Genetic diversity of Ethiopian sorghum reveals signatures of climatic adaptation.

KEY MESSAGE: A large collection of Ethiopian sorghum landraces, characterized by agro-ecology and racial-group, was found to contain high levels of diversity and admixture, with significant SNP associations identified for environmental adaptation. Sorghum [Sorghum bicolor L. (Moench)] is a major staple food crop in Ethiopia, exhibiting extensive genetic diversity with adaptations to diverse agroecologies. The environmental and climatic drivers, as well as the genomic basis of adaptation, are poorly understood in Ethiopian sorghum and are critical elements for the development of climate-resilient crops. Exploration of the genome-environment association (GEA) is important for identifying adaptive loci and predicting phenotypic variation. The current study aimed to better understand the GEA of a large collection of Ethiopian sorghum landraces (n = 940), characterized with genome-wide SNP markers, to investigate key traits related to adaptation to temperature, precipitation and altitude. The Ethiopian sorghum landrace collection was found to consist of 12 subpopulations with high levels of admixture (47%), representing all the major racial groups of cultivated sorghum with the exception of kafir. Redundancy analysis indicated that agroecology explained up to 10% of the total SNP variation, and geographical location up to 6%. GEA identified 18 significant SNP markers for environmental variables. These SNPs were found to be significantly enriched (P < 0.05) for a priori QTL for drought and cold adaptation. The findings from this study improve our understanding of the genetic control of adaptive traits in Ethiopian sorghum. Further, the Ethiopian sorghum germplasm collection provides sources of adaptation to harsh environments (cold and/or drought) that could be deployed in breeding programs globally for abiotic stress adaptation.

High-Throughput Phenotyping of Dynamic Canopy Traits Associated with Stay-Green in Grain Sorghum.

Drought is a recurring phenomenon that puts crop yields at risk and threatens the livelihoods of many people around the globe. Stay-green is a drought adaption phenotype found in sorghum and other cereals. Plants expressing this phenotype show less drought-induced senescence and maintain functional green leaves for longer when water limitation occurs during grain fill, conferring benefits in both yield per se and harvestability. The physiological causes of the phenotype are postulated to be water saving through mechanisms such as reduced canopy size or access to extra water through mechanisms such as deeper roots. In sorghum breeding programs, stay-green has traditionally been assessed by comparing visual scores of leaf senescence either by identifying final leaf senescence or by estimating rate of leaf senescence. In this study, we compared measurements of canopy dynamics obtained from remote sensing on two sorghum breeding trials to stay-green values (breeding values) obtained from visual leaf senescence ratings in multienvironment breeding trials to determine which components of canopy development were most closely linked to the stay-green phenotype. Surprisingly, canopy size as estimated using preflowering canopy parameters was weakly correlated with stay-green values for leaf senescence while postflowering canopy parameters showed a much stronger association with leaf senescence. Our study suggests that factors other than canopy size have an important role in the expression of a stay-green phenotype in grain sorghum and further that the use of UAVs with multispectral sensors provides an excellent way of measuring canopy traits of hundreds of plots grown in large field trials.

Integrating modelling and phenotyping approaches to identify and screen complex traits: transpiration efficiency in cereals.

Following advances in genetics, genomics, and phenotyping, trait selection in breeding is limited by our ability to understand interactions within the plant and with the environment, and to identify traits of most relevance to the target population of environments. We propose an integrated approach that combines insights from crop modelling, physiology, genetics, and breeding to characterize traits valuable for yield gain in the target population of environments, develop relevant high-throughput phenotyping platforms, and identify genetic controls and their value in production environments. This paper uses transpiration efficiency (biomass produced per unit of water used) as an example of a complex trait of interest to illustrate how the approach can guide modelling, phenotyping, and selection in a breeding programme. We believe that this approach, by integrating insights from diverse disciplines, can increase the resource use efficiency of breeding programmes for improving yield gains in target populations of environments.

Investigating successive Australian barley breeding populations for stable resistance to leaf rust.

KEY MESSAGE: Genome-wide association studies of barley breeding populations identified candidate minor genes for pairing with the adult plant resistance gene Rph20 to provide stable leaf rust resistance across environments. Stable resistance to barley leaf rust (BLR, caused by Puccinia hordei) was evaluated across environments in barley breeding populations (BPs). To identify genomic regions that can be combined with Rph20 to improve adult plant resistance (APR), two BPs genotyped with the Diversity Arrays Technology genotyping-by-sequencing platform (DArT-seq) were examined for reaction to BLR at both seedling and adult growth stages in Australian environments. An integrated consensus map comprising both first- and second-generation DArT platforms was used to integrate QTL information across two additional BPs, providing a total of four interrelated BPs and 15 phenotypic data sets. This enabled identification of key loci underpinning BLR resistance. The APR gene Rph20 was the only active resistance region consistently detected across BPs. Of the QTL identified, RphQ27 on chromosome 6HL was considered the best candidate for pairing with Rph20. RphQ27 did not align or share proximity with known genes and was detected in three of the four BPs. The combination of RphQ27 and Rph20 was of low frequency in the breeding material; however, strong resistance responses were observed for the lines carrying this pairing. This suggests that the candidate minor gene RphQ27 can interact additively with Rph20 to provide stable resistance to BLR across diverse environments.

QTL analysis in multiple sorghum populations facilitates the dissection of the genetic and physiological control of tillering.

KEY MESSAGE: A QTL model for the genetic control of tillering in sorghum is proposed, presenting new opportunities for sorghum breeders to select germplasm with tillering characteristics appropriate for their target environments. Tillering in sorghum can be associated with either the carbon supply-demand (S/D) balance of the plant or an intrinsic propensity to tiller (PTT). Knowledge of the genetic control of tillering could assist breeders in selecting germplasm with tillering characteristics appropriate for their target environments. The aims of this study were to identify QTL for tillering and component traits associated with the S/D balance or PTT, to develop a framework model for the genetic control of tillering in sorghum. Four mapping populations were grown in a number of experiments in south east Queensland, Australia. The QTL analysis suggested that the contribution of traits associated with either the S/D balance or PTT to the genotypic differences in tillering differed among populations. Thirty-four tillering QTL were identified across the populations, of which 15 were novel to this study. Additionally, half of the tillering QTL co-located with QTL for component traits. A comparison of tillering QTL and candidate gene locations identified numerous coincident QTL and gene locations across populations, including the identification of common non-synonymous SNPs in the parental genotypes of two mapping populations in a sorghum homologue of MAX1, a gene involved in the control of tiller bud outgrowth through the production of strigolactones. Combined with a framework for crop physiological processes that underpin genotypic differences in tillering, the co-location of QTL for tillering and component traits and candidate genes allowed the development of a framework QTL model for the genetic control of tillering in sorghum.

Association mapping of resistance to Puccinia hordei in Australian barley breeding germplasm.

KEY MESSAGE: "To find stable resistance using association mapping tools, QTL with major and minor effects on leaf rust reactions were identified in barley breeding lines by assessing seedlings and adult plants." Three hundred and sixty (360) elite barley (Hordeum vulgare L.) breeding lines from the Northern Region Barley Breeding Program in Australia were genotyped with 3,244 polymorphic diversity arrays technology markers and the results used to map quantitative trait loci (QTL) conferring a reaction to leaf rust (Puccinia hordei Otth). The F3:5 (Stage 2) lines were derived or sourced from different geographic origins or hubs of international barley breeding ventures representing two breeding cycles (2009 and 2011 trials) and were evaluated across eight environments for infection type at both seedling and adult plant stages. Association mapping was performed using mean scores for disease reaction, accounting for family effects using the eigenvalues from a matrix of genotype correlations. In this study, 15 QTL were detected; 5 QTL co-located with catalogued leaf rust resistance genes (Rph1, Rph3/19, Rph8/14/15, Rph20, Rph21), 6 QTL aligned with previously reported genomic regions and 4 QTL (3 on chromosome 1H and 1 on 7H) were novel. The adult plant resistance gene Rph20 was identified across the majority of environments and pathotypes. The QTL detected in this study offer opportunities for breeding for more durable resistance to leaf rust through pyramiding multiple genomic regions via marker-assisted selection.

Supermodels: sorghum and maize provide mutual insight into the genetics of flowering time.

Nested association mapping (NAM) offers power to dissect complex, quantitative traits. This study made use of a recently developed sorghum backcross (BC)-NAM population to dissect the genetic architecture of flowering time in sorghum; to compare the QTL identified with other genomic regions identified in previous sorghum and maize flowering time studies and to highlight the implications of our findings for plant breeding. A subset of the sorghum BC-NAM population consisting of over 1,300 individuals from 24 families was evaluated for flowering time across multiple environments. Two QTL analysis methodologies were used to identify 40 QTLs with predominately small, additive effects on flowering time; 24 of these co-located with previously identified QTL for flowering time in sorghum and 16 were novel in sorghum. Significant synteny was also detected with the QTL for flowering time detected in a comparable NAM resource recently developed for maize (Zea mays) by Buckler et al. (Science 325:714-718, 2009). The use of the sorghum BC-NAM population allowed us to catalogue allelic variants at a maximal number of QTL and understand their contribution to the flowering time phenotype and distribution across diverse germplasm. The successful demonstration of the power of the sorghum BC-NAM population is exemplified not only by correspondence of QTL previously identified in sorghum, but also by correspondence of QTL in different taxa, specifically maize in this case. The unification across taxa of the candidate genes influencing complex traits, such as flowering time can further facilitate the detailed dissection of the genetic control and causal genes.

QTL for nodal root angle in sorghum (Sorghum bicolor L. Moench) co-locate with QTL for traits associated with drought adaptation.

Nodal root angle in sorghum influences vertical and horizontal root distribution in the soil profile and is thus relevant to drought adaptation. In this study, we report for the first time on the mapping of four QTL for nodal root angle (qRA) in sorghum, in addition to three QTL for root dry weight, two for shoot dry weight, and three for plant leaf area. Phenotyping was done at the six leaf stage for a mapping population (n = 141) developed by crossing two inbred sorghum lines with contrasting root angle. Nodal root angle QTL explained 58.2% of the phenotypic variance and were validated across a range of diverse inbred lines. Three of the four nodal root angle QTL showed homology to previously identified root angle QTL in rice and maize, whereas all four QTL co-located with previously identified QTL for stay-green in sorghum. A putative association between nodal root angle QTL and grain yield was identified through single marker analysis on field testing data from a subset of the mapping population grown in hybrid combination with three different tester lines. Furthermore, a putative association between nodal root angle QTL and stay-green was identified using data sets from selected sorghum nested association mapping populations segregating for root angle. The identification of nodal root angle QTL presents new opportunities for improving drought adaptation mechanisms via molecular breeding to manipulate a trait for which selection has previously been very difficult.

Mapping and characterization of Rf 5: a new gene conditioning pollen fertility restoration in A1 and A2 cytoplasm in sorghum (Sorghum bicolor (L.) Moench).

With an aim to further characterize the cytoplasmic male sterility-fertility restoration system in sorghum, a major fertility restoration gene was mapped along with a second locus capable of partial restoration of pollen fertility. The major fertility restoration gene, Rf(5), was located on sorghum chromosome SBI-05, and was capable of restoring pollen fertility in both A(1) and A(2) male sterile cytoplasms. Depending on the restorer parent, mapping populations exhibited fertility restoration phenotypes that ranged from nearly bimodal distribution due to the action of Rf(5), to a more normalized distribution reflecting the action of Rf(5) and additional modifier/partial restoration genes. A second fertility restoration locus capable of partially restoring pollen fertility in A(1) cytoplasm was localized to chromosome SBI-04. Unlike Rf(5), this modifier/partial restorer gene acting alone resulted in less than 10% seed set in both A(1) and A(2) cytoplasms, and modified the extent of restoration conditioned by the major restorer Rf(5) in A(1) cytoplasm. In examining the genomic regions spanning the Rf(5) locus, a cluster of pentatricopeptide gene family members with high homology to rice Rf (1) and sorghum Rf (2) were identified as potential candidates encoding Rf(5).

Integrating sorghum whole genome sequence information with a compendium of sorghum QTL studies reveals uneven distribution of QTL and of gene-rich regions with significant implications for crop improvement.

A comprehensive analysis was conducted using 48 sorghum QTL studies published from 1995 to 2010 to make information from historical sorghum QTL experiments available in a form that could be more readily used by sorghum researchers and plant breeders. In total, 771 QTL relating to 161 unique traits from 44 studies were projected onto a sorghum consensus map. Confidence intervals (CI) of QTL were estimated so that valid comparisons could be made between studies. The method accounted for the number of lines used and the phenotypic variation explained by individual QTL from each study. In addition, estimated centimorgan (cM) locations were calculated for the predicted sorghum gene models identified in Phytozome (JGI GeneModels SBI v1.4) and compared with QTL distribution genome-wide, both on genetic linkage (cM) and physical (base-pair/bp) map scales. QTL and genes were distributed unevenly across the genome. Heterochromatic enrichment for QTL was observed, with approximately 22% of QTL either entirely or partially located in the heterochromatic regions. Heterochromatic gene enrichment was also observed based on their predicted cM locations on the sorghum consensus map, due to suppressed recombination in heterochromatic regions, in contrast to the euchromatic gene enrichment observed on the physical, sequence-based map. The finding of high gene density in recombination-poor regions, coupled with the association with increased QTL density, has implications for the development of more efficient breeding systems in sorghum to better exploit heterosis. The projected QTL information described, combined with the physical locations of sorghum sequence-based markers and predicted gene models, provides sorghum researchers with a useful resource for more detailed analysis of traits and development of efficient marker-assisted breeding strategies.

Allelic variation of the ß-, γ- and δ-kafirin genes in diverse Sorghum genotypes.

The ß-, γ- and δ-kafirin genes were sequenced from 35 Sorghum genotypes to investigate the allelic diversity of seed storage proteins. A range of grain sorghums, including inbred parents from internationally diverse breeding programs and landraces, and three wild Sorghum relatives were selected to encompass an extensive array of improved and unimproved germplasm in the Eusorghum. A single locus exists for each of the expressed kafirin-encoding genes, unlike the multigenic α-kafirins. Significant diversity was found for each locus, with the cysteine-rich ß-kafirin having four alleles, including the first natural null mutant reported for this prolamin subfamily. This allele contains a frame shift insertion at +206 resulting in a premature stop codon. SDS-PAGE revealed that lines with this allele do not produce ß-kafirin. An analysis of flour viscosity reveals that these ß-kafirin null lines have a difference in grain quality, with significantly lower viscosity observed over the entire Rapid ViscoAnalyser time course. There was less diversity at the protein level within the cysteine-rich γ-kafirin, with only two alleles in the cultivated sorghums. There were only two alleles for the δ-kafirin locus among the S. bicolor germplasm, with one allele encoding ten extra amino acids, of which five were methionine residues, with an additional methionine resulting from a nucleotide substitution. This longer allele encodes a protein with 19.1% methionine. The Asian species, S. propinquum, had distinct alleles for all three kafirin genes. We found no evidence for selection on the three kafirin genes during sorghum domestication even though the δ-kafirin locus displayed comparatively low genetic variation. This study has identified genetic diversity in all single copy seed storage protein genes, including a null mutant for ß-kafirin in Sorghum.

Location of major effect genes in sorghum (Sorghum bicolor (L.) Moench).

Major effect genes are often used for germplasm identification, for diversity analyses and as selection targets in breeding. To date, only a few morphological characters have been mapped as major effect genes across a range of genetic linkage maps based on different types of molecular markers in sorghum (Sorghum bicolor (L.) Moench). This study aims to integrate all available previously mapped major effect genes onto a complete genome map, linked to the whole genome sequence, allowing sorghum breeders and researchers to link this information to QTL studies and to be aware of the consequences of selection for major genes. This provides new opportunities for breeders to take advantage of readily scorable morphological traits and to develop more effective breeding strategies. We also provide examples of the impact of selection for major effect genes on quantitative traits in sorghum. The concepts described in this paper have particular application to breeding programmes in developing countries where molecular markers are expensive or impossible to access.

Molecular mapping and candidate gene identification of the Rf2 gene for pollen fertility restoration in sorghum [Sorghum bicolor (L.) Moench].

The A1 cytoplasmic-nuclear male sterility system in sorghum is used almost exclusively for the production of commercial hybrid seed and thus, the dominant genes that restore male fertility in F(1) hybrids are of critical importance to commercial seed production. The genetics of fertility restoration in sorghum can appear complex, being controlled by at least two major genes with additional modifiers and additional gene-environment interaction. To elucidate the molecular processes controlling fertility restoration and to develop a marker screening system for this important trait, two sorghum recombinant inbred line populations were created by crossing a restorer and a non-restoring inbred line, with fertility phenotypes evaluated in hybrid combination with three unique cytoplasmic male sterile lines. In both populations, a single major gene segregated for restoration which was localized to chromosome SBI-02 at approximately 0.5 cM from microsatellite marker, Xtxp304. In the two populations we observed that approximately 85 and 87% of the phenotypic variation in seed set was associated with the major Rf gene on SBI-02. Some evidence for modifier genes was also observed since a continuum of partial restored fertility was exhibited by lines in both RIL populations. With the prior report (Klein et al. in Theor Appl Genet 111:994-1012, 2005) of the cloning of the major fertility restoration gene Rf1 in sorghum, the major fertility restorer locus identified in this study was designated Rf2. A fine-mapping population was used to resolve the Rf2 locus to a 236,219-bp region of chromosome SBI-02, which spanned ~31 predicted open reading frames including a pentatricopeptide repeat (PPR) gene family member. The PPR gene displayed high homology with rice Rf1. Progress towards the development of a marker-assisted screen for fertility restoration is discussed.

Molecular characterization of the waxy locus in sorghum.

A comparison of approximately 4.5 kb of nucleotide sequence from the waxy locus (the granule-bound starch synthase I [GBSS I] locus) from a waxy line, BTxARG1, and a non-waxy line, QL39, revealed an extremely high level of sequence conservation. Among a total of 24 nucleotide differences and 9 indels, only 2 nucleotide changes resulted in altered amino acid residues. Protein folding prediction software suggested that one of the amino acid changes (Glu to His) may result in an altered protein structure, which may explain the apparently inactive GBSS I present in BTxARG1. This SNP was not found in the second waxy line, RTx2907, which does not produce GBSS I, and no other SNPs or indels were found in the approximately 4 kb of sequence obtained from RTx2907. Using one indel, the waxy locus was mapped to sorghum chromosome SBI-10, which is syntenous to maize chromosome 9; the waxy locus has been mapped to this maize chromosome. The distribution of indels in a diverse set of sorghum germplasm suggested that there are two broad types of non-waxy GBSS I alleles, each type comprising several alleles, and that the two waxy alleles in BTxARG1 and RTx2907 have evolved from one of the non-waxy allele types. The Glu/His polymorphism was found only in BTxARG1 and derived lines and has potential as a perfect marker for the BTxARG1 source of the waxy allele at the GBSS I locus. The indels correctly predicted the non-waxy phenotype in approximately 65% of diverse sorghum germplasm. The indels co-segregated perfectly with phenotype in two sorghum populations derived from crosses between a waxy and a non-waxy sorghum line, correctly identifying heterozygous lines. Thus, these indel markers or sequence-based SNP markers can be used to follow waxy alleles in sorghum breeding programs in selected pedigrees.

QTL analysis of ergot resistance in sorghum.

Sorghum ergot, caused predominantly by Claviceps africana Frederickson, Mantle, de Milliano, is a significant threat to the sorghum industry worldwide. The objectives of this study were firstly, to identify molecular markers linked to ergot resistance and to two pollen traits, pollen quantity (PQ) and pollen viability (PV), and secondly, to assess the relationship between the two pollen traits and ergot resistance in sorghum. A genetic linkage map of sorghum RIL population R931945-2-2 x IS 8525 (resistance source) was constructed using 303 markers including 36 SSR, 117 AFLP , 148 DArT and two morphological trait loci. Composite interval mapping identified nine, five, and four QTL linked to molecular markers for percentage ergot infection (PCERGOT), PQ and PV, respectively, at a LOD >2.0. Co-location/linkage of QTL were identified on four chromosomes while other QTL for the three traits mapped independently, indicating that both pollen and non pollen-based mechanisms of ergot resistance were operating in this sorghum population. Of the nine QTL identified for PCERGOT, five were identified using the overall data set while four were specific to the group data sets defined by temperature and humidity. QTL identified on SBI-02 and SBI-06 were further validated in additional populations. This is the first report of QTL associated with ergot resistance in sorghum. The markers reported herein could be used for marker-assisted selection for this important disease of sorghum.

Resistance gene analogues in sugarcane and sorghum and their association with quantitative trait loci for rust resistance.

Fifty-four different sugarcane resistance gene analogue (RGA) sequences were isolated, characterized, and used to identify molecular markers linked to major disease-resistance loci in sugarcane. Ten RGAs were identified from a sugarcane stem expressed sequence tag (EST) library; the remaining 44 were isolated from sugarcane stem, leaf, and root tissue using primers designed to conserved RGA motifs. The map location of 31 of the RGAs was determined in sugarcane and compared with the location of quantitative trait loci (QTL) for brown rust resistance. After 2 years of phenotyping, 3 RGAs were shown to generate markers that were significantly associated with resistance to this disease. To assist in the understanding of the complex genetic structure of sugarcane, 17 of the 31 RGAs were also mapped in sorghum. Comparative mapping between sugarcane and sorghum revealed syntenic localization of several RGA clusters. The 3 brown rust associated RGAs were shown to map to the same linkage group (LG) in sorghum with 2 mapping to one region and the third to a region previously shown to contain a major rust-resistance QTL in sorghum. These results illustrate the value of using RGAs for the identification of markers linked to disease resistance loci and the value of simultaneous mapping in sugarcane and sorghum.

Markers associated with stalk number and suckering in sugarcane colocate with tillering and rhizomatousness QTLs in sorghum.

Two important factors influencing sugar yield, the primary focus of sugarcane plant breeding programs, are stalk number and suckering. Molecular markers linked to both of these traits are sought to assist in the identification of high sugar yield, high stalk number, low-suckering sugarcane clones. In this preliminary mapping study, 108 progeny from a biparental cross involving two elite Australian sugarcane clones were evaluated at two sites for two years for both stalk number and suckering. A total of 258 DNA markers, including both restriction fragment length polymorphisms (RFLPs) and radio-labelled amplified fragments (RAFs), were scored and evaluated using single-factor analysis. Sixteen (7 RFLPs and 9 RAFs) and 14 (6 RFLPs and 8 RAFs) markers were identified that were significantly associated (P < 0.01) with stalk number and suckering, respectively, across both years and sites. The seven and six RFLP markers associated with stalk number and suckering, respectively, were generated by eight different RFLP probes, of which seven had been mapped in sorghum and (or) sugarcane. Of significant interest was the observation that all seven RFLP probes could be shown to be located within or near QTLs associated with tillering and rhizomatousness in sorghum. This observation highlights the usefulness of comparative mapping between sorghum and sugarcane and suggests that the identification of useful markers for stalk number and suckering in sugarcane would be facilitated by focussing on sorghum QTLs associated with related traits.

Fibrovascularization of porous polyethylene (Medpor) orbital implant in a rabbit model.

PURPOSE: To evaluate the porous polyethylene (Medpor) orbital implant in a rabbit model and compare it with three other currently available porous implants: Bio-Eye coralline hydroxyapatite (HA), FCI(3) synthetic HA, and aluminum oxide (Bioceramic). METHODS: The porous polyethylene implant was examined macroscopically and microscopically (with scanning electron microscopy). Implantation was performed in 10 adult male New Zealand albino rabbits. Each animal underwent enucleation of the right globe under general halothane gas anesthesia, followed by placement of a 12-mm porous polyethylene implant. In 5 animals, the implant was encased in polyglactin 910 (Vicryl mesh); in the other 5, it was left unwrapped. The implants were moistened in saline before placement. Implant vascularization was evaluated by histopathology at 4, 8, 12, 16, and 24 weeks. RESULTS: The porous polyethylene implant was found to have a smoother exterior surface than the Bio-Eye, FCI(3) synthetic HA, and aluminum oxide implants. Rather than a uniform interconnected porous architecture, there was an extensive system of interconnected channels through the implant, ranging in size from 125 to 1000 microm. On high-power examination there was a more solid, woven appearance without any sign of the microcrystals seen in the other porous implants. One rabbit had a retrobulbar hemorrhage after surgery and was euthanized. All the other rabbits tolerated the implant well, and there were no complications. On histopathologic examination, fibrovascularization gradually increased over time. One implant was completely vascularized at 12 weeks, and both implants harvested at 16 weeks were completely vascularized. The implant harvested at 24 weeks showed only partial vascularization (14%). CONCLUSIONS: The porous polyethylene orbital implant represents an alternative implant for use after enucleation or evisceration or for secondary implantation. In our rabbit model, the porous polyethylene implant was well tolerated without complication. Complete fibrovascularization was first seen at 12 weeks. Porous polyethylene orbital implants appear to vascularize more slowly than Bio-Eye coralline HA, FCI(3) synthetic HA, and aluminum oxide implants.

Identifications of two different mechanisms for sorghum midge resistance through QTL mapping.

Sorghum midge is the one of the most damaging insect pests of grain sorghum production worldwide. At least three different mechanisms are involved in midge resistance. The genetic bases of these mechanisms, however, are poorly understood. In this study, for the first time, quantitative trait loci associated with two of the mechanisms of midge resistance, antixenosis and antibiosis, were identified in an RI (recombinant inbred) population from the cross of sorghum lines ICSV745 x 90562. Two genetic regions located on separate linkage groups were found to be associated with antixenosis and explained 12% and 15%, respectively, of the total variation in egg numbers/spikelet laid in a cage experiment. One region was significantly associated with antibiosis and explained 34.5% of the variation of the difference of egg and pupal counts in the RI population. The identification of genes for different mechanisms of midge resistance will be particularly useful for exploring new sources of midge resistance and for gene pyramiding of different mechanisms for increased security in sorghum breeding through marker-assisted selection.