Species coherence in the face of karyotype diversification in holocentric organisms: the case of a cytogenetically variable sedge (Carex scoparia,Cyperaceae)

Background and Aims

The sedge genus Carex, the most diversified angiosperm genus of the northern temperate zone, is renowned for its holocentric chromosomes and karyotype variability. The genus exhibits high variation in chromosome numbers both among and within species. Despite the possibility that this chromosome evolution may play a role in the high species diversity of Carex, population-level patterns of molecular and cytogenetic differentiation in the genus have not been extensively studied.

Methods

Microsatellite variation (11 loci, 461 individuals) and chromosomal diversity (82 individuals) were investigated in 22 Midwestern populations of the North American sedge Carex scoparia and two Northeastern populations.

Key Results

Among Midwestern populations, geographic distance is the most important predictor of genetic differentiation. Within populations, inbreeding is high and chromosome variation explains a significant component of genetic differentiation. Infrequent dispersal among populations separated by >100 km explains an important component of molecular genetic and cytogenetic diversity within populations. However, karyotype variation and correlation between genetic and chromosomal variation persist within populations even when putative migrants based on genetic data are excluded.

Conclusions

These findings demonstrate dispersal and genetic connectivity among widespread populations that differ in chromosome numbers, explaining the phenomenon of genetic coherence in this karyotypically diverse sedge species. More generally, the study suggests that traditional sedge taxonomic boundaries demarcate good species even when those species encompass a high range of chromosomal diversity. This finding is important evidence as we work to document the limits and drivers of biodiversity in one of the world''s largest angiosperm genera.     

Genetic structure in populations of an ancient woodland sedge, Carex sylvatica Hudson, at a regional and local scale

Wood sedge (Carex sylvatica) is a well-known ancient woodland species with a long-term persistent seed bank and a caespitose growth habit. All thirteen isolated Carex sylvatica populations in the Dutch Rhine floodplain (including the river branches Waal and IJssel) were mapped in detail and analysed for genetic variation at a large number of AFLP loci and one microsatellite locus. Across all populations, only 40 % of the sampled individuals (n=216) represented a unique genotype. A high number of the studied patches (spatial clusters of tussocks, 2-10 m in diameter) within populations contained only one or a few genotypes. Identical plants (tussocks) were also found 20-500 m apart and in one case even 1000 m apart. Observed heterozygosity levels (H(O)=0.029) were low, indicating low levels of gene flow, which is in agreement with the selfing nature of other caespitose sedges. Although the number of genotypes in populations is low, these genotypes are genetically very distinct and variation within populations accounted for 55% of the total variation. The absence of a correlation between genetic and geographic distances among populations, and the scattered distribution of genotypes among patches within woodlands, support our hypothesis of rare establishments and subsequent local dispersal within woodlands in this forest floor species, which may benefit from and partly depend on human land use and forest management activities.     

Factors Affecting Revegetation of Carex lacustris and Carex stricta from Rhizomes

The revegetation of sedge meadows has been problematic because natural recolonization does not occur under many circumstances and because planted propagules often fail to reestablish successfully. In this study, detached rhizomes of Carex lacustris Willd. and Carex stricta Lam. were transplanted in both fall (September) and spring (May) into three experimental wetlands to determine the effects of both planting season and hydrology on survival and establishment. Each experimental wetland had the same mean water depth across 5% slopes, but one had a constant water depth (0.5 m) throughout the growing season, another fell from a mean depth of 0.75 m to 0.25 m, and a third rose from a mean depth of 0.25 m to 0.75 m. Initial rhizome survival, shoot growth, and soil characteristics were recorded over 2 years. Neither planting proved successful (6.9% versus 0.5%) for C. stricta, a tussock-forming sedge. For C. lacustris, a sedge with spreading rhizomes, spring planting had greater rhizome survival (53.2% survival) than fall planting (0.7%). Since both species initiate new shoots in the fall, they are susceptible to transplant failure during this season. The highest survival rates (71–100%) and plant production (736.0 and 494.5 g/m²) for C. lacustris occurred near the water’s edge in both the constant and falling basins. In the rising basin, establishment and growth of this species was high at all water depths (71–96%; 399 g/m²). C. lacustris grew optimally at the same elevations where rhizome survival was greatest, suggesting that shoots are more sensitive to early-season than late-season water levels.     

Isolation and characterization of 11 microsatellite loci from Carex macrocephala (Cyperaceae)

Carex macrocephala (Cyperaceae) is a species of great conservation concern along the northwest coast of North America. Using a biotinylated-streptavidin bead protocol, we successfully isolated and characterized 11 microsatellite loci from a C. macrocephala genomic library. Although these loci are polymorphic, the scored populations exhibit a significant deficiency in heterozygosity. Carex macrocephala is a self-pollinating rhizomatously spreading sedge, consequently this may contribute to the observed patterns.     

Transplanting Sedges (Carex spp.) in Southwestern Riparian Meadows

Abstract The success of revegetation efforts in southwestern U.S. riparian meadows has been limited because natural recolonization is often poor and little is known about establishment of transplanted native meadow species such as sedges. To evaluate their potential use in riparian restoration, the survival and growth of transplanted wildlings of three sedge species, Carex lanuginosa (woolly sedge), C. nebrascensis (Nebraska sedge), and C. rostrata (beaked sedge), were assessed. Transplanting occurred during two seasons (summer and fall) using two transplant sizes (295 and 680 cm³) at seven montane meadow sites in Arizona. Survival was similar among species, but shoot numbers were greater for C. lanuginosa (12.7 shoots/wildling) compared with C. nebrascensis (5.5 shoots/wildling) and C. rostrata (7.9 shoots/wildling). Survival was significantly greater for the summer transplant season (55.1%) versus fall (24.1%). Survival and growth were greater for the larger transplant size (46.1% large vs. 33.0% small; 8.1 shoots/wildling large vs. 6.4 small). Wildling survival was related to depth to groundwater for each species. Survival was highest for C. lanuginosa (78.6%), C. nebrascensis (88.2%), and C. rostrata (64.3%) where the groundwater depth was ?48 to ?60, ?28 to ?47, and ?8 to ?27 cm, respectively. These results suggest that restoration will be most successful if transplanting occurs in summer, using large wildling transplants when under stressful conditions such as waterlogged or dry soils, and when species are planted at appropriate groundwater depths.     

Allozyme Variation and the Genetic Structure of Populations of the Rare Sedge Carex misera (Cyperaceae)

Abstract Carex misera is a rare sedge, endemic to rocky outcrops and mountain summits within the southern Appalachian Mountains from northern Georgia to northern North Carolina. We assessed allozyme diversity for 406 individuals from nine populations over most of the geographic range. Twenty-seven putative loci were assayed and eight (30%) were found to be polymorphic. Nei's gene diversity statistics ( H _T =0.043, H _S =0.019, G _ST =0.551) indicated low levels of variation but relatively highly differentiated populations, suggesting little gene flow. Significant deviations from genotypic expectations under Hardy-Weinberg equilibrium, high positive fixation indices, and the existence of small genetic neighborhoods within populations suggest that at least some inbreeding occurs. Cluster analysis of Nei's genetic identity statistics and principal component analysis of allele frequency data showed high similarity among the six southern populations with the two northern populations more differentiated from them and from each other. These results suggest that preservation of the northern populations is necessary to conserve the already low levels of genetic diversity within the species.     

Post-glacial history of the dominant alpine sedge Carex curvula in the European Alpine System inferred from nuclear and chloroplast markers

The alpine sedge Carex curvula ssp. curvula is a clonal, dominant graminoid found in the European Alps, the Carpathians, the Pyrenees and in some of the Balkan Mountains. It is a late-successional species of acidophilous alpine meadows that occurs on sites that were covered by ice during the last glacial maximum (LGM). By applying the amplified fragment length polymorphism (AFLP) fingerprinting and chloroplast DNA (cpDNA) sequencing, we attempted to identify the recolonization routes followed by the species after the last ice retreat. We relied on the genetic diversity of 37 populations covering the entire distributional range of the species. As a wind-pollinated species, C. curvula is characterized by a low level of population genetic differentiation. Nuclear and chloroplast data both support the hypothesis of a long-term separation of Eastern (Balkans and Carpathians) and Western (Alps and Pyrenees) lineages. In the Alps, a continuum of genetic depauperation from the east to the west may be related to a recolonization wave originating in the eastern-most parts of the chain, where the main glacial refugium was likely located. The Pyrenean populations are nested within the western Alps group and show a low level of genetic diversity, probably due to recent long-distance colonization. In contrast to the Alps, we found no phylogeographical structure in the Carpathians. The combination of reduced ice extension during the Würm period and the presence of large areas of siliceous substrate at suitable elevation suggest that in contrast to populations in the Alps, the species in the Carpathians underwent a local vertical migration rather than extinction and recolonization over long distance.     

No positive correlation between species and genetic diversity in European alpine grasslands dominated by Carex curvula

The distributional range of alpine plants experienced dramatic changes during the Quaternary ice ages. These changes offer many opportunities for studying the impact of habitat contraction and fragmentation on both species and genetic diversity. Here, we examined the amplified fragment length polymorphism-based genetic diversity in the sedge Carex curvula All. in relation to the species diversity of siliceous European alpine grasslands in which the sedge is dominant. We found no relationship or even a negative relationship between genetic and species diversity indices. Local species richness was associated with the regional pool size of siliceous alpine species, which was itself dependent on the extant area of suitable habitats for these species. Genetic diversity of C. curvula was primarily shaped by the presumed location of glacial refugia and the routes of post-glacial colonization. We conclude that the two levels of diversity are not positively correlated because genotypes and species do not respond similarly to the spatial dynamics of suitable habitats induced by Quaternary temperature changes.     

Environmental control of flowering and sex expression in Carex flava

The environmental control of flowering and sex expression has been studied under controlled environment conditions in three populations of the sedge Carex flava L. A dual floral induction requirement was demonstrated in all populations. Low temperature (< 12°C) was obligatory for, and short photoperiods strongly enhanced, primary induction and inflorescence initiation. Stem elongation and inflorescence development were promoted by long photoperiods, although most plants developed stunted flower stems also under short day (SD) conditions. Growth vigour, abundance of flowering and primary induction requirements varied widely among the populations, with critical exposure times for full flowering varying from less than 9 to about 12 weeks in SD at 9°C, and from about 9 to more than 15 weeks in long days (LD). Sex expression in the normally male terminal spike was shifted towards femaleness by marginal or incomplete primary induction. Primary induction in LD resulted in a complete change to entirely female inflorescences, whereas marginal induction in SD resulted in a similar sex reversal in some plants. The results are discussed in relation to environmental and hormonal factors known to modify sex expression in flowering plants and the significance of the results to Carex systematics and classification.     

Isolation and characterization of polymorphic microsatellite loci in Asiatic sand sedge,Carex kobomugi Ohwi (Cyperaceae)

Fourteen polymorphic microsatellite loci were isolated from Asiatic sand sedge, Carex kobomugi Ohwi (Cyperaceae), a clonal perennial herb predominant in sandy coast vegetation in Japan. Polymorphism was surveyed for 24 ramets within a single population and for eight distinct individuals from six populations. Within a population, between two and eight alleles were found per locus, and expected heterozygosity ranged from 0.041 to 0.720. Among populations, a higher level of polymorphism was observed. The microsatellite loci were shown to be useful for the study of spatial genetic structure at fine scale and genetic differentiation among populations.     

Genetic analysis of salt-marsh sedge Carex scabrifolia Steud. populations using newly developed microsatellite markers

Nine microsatellite loci were isolated and characterized from the clonal salt-marsh sedge Carex scabrifolia, and genetic diversities within four populations were analyzed. The number of alleles per locus ranged from 2 to 7, with an average of 4.7. The observed and expected heterozygosities ranged from 0.000 to 1.000 and 0.000 to 0.679, respectively. In two populations, almost all polymorphic loci showed a significant excess of heterozygotes due to their high clonal reproduction and low number of genotypes (genets) composing the populations. Thus, the number of identical genets varied greatly among populations and ranged from 1 to 28, irrespective of population size (i.e., population area and ramet number). When attempting to conserve and restore clonal plants such as C. scabrifolia, it is important to preserve sufficient genetic diversity within a population, which can be assessed using genetic markers such as the simple sequence repeat markers described here.     

Allozyme variation and racial differentiation in Swedish Carex lepidocarpa s.l. (Cyperaceae)

Two morphological races have previously been recognized within the sedge, Carex lepidocarpa , in Sweden. These largely allopatric races are accorded specific status, as C. lepidocarpa s.s. and C. jemtlandica , in Scandinavian floras. A study of allozyme variation in populations from 80 Swedish sites supports the morphological evidence for racial differentiation within C. lepidocarpa. The two races differ from each other in terms of allele frequencies at polymorphic loci and also show different levels of within-population genetic diversity. Material that is morphologically referable to C. lepidocarpa s.s. is characterized by relatively high levels of allozyme variation, both within and between populations. Carex lepidocarpa s.s. is widespread in southern Sweden. In contrast, material that is morphologically assignable to C. jemtlandica shows low levels of within-population genetic diversity, and there is little differentiation between the geographically separated isolates of C. jemtlandica in northern Sweden and on the Baltic island of Gotland. The high degree of morphological similarity and moderate levels of genetic differentiation between the two races within C. lepidocarpa indicate that it is more appropriate to recognize the races as subspecies than as species. The low levels of genetic variation in C. jemtlandica , both within and between populations, suggest that C. jemtlandica may have arisen from C. lepidocarpa (or a near ancestor of C. lepidocarpa) as a result of population fragmentation and isolation in glacial refugia, or during the process of post-glacial colonization of Scandinavia. Lack of allozyme evidence for extensive hybridization between the two races of C. lepidocarpa , despite their ability to hybridize freely where their ranges overlap at present, supports the suggestion that the two races have had separate post-glacial histories.     

Identification and ecdysteroid antagonist activity of three oligostilbenes from the seeds of Carex pendula (Cyperaceae)

Methanolic extracts of seeds of several (Carex species were found to antagonise the action of 20-hydroxyecdysone in the Drosophila melanogaster microplate-based B(II) cell bioassay. Bioassay-guided HPLC analysis of seeds of Carex pendula (drooping sedge) provided one previously unknown tetrastilbene (cis-miyabenol A) and two known oligostilbenes (kobophenol B and cis-miyabenol C) as the biologically active compounds (EC50 values were 31, 37 and 19 microM, respectively, vs. 5 x 10(-8) M 20-hydroxyecdysone). The structures and relative stereochemistries of these compounds were deduced by comprehensive ID- and 2D-NMR experiments. These compounds are isolated from Carex pendula for the first time. In vitro experiments with dipteran and lepidopteran ecdysteroid receptor proteins demonstrate that the oligostilbenes are able to compete with radiolabelled ecdysteroid ([3H]ponasterone A) for occupancy of the ligand binding site. IC50/Ki values are similar to the EC50 values obtained in the B(II) bioassay.     

Microcosm tests of the effects of temperature and microbial species number on the decomposition of Carex aquatilis and Sphagnum fuscum litter from southern boreal peatlands

Increased decomposition rates in boreal peatlands with global warming might increase the release of atmospheric greenhouse gases, thereby producing a positive feedback to global warming. How temperature influences microbial decomposers is unclear. We measured in vitro rates of decomposition of senesced sedge leaves and rhizomes (Carex aquatilis), from a fen, and peat moss (Sphagnum fuscum), from a bog, at 14 and 20 degrees C by the three most frequently isolated fungi and bacteria from these materials. Decomposition rates of the bog litter decreased (5- to 17-fold) with elevated temperatures, and decomposition of the sedge litters was either enhanced (2- to 30-fold) or remained unaffected by elevated temperatures. The increased temperature regime always favoured fungal over bacterial decomposition rates (2- to 3-fold). Different physiological characteristics of these microbes suggest that fungi using polyphenolic polymers as a carbon source cause greater mass losses of these litters. Litter quality exerted a stronger influence on decomposition at elevated temperatures, as litter rich in nutrients decomposed more quickly than litter poorer in nutrients at higher temperatures (8.0%-25.7% for the sedge litters vs. 0.2% for the bryophyte litter). We conclude that not all peatlands may provide a positive feedback to global warming. Cautious extrapolation of our data to the ecosystem level suggests that decomposition rates in fens may increase and those in bogs may decrease under a global warming scenario.     

Partitioning of the space in tussocks of the sedge, Carex distorts, during winter, by a spider community

In the flat parts of a coastal plain on the Dutch Wadden island of Schiermonnikoog, the entire spider fauna aggregates in plant tussocks ( Carex distans L.) at the onset of winter. Spiders are known predators of other spiders, therefore we expected an avoidance of predation by spatial separation of the species. The sedge tussocks were divided into several zones, each of which was sampled for spiders. Spatial separation on a macroscale (between tussocks) as well as on a microscale (inside tussocks) was demonstrated.     

Phylogeny of Siberian species of Carex sect. Vesicariae based on nuclear and plastid markers

Carex sect. Vesicariae is a group of about 30 sedge species widespread in temperate and cold regions of Eurasia and North America. In this study we performed a phylogenetic analysis of ten taxa (9 species and one subspecies) found in Siberia based on two plastid sequences (matK and atpF‐H) and the nuclear ribosomal ITS2 sequence. We also developed a novel sequence marker for the genus Carex, an intron of the nuclear hsp90 gene, which was found to be several times more variable than plastid and nuclear ribosomal sequences. Total nucleotide variation within the Vesicariae section was found to be very low, comparable to that within single other sedge species. Phylogenetic analysis supported the existence of two groups within this section: clade A (C. vesicata, C. vesicaria, both subspecies of C. saxatilis and C. pamirica) and clade B (C. rostrata, C. mollissima, C. jacutica, C. rotundata and C. membranacea). Carex rhynchophysa holds an intermediate position between these groups. The main morphological difference between these clades lies in the nature of the transition of the utricle into the beak: it is gradual in clade A and abrupt in clade B. This division is in good accordance with the results of previous allozyme studies and with current taxonomy. These results may become the basis for consideration of subsectional taxa within this section.     

Microsatellite primers in Carex moorcroftii (Cyperaceae), a dominant species of the steppe on the Qinghai-Tibetan Plateau

? Premise of the study: Microsatellite markers were developed in Carex moorcroftii, a rhizomatous clonal sedge that dominates the steppes and meadows of the Qinghai-Tibetan Plateau. ? Methods and Results: Using the combined biotin capture method, 30 microsatellite primer sets were isolated and characterized. Twenty-two of these markers showed polymorphism, and the number of alleles per locus ranged from two to 11 across 96 individuals from four populations. ? Conclusions: These markers provide a useful tool to investigate the clonal structure and mating system of C. moorcroftii.     

Intraclonal variation in defence substances and palatability: a study on Carex and lemmings

Clonal sedges consist of integrated ramets at different development stages. Many of these sedges are important food for herbivores, yet differences in herbivore preferences and defence allocation between ramet development stages have not previously been evaluated. In this study we investigated intraclonal ramet variation in level of plant defence and nutrient compounds and intraclonal ramet preferences by lemmings ( Lemmus trimucronatus ) in field samples of a rhizomatous sedge ( Carex stans ). Plant defence was measured as the level of proteinase inhibitor activity (PIA) and the ratio of PIA to soluble plant proteins (SPP), whereas plant nutrients were measured as the level of soluble plant sugars (SPS) and SPP. Flowering ramets generally had a higher content of defence compared to vegetative ramets, which is consistent with the optimal defence theory predicting that defence compounds are allocated to the ramet stage of the highest fitness value. Compared to vegetative ramets, the flowering ramets had a lower content of SPP and a higher content of SPS. The lemmings showed preference differences between the ramet development stages, and to a large extent the ramet content of defence compounds and nutrient compounds covaried with these preferences in the predicted way. This study shows that defence allocation between ramet development stages of the clonal sedge Carex conforms to predictions of the optimal defence theory.     

Disjunct performance and distribution in the sedge Carex prasina

Efforts to understand species distributions and predict responses to environmental changes depend on specifying how the abiotic environment determines distributions. At landscape scales, it is critical to distinguish effects of environmental factors from other mechanisms such as competition and dispersal limitation. We examined how environmental factors affect the distribution and performance of the sedge Carex prasina across a 10-km² old-growth forest in southern Québec. We isolated the effects of soil characteristics by conducting a greenhouse experiment that assessed the performance of C. prasina on soils from a range of wetland habitats where it could potentially occur. This allowed us to compare how the species’ performance and its distribution across the landscape relate to the same soil characteristics. In the experiment, the biomass and leaf chlorophyll content of C. prasina increased with increasing soil organic matter (OM). Across the landscape, however, the species’ probability of occurrence and abundance decreased with increasing soil OM. C. prasina had similar biomass on soils from sites where it did and did not occur, but it had higher leaf chlorophyll content on soils from sites where it did not occur. We found no evidence that differential performance across environments determines the distribution of this species, as C. prasina tended to occur on soils where it showed reduced performance. Rather, other processes such as competition or dispersal limitation likely override any direct effects of the soil environment on distribution. Our results caution against the common assumption that the environments where a species tends to occur or be most abundant are the environments where it performs best. C. prasina presents a clear example of a species whose performance, at least along edaphic gradients, cannot explain its distribution. This example highlights the importance of distinguishing the relative roles of biotic and abiotic factors that shape species distributions across landscapes.     

Allozyme variation and genetic relationships among species in the Carex willdenowii complex (Cyperaceae)

A taxonomic study by Naczi, Reznicek, and Ford (American Journal of Botany, 85, 434-447, 1998) has determined that three species (Carex willdenowii, C. basiantha, and C. superata) can be recognized within the C. willdenowii complex. To determine the amount of genetic divergence within and between these species, allozyme analyses were conducted on 14 populations distributed from Pennsylvania to eastern Texas. Seventeen loci were surveyed, 13 of which were polymorphic, with all populations being polymorphic at one or more loci. Interspecific genetic identities ranged from 0.560 (C. willdenowii and C. basiantha) to 0.807 (C. basiantha and C. superata). Alleles for the isozymes Aat-1, Dia-1, Idh-2, Mdh-2, Per-1, Pgm-1, and Pgm-2 served to distinguish C. willdenowii from C. basiantha and C. superata. Carex basiantha and C. superata were recognized by alleles of Mdh-2, Pgm-1, and Tpi-2. The genetic identities of populations within species were high and exceeded 0.957. A caespitose growth habit and perigynia in close proximity to the staminate flowers suggest adaptations for selfing and therefore low levels of heterozygosity. Paradoxically, the values for expected heterozygosities (Hexp) were always lower than those obtained by direct count (Hobs): F values were highly negative, indicating heterozygous excess. Disassortative mating and selection are discussed as possible mechanisms for maintaining heterozygous excess within populations.