Phenotypic and QTL analyses of herbage production-related traits in perennial ryegrass (<Emphasis Type="Italic">Lolium perenne</Emphasis> L.)

Phenotypic and genetic evaluation of morphological traits associated with herbage biomass production was undertaken in a perennial ryegrass (Lolium perenne L.) biparental F₁ mapping population (n = 200) with parent plants from cultivars ‘Grasslands Impact’ and ‘Grasslands Samson’. Morphological traits measured on three clonal replicates of the parental genotypes and 200 F₁ progeny in a glasshouse in two separate trials (autumn and spring) included: dry weight (DW), leaf elongation rate (LER), initial tiller number (TNs), final tiller number (TNe), site filling (Fs), tiller weight (TW), leaf lamina length, leaf tip and ligule appearance rates (ALf, ALg) and leaf elongation duration (LED). Principal component analysis of patterns of trait association identified negative correlation between TNs or TNe, and TW as the primary basis for morphological difference and indicated that either high LER or long LED could reduce TN. Plants with higher LER tended to have increased DW. Quantitative trait loci (QTL) were detected on all seven linkage groups (LG) of a perennial ryegrass linkage map for all but three traits. A total of 61 QTL were identified, many of which clustered at 15 shared genome locations. Significant genotype by environment effects were encountered, evidenced both by variation between experiments in genotype rankings and by a general lack of commonality for QTL for the same traits in the different experiments. Only five QTL, for ALf, ALg and TN, were conserved between autumn and spring trials. A QTL for TN and DW on LG6 is a strong candidate for application of MAS in future plant improvement work and was found to be co-linear with QTL for equivalent traits reported on chromosome 2 in rice.     

Freezing tolerance of selected perennial ryegrass (<Emphasis Type="Italic">Lolium perenne</Emphasis> L.) accessions and its association with field winterhardiness and turf traits

Reproductive output and fertility variation in teak were evaluated in five study sites in Western Ghats region of southern India. In four seed production areas (SPA1-4)) and a natural population (NAT) plots with 100 trees were investigated. The 40-year-old seed production areas are located in high rainfall (>2,500 mm) sites in Kerala state and the natural stand is at a drier location in Tamil Nadu. Fruit production per tree was rather high in the natural stand (1500) and quite variable in the seed production areas. SPA 4, located far south from the other sites, had the highest fruit production (almost twice that in the other study sites), high relative population size (N _r = 0.5) and predicted low genetic drift in two consecutive years. Fecundity was very low in SPAs 2 & 3 (200 fruits per tree). Reproductive output was similar in successive years except SPA 1, where the fruit production in the first year was almost five times that in the second year. Fertility variation (computed as sibling coefficient, Ψ) was in general higher in NAT and hence the relative population size was lower than in SPAs. The variation in female fertility in the natural stand was, somewhat surprisingly, similar to that in the seed production areas, while the male fertility variation was higher. In the seed production areas the male fertility variation was lower or similar to the female fertility variation. The fertility variation, Ψ was on average around 3 and correspondingly the relative effective population size, N _r = 0.36 and the variance effective number around 60. Selection for tree diameter would improve predicted diversity of the seed crop as it was positively correlated with reproductive output and negatively correlated with fertility variation among trees.     

Spaced-plant traits related to seed yield in plots of perennial ryegrass (Lolium perenne L.)

Summary Seed yield in perennial ryegrass is low and selection criteria for high seed production are lacking. Indirect selection in spaced plants would be efficient, but spaced-plant traits need to be identified that correlate with seed yield in drilled plots. Spaced plants were studied of nine perennial ryegrass cultivars with a different seed production when grown in plots. Genotypic variation within cultivars for seed yield components and other traits was assessed in four environments in replicated clonal experiments. Extensive genotypic variation for most traits was present within each cultivar. Based on cultivar means of 25 or 50 genotypes, significant differences among cultivars were found for most traits. Cultivar-year interactions were found for most traits, but no significant cultivar-location interactions were generally found. Spaced-plant traits in general showed poor correlation to corresponding traits in drilled plots. Cultivars with contrasting seed yield in plots could not be distinguished on the basis of their spaced-plant traits and therefore no criteria for indirect selection for seed production in drilled plots could be identified in spaced plants.     

Seed yield related to crop development and to yield components in nine cultivars of perennial ryegrass (Lolium perenne L.)

Summary We had previously found differences for seed yield among nine perennial ryegrass cultivars which were not associated with variation for seed weight. To detect the physiological basis of these genetic differences for seed yield, growth analyses were carried out. We related crop development and components of seed yield to seed yield during three years on clay and sandy soil. No significant differences occurred among cultivars for accumulation and partitioning of dry matter or the pattern of tiller production. Seed yield of the cultivars was not associated with ear number or total dry matter yield of the seed crop. Seed yield was more correlated with the number of seeds per unit area than with seed weight. The number of seeds as calculated after harvest from seed yield and seed weight was much lower than the number of seeds as estimated prior to harvest from seed yield components. The number of spikelets differed significantly among the cultivars, but the ranking was different from that for seed yield. The physiological basis of the genetic differences for seed yield is not clear. Implications for breeding perennial ryegrass are discussed.     

Further QTL mapping for yield component traits using introgression lines in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) under drought field environments

To better understand the underlying mechanisms of agronomic traits related to drought resistance and discover candidate genes or chromosome segments for drought-tolerant rice breeding, a fundamental introgression population, BC₃, derived from the backcross of local upland rice cv. Haogelao (donor parent) and super yield lowland rice cv. Shennong265 (recurrent parent) had been constructed before 2006. Previous quantitative trait locus (QTL) mapping results using 180 and 94 BC₃F_6,7 rice introgression lines (ILs) with 187 and 130 simple sequence repeat (SSR) markers for agronomy and physiology traits under drought in the field have been reported in 2009 and 2012, respectively. In this report, we conducted further QTL mapping for grain yield component traits under water-stressed (WS) and well-watered (WW) field conditions during 3 years (2012, 2013 and 2014). We used 62 SSR markers, 41 of which were newly screened, and 492 BC₄F_2,4 core lines derived from the fourth backcross between D123, an elite drought-tolerant IL (BC₃F₇), and Shennong265. Under WS conditions, a total of 19 QTLs were detected, all of which were associated with the new SSRs. Each QTL was only identified in 1 year and one site except for qPL-12-1 and qPL-5, which additively increased panicle length under drought stress. qPL-12-1 was detected in 2013 between new marker RM1337 and old marker RM3455 (34.39 cM) and was a major QTL with high reliability and 15.36% phenotypic variance. qPL-5 was a minor QTL detected in 2013 and 2014 between new marker RM5693 and old marker RM3476. Two QTLs for plant height (qPHL-3-1 and qPHP-12) were detected under both WS and WW conditions in 1 year and one site. qPHL-3-1, a major QTL from Shennong265 for decreasing plant height of leaf located on chromosome 3 between two new markers, explained 22.57% of phenotypic variation with high reliability under WS conditions. On the contrary, qPHP-12 was a minor QTL for increasing plant height of panicle from Haogelao on chromosome 12. Except for these two QTLs, all other 17 QTLs mapped under WS conditions were not mapped under WW conditions; thus, they were all related to drought tolerance. Thirteen QTLs mapped from Haogelao under WS conditions showed improved drought tolerance. However, a major QTL for delayed heading date from Shennong265, qDHD-12, enhanced drought tolerance, was located on chromosome 12 between new marker RM1337 and old marker RM3455 (11.11 cM), explained 21.84% of phenotypic variance and showed a negative additive effect (shortening delay days under WS compared with WW). Importantly, chromosome 12 was enriched with seven QTLs, five of which, including major qDHD-12, congregated near new marker RM1337. In addition, four of the seven QTLs improved drought resistance and were located between RM1337 and RM3455, including three minor QTLs from Haogelao for thousand kernel weight, tiller number and panicle length, respectively, and the major QTL qDHD-12 from Shennong265. These results strongly suggested that the newly screened RM1337 marker may be used for marker-assisted selection (MAS) in drought-tolerant rice breeding and that there is a pleiotropic gene or cluster of genes linked to drought tolerance. Another major QTL (qTKW-1-2) for increasing thousand kernel weight from Haogelao was also identified under WW conditions. These results are helpful for MAS in rice breeding and drought-resistant gene cloning.     

Haplotype analysis of <Emphasis Type=Italic>Viviparous</Emphasis>-<Emphasis Type=Italic>1</Emphasis> gene in CIMMYT elite bread wheat germplasm

Pre-harvest sprouting (PHS) greatly reduces the quality and economic value of wheat grain. In this study, a total of 168 International Maize and Wheat Improvement Center (CIMMYT) wheat germplasm lines were examined to characterize the haplotypes of Vp-1A, Vp-1B and Vp-1D, which are located on the long arms of chromosomes 3A, 3B and 3D, respectively. Among them, five new alleles of Vp-1Aa (the wild allele) were identified on chromosome 3A, and designated as Vp-1Ab, Vp-1Ac, Vp-1Ad, Vp-1Ae and Vp-1Af, respectively. The main difference between Vp-1Aa and the newly identified alleles was in the numbers of CTT repeats located in the third intron, but Vp-1Af also had 6 and 2 bp deletions at positions 2860–2865 bp and 2880–2881 bp, and possessed five SNPs within the same intron region. In the Vp-1B locus, several alleles (Vp-1Ba, Vp-1Bb, Vp-1Bc, Vp-1Bd, Vp-1Be and Vp-1Bf) have already been identified. In the present material only two, the already known allele Vp-1Bc, and a new one, designated as Vp-1Bg, were detected. Compared with Vp-Ba, Vp-1Bg had additional insertion of TCC at position 2372 bp and a 9 bp change from CTGCATC AC to GCATCAGTG at 2417–2425 bp. However, no polymorphism was detected in Vp-1D. The frequencies of Vp-1Aa Vp-1Ab, Vp-1Ac, Vp-1Ad, Vp-1Ae, and Vp-1Af were 65, 10, 11, 4, 5 and 5%, respectively. For Vp-1B, 155 out of the 168 lines were Vp-1Bc; the remaining 13 were Vp-1Bg. Analyses of the germination index (GI) and abscisic acid (ABA) sensitivity showed that genotypes with Vp-1Ab or Vp-1Af showed higher PHS resistance than the ones with other alleles, suggesting that they might be valuable for CIMMYT breeding program or germplasm introduction. The results presented here will underpin the introduction of germplasm from CIMMYT and the improvement of PHS resistance, both in CIMMYT and elsewhere.     

Association of agronomic traits with isozyme loci in perennial ryegrass (Lolium perenne L.)

Summary Inbred lines possessing different genotypes at three isozyme loci, PGI-2, GOT-3 and ACP-1, were produced by two generations of selfing from two contrasting cultivars of perennial ryegrass. Four separate F₂ populations were obtained by selfing or crossing F₁ plants from crosses between cultivar inbred lines. F₂ plants were scored for genotype at the three isozyme loci and for eight agronomic traits. The three isozyme loci showed independent segregation confirming that they belonged to separate linkage groups. A consistent association was observed between water soluble carbohydrate content and genotype at the PGI-2 locus. Some association was also detected between genotype at the ACP-1 locus and heading date. The relevance of these observations to ryegrass breeding programmes is discussed.     

Identification of genetic factors affecting plant density response through QTL mapping of yield component traits in maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

It is generally believed that grain yield per unit area of modern maize hybrids is related to their adaptability to high plant population density. In this study, the effects of two different plant densities (52,500 and 90,000 plants/hm²) on 12 traits associated with yield were evaluated using a set of 231 F_2:3 families derived from two elite inbred lines, Zheng58 and Chang7-2. Evaluation of the phenotypes expressed under the two plant density conditions showed that high plant density condition could decrease the value of 10 measured yield component traits, while the final grain yield per hectare and the rate of kernel production were increased. Twenty-seven quantitative trait loci (QTLs) for 10 traits were detected in both high and low plant density conditions; among them, some QTLs were shown to locate in five clusters. Thirty QTLs were only detected under high plant density. These results suggest that some of the yield component traits perhaps were controlled by a common set of genes, and that kernel number per row, ear length, row number per ear, cob diameter, cob weight, and ear diameter may be influenced by additional genetic mechanisms when grown under high plant density. The QTLs identified in this study provide useful information for marker-assisted selection of varieties targeting increased plant density.     

Heritability estimates of spaced-plant traits in three perennial ryegrass (Lolium perenne L.) cultivars

Summary Seed yield in perennial ryegrass is low and unpredictable. Spaced-plant traits suitable for indirect selection for total seed yield in drilled plots would be very useful. The objectives of this investigation were to evaluate genetic variation for seed yield components and other traits among clones from three perennial ryegrass cultivars differing in seed yield and their open-pollinated progenies. Per cultivar, a random set of 50 genotypes was cloned and on each genotype seed was generated by open pollination. Clonal ramets of the parents were observed for 17 traits in 1986 at two locations. In 1987 and 1988, parents and progenies were observed as single plants in a randomized complete block design with two replications. There was little cultivar-environment interaction for most traits. The parents differed significantly for almost all traits. Half-sib (HS) families differed for only three to five traits. Broad-sense heritabilities (h² _b), based on variance components of the parents, were moderate to high; earliness had the highest h_b ². Narrow-sense heritabilities (h² _n), based on variance components among HS-families, were low to moderate and mostly not significant; for most traits h² _n estimates varied between years and cultivars. Flag leaf width and date of first anthesis showed the highest h² _n. Narrow-sense heritability estimates from parent-offspring regressions (h² _nPO) ranged from non-significant to high, depending on year and cultivar; they were generally higher than the corresponding h² _n estimates. Generally, h² _nPO was highest for earliness, flag leaf width, ear length and the number of spikelets per ear. Breeding methods that capitalize on additive genetic variance, such as mass selection, should result in improvement for these traits.     

QTL mapping of agronomically important traits in sorghum (<Emphasis Type="Italic">Sorghum bicolor</Emphasis> L.)

Sorghum is one of the most important cereal crops; it is used to produce feed, sugar, and biofuel. To investigate genetic tradeoffs between grain and stem sugar production, we evaluated plant height, Brix (the percentage of soluble solids in stalk juice), 100-grain weight and flowering time over 3 years in a recombinant inbred line (RIL) population consisting of 189 individuals derived from a cross between the sweet sorghum cultivar ‘Rio’ and grain sorghum ‘BTx623’. We constructed a genetic linkage map (total length, 1418.71 cM; average distance between markers, 11.26 cM), which consisted of 118 simple sequence repeat (SSR) and 8 insertion-deletion (INDEL) markers. A total of 14 QTLs were detected on chromosomes 1, 3, 6, 7, and 9, which included 6 QTLs for plant height; 4 for Brix; and 2 QTLs for each 100-grain weight and flowering time. Eight QTLs were detected at least in 2 years. These results will be useful for future QTL fine mapping and gene mining for these traits, and useful for the improvement of sorghum through molecular marker-assisted selection.     

Path and correlation analysis of perennial ryegrass (Lolium perenne L.) seed yield components

Maximum perennial ryegrass seed production potential is substantially greater than harvested yields with harvested yields representing only 20% of calculated potential. Similar to wheat, maize and other agriculturally important crops, seed yield is highly dependent on a number of interacting seed yield components. This research was performed to apply and describe path analysis of perennial ryegrass seed yield components in relation to harvested seed yields. Utilising extensive yield components which included subdividing reproductive inflorescences into five size categories, path analysis was undertaken assuming a unidirectional causal‐admissible relationship between seed yield components and harvested seed yield in six commercial seed production fields. Both spikelets per inflorescence and florets per spikelet had a significant (p < .05) direct effect on the overall seed yield; however, total path correlation coefficients showed that inflorescence size had the largest positive influence on seed yield via its indirect effects on other seed yield components. Our results will allow seed producers, seed production researchers and plant breeders alike to more efficiently increase harvested seed yield.     

Identification of coincident QTL for days to heading,spike length and spikelets per spike in <Emphasis Type="Italic">Lolium perenne</Emphasis> L.

Flowering time is a trait which has a major influence on the quality of forage. In addition, flowering and subsequent seed yields are important traits for seed production by grass breeders. In this study, we have identified quantitative trait loci (QTL) for flowering time and morphological traits of the flowering head in an F₁ mapping population in Lolium perenne L (perennial ryegrass), a number of which have not previously been identified in L. perenne mapping studies. QTL for days to heading (DTH) were mapped in both outdoor and glasshouse experiments, revealing three and five QTL for DTH which explained 53% and 42% of the total phenotypic variation observed, respectively. Two QTL for DTH were detected in both environments, although they had contrasting relative magnitudes in each environment. One QTL for spike length and three QTL for spikelets per spike were also identified explaining, a total of 32 and 33% of the phenotypic variance, respectively. Furthermore, the QTL for spike length and spikelets per spike generally coincided with QTL for days to heading, implying co-ordinate regulation by underlying genes. Of particular interest was a region harbouring overlapping QTL for days to heading, spike length and spikelets per spike on the top of linkage group 4, containing the major QTL for spike length identified in this population.     

Genetic analysis of resistance to nematodes in inbred maize (<Emphasis Type=Italic>Zea mays</Emphasis> L.) and maize hybrids

Nematodes cause substantial grain yield loss in susceptible maize (Zea mays L.) cultivars. This study was conducted to estimate general combining ability (GCA), specific combining ability (SCA) and genetic effects associated with nematode resistance in maize. The 30 F ₁ hybrids generated from a 6 × 6 diallel and two local checks were evaluated in 2009 at three sites in Uganda. A split plot design was used with nematode treatments serving as whole plots and the hybrids as subplots but arranged in an 8 × 4 spatially adjusted alpha-lattice design. The experiment was replicated three times. Results showed GCA to be important for the reduction of P. zeae and Meloidogyne spp. densities and increase of root mass, with a contribution of 72 to 93% of the phenotypic variance. Inbreds MP709 and CML206 had the highest GCA for Pratylenchus zeae resistance, whereas for grain yield, it was CML444, CML312 and CML395 that were outstanding. The SCA influenced plant height and grain yield under nematode infestation, contributing 43 and 58% of the phenotypic variance, respectively. Observed reciprocal differences due to maternal effects also played a role in influencing the grain yield under nematode infestation. Overdominance genetic effects explained the non-additive variance recorded for the plant height, grain yield, number of root lesions, and P. zeae and Meloidogyne spp. densities under nematode infestation. The parents MP709, CML206, 5057, and CML444 contributed most of the dominant genes for the P. zeae resistance in all their crosses. The parent CML444 contributed most of the dominant genes for improved grain yield in all of its crosses. The high GCA effects among some parents support their utility in breeding of widely adapted nematode-resistant cultivars. The dominant genes and SCA effects would favour pedigree and various sib tests to improve grain yield under nematode pressure.     

Effects of introgressions from <Emphasis Type="Italic">Festuca pratensis</Emphasis> on winter hardiness of <Emphasis Type="Italic">Lolium perenne</Emphasis>

Previous studies reported that some genotypes with introgressed Festuca chromosome segment(s) in Lolium genome showed enhanced winter hardiness compared to Lolium. The aim of this study was to search comprehensively for the Festuca pratensis chromosome regions affecting winter hardiness-related traits when introgressed into the Lolium perenne genome. Association between F. pratensis introgression and winter hardiness-related traits (fall and winter hardiness indexes, early-spring dry matter yield, and freezing tolerance) were screened in the diploid introgression populations (n = 203) that had some F. pratensis chromosome segments introgressed. Eighty-four intron markers corresponding to unique rice genes randomly distributed across the genome were used for genotyping. Winter hardiness of almost all plants in the introgression populations was lower than that of the F. pratensis and triploid hybrid parents, but the average was higher than that of L. perenne. A significant positive effect of F. pratensis introgression on early-spring dry matter yield was detected on chromosome 7. This quantitative trait locus (QTL) was confirmed by linkage analysis using a backcross population with F. pratensis introgression in the target region of chromosome 7. However, the contribution of the newly identified QTL was rather small (6.7–9.6%), suggesting that superior winter hardiness of F. pratensis compared to L. perenne is conferred by multiple small-effect QTLs. We also detected a previously unreported negative effect of Festuca introgression on winter hardiness. Newly obtained QTL information in this study would contribute to the design of Festuca/Lolium hybrid breeding.     

Improved seed yield in perennial ryegrass (Lolium perenne L.) from two generations of phenotypic selection

Two generations of recurrent phenotypic selection for seed yield per plant under controlled pollination were carried out to combine improved seed yield with improved forage grass performance in perennial ryegrass (Lolium perenne L.). Selected and unselected varieties (AberDartand AberElan respectively) and control varieties were grown for seed in pots in a glasshouse experiment and in two field plot experiments over 5 harvest years. Seed yield components and seed yield were measured. Significant improvements in seed yield of the selected AberDart over the unselected AberElan were observed both in individual plants grown in the glasshouse and in field plots. Detailed observations of the individual components of seed yield showed that the increased seed yield of AberDart can be attributed to a higher proportion of ovules forming seeds (% seedset), greater seed number per tiller and more reproductive tillers per plant. This improvement in seed yield of AberDart was also observed when it was grown in field plots with AberElan, experimental selection lines and commercial varieties. AberDart produced more seeds per tiller than the other commercial varieties and selection lines studied. The implication of this approach for the breeding of perennial ryegrass varieties that combine good agronomic performance and acceptable seed yields is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance of anther-culture response in perennial ryegrass (Lolium perenne L.)

Genetic control of the capacity to respond to anther culture in perennial ryegrass was studied in F₁ offspring from crosses between 11 clones selected for anther-culture response (‘inducers’) and 10 clones selected from breeding material on the basis of good agronomic performance. Large differences in anther-culture response were observed between the two types of parent, with inducer clones producing an average of 10.7 green plants per 100 anthers, compared with only 0.3 for breeding clones. Hybrid populations produced an average of 2.2 green plants per 100 plated anthers, which is 7.3 times the response of the breeding material. This improvement was mainly due to a 9.4-fold increase in the percentage of green plants regenerated from hybrid populations to 4.7% compared to an average of 0.5% from breeding clones. Most of the GCA (general combining ability) in the experiment was contributed by the breeding material, constituting 55.8, 100.0 and 36.7% of genetic variation among hybrids for embryo formation, regeneration and green plant percentage, respectively. Any remaining genetic variation among hybrids was predominantly due to SCA (specific combining ability) effects, with percentages of 44.2 and 63.3%, respectively, for embryo and green plant formation. These results are discussed with respect to the possible exploitation of genes controlling anther-culture response for haploid production in breeding programmes of perennial ryegrass.