George L. Hodnett

Distribution and sequence analysis of the centromere-associated repetitive element CEN38 of Sorghum bicolor (Poaceae).

Fluorescence in situ hybridization (FISH) of a large-insert genomic clone, BAC 22B2, previously suggested that Sorghum bicolor (2n = 20) has the tetraploid architecture A(b)A(b)B(b)B(b). Here, we report on BAC 22B2 subclone pCEN38 (1047-bp insert) as related to sorghum and sugarcane. Mitotic FISH of six different subclones of BAC 22B2 showed that pCEN38 produced the strongest specificity to the A(b) subgenome and signal occurred primarily near centromeres. Southern blots of pCEN38 to 21 crop plants revealed a narrow taxonomic distribution. Meiotic metaphase I FISH positioned pCEN38 sequences near active centromeres. Pachytene FISH revealed that the distributions are trimodal in several B(b) and possibly all sorghum chromosomes. DNA sequencing revealed that the pCEN38 fragment contains three tandemly repeated dimers (<280 bp) of the same sequence family found in sorghum clone pSau3A10, and that each dimer consists of two divergent monomers (<140 bp). Sequence comparisons revealed homology between the pCEN38 monomers and the SCEN 140 bp tandem repeat family of sugarcane. FISH of pCEN38 yielded signal in centromere regions of most but not all sugarcane chromosomes. Results suggest that sugarcane and sorghum share at least one ancestor harboring elements similar to pCEN38 and SCEN and that each species had an ancestor in which the repetitive element was weakly present or lacking.

A Sorghum bicolor × S. macrospermum hybrid recovered by embryo rescue and culture

Although exotic germplasm is extensively used in sorghum improvement programs, Sorghum species classified in sections other than Eu-sorghum have not been utilised as germplasm because of strong reproductive barriers involving pollen-pistil incompatibilities. S. macrospermum is of particular interest to sorghum breeders because of its close phylogenetic relationship and cytogenetic similarities to S. bicolor and its resistance to important sorghum pests and pathogens, such as sorghum midge and sorghum downy mildew. A vegetatively vigorous interspecific hybrid was obtained from a cross between a cytoplasmic male-sterile S. bicolor plant and S. macrospermum by using embryo rescue and in vitro culture techniques. The hybrid was morphologically intermediate to S. bicolor and S. macrospermum in leaf width, leaf pubescence, plant height, inflorescence morphology, chromosome number and nuclear DNA content. It was male-sterile like its ATx623 parent. The hybrid produced no offspring when used as the female parent in a backcross with S. bicolor. This is the first confirmed hybrid between S. bicolor and S. macrospermum, and to our knowledge, it is the first reported hybrid between S. bicolor and any Sorghum species outside the Eu-sorghum section.

Exploring origins, invasion history and genetic diversity of Imperata cylindrica (L.) P. Beauv. (Cogongrass) in the United States using genotyping by sequencing

Imperata cylindrica (Cogongrass, Speargrass) is a diploid C4 grass that is a noxious weed in 73 countries and constitutes a significant threat to global biodiversity and sustainable agriculture. We used a cost‐effective genotyping‐by‐sequencing (GBS) approach to identify the reproductive system, genetic diversity and geographic origins of invasions in the south‐eastern United States. In this work, we demonstrated the advantage of employing the closely related, fully sequenced crop species Sorghum bicolor (L.) Moench as a proxy reference genome to identify a set of 2320 informative single nucleotide and insertion–deletion polymorphisms. Genetic analyses identified four clonal lineages of cogongrass and one clonal lineage of Imperata brasiliensis Trin. in the United States. Each lineage was highly homogeneous, and we found no evidence of hybridization among the different lineages, despite geographical overlap. We found evidence that at least three of these lineages showed clonal reproduction prior to introduction to the United States. These results indicate that cogongrass has limited evolutionary potential to adapt to novel environments and further suggest that upon arrival to its invaded range, this species did not require local adaptation through hybridization/introgression or selection of favourable alleles from a broad genetic base. Thus, cogongrass presents a clear case of broad invasive success, across a diversity of environments, in a clonal organism with limited genetic diversity.

Fluorescent in situ hybridization of 18S and 5S rDNA in papaya (Carica papaya l.) and wild relatives

Abstract Papaya (Carica papaya L.) is cultivated widely as a fruit crop in the tropics. Related species as Vasconcellea cundinamarcensis and V. goudotiana are prospective sources of desirable genes. The requisite germplasm introgression will be facilitated by a better understanding of their cytogenetic relationships to papaya, but the chromosomes of Caricaceae species are very small and similar so conventional karyotyping is difficult and of limited utility. Here, we report on fluorescence in situ hybridization (FISH) of rDNA probes to create several karyological markers that enable chromosomes to be distinguished and to produce information about the genetic relationships among Caricaceae species. Based on the number and the position of rDNA sites, V. cundinamarcensis and V. goudotiana were the closest species while C. papaya was isolated from them. Both Vasconcellea species showed only one pair of 5S site whereas three pairs were found in C. papaya. On the other hand, one 18S site was observed in papaya, whereas four and five 18S sites were found in V. goudotiana and V. cundinamarcensis, respectively. It is possible that the unpaired signal of 18S probe in V. cundinamarcensis is located in a sexual chromosome, however, further studies are required to confirm this hypothesis of sex-chromosome linkage.

Gene Flow and its Consequences in Sorghum spp.

ABSTRACT Gene flow between crops and their weedy or wild relatives can be problematic in modern agricultural systems, especially if it endows novel adaptive genes that confer tolerance to abiotic and biotic stresses. Alternatively, gene flow from weedy relatives to domesticated crops may facilitate ferality through introgression of weedy characteristics in the progeny. Cultivated sorghum (Sorghum bicolor), is particularly vulnerable to the risks associated with gene flow to several weedy relatives, johnsongrass (S. halepense), shattercane (S. bicolor ssp. drummondii) and columbusgrass (S. almum). Johnsongrass and shattercane are common weeds in many sorghum production areas around the world. Sorghum varieties with adaptive traits developed through conventional breeding or novel transgenesis pose agronomic and ecological risks if transferred into weedy/wild relatives. Knowledge of the nature and characteristics of gene flow among different sorghum species is scarce, and existing knowledge is scattered. Here, we review current knowledge of gene flow between cultivated sorghum and its weedy and wild relatives. We further discuss potential avenues for addressing gene flow through genetic, molecular, and field level containment, mitigation and management strategies to facilitate successful deployment of novel traits in this economically important crop species.

Estimating Broad Sense Heritability and Investigating the Mechanism of Genetic Transmission of Cold Tolerance Using Mannitol as a Measure of Post-freeze Juice Degradation in Sugarcane and Energycane (Saccharum spp.)

In approximately 25% of the sugarcane-producing countries worldwide, conventional sugarcane (Saccharum spp. hybrids) is exposed to damaging freezes. A study was conducted during the 2009 and 2010 harvest seasons to compare late-season freeze tolerance among three groups: commercial Louisiana sugarcane genotypes, early generation genotypes selected for cold tolerance in the U.S. Department of Agriculture sugarcane breeding programs at Houma, LA, and Canal Point, FL, and potential energycane genotypes selected for high total biomass per acre. Mannitol concentrations in cane juice following freezing temperatures were determined to evaluate levels of cold tolerance. Genotypes selected for cold tolerance in Houma, LA, had significantly more late-season freeze tolerance than commercial sugarcane genotypes and genotypes selected in Canal Point, FL. Genotypes showing the most cold tolerance were Ho02-146 and Ho02-152, and those that were most highly susceptible were US87-1006 and US87-1003 (early-generation breeding genotypes) and L99-233 (commercial genotype). Broad-sense heritability for late-season cold tolerance in the two-year study was estimated at g(2) = 0.78. The enzymatic mannitol analysis successfully differentiated high-fiber energycane genotypes from those from other sources.

Male sterility induction of sorghum using chemical hybridizing agent TFMSA, trifluoromethanesulfonamide

Abstract: In sorghum, male sterility has been induced mainly using mechanical and genetic means with minimal use of chemical methods. Mechanical sterility induction is limited to producing small quantities of seed and genetic male sterility is limited to specific germplasm. An effective chemical hybridizing agent could be used to produce large amounts of seed and would not be limited by genotypes. The chemical trifluoromethanesulfonamide (TFMSA) was evaluated as a male gametocide in sorghum. In greenhouse conditions, TFMSA induced varying degrees of male sterility contingent on the dosage, timing of application, and sorghum genotype. If applied at or after the flag leaf emergence, panicles were male fertile but male sterile when applied at least 2 d before the flag leaf emerged. In terms of dosage, 2 mg of TFMSA rendered a BTx623 male sterile if applied 2–6 d prior to flag leaf emergence. However, 30 mg of TFMSA applied as much as 34 d before the flag leaf emerged also induced complete sterility of the panicle. There may be a genotypic effect as less TFMSA was necessary to induce complete male sterility in BTx623 than in BTxArg-1. No phytotoxic effects or reductions in female fertility were observed in dosages of TFMSA up to 40 mg. Germination of hybrid seed made on treated plants was equal to that of the parents and the hybrid seedlings were phenotypically normal. The results indicate that male sterility can be induced effectively in sorghum using TFMSA at appropriate dosages and application timings.