Simple Sequence Repeat Analysis of Genetic Diversity in Primary Core Collection of Peach （Prunus persica）

In this study, the genetic diversity of 51 cultivars in the primary core collection of peach （Prunus persica （L.） Batsch） was evaluated by using simple sequence repeats （SSRs）. The phylogenetic relationships and the evolutionary history among different cultivars were determined on the basis of SSR data. Twenty-two polymorphic SSR primer pairs were selected, and a total of 111 alleles were identified in the 51 cultivars, with an average of 5 alleles per locus. According to traditional Chinese classification of peach cultivars, the 51 cultivars in the peach primary core collection belong to six variety groups. The SSR analysis revealed that the levels of the genetic diversity within each variety group were ranked as Sweet peach 〉 Crisp peach 〉 Flat peach 〉 Nectarine 〉 Honey Peach 〉 Yellow fleshed peach. The genetic diversity among the Chinese cultivars was higher than that among the introduced cultivars. Cluster analysis by the unweighted pair group method with arithmetic averaging （UPGMA） placed the 51 cultivars into five linkage clusters. Cultivar members from the same variety group were distributed in different UPGMA clusters and some members from different variety groups were placed under the same cluster. Different variety groups could not be differentiated in accordance with SSR markers. The SSR analysis revealed rich genetic diversity in the peach primary core collection, representative of genetic resources of peach.     

Genetic Diversity and Characterization of a Core Collection of Malus Germplasm Using Simple Sequence Repeats (SSRs)

Simple sequence repeats (SSRs) were used to assess genetic diversity and study genetic relatedness in a large collection of Malus germplasm. A total of 164 accessions from the Malus core collection, maintained at the University of Illinois, were genotyped using apple SSR markers. Each of the accessions was genotyped using a single robust SSR marker from each of the 17 different linkage groups in Malus. Data were subjected to principal component analysis, and a dendrogram was constructed to establish genetic relatedness. As expected, this diverse core collection showed high allelic diversity; moreover, this allelic diversity was higher than that previously reported. Cluster analysis revealed the presence of four distinct clusters of accessions in this collection.     

Genetic Diversity and Identity of Chinese Loquat Cultivars/Accessions (<Emphasis Type="Italic">Eriobotrya japonica</Emphasis>) Using Apple SSR Markers

Loquat (Eriobotrya japonica) is an underutilized fruit crop that originated in China and for which only a small number of molecular markers are available. This number can be increased by identifying apple SSRs that are transferable to loquat cultivars/accessions to provide new insight into the level of genetic diversity within loquat and synteny with apple. We evaluated 71 apple SSR markers distributed across 17 linkage groups, and identified 39 SSRs transferable to loquat. Testing 54 loquat accessions, from Japan, Spain, four provinces in China, and two wild species gave a total of 155 different alleles with a mean value of 3.38 per locus. The mean effective number of alleles was 2.21, and the mean observed heterozygosity was 0.47. These values indicate a high degree of genetic diversity in the set of Chinese loquat accessions analyzed. Unweighted pair-group method analysis based on simple matching coefficent clustered the accessions into two groups, cultivated and wild loquat. The cultivated loquat can be subdivided into three subgroups which generally reflect their geographic origin in China. The Spanish cultivars clustered with those of the Jiangsu and Zhejiang provinces. A core set of five SSR markers could distinguish most accessions.     

Genetic diversity in European Pisum germplasm collections

The distinctness of, and overlap between, pea genotypes held in several Pisum germplasm collections has been used to determine their relatedness and to test previous ideas about the genetic diversity of Pisum. Our characterisation of genetic diversity among 4,538 Pisum accessions held in 7 European Genebanks has identified sources of novel genetic variation, and both reinforces and refines previous interpretations of the overall structure of genetic diversity in Pisum. Molecular marker analysis was based upon the presence/absence of polymorphism of retrotransposon insertions scored by a high-throughput microarray and SSAP approaches. We conclude that the diversity of Pisum constitutes a broad continuum, with graded differentiation into sub-populations which display various degrees of distinctness. The most distinct genetic groups correspond to the named taxa while the cultivars and landraces of Pisum sativum can be divided into two broad types, one of which is strongly enriched for modern cultivars. The addition of germplasm sets from six European Genebanks, chosen to represent high diversity, to a single collection previously studied with these markers resulted in modest additions to the overall diversity observed, suggesting that the great majority of the total genetic diversity collected for the Pisum genus has now been described. Two interesting sources of novel genetic variation have been identified. Finally, we have proposed reference sets of core accessions with a range of sample sizes to represent Pisum diversity for the future study and exploitation by researchers and breeders.     

Assessing genetic diversity in <Emphasis Type="Italic">Gossypium arboreum</Emphasis> L. cultivars using genomic and EST-derived microsatellites

The cultivated diploid, Gossypium arboreum L., (A genome) is an invaluable genetic resource for improving modern tetraploid cotton (G. hirsutum L. and G. barbadense L.) cultivars. The objective of this research is to select a set of informative and robust microsatellites for studying genetic relationships among accessions of geographically diverse G. arboreum cultivars. From more than 1,500 previously developed simple sequence repeat (SSR) markers, 115 genomic (BNL) and EST-derived (MUCS and MUSS) markers were used to evaluate the allelic diversity of a core panel of G. arboreum accessions. These SSR data enabled advanced genome analyses. A set of 25 SSRs were selected based both upon their high level of informativeness (PIC ≥ 0.50) and the production of clear PCR bands on agarose gels. Subsequently, 96 accessions representing a wide spectrum of diversity of G. arboreum cultivars were analyzed with these markers. The 25 SSR loci revealed 75 allelic variants (polymorphisms) ranging from 2 to 4 alleles per locus. The Neighborjoining (NJ) method, based on genetic dissimilarities, revealed that cultivars from geographically adjacent countries tend to cluster together. Outcomes of this research should be useful in decreasing redundancy of effort and in constructing a core collection of G. arboreum, important for efficient use of this genetic resource in cotton breeding.     

Genetic diversity and population structure in cultivated sunflower and a comparison to its wild progenitor, <Emphasis Type="Italic">Helianthus annuus</Emphasis> L

Crop germplasm collections are valuable resources for ongoing plant breeding efforts. To fully utilize such collections, however, researchers need detailed information about the amount and distribution of genetic diversity present within collections. Here, we report the results of a population genetic analysis of the primary gene pool of sunflower (Helianthus annuus L.) based on a broad sampling of 433 cultivated accessions from North America and Europe, as well as a range-wide collection of 24 wild sunflower populations. Gene diversity across the cultivars was 0.47, as compared with 0.70 in the wilds, indicating that cultivated sunflower harbors roughly two-thirds of the total genetic diversity present in wild sunflower. Population structure analyses revealed that wild sunflower can be subdivided into four genetically distinct population clusters throughout its North American range, whereas the cultivated sunflower gene pool could be split into two main clusters separating restorer lines from the balance of the gene pool. Use of a maximum likelihood method to estimate the contribution of the wild gene pool to the cultivated sunflower germplasm revealed that the bulk of the cultivar diversity is derived from two wild sunflower population genetic clusters that are primarily composed of individuals from the east-central United States, the same general region in which sunflower domestication is believed to have occurred. We also identified a nested subset of accessions that capture as much of the allelic diversity present within the sampled cultivated sunflower germplasm collection as possible. At the high end, a core set of 288 captured nearly 90% of the alleles present in the full set of 433, whereas a core set of just 12 accessions was sufficient to capture nearly 50% of the total allelic diversity present within this sample of cultivated sunflower.     

Molecular characterization and genetic diversity analysis of soybean (Glycine max (L.) Merr.) germplasm accessions in India

Molecular characterization and genetic diversity among 82 soybean accessions was carried out by using 44 simple sequence repeat (SSR) markers. Of the 44 SSR markers used, 40 markers were found polymorphic among 82 soybean accessions. These 40 polymorphic markers produced a total of 119 alleles, of which five were unique alleles and four alleles were rare. The allele number for each SSR locus varied between two to four with an average of 2.97 alleles per marker. Polymorphic information content values of SSRs ranged from 0.101 to 0.742 with an average of 0.477. Jaccard’s similarity coefficient was employed to study the molecular diversity of 82 soybean accessions. The pairwise genetic similarity among 82 soybean accessions varied from 0.28 to 0.90. The dendrogram constructed based on genetic similarities among 82 soybean accessions identified three major clusters. The majority of genotypes including four improved cultivars were grouped in a single subcluster IIIa of cluster III, indicating high genetic resemblance among soybean germplasm collection in India.

Electronic supplementary material

The online version of this article (doi:10.1007/s12298-014-0266-y) contains supplementary material, which is available to authorized users.     

Genetic Diversity of Soybean and the Establishment of a Core Collection Focused on Resistance to Soybean Cyst Nematode

Soybean cyst nematode （SCN; Heterodera glycines） Is one of the most Important pests affecting soybean production. The best method of control of SCN is through the development of resistant cultlvars. However, limited progress has been made in soybean breeding In China because most modern cultlvars have no resistance to SCN. The distribution and phenotype of 432 immune or highly resistant Chinese accessions were surveyed and a primary core collection was selected as a representative sample for further analyses. Using evenly distributed simple sequence repeat markers, five selection methods were applied to the primary core collection and the optimal method was chosen to establish a core collection, which consisted of 28 accessions. These encompassed 70.8% of the ailelic variation present in the overall resistant collection. The 28 accessions differed from the reference resistant accessions at the genomlc level, Indicating that Chinese resistant accessions are distinct from known resistant accessions. This applied core collection provides a rational framework for undertaking diversity surveys, using genetic variation for the investigation of complex traits and for the discovery of novel traits.     

Assessment of Genetic Diversity in <Emphasis Type="Italic">Secale cereale</Emphasis> Based on SSR Markers

The primary aim of this study was to estimate genetic diversity among Secale cereale L. accessions using 22 previously published simple sequence repeat (SSR) markers. The plant material included 367 rye accessions comprising historical and contemporary cultivars, cultivated materials, landraces, and breeding strains from the Polish breeding company Danko. The studied accessions represented a wide geographical diversity. Several methods were employed to analyze genetic diversity among the Secale cereale L. accessions and to determine population structure: principal coordinate analysis (PCoA), neighbor-joining (NJ), and Bayesian clustering. We also defined a core collection of 25 rye accessions representing over 93 % of SSR alleles. The results of these analyses showed that accessions from the rye gene bank are clearly divergent in comparison with materials received directly from European breeding companies. Our findings suggest also that the genetic pool of current rye cultivars is becoming narrower during breeding processes. The selected panel of SSR markers performed well in detection of genetic diversity patterns and can be recommended for future germplasm characterization studies in rye.     

基于SSR标记的谷子遗传多样性研究

用21个分布在谷子9条染色体上的SSR标记,对120份来自于核心种质的谷子材料进行遗传多样性研究。21个标记共检查出305个等位变异,各标记检测出的等位变异数在3～26个之间,平均每个位点检测出的等位变异数为14.5个;21个位点的平均多态信息量(PIC)为0.809。基于21个SSR标记的分子鉴定,计算了120份材料间的遗传相似系数,其变化范围为0.8393～0.9672,平均值为0.8906。根据计算的遗传距离,对120份谷子材料进行UPGMA聚类,在遗传相似系数0.8865处这些材料被划分为4个类群,分类结果与这些谷子来源地生态类型总体上表现一致,分别为西北内陆类群、黄土高原内蒙古高原类群、华北平原类群以及华北平原近年育成种类群。     

Unravelling genetic diversity and cultivar parentage in the Danish apple gene bank collection

Characterization of apple germplasm is important for conservation management and breeding strategies. A set of 448 Malus domestica accessions, primarily of local Danish origin, were genotyped using 15 microsatellite markers. Ploidy levels were determined by flow cytometry. Special emphasis was given to pedigree reconstruction, cultivar fingerprinting and genetic clustering. A reference set of cultivars, mostly from other European countries, together with a private nursery collection and a small set of Malus sieversii, Malus sylvestris and small-fruited, ornamental Malus cultivars, was also included. The microsatellite markers amplified 17–30 alleles per loci with an average degree of heterozygosity at 0.78. We identified 104 (23%) duplicate genotypes including colour sports. We could infer first-degree relationships for many cultivars with previously unknown parentages. STRUCTURE analysis provided no evidence for a genetic structure but allowed us to present a putative genetic assembly that was consistent with both PCA analysis and parental affiliation. The Danish cultivar collection contains 10% duplicate genotypes including colour sports and 22% triploids. Many unique accessions and considerable genetic diversity make the collection a valuable resource within the European apple germplasm. The findings presented shed new light on the origin of Danish apple cultivars. The fingerprints can be used for cultivar identification and future management of apple genetic resources. In addition, future genome-wide association studies and breeding programmes may benefit from the findings concerning genetic clustering and diversity of cultivars.     

Microsatellite marker-mediated analysis of the EMBRAPA Rice Core Collection genetic diversity

The objectives of this study were to determine the genetic structure of 242 accessions from the EMBRAPA Rice Core Collection (ERiCC), to create a mini-core collection and to develop a multiplex panel of fluorescent labeled simple sequence repeats (SSRs). Eighty-six SSRs were used to identify 1,066 alleles, with an average number of 12.4 alleles/locus and average polymorphism information content (PIC)/locus of 0.75. A model-based clustering method recognized the structure of the accessions on two levels, according to their cultivation system and origin. The most divergent subgroup identified was the worldwide lowland accessions, with the highest values for gene diversity (0.75), average Rogers distance modified by Wright (0.80), average number of alleles/locus (11.7) and private alleles (132). A mini-core was assembled with the most divergent 24 lowland and upland accessions. This mini-core displayed an average distance of 0.86, an average number of alleles/locus of 8.4 and an average PIC/locus of 0.8. From the 86 SSRs, 24 were selected to compose six multiplex panels in order to optimize the process of rice genotyping. This set of markers distinguished all 242 accessions, and showed an average PIC of 0.80 and an average number of alleles/locus of 15.4, higher than the entire set of 86 SSRs. Since the heterogeneity found in lines and cultivars of ERiCC was higher than expected, it is necessary to analyze pooled DNA samples to get a better estimate of genetic variability. The SSR characterization of ERiCC clearly indicates that there is high genetic variability in rice accessions stored in genebanks worldwide which can be promptly explored by rice pre-breeding programs.     

The origin of cultivated <Emphasis Type="Italic">Coffea arabica</Emphasis> L. varieties revealed by AFLP and SSR markers

Molecular markers were used to assess polymorphism between and within the genetic bases of coffee (i.e. Typica and Bourbon) spread from Yemen since the early 18th century that have given rise to most arabica cultivars grown world-wide. Eleven Coffea arabica accessions derived from the disseminated bases were evaluated by amplified fragment length polymorphism (AFLP) using 37 primer combinations and simple-sequence repeats (SSRs) produced by six microsatellites. Four cultivars growing in Yemen and 11 subspontaneous accessions collected in the primary centre of diversity of the species were included in the study in order to define their relationship with the accessions derived from the genetic bases of cultivars. One hundred and seven AFLP markers were used to calculate genetic distances and construct a dendrogram. The accessions derived from the disseminated bases were grouped separately, according to their genetic origin, and were distinguished from the subspontaneous accessions. The Yemen cultivars were classified with the Typica-derived accessions. Except for one AFLP marker, all AFLP and SSR markers present in the cultivated accessions were also detected in the subspontaneous accessions. Polymorphism among the subspontaneous accessions was much higher than among the cultivated accessions. It was very low within the genetic bases, confirming the historical documentation on their dissemination. The results enabled a discussion of the genetic diversity reductions that successively occurred during the dissemination of C. arabica from its primary centre of diversity.     

Genetic structure and core collection of the World Olive Germplasm Bank of Marrakech: towards the optimised management and use of Mediterranean olive genetic resources

The conservation of cultivated plants in ex-situ collections is essential for the optimal management and use of their genetic resources. For the olive tree, two world germplasm banks (OWGB) are presently established, in Córdoba (Spain) and Marrakech (Morocco). This latter was recently founded and includes 561 accessions from 14 Mediterranean countries. Using 12 nuclear microsatellites (SSRs) and three chloroplast DNA markers, this collection was characterised to examine the structure of the genetic diversity and propose a set of olive accessions encompassing the whole Mediterranean allelic diversity range. We identified 505 SSR profiles based on a total of 210 alleles. Based on these markers, the genetic diversity was similar to that of cultivars and wild olives which were previously characterised in another study indicating that OWGB Marrakech is representative of Mediterranean olive germplasm. Using a model-based Bayesian clustering method and principal components analysis, this OWGB was structured into three main gene pools corresponding to eastern, central and western parts of the Mediterranean Basin. We proposed 10 cores of 67 accessions capturing all detected alleles and 10 cores of 58 accessions capturing the 186 alleles observed more than once. In each of the 10 cores, a set of 40 accessions was identical, whereas the remaining accessions were different, indicating the need to include complementary criteria such as phenotypic adaptive and agronomic traits. Our study generated a molecular database for the entire OWGB Marrakech that may be used to optimise a strategy for the management of olive genetic resources and their use for subsequent genetic and genomic olive breeding.     

Genetic variation in Miscanthus × giganteus and the importance of estimating genetic distance thresholds for differentiating clones

Miscanthus × giganteus (Mxg) is an important bioenergy feedstock crop, however, genetic diversity among legacy cultivars may be severely constrained. Only one introduction from Japan to Denmark of this sterile, triploid, vegetatively propagated crop was recorded in the 1930s. We sought to determine if the Mxg cultivars in North America were all synonyms, and if they were derived from the European introduction. We used 64 nuclear and five chloroplast simple sequence repeat (SSR) markers to estimate genetic similarity for 27 Mxg accessions from North America, and compared them with six accessions from Europe, including the species’ type‐specimen. A subset of accessions was also evaluated by restriction‐site associated DNA sequencing (RAD‐seq). In addition, we assessed the potential of new crosses to increase Mxg genetic diversity by comparing eight new triploid Mxg progeny grown from seed, along with samples of the parental species M. sacchariflorus and M. sinensis. Estimates of genotyping error rates were essential for distinguishing between experimental error and true genotypic differences among accessions. Given differences in estimated error rates and costs per marker for SSRs and RAD‐seq, the former is currently more cost‐effective for determining if two accessions are genetically identical. We concluded that all of the Mxg legacy cultivars were derived via vegetative propagation from a single genet. In contrast with the Mxg legacy cultivars, genetic similarity to the type‐specimen of eight new triploid Mxg progeny ranged from 0.46 to 0.56. Though genetic diversity among the Mxg legacy cultivars is critically low, new crosses can provide much‐needed variation to growers.     

Comparative analysis on the genetic relatedness of Sorghum bicolor accessions from Southern Africa by RAPDs,AFLPs and SSRs

In order to get an overview on the genetic relatedness of sorghum (Sorghum bicolor) landraces and cultivars grown in low-input conditions of small-scale farming systems, 46 sorghum accessions derived from Southern Africa were evaluated on the basis of amplified fragment length polymorphism (AFLPs), random amplified polymorphic DNAs (RAPDs) and simple sequence repeats (SSRs). By this approach all sorghum accessions were uniquely fingerprinted by all marker systems. Mean genetic similarity was estimated at 0.88 based on RAPDs, 0.85 using AFLPs and 0.31 based on SSRs. In addition to this, genetic distance based on SSR data was estimated at 57 according to a stepwise mutation model (Deltamu-SSR). All UPGMA-clusters showed a good fit to the similarity estimates (AFLPs: r = 0.92; RAPDs: r = 0.88; SSRs: r = 0.87; Deltamu-SSRs: r = 0.85). By UPGMA-clustering two main clusters were built on all marker systems comprising landraces on the one hand and newly developed varieties on the other hand. Further sub-groupings were not unequivocal. Genetic diversity (H, DI) was estimated on a similar level within landraces and breeding varieties. Comparing the three approaches to each other, RAPD and AFLP similarity indices were highly correlated (r = 0.81), while the Spearman's rank correlation coefficient between SSRs and AFLPs was r = 0.57 and r = 0.51 between RAPDs and SSRs. Applying a stepwise mutation model on the SSR data resulted in an intermediate correlation coefficient between Deltamu-SSRs and AFLPs (r = 0.66) and RAPDs ( r = 0.67), respectively, while SSRs and Deltamu-SSRs showed a lower correlation coefficient (r = 0.52). The highest bootstrap probabilities were found using AFLPs (56% on average) while SSR, Deltamu-SSR and RAPD-based similarity estimates had low mean bootstrap probabilities (24%, 27%, 30%, respectively). The coefficient of variation (CV) of the estimated genetic similarity decreased with an increasing number of bands and was lowest using AFLPs.     

Genetic analysis of Spanish melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.) germplasm using a standardized molecular-marker array and geographically diverse reference accessions

Genetic relationships among 125 Spanish melon (Cucumis melo L.) accessions from a Spanish germplasm collection were assessed using a standard molecular-marker array consisting of 34 random amplified polymorphic DNA (RAPD) markers bands (19 primers) and 72 reference accessions drawn from previous studies. The reference accession array consisted of a broad range [Japanese (19) Crete (17), African (15), and USA and Europe (US/EU, 21)] of horticultural groupings (Group Cantalupensis, Group Conomon, Group Inodorus, Group Flexuosus, and Group Chito), and of melon market classes (e.g., Charentais, U.S. Western and European Shipper types, Ogen, and Galia, Honeydew, and Casaba). Spanish melon accessions (largely Casaba, Group Inodorus) were genetically distinct from the reference accessions and other Group Inodorus melons of different origins. Most African accessions showed common genetic affinities, and grouped with the Group Chito and the Group Conomon accessions examined. Those accession groupings were distinct from all other accessions belonging to Group Cantalupensis, Flexuosus, and Inodorus accessions originating from Crete, Japan, Europe, and the U.S. Genetic diversity was highest in accessions of African origin and lowest in accessions of Spanish origin. Additional RAPD markers (49 primers, 141 bands) and 22 selected agronomic traits (quantitative and qualitative) were then used to assess the genetic diversity among Spanish accessions. While cluster analysis using fruit characteristics grouped accessions into cultivars, RAPD-based genetic-distance estimate did not provide consistent accession groupings either by cultivar or geographic origin. While the highest level of polymorphism was detected among melons originating from the central region of Spain, and in the Rochet cultivar, accessions from the Andalucía region and Green cultivars were comparatively less diverse. These results indicate that the Spanish melon accessions could be used to broaden the genetic base of local and foreign Casaba germplasm, to enhance the genetic diversity of U.S and European commercial melon germplasm, and to delineate collection strategies for acquisition of additional Spanish landraces.Communicated by C. MöllersMention of trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the USDA and does not imply its approval to the exclusion of other products that may be suitable.     

Molecular genetic diversity of the Turkish national hazelnut collection and selection of a core set

European hazelnut (Corylus avellana L.) is an economically and nutritionally important nut crop with wild and cultivated populations found throughout Europe and in parts of Asia. This study examined the molecular genetic diversity and population structure of 402 genotypes including 143 wild individuals, 239 landraces, and 20 cultivars from the Turkish national hazelnut collection using simple sequence repeat (SSR) markers. A total of 30 SSR markers yielded 407 polymorphic fragments. Diversity analysis of the Turkish hazelnut genotypes indicated that they fell into three subpopulations according to ad hoc statistics and neighbor-joining algorithm. Although all cultivars clustered together, they overlapped with the wild accessions and landraces. Thus, the dendrogram, principal coordinate, and population structure analyses suggest that they share the same gene pool. A total of 78 accessions were selected as a core set to encompass the molecular genetic and morphological diversity present in the national collection. This core set should have priority in preservation efforts and in trait characterization.     

Genetic Diversity,Population Structure,and Resistance to Phytophthora capsici of a Worldwide Collection of Eggplant Germplasm

Eggplant (Solanum melongena L.) is an important solanaceous crop with high phenotypic diversity and moderate genotypic diversity. Ninety-nine genotypes of eggplant germplasm (species (S. melongena, S. incanum, S. linnaeanum and S. gilo), landraces and heirloom cultivars) from 32 countries and five continents were evaluated for genetic diversity, population structure, fruit shape, and disease resistance to Phytophthora fruit rot. Fruits from each line were measured for fruit shape and evaluated for resistance to two Phytophthora capsici isolates seven days post inoculation. Only one accession (PI 413784) was completely resistant to both isolates evaluated. Partial resistance to Phytophthora fruit rot was found in accessions from all four eggplant species evaluated in this study. Genetic diversity and population structure were assessed using 22 polymorphic simple sequence repeats (SSRs). The polymorphism information content (PIC) for the population was moderate (0.49) in the population. Genetic analyses using the program STRUCTURE indicated the existence of four genetic clusters within the eggplant collection. Population structure was detected when eggplant lines were grouped by species, continent of origin, country of origin, fruit shape and disease resistance.