Molecular mapping of four ovule lethal mutants in soybean

Morphological Characterization and Integrated Transcriptome and Proteome Analysis of Organ Development Defective 1 (odd1) Mutant in Cucumis sativus L

Cucumber (Cucumis sativus L.) is an economically important vegetable crop with the unique growth habit and typical trailing shoot architecture of Cucurbitaceae. Elucidating the regulatory mechanisms of growth and development is significant for improving quality and productivity in cucumber. Here we isolated a spontaneous cucumber mutant organ development defective 1 (odd1) with multiple morphological changes including root, plant stature, stem, leaf, male and female flowers, as well as fruit. Anatomical and cytological analyses demonstrated that both cell size and number decreased, and the shoot apical meristem (SAM) was smaller in odd1 compared with WT. Pollen vigor and germination assays and cross tests revealed that odd1 is female sterile, which may be caused by the absence of ovules. Genetic analysis showed that odd1 is a recessive single gene mutant. Using the MutMap strategy, the odd1 gene was found to be located on chromosome 5. Integrated profiling of transcriptome and proteome indicated that the different expression genes related to hormones and SAM maintenance might be the reason for the phenotypic changes of odd1. These results expanded the insight into the molecular regulation of organ growth and development and provided a comprehensive reference map for further studies in cucumber.

The endogenous transposable element Tgm9 is suitable for generating knockout mutants for functional analyses of soybean genes and genetic improvement in soybean

In soybean, variegated flowers can be caused by somatic excision of the CACTA-type transposable element Tgm9 from Intron 2 of the DFR2 gene encoding dihydroflavonol-4-reductase of the anthocyanin pigment biosynthetic pathway. DFR2 was mapped to the W4 locus, where the allele containing Tgm9 was termed w4-m. In this study we have demonstrated that previously identified morphological mutants (three chlorophyll deficient mutants, one male sterile-female fertile mutant, and three partial female sterile mutants) were caused by insertion of Tgm9 following its excision from DFR2. Analyses of Tgm9 insertion sites among 105 independent mutants demonstrated that Tgm9 hops to all 20 soybean chromosomes from its original location on Chromosome 17. Some genomic regions are prone to increased Tgm9-insertions. Tgm9 transposed over 25% of the time into exon or intron sequences. Tgm9 is therefore suitable for generating an indexed insertional mutant collection for functional analyses of most soybean genes. Furthermore, desirable Tgm9-induced stable knockout mutants can be utilized in generating improved traits for commercial soybean cultivars.

Transposon Tagging of a Male-Sterility, Female-Sterility Gene, St8, Revealed that the Meiotic MER3 DNA Helicase Activity Is Essential for Fertility in Soybean

The W4 locus in soybean encodes a dihydroflavonol-4-reductase (DFR2) that regulates pigmentation patterns in flowers and hypocotyl. The mutable w4-m allele that governs variegated flowers has arisen through insertion of a CACTA-type transposable element, Tgm9, in DFR2. In the w4-m line, reversion from variegated to purple flower indicates excision of Tgm9, and its insertion at a new locus. Previously, we have identified a male-sterile, female-sterile mutant among the selfed progenies of a revertant plant carrying only purple flowers. Co-segregation between Tgm9 and the sterility phenotype suggested that the mutant was generated by insertion of Tgm9 at the St8 locus. The transposon was localized to exon 10 of Glyma.16G072300 that shows high identity to the MER3 DNA helicase involved in crossing over. Molecular analysis of fertile branches from two independent revertant plants confirmed precise excision of Tgm9 from the st8 allele, which restored fertility. In soybean, the gene is expressed in flower-buds, trifoliate leaves and stem. Phylogenetic analysis placed St8 in a clade with the Arabidopsis and rice MER3 suggesting that St8 is most likely the orthologous MER3 soybean gene. This study established the utility of Tgm9 in gene identification as well as in forward and reverse genetics studies.

A candidate male-fertility female-fertility gene tagged by the soybean endogenous transposon, Tgm9

In soybean, the W4 gene encoding dihydroflavonol-4-reductase controls anthocyanin pigment biosynthesis in flowers. The mutant allele, w4-m, is characterized by variegated flowers and was evolved from the insertion of an endogenous transposable element, Tgm9, in intron II of the W4 gene. In the w4-m mutant line, reversion of the unstable allele from variegated to normal purple flower in revertants would indicate Tgm9's excision accompanied by its insertion into a second locus. We identified a male-sterile, female-sterile mutant from such germinal revertant bearing purple flowers. The objectives of our investigation were to map the sterility locus, identify candidate genes for the male-fertile, female-fertile phenotype, and then determine if sterility is associated with the insertion of Tgm9 in the sterility locus. We used bulked segregant analysis to map the locus to molecular linkage group J (chromosome 16). Fine mapping enabled us to flank the locus to a 62-kb region that contains only five predicted genes. One of the genes in that region, Glyma16g07850.1, codes for a helicase. A rice homolog of this gene has been shown to control crossing over and fertility phenotype. Thus, Glyma16g07850.1 is most likely the gene regulating the male and female fertility phenotype in soybean. DNA blot analysis of the segregating individuals for Tgm9 showed perfect association between sterility and the presence of the transposon. Most likely, the sterility mutation was caused by the insertion of Tgm9. The transposable element should facilitate identification of the male- and female-fertility gene. Characterization of the fertility gene will provide vital molecular insight on the reproductive biology of soybean and other plants.

Segregation distortion in a region containing a male-sterility, female-sterility locus in soybean

In diploid segregation, each alternative allele has a 50% chance of being passed on to the offspring. Mutations in genes involved in the process of meiotic division or early stages of reproductive cell development can affect allele frequency in the gametes. In addition, competition among gametes and differential survival rates of gametes can lead to segregation distortion. In a recent transformation study, a male-sterile, female-sterile (MSFS) mutant was identified in the soybean cultivar, Williams. The mutant in heterozygous condition segregated 3 fertile:1 sterile in the progeny confirming monogenic inheritance. To map the lesion, we generated an F(2) mapping population by crossing the mutant (in heterozygous condition) with Minsoy (PI 27890). The F(2) progeny showed strong segregation distortion against the MSFS phenotype. The objectives of our study were to molecularly map the gene responsible for sterility in the soybean genome, to determine if the MSFS gene is a result of T-DNA insertion during Agrobacterium-mediated transformation, and to map the region that showed distorted segregation. The fertility/sterility locus was mapped to molecular linkage group (MLG) D1a (chromosome Gm01) using bulked segregant analysis. The closest marker, Satt531, mapped 9.4cM from the gene. Cloning of insertion sites for T-DNA in the mutant plants revealed that there are two copies of T-DNA in the genome. Physical locations of these insertion sites do not correlate with the map location of the MSFS gene, suggesting that MSFS mutation may not be associated with T-DNA insertions. Segregation distortion was most extreme at or around the st_A06-2/6 locus suggesting that sterility and segregation distortion are tightly linked attributes. Our results cue that the distorted segregation may be due to a gamete elimination system.

Fluorescence in situ hybridization-based karyotyping of soybean translocation lines

Soybean (Glycine max [L.] Merr.) is a major crop species and, therefore, a major target of genomic and genetic research. However, in contrast to other plant species, relatively few chromosomal aberrations have been identified and characterized in soybean. This is due in part to the difficulty of cytogenetic analysis of its small, morphologically homogeneous chromosomes. The recent development of a fluorescence in situ hybridization -based karyotyping system for soybean has enabled our characterization of most of the chromosomal translocation lines identified to date. Utilizing genetic data from existing translocation studies in soybean, we identified the chromosomes and approximate breakpoints involved in five translocation lines.

Excision of an active CACTA-like transposable element from DFR2 causes variegated flowers in soybean [Glycine max (L.) Merr.]

Active endogenous transposable elements, useful tools for gene isolation, have not been reported from any legume species. An active transposable element was suggested to reside in the W4 locus that governs flower color in soybean. Through biochemical and molecular analyses of several revertants of the w4-m allele, we have shown that the W4 locus encodes dihydroflavonol-4-reductase 2 (DFR2). w4-m has arisen through insertion of Tgm9, a 20,548-bp CACTA-like transposable element, into the second intron of DFR2. Tgm9 showed high nucleic acid sequence identity to Tgmt*. Its 5' and 3' terminal inverted repeats start with conserved CACTA sequence. The 3' subterminal region is highly repetitive. Tgm9 carries TNP1- and TNP2-like transposase genes that are expressed in the mutable line, T322 (w4-m). The element excises at a high frequency from both somatic and germinal tissues. Following excision, reinsertions of Tgm9 into the DFR2 promoter generated novel stable alleles, w4-dp (dilute purple flowers) and w4-p (pale flowers). We hypothesize that the element is fractured during transposition, and truncated versions of the element in new insertion sites cause stable mutations. The highly active endogenous transposon, Tgm9, should facilitate genomics studies specifically that relate to legume biology.

Isolation of genes from female sterile flowers in Medicago sativa

A better knowledge of female sporogenesis and gametogenesis could have several practical applications, from commercial hybrid seed production to gene containment in GM crops. With the purpose of isolating genes involved in the megasporogenesis process, the cDNA-AFLP technique was employed to isolate transcript-derived fragments (TDF) differentially expressed between female-fertile and female-sterile full-sib alfalfa plants. This female sterility trait involves female-specific arrest of sporogenesis at early prophase associated with ectopic, massive callose deposition within the nucellus. Ninety-six TDFs were generated and BLAST analyses revealed similarities with genes involved in different Gene Ontology categories. Three TDFs were selected based on their putative functions: showing high similarity to a soybean flower-expressed beta 1,3-glucanase, to an Arabidopsis thaliana MAPKKK, and to an A. thaliana eukaryotic initiation translation factor eIF4G III, respectively. The full length mRNA sequences were obtained. RT-PCR and in situ hybridizations were performed to confirm differential expression during flower development. The genomic organization of the three genes was assessed through sequencing and Southern experiments. Sequence polymorphisms were found between sterile and fertile plants. Our approach based on differential display and bulked segregant analysis was successful in isolating genes that were differentially expressed between fertile and sterile alfalfa plants.

The male sterility locus ms3 is present in a fertility controlling gene cluster in soybean

Soybean [Glycine max (L.) Merr.] is self-pollinated. To produce large quantities of hybrid seed, insect-mediated cross-pollination is necessary. An efficient nuclear male-sterile system for hybrid seed production would benefit from molecular and/or phenotypic markers linked to male fertility/sterility loci to facilitate early identification of phenotypes. Nuclear male-sterile, female-fertile ms3 mutant is a single recessive gene and displays high outcrossed seed set with pollinators. Our objective was to map the ms3 locus. A segregating population of 150 F(2) plants from Minsoy (PI 27890) x T284H, Ms3ms3 (A00-68), was screened with 231 simple sequence repeat markers. The ms3 locus mapped to molecular linkage group (MLG) D1b (Gm02) and is flanked by markers Satt157 and Satt542, with a distance of 3.7 and 12.3 cM, respectively. Female-partial sterile-1 (Fsp1) and the Midwest Oilseed male-sterile (msMOS) mutants previously were located on MLG D1b. msMOS and Fsp1 are independent genes located very close to each other. All 3 genes are located in close proximity of Satt157. We believe that this is the first report of clustering of fertility-related genes in plants. Characterization of these closely linked genes may help in understanding the evolutionary relationship among them.

Molecular mapping of 36 soybean male-sterile, female-sterile mutants

Mutability of the w(4) flower color locus in soybean [Glycine max (L.) Merr.] is conditioned by an unstable allele designated w(4)-m. Germinal revertants, purple-flower plants, recovered among self-pollinated progeny of mutable flower plants were associated with the generation of necrotic root, chlorophyll-deficiency, and sterility mutations. Thirty-seven male-sterile, female-sterile mutant lines were generated from 37 independent reversion events at the w(4)-m locus. The first germinal revertant study had one male-sterile, female-sterile mutant (st8, T352), located on Molecular Linkage Group (MLG) J. The second study had 36 germinal-revertant derived sterility mutants descended from four mutable categories of w(4)-m. The mutable categories were designated; (1) low frequency of early excisions, (2) low frequency of late excisions, (3) high frequency of early excisions, and (4) high frequency of late excisions. The objectives of the present study were to; (1) molecularly map the 36 male-sterile, female-sterile mutants, and to (2) compare map locations of these mutants with T352 (st8), identified from the first germinal revertant study. Thirty-three of 36 male-sterile, female-sterile mutations were derived from germinal reversions that were classified in the late excision categories. Thirty-five male-sterile mutants mapped to the st8 region on MLG J. The only exception mapped to MLG G. Most likely mutants were generated through insertion of a putative transposon that was excised from the w(4) locus. The location of 36 of 37 mutations to a single chromosomal region suggests preference for sequence-dependent insertion.

Genome structure in soybean revealed by a genomewide genetic map constructed from a single population

A complete genetic linkage map of the soybean, in which sequence-based (SB) genetic markers are evenly distributed genomewide, was constructed from an F(12) population composed of 113 recombinant inbred lines derived from an interspecific cross involving Korean genotypes Hwangkeum and IT182932. Several approaches were employed for the development of 112 novel SB markers targeting both the gaps and the ends of the linkage groups (LGs). The resultant map harbored 20 well-resolved LGs presumed to correspond to the 20 pairs of soybean chromosomes. The map allowed us to identify the important chromosomal structures that were not observed in the integrated genetic maps, to identify the new potentially gene-rich regions, to detect segregation distortion regions within the whole genome, and to extend the ends of the LGs. The results will facilitate the further discovery of agronomically relevant genetic loci in the heretofore neglected chromosomal regions and should also provide some important links between the soybean genetic, physical, and genome sequence maps in the regions.

Allelism and molecular mapping of soybean necrotic root mutants

Mutability of the w4 flower color locus in soybean, Glycine max (L.) Merr., is conditioned by an allele designated w4-m. Germinal revertants recovered among self-pollinated progeny of mutable plants have been associated with the generation of necrotic root mutations, chlorophyll-deficiency mutations, and sterility mutations. A total of 24 necrotic root mutant lines were generated from a total of 24 independent reversion events at the w4-m locus. The initial mutable population included 4 mutable categories for w4-m, designated (1) low frequency of early excisions, (2) low frequency of late excisions, (3) high frequency of early excisions, and (4) high frequency of late excisions. These mutable categories were based upon flower phenotype, i.e., somatic tissue. A total of 22 of 24 necrotic root mutations occurred from germinal reversions classified in the high frequency of excision categories. Of these 22 mutants, 14 came from early excisions and 8 came from late excisions. These necrotic root mutants were allelic to 6 previously identified necrotic root mutants derived from the study of germinal revertants, i.e., gene tagging studies, chemical mutagenesis, and "spontaneous" occurrences from genetic crosses. Thus, all 30 necrotic root mutants in soybean are allelic. An F2 mapping population from the cross of Minsoy (Rn1 Rn1) x T328 (rn1 rn1) was used to map the Rn1 locus using simple sequence repeat (SSR) markers. The Rn1 locus was located between Satt288 and Satt612 on molecular linkage group G.

Broadening the application of evolutionarily based genetic pest management

Insect- and tick-vectored diseases such as malaria, dengue fever, and Lyme disease cause human suffering, and current approaches for prevention are not adequate. Invasive plants and animals such as Scotch broom, zebra mussels, and gypsy moths continue to cause environmental damage and economic losses in agriculture and forestry. Rodents transmit diseases and cause major pre- and postharvest losses, especially in less affluent countries. Each of these problems might benefit from the developing field of Genetic Pest Management that is conceptually based on principles of evolutionary biology. This article briefly describes the history of this field, new molecular tools in this field, and potential applications of those tools. There will be a need for evolutionary biologists to interact with researchers and practitioners in a variety of other fields to determine the most appropriate targets for genetic pest management, the most appropriate methods for specific targets, and the potential of natural selection to diminish the effectiveness of genetic pest management. In addition to producing environmentally sustainable pest management solutions, research efforts in this area could lead to new insights about the evolution of selfish genetic elements in natural systems and will provide students with the opportunity to develop a more sophisticated understanding of the role of evolutionary biology in solving societal problems.

Molecular mapping of k2 Mdh1-n y20, an unstable chromosomal region in soybean [Glycine max (L.) Merr.]

In the soybean genome, a chromosomal region covering three tightly linked genes, k2, Mdh1-n, and y20, was found very unstable. It was suspected that the instability of the k2 Mdh1-n y20 chromosomal region was caused by a non-autonomous transposable element residing adjacent to or in this region. In this study, we located and mapped this region with simple sequence repeat (SSR) markers on the soybean integrated map using five mapping populations. The k2 Mdh1-n y20 chromosomal region was located on molecular linkage group H. The integrated map from five mapping populations consisted of 13 loci in the order Satt541, Satt469, Sat_122, Satt279, Satt253, Satt314, Mdh1-n,y20, k2, Satt302, Satt142, Satt181, and Satt434. The k2 Mdh1-n y20 chromosomal region was very close to Satt314, Satt253, and Satt279. The genetic distance between the Mdh1-n gene and Satt314 was less than 1 cM. The results of the mapping study were consistent with the results from previous studies that the Mdh1-n mutation in T261 (k2 Mdh1-n) and the Mdh1-n y20 mutation in T317 (Mdh1-n y20) were caused by deletions. In addition, another putative deletion was found in the genome of T261 which covered three SSR markers (Satt314, Satt253, and Satt279).

Genetic analysis and molecular mapping of a pale flower allele at the W4 locus in soybean

In soybean (Glycine max (L.) Merr.), the w4-mutable line that harbors the w4-m allele was identified in 1983. It was proposed that this line contained an autonomous transposable element at the W4 locus, which is a major locus controlling the biosynthesis of anthocyanin. The w4-m allele can revert to the W4 allele that produces the wild-type phenotype, or sometimes to other alleles that produce intermediate phenotypes. Mutant plants that produce pale flowers were identified among the progeny of a single germinal revertant event from the w4-mutable line. Through genetic analysis, we established that the pale-flower mutation was conditioned by a new allele (w4-p) at the W4 locus. The w4-p allele is dominant to the w4 allele but recessive to the W4 allele, and the w1 allele has an epistatic effect on the w4-p allele. The pale-mutant line (w4-pw4-p) was designated as Genetic Type Collection number T369. An F2 mapping population derived from the cross of Minsoy (W4W4) x T369 (w4-pw4-p) was used to map the W4/w4-p locus, using simple sequence repeat (SSR) markers. The W4 locus was located at one end of molecular linkage group D2, 2.3 cM from the SSR marker Satt386 and close to the nearby telomere.