Endophytic fungi alter foraging and dispersal by desert seed-harvesting ants

Endophytic fungi are thought to interact mutualistically with host plants by producing alkaloid metabolites that deter herbivory. Since such fungi are transmitted via seed in some grasses, the presence of endophytes may also protect plants from seed predators. We conducted seed choice experiments for two dominant seed harvesting ants, Pogonomyrmex rugosus in the Sonoran desert and Pogonomyrmex occidentalis at a higher elevation, riparian zone in Arizona, USA. Non-infected fescue (Festuca arundinacea) seeds and seeds infected with the endophytic fungus, Acremonium coenophialum, were presented to ant colonies in three different populations. Infected seeds were harvested less frequently than non-infected seed for the two populations of Pogonomyrmex rugosus but not for the population of Pogonomyrmex occidentalis. We also a conducted seed dispersal experiment for one population of Pogonomyrmex rugosus. Of the seeds that were harvested, most of the colonies discarded more infected seeds into refuse piles than expected by chance. Seeds discarded into refuse piles have greater germination success than surrounding areas. The most important interaction of endophytes and grasses may be deterrence of seed predation and enhancing the probability of germinating in favorable sites, since these processes directly increase plant fitness.     

Biochemical and morphological evidence for host race evolution in desert mistletoe,Phoradendron californicum (Viscaceae)

Allozymes and morphological characters were used to test whether host race evolution—the genetic divergence of parasitic populations caused by adaptation to different host species—has occurred in desert mistletoe,Phoradendron californicum. Populations ofPhoradendron californicum from two hosts,Acacia greggii andProsopis glandulosa, were surveyed from the Mojave and Colorado deserts. Electrophoretic data indicated genetic differentiation of mistletoes occurring on these hosts. Three of four morphological characters (internode length, main shoot lateral shoot diameter ratio and berry color) also showed significant host-specific differentiation. These data support the hypothesis that host race formation has occurred or is occurring in this parasitic angiosperm.     

The scale-dependent importance of habitat factors and dispersal limitation in structuring Great Lakes shoreline plant communities

Niche-based and neutral models of community structure posit distinct mechanisms underlying patterns in community structure; correlation between species’ distributions and habitat factors points to niche assembly while spatial pattern independent of habitat suggests neutral assembly via dispersal limitation. The challenge is to disentangle the relative contributions when both processes are operating, and to determine the scales at which each is important. We sampled shoreline plant communities on an island in Lake Michigan, varying the extent and the grain of sampling, and used both distance-based correlation methods and variance partitioning to quantify the proportion of the variation in plant species composition that was attributable to habitat factors and to spatial configuration independent of habitat. Our results were highly scale dependent. We found no distance decay of plant community similarity at the island scale (1−33 km). All of the explained variation (32%) in species composition among samples at this scale was attributed to habitat factors. However, at a site intensively sampled at a smaller scale (5−1,200 m), similarity of species composition did decay with distance. Using a coarse sampling grain (transects), habitat factors explained 40% of the variation, but the purely spatial component explained a comparable 22%. Analyzing plots within transects revealed variation in species composition that was still jointly determined by habitat and spatial factors (18 and 11% of the variance, respectively). For both grain sizes, most of the habitat component was spatially structured, reflecting an abrupt alongshore transition from sandy dunes to cobble beach. Space per se explained more variation in species composition at a second site where the habitat transition was more gradual; here, habitat acted as a less selective filter, allowing the signal of dispersal limitation to be detected more readily. We conclude that both adaptation to specific habitat factors and habitat-independent spatial position indicative of dispersal limitation determine plant species composition in this system. Our results support the prediction that dispersal limitation—a potentially, but not necessarily, neutral driver—is relatively more important at smaller scales.     

Distribution patterns and dispersal strategies of Galiciales

In Caliciales both passive and active spore dispersal occurs. A review of the distribution of 162 species is given. Species with small spores have wider distribution than species with large spores, irrespective of dispersal strategy. In antitropical species there is only rarely a differentiation between Northern and Southern Hemisphere populations. It is concluded that in several small-spored genera long-distance dispersal has been instrumental in dispersing the species and range extensions have been much more frequent than in large-spored species for which vicariance explanations are more appropriate. Comparisons with bryophyte distributions show differences in dispersal strategies and distribution patterns.     

Trade-offs between seed dispersal and dormancy in an amphi-basicarpic cold desert annual

Background and Aims

Several studies have demonstrated trade-offs between depth of seed dormancy and dispersal ability for diaspore-dimorphic species. However, relatively little is known about trade-offs between these two life history traits for a species that produces more than two diaspore morphs. The aim of this study was to investigate the relationship between seed dormancy and dispersal in Ceratocarpus arenarius, an amphi-basicarpic cold desert annual that produces a continuum of dispersal unit morphs.

Methods

A comparison was made of dispersal and dormancy breaking/germination responses of dispersal units from ground level (a), the middle of the plant canopy (c) and the top of the plant canopy (f). Various features of the morphology and mass of dispersal units and fruits (utricles) were measured. The role of bracteoles in diaspore dispersal by wind, settlement onto the soil surface and dormancy/germination was determined by comparing responses of intact dispersal units and fruits. Movement of dispersal units by wind and animals, seed after-ripening, germination phenology and the presence of water-soluble germination inhibitors in bracteoles were tested using standard procedures.

Key Results

Dispersal units a, c and f differed in morphology and mass; in the majority of cases, extremes were exhibited by a and f, with c being intermediate. Overall, relative dispersal ability was f > c > a, whereas relative intensity of dormancy was a > c > f. Bracteoles increased dispersal distance by wind, enhanced settlement of diaspores onto the soil surface and mechanically inhibited germination.

Conclusions

The results provide evidence for a model in which there is a continuous inverse-linear relationship between diaspore dispersal ability and depth of dormancy. Thus, dispersal unit heteromorphism of C. arenarius results in a continuum, from no dispersal ability/high dormancy (dispersal unit a) to high dispersal ability/low dormancy (unit f), which may be a bet-hedging strategy in the cold desert environment.     

The relationship between sit and wait foraging strategy and dispersal in the desert scorpion,Scorpio maurus palmatus

We examine the role of the sit and wait foraging strategy and of the stability and predictability of the biotic environment in shaping population biology and life history traits of the desert scorpion, Scorpio maurus palmatus. The study was carried out in a loessial plain in the Negev desert, Israel. We used a grid system (5x80 m) within the Hammada scoparia plant association. The area was divided into 1,600 squares (50x50 cm) and the presence (within ±5 cm) and width of each scorpion burrow was recorded for eight years (1973–1980). It was found that: (1) Dispersal is a common phenomenon for all S. maurus palmatus age groups and takes place only after a heavy rainfall when soil moisture is high. (2) Dispersion patterns change from clumped to random as a function of age. (3) Dispersal is age specific and density dependent. (4) During the dry seasons a high percentage of the population goes into the resting stage. In order to integrate the above findings, a graphical model was constructed with the main assumption that the S. maurus palmatus prey availability relationship is a decreasing function between the scorpion burrow and its food resources. Data and model analysis led to the conclusion that the desert scorpion, S. maurus palmatus, is confronted with a relatively predictable physical environmeent and unpredictable biological environment. Two alternative strategies are available for S. maurus palmatus to overcome prey unpredictability; dispersal during the rainy season and inactivity during the dry seasons. We suggest that many other burrowing desert animals may face the problem of unpredictability of biological resources. Thus it seems that the S. maurus palmatus biological relationship model can be applied, with modifications, to a wide spectrum of desert burrowing animals.     

Distribution and dispersal in populations capable of resource depletion

Summary Theoretical predictions from a simulation model of insect distributions and dispersal among isolated food plants have been tested with data gathered from 13 Cinnabar moth populations. Agreement with the predictions was good. Egg batch size was equal to or slightly larger than the number which could be supported by the average food plant. Egg batch distribution was more clumped when density was high and when egg batch size was small relative to food plant size. The tendency for larval dispersal was lower in populations in areas where plants were widely spaced. These findings indicate that the Cinnabar moth has the genetic or phenotypic flexibility to adjust egg batch size, egg distribution, and larval dispersal to characteristics of the habitat. A hypothesized model is proposed to describe the interactions between larval dispersal, food plant response to defoliation, and population fluctuations for the Cinnabar moth and its food plant, tansy ragwort.     

Timing and spatial patterning of seed dispersal and redistribution in a South American warm desert

We measured newly-produced seeds entering the soil (Potential Seed Bank) to assess the timing and spatial patterning of Phase I dispersal in the central Monte desert, Argentina. Rates of forb- (6.13 mg m^-2 d^-1) and shrub-seed input (48.9 mg m^-2 d^-1) were maximum in early summer. The rate of grass-seed input, instead, was similar in early and late summer (7 to 8 mg m^-2 d^-1). About 90% of forb- and shrub-seed mass entered the habitat through protected (i.e., under canopy) areas, whereas 70% of grass-seed mass did so through exposed areas. Adult plant location and the uneven impact of wind on shrub, forb and grass seeds may explain such patterns.We also compared the Potential Seed Bank with the soil seed bank in the following spring (Realized Seed Bank). Seeds that form transient banks in other ecosystems (e.g., shrub seeds of the genus Larrea, or perennial grass seeds like those of Pappophorum and Trichloris) prevailed in the Potential Seed Bank. Some annual forb seeds, instead, appeared to form a more persistent seed bank, and prevailed in the Realized Seed Bank (e.g., Chenopodium). Horizontal redistribution did not affect the spatial patterning of forb and shrub seeds, but produced a more homogeneous distribution of grass seeds in the habitat. The impact of wind could explain the redistribution pattern of grass seeds. Finally, we found almost 80% of total seeds in the top 2 cm of soil. The smallest grass and forb seeds (Sporobolus and Descurainia) as well as some medium-sized and large forb seeds (e.g., Glandularia, Sphaeralcea, Phacelia) were able to reach deeper soil layers in the central Monte desert.     

Repeat intercontinental dispersal and Pleistocene speciation in disjunct Mediterranean and desert Senecio (Asteraceae)

To explore the biogeographic history of Mediterranean/arid plant disjunctions, Old and New World Senecio sect. Senecio were analyzed phylogenetically using nuclear ribosomal DNA sequences (ITS). A clade corresponding to sect. Senecio was strongly supported. Area optimization indicated this clade to be of southern African origin. The Mediterranean and southern African floras were not distinguishable as sources of the main New World lineage, estimated to have become established during the middle Pliocene. Another previously suspected recent dispersal to the New World from the Mediterranean was confirmed for the recently recognized disjunction in S. mohavensis. The loss of suitable land connections by the Miocene means that both New World lineages must represent long-distance dispersal, providing the first evidence of repeat intercontinental dispersal in a Mediterranean group. In contrast, migration within Africa may have utilized an East African arid corridor. Recent dispersal to northern Africa is supported for S. flavus, which formed part of a distinct southern African lineage. Novel pappus modifications in both disjunct species may have enabled dispersal by birds. An estimated early Pliocene origin of sect. Senecio coincides with the appearance of summer-dry climate. However, diversification from 1.6 BP highlights the importance of Pleistocene climate fluctuations for speciation.     

Directed dispersal of mistletoe (Plicosepalus acaciae) by Yellow-vented Bulbuls (Pycnonotus xanthopygos)

We studied avian dispersal of seeds from the hemiparasitic mistletoe Plicosepalus acaciae (Loranthaceae) to its tree hosts Acacia raddiana and A. tortilis in the Syrian–African Rift (Arava) valley, Israel. The Yellow-vented Bulbul (Pycnonotus xanthopygos) was the sole avian visitor observed feeding on mistletoe fruits. Bulbuls consumed mistletoe fruits whenever they were available, but the fruits only constituted a significant portion of the diet (71% of foraging attempts) when they were most abundant. These birds are potentially good dispersal vectors of P. acaciae because they swallowed the fruit whole and defecated viable seeds that were covered in a viscid pulp, which allowed the seeds to adhere to substrates when voided. In addition, bulbuls spent a large proportion (66–93%) of total observation time perched in Acacia trees, allowing for directed dispersal. Ephemeral river valleys (wadis) with high mistletoe infection were adjacent to those containing no infections, demonstrating that mistletoe dispersal is common within, but not among wadis. This is consistent with the flight behaviour in bulbuls, which do not typically move among wadis. We combined data on bulbul movements between Acacia trees with transit times of mistletoe seeds to create a hypothetical seed shadow as a function of distance from the parent mistletoe plant. Because they are directed dispersers, the movement patterns of bulbuls may explain the current distribution of P. acaciae in the Arava valley.     

Age dependent dispersal is not a simple process: Density dependence, stability, and chaos

I present both discrete and continuous models for a single species with overlapping generations, density dependence, and movement rates that vary with age. In the discrete time and space model, I show that if the strongly density dependent age class(es) are stationary, the spatially structured model may exhibit instabilities or even chaos which are not present in the corresponding model without spatial structure. I argue that the conditions which lead to these diffusive instabilities are likely to be met in natural populations. Thus it is important to consider the interaction between age structure and spatial structure in both experimental and theoretical work. In particular, the conditions leading to chaos are more common than would be predicted in models which ignore structure.     

Chloroplast DNA evidence for introgression and long distance dispersal in the desert annualSenecio flavus (Asteraceae)

Phylogenetic analysis of chloroplast DNA (cpDNA) restriction site variation supports a close genetic relationship between the Southwest AsianSenecio flavus subsp.breviflorus and the North AmericanS. mohavensis. The intercontinental disjunct distribution of these two desert annuals may have originated via long distance dispersal. The chloroplast genomes of the Southern and North AfricanS. flavus subsp.flavus and subsp.breviflorus differ by at least ten restriction sites, while at most two restriction sites differentiate the cpDNA genomes of subsp.breviflorus and the outgroupS. squalidus. This suggests that the cpDNA genome ofS. flavus subsp.breviflorus may have resulted from introgression and chloroplast capture with a Mediterranean species related toS. squalidus. This hypothesized introgression could account for the morphological distinctiveness and duplicated isozyme loci ofS. flavus subsp.breviflorus relative to subsp.flavus.     

Distribution of aquaporins in the colon of Octodon degus,a South American desert rodent

Octodon degus is a desert rodent of northern Chile, adapted to survive with a limited supply of water. This rodent has a high degree of fecal dehydration, related to colon water absorption. With the hypothesis that aquaporins (AQPs) might be present in the colon epithelium of O. degus and involved in fluid absorption, we studied colon water absorption in vivo and the distribution of AQPs and Na(+) transporters by immunocytochemistry. AQP-1 was found in apical and basolateral membranes of surface-absorptive and crypt epithelial cells. AQP-8 was found in the cytoplasm of enterocytes of surface colon. AQP-3 immunolabeling, on the other hand, was absent from the epithelium but present in a subepithelial fibroblast layer, pericryptal cells, and muscularis mucosae. The hydration state did not modify the amount of immunostaining for any of the AQPs. Colon water absorption was markedly decreased by the mercurial agent p-chloromercuribenzenesulfonic acid and was not affected by water deprivation. The NHE3 isoform of Na(+)/H(+) exchanger and alpha-1 subunit of the Na(+)-K(+)-ATPase were found in apical and basolateral membranes of surface-absorptive cells, respectively. These results suggest that colon water absorption is mostly transcellular and mediated by water channels like AQP-1. Apical Na(+)/H(+) exchanger and basolateral Na(+)-K(+)-ATPase in surface cells could be part of the Na(+) absorption pathway. It is hypothesized that this transport is necessary to provide an osmotic gradient for water absorption. The roles of AQP-8 and AQP-3 in water absorption remain to be established.     

Distribution, frequency and host patterns of European mistletoe (Viscum album subsp. album) in the major city of Lodz, Poland

The frequency of parasitism of the European mistletoe, Viscum album L. subsp. album, in the city of Lodz, a typical major city in Poland, was investigated. The infection prevalence and intensity of the mistletoe Viscum album subsp. album on its main host, Acer saccharinum as a function of host size was also investigated. The parasite showed a strong preference for alien, planted tree species (i.e. A. saccharinum, Populus×canadensis and Robinia pseudoacacia). In 2009–2011, V. album subsp. album was observed on 28 host taxa of trees and shrubs, which represents the highest diversity of host trees in a single locality in the Poland. Within the studied area 2147 trees were infested by mistletoe. The distribution of mistletoes (V. album subsp. album) among A. saccharinum hosts is significantly aggregated. The intensity of mistletoe infection in the silver maple trees was affected by the individual tree characteristics, such as the height of the tree. The overall level of aggregation as indicated by the variance to mean ratio of mistletoe numbers per host fell within the midrange of values found in other published studies of host-mistletoe interactions. The higher mistletoe infection prevalence in taller trees results from differential dispersal of mistletoe seeds to tall trees as well as differential survival of established mistletoes on tall trees. The incidence of mistletoe was higher in city centre (zone of high density development) than it was on the outskirts of a city (outer marginal zone). It was found that the abundant occurrence of mistletoe was recorded in the stands of increased nitrogen input, while other stands have little or no mistletoe infection present. Thus, this mistletoe species uses both passive and active uptake, which may be a selective advantage in a nutrient-poor environment or on a nutrient-deficient host species.     

Characterization and biosynthesis of mistletoe lignin

Mistletoe lignin was a typical angiosperm one based on the spectral (UV, IR, ¹³C-NMR) and functional group analyses, and on degradation products (nitrobenzene oxidation and acidolysis), the analytical results of which were compared with those of the host lignin. l-Phenylalanine-[U-¹⁴C] was efficiently incorporated into mistletoe lignin. Phenylalanine ammonia-lyase and cinnamate-4-hydroxylase were detected by incubation of the tissue slices under illumination. It was also found that O-methyltransferase activity of the crude homogenate catalysed the methylation of 5-hydroxyferulic but not the methylation of caffeic acid. However, the latter methylation activity could be recovered by purification. These results indicate that mistletoe lignin is synthesized independently from that of its host.     

Primary structure, isoforms, and molecular modeling of a chitin-binding mistletoe lectin

From mistletoe Viscum album L. extracts three chitin-binding lectin isoforms, cbML1, cbML2, and cbML3, were isolated and their primary structure determined. All three cbML isoforms are composed of two protein chains of 48 or 49 amino acid residues, linked by an intermolecular disulfide bond. The sequence of each single cbML chain is characterized by a relatively high number of cysteine and glycine residues, 9 and 6, respectively, and contains four intramolecular disulfide bridges. On the basis of the combined interpretation of sequencing and MALDI MS data, the following results for the three cbML isoforms were obtained: the first one consists of two identical truncated polypeptide chains (1--48), the second is a heterodimer, containing one truncated (1--48) and one full-length chain (1--49), and the third is composed of two full length chains (1--49). The cbML sequence shows 55% identity to hevein, a single-chain chitin-binding protein of 43 amino acids, one of the most predominant proteins in natural rubber latex. On the basis of the NMR data on hevein from Hevea brasiliensis the three-dimensional structure of cbML3 was modelled. The 26 sequence changes between cbML3 and hevein were accommodated with only little perturbation in the main chain folding. A comparison of the primary structures of cbML3 and hevein is shown and the effects of the sequence changes are discussed. Differences have been identified in the loop region of the molecule and the potential interface region of cbML3 supporting the dimer formation. The high-affinity chitin-binding site seems to be highly conserved.