Genetic linkage mapping of an annual × perennial ryegrass population

Annual (Lolium multiflorum Lam.) and perennial (L. perenne L.) ryegrass are two common forage and turfgrass species grown throughout the world. Perennial ryegrass is most commonly used for turfgrass purposes, and contamination by annual ryegrass, through physical seed mixing or gene flow, can result in a significant reduction in turfgrass quality. Seed certifying agencies in the United States currently use a test called seedling root fluorescence (SRF) to detect contamination between these species. The SRF test, however, can be inaccurate and therefore, the development of additional markers for species separation is needed. Male and female molecular-marker linkage maps of an interspecific annual × perennial ryegrass mapping population were developed to determine the map location of the SRF character and to identify additional genomic regions useful for species separation. A total of 235 AFLP markers, 81 RAPD markers, 16 comparative grass RFLPs, 106 SSR markers, 2 isozyme loci and 2 morphological characteristics, 8-h flowering, and SRF were used to construct the maps. RFLP markers from oat and barley and SSR markers from tall fescue and other grasses allowed the linkage groups to be numbered, relative to the Triticeae and the International Lolium Genome Initative reference population P150/112. The three-generation population structure allowed both male and female maps to be constructed. The male and female maps each have seven linkage groups, but differ in map length with the male map being 537 cm long and the female map 712 cm long. Regions of skewed segregation were identified in both maps with linkage groups 1, 3, and 6 of the male map showing the highest percentage of skewed markers. The (SRF) character mapped to linkage group 1 in both the male and female maps, and the 8-h flowering character was also localized to this linkage group on the female map. In addition, the Sod-1 isozyme marker, which can separate annual and perennial ryegrasses, mapped to linkage group 7. These results indicate that Lolium linkage groups 1 and 7 may provide additional markers and candidate genes for use in ryegrass species separation.Communicated by C. Möllers     

Does selfing provide reproductive assurance in the perennial herb Bulbine vagans (Asphodelaceae)?

Selection for selfing to provide reproductive assurance depends on the balance between increased reproductive output when pollinators or potential mates are scarce and the extent that inbreeding depression erodes such fertility gains. We use glasshouse and field experiments to examine the benefits of autonomous and facilitated selfing in Bulbine vagans . Autonomous selfing was delayed until after opportunities for outcrossing and reproductive output was 0.67 relative to manual selfing and open pollination. Values less than one probably reflected insufficient autonomous deposition of self pollen. In the field, reproductive output of emasculated flowers was 0.50 relative to intact flowers that could both outcross and self, indicating that outcross pollen was limited and that selfing boosted reproductive output. Because all pollen was removed from anthers before intact flowers closed, facilitated selfing rather than autonomous selfing occurred. In the glasshouse, inbreeding depression was 0.45, but under natural conditions would probably exceed 0.5. Values greater than 0.5 negate the automatic gene transmission advantage afforded by selfing and increasingly erode the benefits of reproductive assurance. We conclude that in B. vagans delayed and facilitated selfing can confer reproductive assurance, providing the latter does not usurp ovules that could be outcrossed.     

Molecular cloning and genetic mapping of perennial ryegrass casein protein kinase 2 α-subunit genes

The α-subunit of the casein protein kinase CK2 has been implicated in both light-regulated and circadian rhythm-controlled plant gene expression, including control of the flowering time. Two putative CK2α genes of perennial ryegrass (Lolium perenne L.) have been obtained from a cDNA library constructed with mRNA isolated from cold-acclimated crown tissue. The genomic organisation of the two genes was determined by Southern hybridisation analysis. Primer designs to the Lpck2a-1 and Lpck2a-2 cDNA sequences permitted the amplification of genomic products containing large intron sequences. Amplicon sequence analysis detected single nucleotide polymorphisms (SNPs) within the p150/112 reference mapping population. Validated SNPs, within diagnostic restriction enzyme sites, were used to design cleaved amplified polymorphic sequence (CAPS) assays. The Lpck2a-1 CAPS marker was assigned to perennial ryegrass linkage group (LG) 4 and the Lpck2a-2 CAPS marker was assigned to LG2. The location of the Lpck2a-1 gene locus supports the previous conclusion of conserved synteny between perennial ryegrass LG4, the Triticeae homoeologous group 5L chromosomes and the corresponding segment of rice chromosome 3. Allelic variation at the Lpck2a-1 and Lpck2a-2 gene loci was correlated with phenotypic variation for heading date and winter survival, respectively. SNP polymorphism may be used for the further study of the role of CK2α genes in the initiation of reproductive development and winter hardiness in grasses.     

Can perennial C4 grasses attain high efficiencies of radiant energy conversion in cool climates?

Although C₄ plants are considered to have higher conversion efficiency and productivity than C₃, plants, this advantage may not be realized under sub-optimal conditions. Two perennial C₄ rhizomatous grasses of cool temperate origin, Miscanthus × giganteus and Spartina cyno-suroides, have been suggested as potential fuel crops for north-western Europe. The conversion efficiencies of these species were examined for 2 years in fertilized, irrigated, replicated plots in south-eastern England. In the second year, the energy conversion efficiencies for shoot and total biomass production were 0.040 and 0.051, respectively, for S. cynosuroides, with significantly higher values of 0.060 and 0.078, respectively, for M. × giganteus. The M. × giganteus crop attained shoot productivity of 2.87 kg m^?2 between April and September, exceeding the highest values typically obtained with intensively managed C₃, crops. Canopy development was early, and high interception and conversion efficiencies were maintained over most of the growing season. This study provides the evidence that the superior potential light conversion efficiencies associated with C₄ photosynthesis can be realized under cool temperate conditions and that such climatic conditions do not Inherently impair the C₄ process.     

How does herbivory affect individuals and populations of the perennial herb Paeonia officinalis?

In this study we quantified variability in foliage herbivory and pre-dispersal seed predation and its effects on plant performance and demography in populations of a rare and protected perennial herb, Paeonia officinalis. An individual-based survey was performed during four years in four populations, which contained plants in both open habitat and woodland. We detected marked spatial and temporal variation among and within populations in foliage herbivory (by insects) and pre-dispersal seed predation (by insects, rodents and Roe Deer). Foliage herbivory decreased with plant demographic stages in open habitats, from seedlings to reproductive individuals, but no significant trend was detected in woodland habitats. This may be due to different demographic origin of larger vegetative plants in this habitat. Depending on demographic stage, herbivory was higher in open habitats or not significantly different between habitats. This suggests differences in herbivore abundance in different habitats within sites. Pre-dispersal seed predation remained weak and did not depend on habitat. We did not detect any consequence of foliage herbivory on seedling mortality and individual growth in our study. Our results illustrate the need to investigate plant-herbivore interactions over several years in distinct populations in order to more accurately evaluate herbivore impact on plant population dynamics.     

Does polyembryony confer a competitive advantage to the invasive perennial vine Vincetoxicum rossicum (Apocynaceae)?

Determining which traits may allow some introduced plant species to become invasive in their new environment continues to be a key question in invasion biology. Vincetoxicum rossicum is an invasive, perennial vine colonizing natural and seminatural habitats primarily in the northeastern United States and southeastern Canada. More than half its seeds exhibit polyembryony, a relatively uncommon condition in which a single seed produces multiple seedlings. For evaluating the potential consequences of polyembryony on invasiveness, V. rossicum plants derived from seeds of three embryonic classes-singlets, doublets, and triplets (one, two, and three seedlings per seed, respectively)-were paired in all combinations intraspecifically and with the co-occurring native herbs Solidago canadensis and Asclepias syriaca in a greenhouse study. Vincetoxicum rossicum biomass was 25-55% greater and follicle production 55-100% greater under intraspecific competition compared with interspecific competition. However, within a competitive environment, follicle production varied little. Regardless of competitive environment, V. rossicum originating from seeds with a greater number of embryos typically performed no better than plants arising from seed with fewer embryos (singlets = doublets = triplets)-except intraspecifically where doublets outperformed singlets, and with S. canadensis where triplets outperformed singlets. Our findings suggest that overall performance and fitness of V. rossicum is higher in monocultures than in mixed stands and that its ability to invade new habitats may not be attributable to the production of polyembryonic seeds.     

Pollination ecology of Penstemon roseus (Plantaginaceae), an endemic perennial shifted toward hummingbird specialization?

Summary Pink-flowered tubular Penstemon roseus (Plantaginaceae), which has shifted partially to hummingbird pollination, blooms on high-elevation slopes in the mountains in Tlaxcala, Mexico. We studied the interactions between pollinator visitation rates to flowers, pollen removal and deposition, flower size, and nectar removal frequency on seed production in P. roseus. We combine observational and experimental studies in two contrasting natural populations. Our manual pollinations revealed that P. roseus is fully self-compatible. Autonomous self- and manual self-pollinated flowers matured as many seeds as when outcrossed, but outcrossing seems to become better than selfing as the flowering season progressed. Early in the season flowers that were bagged and hand-selfed, hand-outcrossed, or autonomously selfed, or unbagged and naturally pollinated had equal seed set in all four treatments. But later in the season, outcross pollen gave approximately twice as much seed set as the two self-treatments. Low levels of pollen receipt and pollen removal were consistent with the long time elapsed for a given plant to be visited by hummingbirds, which suggests pollen shortage in both sites. Despite differences in pollinator visitation rates to flowers, probability of flower visitation, removal and deposition of pollen, and nectar production rates between populations, we found that total nectar production had no effect on seed production at either site. The daily nectar secretion rate of 0.3–0.65 mg sugar per flower per 1–3 days was low relative to other hummingbird-adapted Penstemon species (typical range: 1.5–5 mg sugar per flower), and it might be intermediate between hummingbird- and bee-adapted Penstemon flowers. Our results support the hypothesis about a shift toward hummingbird pollination, and provide an example of a ‘despecialized’ Penstemon species, which attracts high-energy pollinators (hummingbirds) and profits from outcrossing, but retains bee-syndrome floral traits and low sugar production rates.     

Development of microsatellite markers for the endangered grassland perennial herb Vincetoxicum atratum (Apocynaceae–Asclepiadoideae)

Eight microsatellite markers were developed for the endangered grassland perennial herb Vincetoxicum atratum. The number of alleles ranged from 4 to 14, and the expected heterozygosities were from 0.575 to 0.933 in a population of V. atratum. Five of the eight loci did not significantly deviated from the Hardy–Weinberg equilibrium. All eight loci were tested for cross-species amplification in five other species of Vincetoxicum in Japan. These microsatellite loci will be useful for conservation genetics of V. atratum and other species of Vincetoxicum.     

Split sex ratios in perennial social Hymenoptera: a mixed evolutionary stable strategy from the queens' perspective?

In social Hymenoptera, relatedness asymmetries due to haplodiploidy often generate conflicts of genetic interest between queens and workers. Split sex ratios are common in ant populations and may result from such conflicts, with workers favoring the production of males in some colonies and of gynes in others. Such intercolonial differences may result from variations in relatedness asymmetries among colony members, but several examples are now known in which this hypothesis does not hold. We develop here a simple model assuming monogynous, monoandrous, worker-sterile, perennial colonies without dispersal restrictions. Workers may eliminate eggs of either sex and determine the caste of the female brood, but the queen controls the number of eggs of each sex she lays. In such conditions, we demonstrate that split sex ratios can result from queens adopting a mixed evolutionary stable strategy (ESS), with one option being to put a strict limit to the number of diploid eggs available and the alternative one to provide diploid eggs ad lib. In the former situation, workers should raise all diploid eggs as workers and release only male sexuals. In the latter, workers should adjust the caste ratio so as to reach the maximum sexual productivity for the colony, which is entirely invested into gynes. For a particular relative investment in gynes at the population level, between 0.5 (ESS under full queen control) and 0.75 (ESS under full worker control), an equilibrium is reached at which both strategies yield an equal genetic payoff to the queen. Male-specialized colonies are predicted to be equally abundant but less populous and less productive than gyne-specialized ones. Available data on the monogyne form of the fire ant, Solenopsis invicta, suggest that this model may apply in this case, although more specific studies are required to test these predictions.     

Does grazing reduce survival of indigenous perennial grasses of the semi-arid woodlands of western New South Wales?

Abstract Exclosures were used to examine the impact of grazing upon the mortality patterns of populations of six indigenous grass species. The experiment compared unfenced areas with areas from which either sheep only or sheep, rabbits and kangaroos were excluded. There were large interspecific differences in mortality patterns, with Stipa nitida and Aristida browniana having relatively high mortality rates and Eragrostis eriopoda having relatively low mortality rates. Grazing-induced mortality was observed in treatment areas that were grazed by sheep, rabbits and kangaroos and in areas grazed only by rabbits and kangaroos. The short-lived S. nitida appears less likely to suffer grazing-induced mortality than species of intermediate longevity such as Monachather paradoxa. These observations help explain the decline in endemic perennial grasses that has taken place in the vegetation of western New South Wales since European settlement. Management of these rangelands to encourage these grasses must consider total grazing pressure and not simply the impact of livestock.     

Monocropping decouples plant–bacteria interaction and strengthens phytopathogenic fungi colonization in the rhizosphere of a perennial plant species

Plant and Soil - Whether Arbuscular mycorrhizal fungi (AMF) influence community composition by changing plant adaptation to resource limitation remains unclear. This study examined how AMF affect...     

Identification of quantitative trait loci controlling winter hardiness in an annual × perennial ryegrass interspecific hybrid population

Winter hardiness is a quantitative trait and the lack of it limits geographic distribution of ryegrass. Improving winter hardiness is an important breeding goal in ryegrass breeding programs. An understanding of the genetic basis for the component traits of winter hardiness would allow more efficient selection. A three-generation interspecific population of an annual × perennial ryegrass consisting of 152 progenies was used to map quantitative trait loci (QTL) that control winter hardiness-related traits including fall growth (FG), freezing tolerance (FT), and winter survival (WS) over 2 years. A total of 39 QTL were identified for the three traits from both the female parental (MFA) and the male parental (MFB) maps, of which 13 were for FG, 6 for FT, and 20 for WS. The proportion of phenotypic variation explained by individual QTL ranged from 10.4 to 22.1%. Both FG and FT were positively correlated with WS. Common QTL were detected between FG, FT, and WS. The QTL associated with WS on linkage groups (LGs) 4 and 5, and the QTL for FT on LG 5 were consistently identified over years and maps. These consistent QTL might serve as potential tools for marker-assisted selection to improve ryegrass winter hardiness.     

Soil organic carbon dynamics 75 years after land-use change in perennial grassland and annual wheat agricultural systems

The dynamics of roots and soil organic carbon (SOC) in deeper soil layers are amongst the least well understood components of the global C cycle, but essential if soil C is to be managed effectively. This study utilized a unique set of land-use pairings of harvested tallgrass prairie grasslands (C₄) and annual wheat croplands (C₃) that were under continuous management for 75 years to investigate and compare the storage, turnover and allocation of SOC in the two systems to 1 m depth. Cropland soils contained 25 % less SOC than grassland soils (115  and 153 Mg C ha^?1, respectively) to 1 m depth, and had lower SOC contents in all particle size fractions (2000–250, 250–53, 53–2 and <2 μm), which nominally correspond to SOC pools with different stability. Soil bulk δ¹³C values also indicated the significant turnover of grassland-derived SOC up to 80 cm depth in cropland soils in all fractions, including deeper (>40 cm) layers and mineral-associated (<53 μm) SOC. Grassland soils had significantly more visible root biomass C than cropland soils (3.2 and 0.6 Mg ha^?1, respectively) and microbial biomass C (3.7 and 1.3 Mg ha^?1, respectively) up to 1 m depth. The outcomes of this study demonstrated that: (i) SOC pools that are perceived to be stable, i.e. subsoil and mineral-associated SOC, are affected by land-use change; and, (ii) managed perennial grasslands contained larger SOC stocks and exhibited much larger C allocations to root and microbial pools than annual croplands throughout the soil profile.     

Determinants of annual–perennial plant zonation across a salt–fresh marsh interface: a multistage assessment

Vegetation zonation patterns in coastal marshes are hypothesized to be the result of both physical stress and competitive interactions. How these patterns may be driven by these factors at different life history stages remains poorly understood. We investigated the relative importance of species tolerance (response to physical stress) and competitive ability in determining the distributions of two dominant marsh species across a salt–fresh marsh interface in the Yellow River Estuary, China. There is a steep gradient in salinity across this interface and Suaeda salsa, an annual, dominates the saline side of the interface, while Phragmites australis, a perennial species, dominates the freshwater side. Using a series of field transplants, we examined the roles of physical stress and competition in mediating this zonation at different life history stages. Suaeda salsa performed well in its home zone, but seedling emergence, seedling survival, adult survival and adult growth were significantly suppressed by competition in the freshwater P. australis zone. Emergence, survival and growth of P. australis were inhibited in the saline S. salsa zone, regardless of neighbor treatments, but it performed well in its home zone. The magnitude of the competitive effect on the performance of S. salsa differed among the life history stages. Competition from P. australis had a much stronger effect on S. salsa seedling emergence and adult growth than on seedling survival and adult survival. Our results reveal that both physical stress and competition contributed to the observed zonation patterns in this marsh system. However, for S. salsa, the effect of competition varied with life-history stage. Insight into these ecological processes is critical to understanding how the zonation pattern in the marsh system is formed and maintained.     

Species-specific herbivore–herbivore interactions and plant responses to sequential attack by multiple herbivores on a perennial woody plant

When plants are sequentially attacked by multiple herbivores, herbivore identity and host specialization can greatly influence the patterns of herbivore–herbivore and plant–herbivore interactions. However, how prior herbivory and the resulting induced plant responses potentially affect subsequent herbivores deserves further investigation. In this study, we conducted a common-garden experiment that manipulated sequential herbivory by the specialist caterpillar Gadirtha fusca Pogue (Lepidoptera: Nolidae) and the generalist caterpillar Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) on Chinese tallow, Triadica sebifera (L.) Small (Euphorbiaceae). We tested how prior exposure to herbivores with different levels of host specialization affected the performance of subsequently arriving con- and heterospecifics, as well as plant growth and defense responses under subsequent herbivory. We found that prior exposure to the specialist G. fusca facilitated the performance of subsequent conspecifics, resulting in a significant decrease in the growth (height and stem diameter at ground level) of tallow plants. However, prior exposure to the generalist S. litura did not affect the feeding of subsequent con- or heterospecifics or the growth of tallow plants. Sequential herbivory by specialist and generalist conspecifics resulted in lower levels of tannins and flavonoids, respectively, in leaves of tallow plants, whereas sequential herbivory by the two species did not affect the levels of tannins or flavonoids, compared to a single damage event. We conclude that herbivore species-specific plant responses appear to be more important than herbivore identity or specialization in determining herbivore–herbivore interactions and plant responses to sequential herbivore attack.     

QTL analyses of fiber components and crude protein in an annual × perennial ryegrass interspecific hybrid population

Annual (Lolium multiflorum Lam.) and perennial (Lolium perenne L.) ryegrasses are two important forage and turfgrass species. Improving the digestibility of forage by decreasing fiber content is a major goal in forage crop breeding programs. An annual × perennial ryegrass interspecific hybrid population was used to map quantitative trait loci (QTLs) for fiber components, neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL), and crude protein (CP). Samples were harvested three times in August and September 2003 and August 2004, respectively. Simple interval mapping was used to detect QTLs from both the male and female parental maps previously developed for the population. Fiber components were all correlated positively with each other and were negatively correlated with CP. The largest correlations were between NDF and ADF with r = 0.86, 0.72, and 0.82 for each of the three harvests. All four traits showed intermediate broad-sense heritability values ranging from 0.35 to 0.72. A total of 63 QTLs were detected for the four traits measured over the three harvests from both the female and male maps. Coincident QTLs were detected on linkage groups (LGs) 2, 6, and 7 for NDF, LGs 1, 2, and 7 for ADF, LGs 6 and 7 for ADL, and LG 2 for CP, respectively. Coincident QTLs were also detected on LGs 2, 6, and 7 for NDF and ADF, providing evidence of the genetic basis of the observed high level of phenotypic correlation. The QTLs on LGs 2, 6, and possibly 7 for fiber components were co-located on the same LG as several lignin biosynthetic genes from perennial ryegrass.     

Does grazing mediate soil carbon and nitrogen accumulation beneath C4, perennial grasses along an environmental gradient?

An experiment was conducted to evaluate the influence of long-term (>25 yrs) grazing on soil organic carbon (SOC) and total soil nitrogen (N) accumulation beneath individual plants of three perennial grasses along an environmental gradient in the North American Great Plains. The zone of maximum SOC and N accumulation was restricted vertically to the upper soil depth (0-5 cm) and horizontally within the basal area occupied by individual caespitose grasses, which contributed to fine-scale patterning of soil heterogeneity. Long-term grazing mediated SOC and N accumulation in the tall-, mid- and shortgrass communities, but the responses were community specific. SOC and N were lower beneath Schizachyrium scoparium plants in long-term grazed sites of the tall- and midgrass communities, but higher beneath Bouteloua gracilis plants in the long-term grazed site of the shortgrass community. SOC, but not N, was greater in soils beneath compared to between S. scoparium plants in an abandoned field seeded in 1941, indicating that this caespitose grass accumulated SOC more rapidly than N. SOC and N were greater in the 0-5 cm soil depth beneath a caespitose grass (S. scoparium) compared to a rhizomatous grass (Panicum virgatum) in the tallgrass community, with no significant accumulation of either SOC or N beneath P. virgatum plants. Grazing appears to indirectly mediate nutrient accumulation beneath caespitose grasses along the environmental gradient by modifying the size class distribution of plants. Populations with a greater proportion of large plants have a greater potential for biomass incorporation into soils and may more effectively capture redistributed organic matter from between plant locations. Contrasting plant responses to grazing at various locations along the environmental gradient conform to the predictions of the generalized grazing model, as the selection pressures of grazing and aridity may have also influenced the ability of caespitose grasses to accumulate nutrients in soils beneath them by mediating grazing resistance, competitive ability and population structure.     

Wild bees respond complementarily to ‘high-quality’ perennial and annual habitats of organic farms in a complex landscape

Agricultural intensification leads to large-scale loss of habitats offering food and nesting sites for bees. This has resulted in a severe decline of wild bee diversity and abundance during the past decades. There is an urgent need for cost-effective conservation measures to mitigate this decline. We analysed the impact of five different high-quality habitats on species richness and abundance of wild bees in a complex landscape of north-western Switzerland at six sites. The five habitat types included 45 plots situated on eight organic farms and were composed of 16 low-input meadows, six low-input pastures, seven herbaceous strips adjacent to hedges, five sown flower strips and eleven organic cereal fields. All of them are financially subsidised by the Swiss agri-environmental scheme. Wild bees were sampled between the end of April and end of August 2014 by using trio-pan traps and complementary sweep netting on these five habitat types. On 45 plots we recorded 3973 bee specimens, belonging to 91 species, 16 of which are red listed, revealing a high bee species richness in the study area. Wild bee species richness and abundance were best explained by habitat type, number of flowering plants and site. A strong relationship of increasing number of flowering plants and bee species richness and abundance was found. Grassland habitats, especially low-input meadows, harboured the highest species richness and abundances. Organic cereal fields showed a potential to conserve bee species relevant to nature conservation (harbouring exclusively two red list species and four rare species). Ordination analysis of the bee communities showed a relative dissimilarity between the habitat types and indicates their complementary effects to benefit the diversity of wild bees. Our results demonstrate that a matrix of low-input habitats are needed to sustain rich assemblages of wild bees in agroecosystems.