pest species attacking fruit and nut substrates parallel those in North America. Chymomyza amoena has maintained behavioral constancy for interspecies dependency and continues to be the principal drosophilid breeding in parasitized fruits and nuts in both North America and Europe.     

EFFECTS OF SOIL NITROGEN ON POLLEN PRODUCTION,POLLEN GRAIN SIZE,AND POLLEN PERFORMANCE IN CUCURBITA PEPO (CUCURBITACEAE)

To determine the effects of soil nitrogen on pollen production, pollen size, and pollen performance, two cultivars of zucchini (Cucurbita pepo) were grown under two nitrogen regimes in an experimental garden. The two cultivars were true breeding for alternative alleles for a one gene trait, ovary color. The soil nitrogen treatment had a significant effect on most measures of reproductive output through the female function. The nitrogen treatment did not affect the number of staminate flowers per plant but did have an effect on the number of pollen grains per staminate flower and the mean pollen grain size. A pollen mixture experiment revealed that pollen produced by plants in the high nitrogen treatment sired significantly more seeds than pollen from low nitrogen plants. Moreover, we found that the high nitrogen pollen sired even a greater percentage of seeds in the region of the fruit (ovary) previously shown to be fertilized by the fastest growing pollen tubes. Thus, the difference in the number of seeds sired by pollen from the two nitrogen treatments is due to differences in pollen performance. We conclude that spatial heterogeneity in soil nitrogen can influence the paternity of seeds in a plant population.