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Integrated Consensus Map of Cultivated Peanut and Wild Relatives Reveals Structures of the A and B Genomes of Arachis and Divergence of the Legume Genomes
Abstract and Figures
The complex, tetraploid genome structure of peanut (Arachis hypogaea) has obstructed advances in genetics and genomics in the species. The aim of this study is to understand the genome structure of Arachis by developing a high-density integrated consensus map. Three recombinant inbred line populations derived from crosses between the A genome diploid species, Arachis duranensis and Arachis stenosperma; the B genome diploid species, Arachis ipaënsis and Arachis magna; and between the AB genome tetraploids, A. hypogaea and an artificial amphidiploid (A. ipaënsis × A. duranensis)(4×), were used to construct genetic linkage maps: 10 linkage groups (LGs) of 544 cM with 597 loci for the A genome; 10 LGs of 461 cM with 798 loci for the B genome; and 20 LGs of 1442 cM with 1469 loci for the AB genome. The resultant maps plus 13 published maps were integrated into a consensus map covering 2651 cM with 3693 marker loci which was anchored to 20 consensus LGs corresponding to the A and B genomes. The comparative genomics with genome sequences of Cajanus cajan, Glycine max, Lotus japonicus, and Medicago truncatula revealed that the Arachis genome has segmented synteny relationship to the other legumes. The comparative maps in legumes, integrated tetraploid consensus maps, and genome-specific diploid maps will increase the genetic and genomic understanding of Arachis and should facilitate molecular breeding.
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Supplementary resource (1)
Data
January 2013
... Such markers are especially helpful in the mapping of stress-related genes' QTLs (quantitative trait loci) (Younis et al. 2020). When dealing with quantitative features like HT, molecular markers that allow for the exact and time-saving recovery of desired genotypes become essential (Shirasawa et al. 2013). ...
... The grain quality of the NILs was much better than the susceptible parent 'Tohoku 168'. The chromosomal region in question had a strong QTL that influenced more than 20% of the PV and was responsible for higher grain quality under HS (Shirasawa et al. 2013). Table 2.1 contains a non-exhaustive list of DNA markers related to various HT/component features found in various crops. ...
... SNPs qREC-1-1,qREC-1-2, qREC-1-3,qREC-2-1, qREC-3-1,qREC-9-1, qREC-12-1,qCC-1-4, qCC-1-5,qCC-2-2, qFv/Fm ─ 5-1, andqFv/Fm 12-2 Singh et al. (2017) The near isogenic lines (NILs) developed by introgressing desirable alleles into the heat-sensitive lines showed a considerable reduction in the heat-induced injuries (Kobayashi et al. 2013). Recently, a major QTL controlling better grain quality under heat stress has been transferred from "Kokoromachi" to "Tohoku 168" using marker-assisted backcrossing (Shirasawa et al. 2013). The developed NILs had better grain quality with better heat tolerance over the susceptible parent. ...
This chapter addresses the adverse effect of heat stress on plant growth, genes associated with heat stress tolerance and adaptive strategies that can be used to create heat-tolerant plants. CRISPR/Cas9 seems a promising approach regarding stress tolerance. The modified versions of CRISPR/Cas9 like CRISPRi, CRISPRa, base editing and CRISPR multiplexing offers more and more specificity and advanced editing options and minimizes the off-target effect. The versatility of CRISPR/Cas9 brings a new revolution in the field of plant science to alleviate abiotic stress like heat stress.KeywordsHeat stressCRISPR/Cas 9CRISPRiCRISPRaBase editingCRISPR multiplexing
... This low level of diversity has been a major impediment for mining a large number of polymorphic markers for constructing high-density genetic maps and QTL analyses (Pandey et al. 2017;Qin et al. 2012;Varshney et al. 2014). To date, only a few genetic maps containing more than 1,000 SSR markers have been reported (Huang et al. 2016;Shirasawa et al. 2013). ...
Key message
We identified two stable and homologous major QTLs for sucrose content in peanut, and developed breeder-friendly molecular markers for marker-assisted selection breeding.
Abstract
Sucrose content is a crucial quality trait for edible peanuts, and increasing sucrose content is a key breeding objective. However, the genetic basis of sucrose content in peanut remains unclear, and major quantitative trait loci (QTLs) for sucrose content have yet to be identified. In this study, a high-density genetic map was constructed based on whole-genome re-sequencing data from a peanut RIL population. This map consisted of 2,042 bins and 24,142 SNP markers, making it one of the most comprehensive maps to date in terms of marker density. Two major QTLs (qSCA06.2 and qSCB06.2) were identified, explaining 31.41% and 24.13% of the phenotypic variance, respectively. Notably, these two QTLs were located in homologous genomic regions between the A and B subgenomes. The elite allele of qSCA06.2 was exclusive to Valencia-type, while the elite allele of qSCB06.2 existed in other peanut types. Importantly, the distribution of alleles from two homologous QTLs in the RIL population and diverse germplasm accessions consistently demonstrated that only the combination of elite allelic genotypes from both QTLs/genes resulted in a significantly dominant phenotype, accompanied by a substantial increase in sucrose content. The newly developed diagnostic markers for these QTLs were confirmed to be reliable and could facilitate future breeding efforts to enhance sucrose content using marker-assisted selection techniques. Overall, this study highlights the co-regulation of sucrose content by two major homologous QTLs/genes and provides valuable insights into the genetic basis of sucrose in peanuts.
... SSR markers were concluded to be useful for peanut germplasm analysis, diversity studies, linkage mapping, and phylogenetic relationships (Cuc et al. 2008, Hong et al. 2021. Two integrated consensus genetic maps with markers, such as EST-SSRs, transcriptome-SSRs, and g-SSRs, were constructed for cultivated peanut and wild relatives (Shirasawa et al. 2013, Lu et al. 2018). Additionally, a highly informative set of SSR markers was used to screen parental accessions of peanut mapping populations for molecular breeding (Pandey et al. 2012) and SSR markers and intron sequences were used to study phylogenetic relationships of cultivated peanut and wild species of Arachis . ...
... This helps to identify true QTLs with their genetic effects and mining a large number of candidate genes located in that QTL (Shinozuka et al. 2012;Zhang et al. 2014). The basic requirements for the execution of QTL meta-analysis are: a consensus map with high-density markers (Lu et al. 2018;Hong et al. 2010;Shirasawa et al. 2013) and many QTLs controlling single trait identified under different genetic and environment backgrounds (Goffinet and Gerber 2000). For efficient introgression of MQTLs in MAS, it should have smallest CIs with consistent and large effect for the trait. ...
A large number of QTLs controlling important traits in different crops have been mapped but their deployment in the marker assisted selection (MAS) is highly limited. The possible reason is lack of availability of stable QTLs over diverse genetic background and environment. QTL meta-analysis provides an opportunity to compile QTLs from different studies and identify consistent QTLs across genetic background and environments. These meta-QTLs can be targeted for further fine mapping and used for MAS after validation. QTL meta-analysis involve library search for reported QTL studies for a particular trait in a crop and subsequently a consensus linkage map is constructed projecting QTLs from different studies using simple scaling rule. The Confidence Interval (CI) of QTL is estimated on the consensus map using the formula as CI (95): ¼ 530/(Nλ), where CI (95) is the confidence interval at 95 percent probability level, N is the size of mapping population and λ is the proportion of phenotypic variance explained by the concerned QTL. Finally, detected QTLs are projected for cluster analysis to determine the exact number and positions of true QTLs. The utility of this approach will help to detect the stable meta-QTLs that will be further helpful to identify candidate genes and their application in MAS program.
... LGs. Later, with 897 marker loci (895 SSRs and 2 CAPS) mapped on 20 linkage groups spanning a total map distance of 3607.97 cM, followed by 3693 marker loci mapped on 20 LG with total map distance spanning 2651 cM (Gautami et al., 2012b;Shirasawa et al., 2013). ...
Groundnut is an oilseed crop with global importance to food and nutritional security. It is a source of livelihood for millions of smallholder growers of the Asia and Sub-Saharan Africa. However, present climate change accompanied with rising CO2 levels, erratic rainfall, and rising and fluctuating atmospheric temperature accompanied with various biotic constraints limits yield potential and quality of produce. In addition, genetic architecture and tetraploid nature of the cultivated peanut have resulted in low genetic diversity for several economically important traits. Despite, significant achievement in yield, quality improvement and tolerance against biotic and abiotic factors have been made by conventional approaches but the process is time-consuming, laborious, and have other limitations. Recent advances in genomics coupled with genetic resources have raised the status of peanut to “genomic resource-rich oilseed crop.” So, future use of a holistic approach including the use of genomics information and tools in crop improvement programs is very much essential for further achieving advancement in peanut yield. Among all the genomic tools, molecular markers are the most valuable in characterizing and harnessing available genetic variability. Moreover, even molecular breeding in peanuts is in the nascent stage, and identification of some major quantitative trait locus (QTLs) for only a few traits has been reported. Nevertheless, availability of reference genome of diploid progenitors and allotetraploid cultivar, a high-density genotyping array with 58K single nucleotide polymorphisms (SNPs) has greatly facilitated high-resolution mapping in peanuts. Affordability of sequencing also encouraged initiation of sequence-based trait mapping such as QTL-seq for dissecting economically important traits. Few successful examples are available in peanuts regarding the development of diagnostic markers and their deployment in crop breeding programs. The availability of different cost-effective genomic tools (SNPs, whole genome sequencing, KASPar, GBS, etc.) and more powerful multi-parent mapping populations are expected to speed up the mapping of complex traits of groundnut in the coming years. This chapter provides an overview of the current developments and future prospects of molecular marker development and their applications in groundnut crops.
... 14 Genomics-Enabled Breeding for Heat and Drought Stress Tolerance in Crop Plants 309 The near isogenic lines (NILs) developed by introgressing desirable alleles into the heat-sensitive lines showed a considerable reduction in the heat-induced injuries (Kobayashi et al. 2013). Recently, a major QTL controlling better grain quality under heat stress has been transferred from "Kokoromachi" to "Tohoku 168" using marker-assisted backcrossing (Shirasawa et al. 2013). The developed NILs had better grain quality with better heat tolerance over the susceptible parent. ...
High temperature is one of the major abiotic stresses causing huge yield losses in all crop plants. The challenges posed by global warming are the major antagonistic factors to realize seed yield potential of a genotype. There is a need to generate allelic variation in the existing gene pool for high-temperature tolerance. Induced mutagenesis holds great potential to cause lesions ranged from single base pair to large deletions resulting into development of spectrum of new gene combinations for high temperature tolerance. Advances in scientific methods, especially related to quantifying existing thermotolerance at seedling and reproductive stages, understanding the function of each genetic loci and their position on a chromosome, and deciphering biochemical pathways to analyze the effect of these genetic loci made it possible to measure genetic value of the mutant genes. Substantial efforts have been directed to generate variability in cereal crops such as wheat, rice, maize, and barley in the coded fraction of genome for heat stress tolerance which was exploited to decipher functional characterization of genetic loci at morphological, physiological, biochemical, and molecular levels as well as direct improvement of crop cultivars for warm locations. In wheat; mutations for stay green, thousand kernel weight, small heat shock protein, and stable meiosis; in rice; spikelet fertility, characters at seedling and reproductive stage, chlorophyllide a oxygenase; in maize; EF-Tu factor; in tomato; MAPK gene and mutations for brassinosteroids in barley have been found useful to develop heat-tolerant crop plants. A total of 14 heat-tolerant varieties have been developed through mutation breeding. Besides, precise mutagenesis techniques such as TILLING and CRISPR-cas9 have been found to be useful in developing heat-tolerant crop plants.KeywordsHigh-temperature toleranceMutantMutation breedingStay green mutantsHSPEF-Tu factorMAPKBrassinosteroidsWheatRiceMaizeBarleyTomatoTILLINGCRISPR
The root-knot nematodes (RKN) ( Meloidogyne graminicola ) are a devastating threat to rice worldwide. The cultivated germplasm is either susceptible or moderately resistant to rice RKN. Therefore, there is a need to identify resistance sources against M. graminicola as an eco-friendly management strategy. The present study evaluated the host response of Oryza sativa genotypes comprising basmati, non-basmati improved varieties, their advanced breeding lines (83) and Oryza glaberrima accessions (42) against M. graminicola in the nematode-infested plot for two consecutive years. All O. sativa genotypes exhibited susceptible responses, while O. glaberrima accessions showed variable levels of resistance. Three of the O. glaberrima accessions (IRGC102196, IRGC102538 and IRGC102557) were highly resistant. M. graminicola significantly affected plant growth parameters in susceptible genotypes compared to resistant O. glaberrima accessions. The results were supported by histopathological studies that showed apparent giant cell formation in PR121 while penetration and development of M. graminicola juveniles were low in the O. glaberrima acc. IRGC102196. In silico analysis indicated that none of the reported nematode resistance genes from different crops had homology with the rice genome. The two anti-nematode genes ( Oryzacystatin-I and Oryzacystatin-II ) from O. sativa japonica revealed homology with O. sativa cv. PR121 and O. glaberrima acc. IRGC102206. Comparative analysis of these genes between PR121 and O. glaberrima acc. IRGC102206 resulted in the identification of SNPs/InDels that could be associated with nematode resistance. The identified SNPs/InDels could be validated, and further molecular studies are needed to provide insights into the resistance mechanism against rice RKN.
Forage peanut (Arachis pintoi) is an important leguminous forage that has gained popularity due to increased livestock productivity. Furthermore, the species helps with soil fertility and the restoration of degraded areas. However, A. pintoi has a limited number of molecular markers. The objective of this study was to create and characterize gene-derived microsatellite markers as well as to test their transferability to the peanut (Arachis hypogaea) and other six wild Arachis species. A total of 4461 putative simple sequence repeats (SSR) were identified, and PCR primer pairs were designed for 999 SSR regions after filtering out primers with the same annealing site and searching for sequences related to open reading frames (ORFs). The dinucleotide motif was the most common (628; 62.86%). For validation, 186 primer pairs were chosen at random, of which 63 (33.87%) were polymorphic, with an average of 7.37 alleles per locus. Polymorphic information content (PIC = 0.70) and discriminatory power (D = 0.80) were both high on average. The functional annotation discovered 120 sequences that were assigned to 87 gene ontology functional groups divided into three main categories: molecular function (27 sub-categories), cellular components (21 sub-categories), and biological process (39 sub-categories). Thirty-three SSRs were tested for transferability to peanut and six other wild Arachis species, resulting in variable cross-species amplification (63.64 to 100%). Here, we present the first gene-derived SSR for A. pintoi. These new informative microsatellites may be linked to agronomically important genes to be used in genetic studies.
Groundnut improvement task are generally engrossed to breed foliar fungal diseases resistant varieties. Early and late leaf spot diseases of groundnut cause significant loss in yield and quality of groundnut seeds. Simple Sequence Repeat (SSR) markers represents high polymorphic amplification, while Inter-simple sequence (ISSR) is being used widely for diversity assessment due to their large size and reproducible results. Current investigation was carried out with 96 groundnut genotypes for morpho-physiological characterization, disease indexing for early and late leaf spot diseases under field conditions and by employing SSR and ISSR markers for diversity assessment. In correlation analysis, kernel weight was found positively and highly significantly correlated with harvest index and pod weight (r = 0.906) at 1% level of significance. While, pod weight is negatively correlated with early leaf spot at 30 days (r = -0.401) and at 45 days (r = -0.410), late leaf spot at 70 days (r = -0.342) and at 85 days (r = -0.309) at 1% significant level. With the use of SSR markers, total 16 alleles were recognized with a mean of 3.20 alleles per locus. The gene multiplicity differed from 0.6074 to 0.6491 for the markers Seq5D05 and PM137 respectively with an average of 0.6289. Total 18 alleles were produced by ISSR markers with an average of 3.6 alleles per locus. In our study, the gene diversity varied from 0.5877 to 0.6625 for the markers ISSR3 and ISSR21 respectively with an average of 0.6164. Diverse group of groundnut germplasm has been identified on the basis of SSR and ISSR markers along with morphological characterization.
Peanut (Arachis hypogaea L.) is an allotetraploid (2n = 4x = 40) that has been cultivated for thousands of years. Areas of production range from subsistence farming to large scale commercial operations and are in all continents except Antarctica. Total worldwide peanut production is approximately 45 million MT per year, with an average yield of 2,600kg/ha. The genus Arachis contains 82 described species that contain a vast reservoir of useful traits that are in some cases not readily accessible. One or more chromosome doubling events formed A. hypogaea effectively isolating the cultivated peanut (2n=4x=40) from its wild relatives (2n=2x=20). This type of reproductive isolation created a significant genetic bottleneck that has resulted in a narrow genetic base in A. hypogaea and left the cultigen without access to many of the alleles needed for resistance to many biotic and abiotic stresses. However, these can still be found in the related germplasm. Based on the importance of this crop to many areas of the world, this article will review some of unique taxonomic characteristics of the genus and how it has led to the susceptibility to both biotic and abiotic stressors. It will also discuss how cultivated peanut can be enhanced using exotic and wild germplasm including some of the methods that have been used successfully in the past as well as discuss several examples of how exotic and wild germplasm has been used successfully to incorporate biotic resistance traits.
Ore mineral and host lithologies have been sampled with 89 oriented samples from 14 sites in the Naica District, northern Mexico. Magnetic parameters permit to charac- terise samples: saturation magnetization, density, low- high-temperature magnetic sus- ceptibility, remanence intensity, Koenigsberger ratio, Curie temperature and hystere- sis parameters. Rock magnetic properties are controlled by variations in titanomag- netite content and hydrothermal alteration. Post-mineralization hydrothermal alter- ation seems the major event that affected the minerals and magnetic properties. Curie temperatures are characteristic of titanomagnetites or titanomaghemites. Hysteresis parameters indicate that most samples have pseudo-single domain (PSD) magnetic grains. Alternating filed (AF) demagnetization and isothermal remanence (IRM) ac- quisition both indicate that natural and laboratory remanences are carried by MD-PSD spinels in the host rocks. The trend of NRM intensity vs susceptibility suggests that the carrier of remanent and induced magnetization is the same in all cases (spinels). The Koenigsberger ratio range from 0.05 to 34.04, indicating the presence of MD and PSD magnetic grains. Constraints on the geometry of the intrusive source body devel- oped in the model of the magnetic anomaly are obtained by quantifying the relative contributions of induced and remanent magnetization components.
Background
Cultivated peanut or groundnut (Arachis hypogaea L.) is an important oilseed crop with an allotetraploid genome (AABB, 2n = 4x = 40). Both the low level of genetic variation within the cultivated gene pool and its polyploid nature limit the utilization of molecular markers to explore genome structure and facilitate genetic improvement. Nevertheless, a wealth of genetic diversity exists in diploid Arachis species (2n = 2x = 20), which represent a valuable gene pool for cultivated peanut improvement. Interspecific populations have been used widely for genetic mapping in diploid species of Arachis. However, an intraspecific mapping strategy was essential to detect chromosomal rearrangements among species that could be obscured by mapping in interspecific populations. To develop intraspecific reference linkage maps and gain insights into karyotypic evolution within the genus, we comparatively mapped the A- and B-genome diploid species using intraspecific F2 populations. Exploring genome organization among diploid peanut species by comparative mapping will enhance our understanding of the cultivated tetraploid peanut genome. Moreover, new sources of molecular markers that are highly transferable between species and developed from expressed genes will be required to construct saturated genetic maps for peanut.
Results
A total of 2,138 EST-SSR (expressed sequence tag-simple sequence repeat) markers were developed by mining a tetraploid peanut EST assembly including 101,132 unigenes (37,916 contigs and 63,216 singletons) derived from 70,771 long-read (Sanger) and 270,957 short-read (454) sequences. A set of 97 SSR markers were also developed by mining 9,517 genomic survey sequences of Arachis. An SSR-based intraspecific linkage map was constructed using an F2 population derived from a cross between K 9484 (PI 298639) and GKBSPSc 30081 (PI 468327) in the B-genome species A. batizocoi. A high degree of macrosynteny was observed when comparing the homoeologous linkage groups between A (A. duranensis) and B (A. batizocoi) genomes. Comparison of the A- and B-genome genetic linkage maps also showed a total of five inversions and one major reciprocal translocation between two pairs of chromosomes under our current mapping resolution.
Conclusions
Our findings will contribute to understanding tetraploid peanut genome origin and evolution and eventually promote its genetic improvement. The newly developed EST-SSR markers will enrich current molecular marker resources in peanut.
Background
Cultivated peanut (Arachis hypogaea) is an allotetraploid species whose ancestral genomes are most likely derived from the A-genome species, A. duranensis, and the B-genome species, A. ipaensis. The very recent (several millennia) evolutionary origin of A. hypogaea has imposed a bottleneck for allelic and phenotypic diversity within the cultigen. However, wild diploid relatives are a rich source of alleles that could be used for crop improvement and their simpler genomes can be more easily analyzed while providing insight into the structure of the allotetraploid peanut genome. The objective of this research was to establish a high-density genetic map of the diploid species A. duranensis based on de novo generated EST databases. Arachis duranensis was chosen for mapping because it is the A-genome progenitor of cultivated peanut and also in order to circumvent the confounding effects of gene duplication associated with allopolyploidy in A. hypogaea.
Results
More than one million expressed sequence tag (EST) sequences generated from normalized cDNA libraries of A. duranensis were assembled into 81,116 unique transcripts. Mining this dataset, 1236 EST-SNP markers were developed between two A. duranensis accessions, PI 475887 and Grif 15036. An additional 300 SNP markers also were developed from genomic sequences representing conserved legume orthologs. Of the 1536 SNP markers, 1054 were placed on a genetic map. In addition, 598 EST-SSR markers identified in A. hypogaea assemblies were included in the map along with 37 disease resistance gene candidate (RGC) and 35 other previously published markers. In total, 1724 markers spanning 1081.3 cM over 10 linkage groups were mapped. Gene sequences that provided mapped markers were annotated using similarity searches in three different databases, and gene ontology descriptions were determined using the Medicago Gene Atlas and TAIR databases. Synteny analysis between A. duranensis, Medicago and Glycine revealed significant stretches of conserved gene clusters spread across the peanut genome. A higher level of colinearity was detected between A. duranensis and Glycine than with Medicago.
Conclusions
The first high-density, gene-based linkage map for A. duranensis was generated that can serve as a reference map for both wild and cultivated Arachis species. The markers developed here are valuable resources for the peanut, and more broadly, to the legume research community. The A-genome map will have utility for fine mapping in other peanut species and has already had application for mapping a nematode resistance gene that was introgressed into A. hypogaea from A. cardenasii.
Late leaf spot (LLS) and rust have the greatest impact on yield losses worldwide in groundnut (Arachis hypogaea L.). With the objective of identifying tightly linked markers to these diseases, a total of 3,097 simple sequence repeats (SSRs) were screened on the parents of two recombinant inbred line (RIL) populations, namely TAG 24 × GPBD 4 (RIL-4) and TG 26 × GPBD 4 (RIL-5), and segregation data were obtained for 209 marker loci for each of the mapping populations. Linkage map analysis of the 209 loci resulted in the mapping of 188 and 181 loci in RIL-4 and RIL-5 respectively. Using 143 markers common to the two maps, a consensus map with 225 SSR loci and total map distance of 1,152.9 cM was developed. Comprehensive quantitative trait locus (QTL) analysis detected a total of 28 QTL for LLS and 15 QTL for rust. A major QTL for LLS, namely QTLLLS01 (GM1573/GM1009-pPGPseq8D09), with 10.27–62.34% phenotypic variance explained (PVE) was detected in all the six environments in the RIL-4 population. In the case of rust resistance, in addition to marker IPAHM103 identified earlier, four new markers (GM2009, GM1536, GM2301 and GM2079) showed significant association with the major QTL (82.96% PVE). Localization of 42 QTL for LLS and rust on the consensus map identified two candidate genomic regions conferring resistance to LLS and rust. One region present on linkage group AhXV contained three QTL each for LLS (up to 67.98% PVE) and rust (up to 82.96% PVE). The second candidate genomic region contained the major QTL with up to 62.34% PVE for LLS. Molecular markers associated with the major QTL for resistance to LLS and rust can be deployed in molecular breeding for developing groundnut varieties with enhanced resistance to foliar diseases.
Electronic supplementary material
The online version of this article (doi:10.1007/s11032-011-9661-z) contains supplementary material, which is available to authorized users.
The sexual transfer of genes between taxa possessing different structural karyotypes must involve the passage of genes through a chromosomal sterility barrier. Yet little is known about the effects of structural differences on gene introgression within or adjacent to the rearranged chromosomal fragments or about the patterns of introgression in collinear regions. Here, we employ 197 mapped molecular markers to study the effects of chromosomal structural differences on introgression in backcrossed progeny of the domesticated sunflower, Helianthus annuus, and its karyotypically divergent wild relative, H. petiolaris. Forty percent of the genome from the seven collinear linkages introgressed, whereas only 2.4% of the genome from the 10 rearranged linkages was transferred. Thus, chromosomal rearrangements appear to provide an effective mechanism for reducing or eliminating introgression in rearranged chromosomal segments. On the other hand, observations that 60% of the markers from within the collinear portion of the genome did not introgress suggests that genic factors also resist introgression in Helianthus. That is, selection against H. petiolaris genes in concert with linkage may have reduced or eliminated parts of the genome not protected by structural changes. Thus, barriers to introgression in Helianthus appear to include both chromosomal structural and genic factors.
An RFLP linkage map of peanut has been developed for use in genetic studies and breeding programs aimed at improving the cultivated species (Arachis hypogaea L.). An F2 population derived from the interspecific hybridization of two related diploid species in the sectionArachis (A. stenosperma ×A. cardenasii) was used to construct the map. Both random genomic and cDNA clones were used to develop the framework of the map. In addition, three cDNA clones representing genes coding for enzymes involved in the lipid biosynthesis pathway have been mapped in peanut. Of the 100 genomic and 300 cDNA clones evaluated, 15 and 190, respectively, revealed polymorphisms among the parents of our mapping population. Unfortunately, a large number of these produced complex banding patterns that could not be mapped. Of the 132 markers analyzed for segregation, 117 are distributed among 11 linkage groups, while 15 have not yet been associated with any other marker. A total map distance of approximately 1063 cM has been covered to-date.
Arachis glandulifera Stalker is a diploid (2n = 2x = 20) taxon in section Arachis native to eastern Bolivia. Plants of A. glandulifera have longer lateral branches than other taxa of section Arachis, an upright mainstem, prostrate lateral branches, and larger flowers and seeds than other wild species in the section. The pods are greatly reticulated. Glandular trichomes are present on vegetative plant parts and the peg. Intraspecific hybrids among four accessions are fertile and uniformly have ten bivalents in pollen mother cells. Three accessions had nearly identical karyotypes, while a fourth had subtelocentric chromosomes 6 and 9. Hybrids between A. glandulifera and two other diploid species of section Arachis were male-sterile, and chiasmata frequencies ranged between 5.8 and 12.1 per cell. Attempts to hybridize the species with A. hypogaea failed. A new species description and D genomic classification are proposed for A. glandulifera, which is different from previously described A and B genomes of section Arachis.
