Clonal diversity and distribution in Stenocereus eruca (Cactaceae), a narrow endemic cactus of the Sonoran Desert

Stenocereus eruca (Cactaceae), a prostrate cactus endemic to the Sonoran Desert, is thought to be highly clonal. We examined its clonal diversity and distribution: (1) at the population level, in four distinct populations along its distribution range; and (2) at a micro scale level, within a single population. Our objective was to evaluate the importance of sexual versus clonal recruitment through the use of RAPD markers. Contrary to previous field observations, clonal diversity was relatively high across the distribution range. This finding suggests that sexual recruitment is an important regeneration mechanism. The proportions of distinguishable genotypes (G/N = 0.83) and genotypic diversity (D = 0.987) were greater than in other clonal cacti, suggesting that clonal propagation is not the major regeneration mechanism. Autocorrelation analyses revealed a spatial genetic structure that may be the result of restricted gene flow (via pollen or seeds) and clonal propagation. A molecular variance analysis (AMOVA) indicated that most of the variation (66.3%) was found within and not across populations. Future studies on pollen and seed dispersal are needed to understand the role of the clonal habit in the mating system of S. eruca.     

Bacterial succession in necrotic tissue of agria cactus (Stenocereus gummosus).

The bacterial communities associated with the development of necroses in injured agria cactus tissue were examined in the field by using both human-induced injuries and cactus tissue inoculated with cactophilic bacteria. Whole-cell bacterial fatty acids were used to determine when and where each of 23 detected species occurred. This information was then used to describe successional patterns of bacterial colonization. Although the number of bacterial species in human-induced injuries reached a maximum on day 16, the Shannon-Weaver diversity index increased to a plateau, which reflects a stable bacterial community. The potential of the bacterial community to macerate the injured cactus tissue was also examined, and the results indicate that the bacteria initially colonizing the newly injured cactus tissue were more likely to contain pectinolytic, proteolytic, and lipolytic enzymes than were the bacteria entering the injuries once tissue maceration had already begun. The cactophilic fruit fly Drosophila mojavensis has been previously shown to carry bacteria to newly injured cactus tissue. In these studies, exclusion of these insects did not significantly affect bacterial succession or community structure. This supports our contention that bacteria colonize injured tissue primarily by passive transport, e.g., on wind-blown particles.     

Analysis of the community structure of yeasts associated with the decaying stems of cactus. I.Stenocereus gummosus

Yeast communities of decayingStenocereus gummosus were analyzed for spatial, temporal, and physiological characteristics. Analysis of random samples within plants, between plants, and between localities shows that the species proportions of the yeast community are relatively constant within plants and between localities, but that there is significant variability between rotting plants. It is suggested that the increased variability between plants represents sampling of different stages of succession. The physiological abilities of the yeast community also show a relatively constant pattern within plants and between localities yet more variability between plants.The variablity profiles of species proportions and community physiological characters are demonstrated to be correlated within and between plants. This observation is an extension of the Kluge-Kerfoot phenomenon to the level of the community. The correlation of within and between plant variability profiles is suggested to be a result of the temporal and spatial availability of resources during the stages of rotting plant succession. The community structure is thus postulated to result from a set of possible future resource states of the habitat.     

Microbial colonization of injured cactus tissue (Stenocereus gummosus) and its relationship to the ecology of cactophilic Drosophila mojavensis.

Necrotic tissue of agria cactus (Stenocereus gummosus) serves as a feeding and breeding substrate for Drosophila mojavensis. This fly species is one of the four endemic Drosophila species in the Sonoran Desert. Freeze injuries were created in arms of agria cactus in Mexico to study the events of microbial colonization. Facultative anaerobic bacteria were the first microbes to be detected, and the exclusion of large arthropods by covering the injuries with netting did not affect bacterial colonization. Yeast growth lagged behind bacterial growth by 2 days, and excluding arthropods delayed the detection of yeasts by an additional 2 days. Thus, insects (such as Drosophila species) and other arthropods do play a role in the colonization of agria rots by yeasts. All injuries were attractive to D. mojavensis within 5 days, and these flies were shown to be carrying significant densities of both bacteria and yeasts. Analysis of the volatile compounds present in the developing rots over time indicated that the volatile pattern is dynamic. Ethanol and acetic acid were the two volatile substances most likely responsible for the initial attraction of the injuries for Drosophila species.     

The genetic structure of a columnar cactus with a disjunct distribution: Stenocereus gummosus in the Sonoran desert

Stenocereus gummosus is a columnar cactus endemic to the Sonoran desert that exhibits a disjunct distribution: it is widely distributed in Baja California and restricted to a small coastal area in mainland Sonora. In this paper, we examine the genetic structure and the mating system of this species in order to explore the origin of the disjunction and describe aspects of the pollination biology. Flowers are nocturnal, pollinated mainly by sphingids and self-incompatible. Polymorphism for allozymes (11 loci) was relatively high (P=75%) but moderate levels of heterozygosity were detected (H(o)=0.103 and H(e)=0.261). Sonoran populations exhibited higher levels of genetic variation than peninsular populations. H(e) declined with latitude when just peninsular and two island populations are included. Substantial levels of inbreeding within populations (f=0.60), moderate differentiation among populations (theta=0.10), and no evidence of isolation by distance were detected. The neighbor-joining phenogram showed Sonoran and island populations nested within peninsular populations. Mainland populations showed greater genetic similarity to island populations, supporting a dispersal hypothesis for the origin of the disjunction. Future studies using DNA markers are suggested in order to better understand the forces that have shaped the genetic structure of this species.     

Purification and partial characterization of two lectins from the cactus Machaerocereus eruca

Two lectins (MEAI and MEAII) were isolated from the cactus Machaerocereus eruca by affinity chromatography on mucin-Sepharose and partially characterized with respect to their biochemical and carbohydrate binding properties. Both are oligomeric glycoproteins consisting of 35 kDa monomers. Amino acid analysis indicates that both lectins have similar composition with high amounts of glycine, glutamic acid and serine. MEAI and MEAII contain approximately 36 and 24% (w/w) of carbohydrates, respectively. They agglutinate erythrocytes from several animal species. Binding specificity was directed to galactose-containing oligosaccharides and glycopeptides. The M. eruca lectins are the first lectins to be isolated from a species belonging to the plant family of Cactaceae.     

Reproductive biology and the process of domestication of the columnar cactus Stenocereus Stellatus in Central Mexico

Pollination biology, breeding system, and floral phenology of the columnar cactus Stenocereus stellatus were studied in wild, wild managed in situ and cultivated populations of central Mexico, in order to examine whether these aspects have been modified under domestication and whether they determine reproductive barriers between wild and manipulated individuals. Individuals of both wild and manipulated populations are self-incompatible, indicating that artificial selection has not modified the breeding system. Their pollination biology is also similar. Anthesis is mainly nocturnal, with a peak of nectar production between 0200 and 0400 when the stigma presents maximum turgidity. Nocturnal visitors are the effective pollinators. Nearly 75% of flowers exposed for nocturnal pollination set fruit, while none of the flowers exposed for diurnal pollination produced fruits. The bats Leptonycteris curasoae, L. nivalis, and Choeronycteris mexicana (Glossophaginae) are the most likely pollinators, and their time of foraging is synchronized with the time of nectar production and stigma receptivity in S. stellatus. Bats potentially move pollen over a considerable distance, so there is apparently no spatial isolation to prevent pollen exchange between wild and cultivated populations. Phenological studies showed that there are also no apparent temporal barriers. However, manual cross pollination failed between some domesticated and wild phenotypes, suggesting that gene flow between wild and cultivated populations might be limited by pollen incompatibility.     

Reproductive Consequences of Clonal Growth in Stenocereus eruca, a Rare Clonal Cactus of the Sonoran Desert

Stenocereus eruca is a prostrated, self-incompatible cactus endemic to the Sonoran Desert that regenerates primarily through clonal propagation. Clonal growth is expected to affect mate availability by influencing the number and spatial distribution of mating types. In this paper we examine the role of clonal growth on female fecundity through a series of pollination experiments in a population of S. eruca. We set up a pollen supplementation experiment using five distance treatments with pollen collected at 1, 10, 100, 1000 and 25000 m from receptor flowers during the years 2001 and 2002 and evaluated genetic sifmilarities between pairs of receptor-donor ramets through RAPD markers. Our data on fruit set, number of seeds/fruit, germination and overall fecundity revealed that S. eruca show a significant reduction in female fecundity when pollination occurs between ramets located at short distances (1 and 10 m), while genetic data showed high levels of similarity at those distances. The reduction in female fecundity is apparently a consequence of geitonogamy and inbreeding depression. Our data suggest that clonal growth and geitonogamy are likely to be partially responsible for the low levels of sexual reproduction and seedling recruitment observed in populations of S. eruca. Co-ordinating editor: H. Kudoh     

Analysis of the community structure of yeasts associated with the decaying stems of cactus. III.Stenocereus thurberi

Yeast communities in necroses of organpipe cactus (Stenocereus thurberi) were surveyed at 3 localities in Arizona. Quantitative analysis of random samples allows comparisons of the types and numbers of yeasts at 3 levels: within plants, between plants within a locality, and between localities. The analysis shows that the major source of variability is between plants. This pattern is identical with the pattern shown by agria cactus (Stenocereus gummosus) and is thought to be due to sampling different successional stages. No significant differences in estimates of the effective number of yeast species (ENS) in agria and organpipe samples were found. Comparisons of agria, organpipe, and prickly pear (Opuntia) cacti support the hypothesis that cactus chemistry is an important determinant of the yeast community structure which, in turn, is an important determinant of the diversity ofDrosophila species which utilize necrotic cacti as feeding and breeding substrates.     

Effects of plant size and weather on the flowering phenology of the organ pipe cactus (Stenocereus thurberi)

Background and Aims

Flowering phenology is a critical life-history trait that influences reproductive success. It has been shown that genetic, climatic and other factors such as plant size affect the timing of flowering and its duration. The spatial and temporal variation in the reproductive phenology of the columnar cactus Stenocereus thurberi and its association with plant size and environmental cues was studied.

Methods

Flowering was monitored during 3 years in three populations of S. thurberi along a latitudinal gradient. Plant size was related to phenological parameters. The actual and past weather were used for each site and year to investigate the environmental correlates of flowering.

Key Results

There was significant variation in the timing of flowering within and among populations. Flowering lasted 4 months in the southern population and only 2 months in the northern population. A single flowering peak was evident in each population, but ocurred at different times. Large plants produced more flowers, and bloomed earlier and for a longer period than small plants. Population synchrony increased as the mean duration of flowering per individual decreased. The onset of flowering is primarily related to the variance in winter minimum temperatures and the duration to the autumn–winter mean maximum temperature, whereas spring mean maximum temperature is best correlated with synchrony.

Conclusions

Plant size affects individual plant fecundity as well as flowering time. Thus the population structure strongly affects flowering phenology. Indications of clinal variation in the timing of flowering and reproductive effort suggest selection pressures related to the arrival of migrating pollinators, climate and resource economy in a desert environment. These pressures are likely to be relaxed in populations where individual plants can attain large sizes.Key words: Flowering phenology, optimal timing, plant size, Sonoran Desert, Stenocereus thurberi, temperature     

In vitro propagation and cryopreservation of Romanian endemic and rare Hypericum species

Efficient micropropagation and cryopreservation of Hypericum richeri ssp. transsilvanicum, an endemic species in Romania, and Hypericum umbellatum, a rare and endangered Daco-Balkan species, was achieved. The effects of type of explant and cytokinin on in vitro plant regeneration were investigated. Shoot organogenesis was achieved in all explants, but stem nodes regenerated best. Organogenesis from nodal segments was promoted by incubating these explants on Murashige and Skoog (MS) medium in the presence of cytokinins (6-benzyladenine, thidiazuron, kinetin or 6-??,??-dimethylallylaminopurine), each tested at four concentrations. The best morphogenic response for both Hypericum species (number of shoots per explant, shoot length, axillary branching of shoot, and frequency of shoot organogenesis) was observed when explants were incubated on MS medium containing 0.44 or 1.11???M 6-benzyladenine. Root induction was achieved only when regenerated shoots were transferred to fresh medium with or without auxin. Maximum rooting was recorded on MS medium supplemented with 2.45???M indole-3-butyric acid. Plantlets grown in vitro were successfully acclimatized in the greenhouse and showed normal development. Shoot tips and axillary buds excised from the in vitro regenerated plants were successfully cryopreserved in liquid nitrogen by the droplet-vitrification method. Following preculture in 0.25?M sucrose, dehydration and cryopreservation, the highest regeneration rates were obtained in both species by using axillary buds (68?% for H. richeri ssp. transsilvanicum and 71?% for H. umbellatum).     

Measuring the effects of invasive species on the demography of a rare endemic plant

The impacts of invasive species are among the greatest threats to the persistence of native species and communities. Yet most work on rare plants has focused on issues such as habitat fragmentation and genetic diversity, while few studies have quantified the impacts of invasive plants on native ones or investigated the underlying mechanisms of those impacts. I used removal experiments to assess the effects of invasive grasses on the seedling and adult demography of an endangered California endemic, Oenothera deltoidesssp. howellii. Invasive plant removal significantly increased O. deltoides seedling recruitment, but had no effect on adult plants. Differences in seedling recruitment were primarily driven by greatly increased seedling emergence rates in removal plots, although there was also some evidence of higher seedling survivorship with invasives removal. Differences in habitat type strongly influenced both the effects of removal treatments and O. deltoides demography, with areas that support natural recruitment showing weaker treatment effects and higher overall recruitment, but lower adult survivorship, compared to those under restoration through planting. These results indicate that inhibition of germination due to reduced soil disturbance, rather than resource competition, appears to be the strongest impact of invasive plants on this rare endemic. Although previous work has documented the importance of changed disturbance regimes in generating invasion impacts, invader effects on rare plants are generally presumed to result from resource competition. Studies like this one highlight the need for a greater emphasis on understanding the mechanisms by which invasive plants impact native ones, and the importance of such information in designing conservation and management strategies.     

Factors affecting germination and establishment success of an endemic cactus of the Chihuahuan Desert

Plant Ecology - Seeds and seedlings are the most critical stages of cacti life cycles. From the thousands of seeds produced in a reproductive season, only a small fraction successfully germinate,...     

Distribution of the related weevil species Sitophilus oryzae and S. zeamais in Brazil

The genus Sitophilus （Coleoptera： Curculionidae） encompasses species of great economic importance as stored grain pests worldwide. Among these species, the maize and the rice weevils （Sitophilus zeamais and Sitophilus oryzae, respectively） are partic- ularly important in warmer climates. These two weevils exhibit closely morphological and ecological resemblance making difficult their proper identification and recognition of their distribution in grain-producing regions. Both species are recorded in South America and particularly in Brazil, but their respective distribution and prevalence were not yet assessed in the region. Therefore, several insect samples throughout Brazil were collected and subjected to morphological identification using male genitalia and also using molec- ular identification with species-specific primers designed for clear recognition of both the species. The primers were designed for the specific amplification of a gene fragment of the cytochrome oxidase subunit I, which exhibited high specificity during our prelimi- nary experiments with insects from six populations of known species （either S. zeamais or S. oryzae）. Both identification strategies provided the same results indicating preva- lence of the maize weevil S. zeamais throughout the country. Two hypotheses may explain such prevalence： （i） the likely host preference ofS. zeamais for maize because this is the most cultivated cereal in Brazil, and （ii） the prevalence ofS. zeamais in tropical regions as compared with S. oryzae, which is more disseminated in subtropical and temperate regions.     

Linear modelling of biopolymer systems and related mechanical properties

A numerical method is proposed to assess the role of random microstructure on the effective Young’s modulus of a two-phase biopolymer composite material. An Ising model coupled to a Monte Carlo (MC) technique is used to generate virtual microstructures representing realistic starch–zein blends having random microstructure. The motivation here was to generate virtual microstructures that can be used in a numerical model to allow a continuous variation of both phase fraction and interface length. From the Pair Correlation Function (PCF), the minimum requirement for the Representative Volume Element (RVE) is established based on geometrical considerations. Finite element analysis allowed the prediction of the effective Young’s modulus as function of the phase ratio for the studied microstructures. The predicted trend is found close to that of Confocal Laser Scanning Microscopy (CLSM) microstructures of starch-based blends used as a case study. The comparison between the predicted results and the most popular analytical expressions points out that only the Hashin–Shrickman bounds are the most close bounds to the evolution of the effective Young’s modulus as function of second phase ratio.When implementing the intrinsic properties of starch and zein and considering virtual microstructures, analytical and numerical models exhibit the same trend. However, the comparison with the 3-p bending results suggests instead, a non-linear trend that can be inferred to the presence of imperfect starch–zein interface properties.     

Conservation implications of genetic variation in three rare species endemic to Florida rosemary scrub

Habitat conversion and fire suppression during the last 50 yr have greatly reduced and altered Florida scrub vegetation, resulting in threats to the persistence of its unique flora. As part of a larger conservation project, we investigated patterns of isozyme variation in three rare perennial scrub plants with overlapping ranges endemic to Florida rosemary scrub on the Lake Wales Ridge. All three species have low levels of genetic variation, comparable to or lower than those generally reported for rare plants with restricted geographic ranges. Liatris ohlingerae has more than twice the expected heterozygosity of the other two species, with little population differentiation. In contrast, Hypericum cumulicola has highly differentiated populations with little apparent interpopulation gene flow and heterozygote deficiencies indicative of inbreeding. Eryngium cuneifolium, the species with the narrowest range and fewest populations, has intermediate values for genetic parameters. Although the three species have narrow and overlapping geographic ranges and similar habitat specificity, we discuss how optimal conservation of each species differs.