K. F. Schertz

Convergent domestication of cereal crops by independent mutations at corresponding genetic Loci.

Independent domestication of sorghum, rice, and maize involved convergent selection for large seeds, reduced disarticulation of the mature inflorescence, and daylength-insensitive flowering. These similar phenotypes are largely determined by a small number of quantitative trait loci (QTLs) that correspond closely in the three taxa. The correspondence of these QTLs transcends 65 million years of reproductive isolation. This finding supports models of quantitative inheritance that invoke relatively few genes, obviates difficulties in map-based cloning of QTLs, and impels the comparative mapping of complex pheno-types across large evolutionary distances, such as those that separate humans from rodents and domesticated mammals.

A detailed RFLP map of Sorghum bicolor x S. propinquum, suitable for high-density mapping, suggests ancestral duplication of Sorghum chromosomes or chromosomal segments

The first “complete” genetic linkage map of Sorghum section Sorghum is described, comprised of ten linkage groups putatively corresponding to the ten gametic chromosomes of S. bicolor and S. propinquum. The map includes 276 RFLP loci, predominately detected by PstI-digested S. bicolor genomic probes, segregating in 56 F2 progeny of a cross between S. bicolor and S. propinquum. Although prior cytological evidence suggests that the genomes of these species are largely homosequential, a high level of molecular divergence is evidenced by the abundant RFLP and RAPD polymorphisms, the marked deviations from Mendelian segregation in many regions of the genome, and several species-specific DNA probes. The remarkable level of DNA polymorphism between these species will facilitate development of a high-density genetic map. Further, the high level of DNA polymorphism permitted mapping of multiple loci for 21 (8.2%) DNA probes. Linkage relationships among eight (38%) of these probes suggest ancestral duplication of three genomic regions. Mapping of 13 maize genomic clones in this cross was consistent with prior results. Mapping of heterologous cDNAs from rice and oat suggests that it may be feasible to extend comparative mapping to these distantly-related species, and to ultimately generate a detailed description of chromosome rearrangements among cultivated Gramineae. Limited investigation of a small number of RFLPs showed several alleles common to S. bicolor and S. Halepense (“johnson-grass”), but few alleles common to S. propinquum and S. halepense, raising questions about the origin of S. halepense.

Alignment of genetic maps and QTLs between inter- and intra-specific sorghum populations

To increase the value of associated molecular tools and also to begin to explore the degree to which interspecific and intraspecific genetic variation in Sorghum is attributable to corresponding genetic loci, we have aligned genetic maps derived from two sorghum populations that share one common parent (Sorghum bicolor L. Moench accession BTx623) but differ in morphological and evolutionarily distant alternate parents (S. propinquum or S. bicolor accession IS3620C). A total of 106 well-distributed DNA markers provide for map alignment, revealing only six nominal differences in marker order that are readily explained by sampling variation or mapping of paralogous loci. We also report a total of 61 new QTLs detected from 17 traits in these crosses. Among eight corresponding traits (some new, some previously published) that could be directly compared between the two maps, QTLs for two (tiller height and tiller number) were found to correspond in a non-random manner (P<0.05). For several other traits, correspondence of subsets of QTLs narrowly missed statistical significance. In particular, several QTLs for leaf senescence were near loci previously mapped for ‘stay-green’ that have been implicated by others in drought tolerance. These data provide strong validation for the value of molecular tools developed in the interspecific cross for utilization in cultivated sorghum, and begin to separate QTLs that distinguish among Sorghum species from those that are informative within the cultigen (S. bicolor).

Patterns of allozyme variation in cultivated and wild Sorghum bicolor.

SummaryPatterns of allozyme variation were surveyed in collections of cultivated and wild sorghum from Africa, the Middle East, and Asia. Data for 30 isozyme loci from a total of 2067 plants representing 429 accessions were analyzed. Regional levels of genetic diversity in the cultivars are greater in northern and central Africa compared to southern Africa, the Middle East, or Asia. The spatial distribution of individual alleles at the most variable loci was studied by plotting allele frequencies on geographic maps covering the distribution of sorghum. Generally, many of the alleles with frequencies below 0.25 are localized in specific portions of the range and are commonly present in more than one race in that region. Several alleles occur in both wild and cultivated sorghum of one region and are absent from sorghum elsewhere, suggesting local introgression between the wild and cultivated forms. Although the same most common allele was found in the wild and cultivated gene pools at 29 of the 30 loci, phenetic analyses separated the majority of wild collections from the cultivars, indicating that the two gene pools are distinct. Wild sorghum from northeast and central Africa exhibits greater genetic similarities to the cultivars compared to wild sorghum of northwest or southern Africa. This is consistent with the theory that wild sorghum of northeast-central Africa is ancestral to domesticated sorghum. Wild sorghums of race arundinaceum of northwest Africa and race virgatum from Egypt are shown to be genetically distinct from both other forms of wild sorghum and from the cultivars. Suggestions for genetic conservation are presented in light of these data.

Genetic mapping of Sorghum bicolor (L.) Moench QTLs that control variation in tillering and other morphological characters

Abstract Grain yield of Sorghum bicolor (L.) Moench is significantly influenced by genetically controlled variation in the number of tillers, plant height, time of anthesis, and various other morphological and physiological characters. In this study, a minimum of 27 unique QTLs that control variation in nine morphological traits, including the presence versus the absence and the height of basal tillers, were mapped, and the percentage of additive genetic variance explained by the QTLs was determined in a population of 137 recombinant inbred lines in two environments. Four QTLs explained from 86.3% to 48.9% (depending upon the environment) of the additive genetic variance in the number of basal tillers with heads, and seven QTLs explained from 85.9% to 47.9% of the additive genetic variance in panicle width. It is unlikely that different alleles were segregating in the mapping population at any of the major dwarfing loci, but five QTLs that explained from 65.8% to 52.0% of the additive genetic variance in main-culm height were mapped. QTLs controlling variation in height of the tallest basal tiller, number of basal tillers per basal-tillered plant, panicle length, leaf angle, maturity, and awn length also were mapped. Three or more QTLs were mapped in linkage groups A, E, G, and I, while none were mapped in linkage groups B and D. Several of the QTLs mapped in this study are likely candidates for marker-assisted selection in breeding programs.

A RFLP linkage map of Sorghum bicolor (L.) Moench

A RFLP linkage map of sorghum composed principally of markers detected with sorghum low-copy-number nuclear DNA clones has been constructed. The map spans 1789 cMs and consists of 190 loci grouped into 14 linkage groups. The 10 largest linkage groups consist of from 10 to 24 markers and from 103 to 237 cMs, and the other 4 linkage groups consist of from 2 to 5 markers and from 7 to 62 cMs. The map was derived in Sorghum bicolor ssp. bicolor by analysis of a F2 population composed of 50 plants derived from a cross of IS 3620C, a guinea line, and BTx 623, an agronomically important inbred line derived from a cross between a zera zera (a caudatum-like sorghum) and an established kafir line. The restriction fragment length polymorphism (RFLP) frequency detected in this population using polymerase chain reaction (PCR)-amplifiable low-copy-number sorghum clones and five restriction enzymes was 51%. A minimal estimate of the number of clones that detect duplicate sequences is 11 %. Null alleles occurred at 13% of the mapped RFLP loci.

RFLP-based assay of Sorghum bicolor (L.) Moench genetic diversity

Sixty-two single-copy sorghum DNA clones were used to compare restriction fragment patterns of 53 sorghum accessions from Africa, Asia and the United States. Included were accessions from five morphological races of the cultivated subspecies bicolor, and four races of the wild subspecies verticilliflorum. From two to twelve alleles were detected with each probe. There was greater nuclear diversity in the wild subspecies (255 alleles in ten accessions) than in the domestic accessions (236 alleles in 37 accessions). Overall, 204 of the 340 alleles (60%) that were detected occurred in both subspecies. Phylogenetic analysis using parsimony separated the subspecies into separate clusters, with one group of intermediate accessions. Though exceptions were common, especially for the race bicolor, accessions classified as the same morphological race tended to group together on the basis of RFLP similarities. Selection for traits such as forage quality may have led to accessions genetically more similar to other races being classified as bicolors, which have a loose, small-grained panicle similar to wild races. Population statistics, calculated using four nuclear and four cytoplasmic probes that detect two alleles each, revealed a low but significant amount of heterozygosity, and showed little differentiation in alleles in the wild and cultivated subspecies. Outcrossing with foreign pollen appears to have been more important than migration via seed dispersal as a mechanism for gene flow between the wild and domestic accessions included in this study.

A chloroplast DNA deletion located in RNA polymerase gene rpoC2 in CMS lines of sorghum

SummaryFertile lines of sorghum (Sorghum bicolor) were shown to differ from cytoplasmic male sterile (CMS) lines by the presence of a 3.8 kb HindIII chloroplast DNA fragment in the former and a smaller (3.7 kb) fragment in the latter. DNA/DNA hybridization studies showed that these two fragments are homologous. Fertile plants from S. versicolor, S. almum, S. halepense, and Sorghastrum nutans (Yellow Indiangrass) also have the 3.8 kb fragment, and CMS lines studied containing A1, A2 and A3 cytoplasms have the 3.7 kb fragment. The size difference between the two fragments was localized to a 1.0 kb SacI-HindIII fragment by restriction mapping. A r65 by deletion, which is flanked by a 51 by tandem repeat, was identified in the CMS lines by sequencing the clones. Comparison of the two sequences with those from maize, rice, tobacco, spinach, pea, and liverwort revealed that the deleted sequence is located in the middle of the RNA polymerase β″ subunit encoded by the gene rpoC2. The amino acid sequence deleted in the CMS lines is in a monocot-specific region which contains two protein motifs that are characteristic of several transcriptional activation factors, namely, a leucine zipper motif and an acidic domain capable of forming an amphipathic α-helix. Further studies designed to determine whether or not the deletion is involved in CMS of sorghum are underway.

Molecular mapping of the rf1 gene for pollen fertility restoration in sorghum (Sorghum bicolor L.)

Abstract We report the molecular mapping of a gene for pollen fertility in A1 (milo) type cytoplasm of sorghum using AFLP and SSR marker analysis. DNA from an F2 population comprised of 84 individuals was screened with AFLP genetic markers to detect polymorphic DNAs linked to fertility restoration. Fifteen AFLP markers were linked to fertility restoration from the initial screening with 49 unique AFLP primer combinations (+3/+3 selective bases). As many of these AFLP markers had been previously mapped to a high-density genetic map of sorghum, the target gene (rf1) could be mapped to linkage group H. Confirmation of the map location of rf1 was obtained by demonstrating that additional linkage group-H markers (SSR, STS, AFLP) were linked to fertility restoration. The closest marker, AFLP Xtxa2582, mapped within 2.4 cM of the target loci while two SSRs, Xtxp18 and Xtxp250, flanked the rf1 locus at 12 cM and 10.8 cM, respectively. The availability of molecular markers will facilitate the selection of pollen fertility restoration in sorghum inbred-line development and provide the foundation for map-based gene isolation.

Allozyme variation among the spontaneous species of Sorghum section Sorghum (Poaceae).

SummaryA survey of allozyme variation among the spontaneous taxa of Sorghum section Sorghum was undertaken. Eight plants each of 90 accessions representing the diploid S. bicolor (ssp. arundinaceum and drummondii) and the tetraploids S. almum and S. halepense were analyzed for 17 enzyme systems encoded by 30 loci. Low levels of variation were found within and among accessions, although there was more variation than is typical of inbreeding species. We found an average of 3.2 alleles per locus in ssp. arundinaceum, with a mean expected heterozygosity for the accessions of 0.034 and total panmictic heterozygosity of 0.154. An analysis of the apportionment of genetic variation among accessions of ssp. arundinaceum indicated that 26% of the variation occurs within accessions and 74% among accessions. Cultivated sorghum contains far less allozymic variation than ssp. arundinaceum, its presumed progenitor. This is consistent with the prediction that cultivated sorghum experienced a loss of genetic variation during domestication. For the most part, cultivated sorghum contains a subset of the allozymes found in ssp. arundinaceum. Principal component analysis revealed continuous variation among the accessions and geographic regions, with accessions failing to segregate into discrete clusters. However, accessions of race virgatum of ssp. arundinaceum occupied one end of the continuum and were, in that sense, distinguished from the other accessions. Similarly, most accessions of S. halepense and S. almum occupied the central portion of the continuum. The allozymic data presented here are consistent with the hypothesized origin of S. halepense via autopolyploidy or segmental allopolyploidy.