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.     

Relationships between flowering phenology,plant size and reproductive success in shape Lotus corniculatus (Fabaceae)

Over three years the flowering phenology of individuals of Lotus corniculatus has been studied in relation to fruit set and seed predation to determine the relationships between four components of flowering time, plant size and reproductive success. Timings of first and peak flowering, and duration and synchrony of flowering differed between individuals in the same years. Between years, timing of first flowering was highly correlated for the same individuals, and was closely correlated with plant size and duration of flowering–larger plants flowered earlier and for a longer period. Peak flowering and synchrony were not correlated between-years for individuals.Fruit production and seed predation were correlated with some of the components of flowering phenology in some years, but not in others. The inconstancy of these relationships suggests that directional or stabilising selection is not acting consistently on the aspects of reproductive success studied in this work. The inconstancy of selection may result in the rather asynchronous flowering phenologies of individuals of L. corniculatus observed.We emphasize the importance of studying different components of flowering phenology in relation to individual plant size over several seasons. This work has shown that plant size not only has a direct effect on individual plant fecundity but also can influence flowering time and hence indirectly affect reproductive output.     

Geographic variation in reproductive success of Stenocereus thurberi (Cactaceae): Effects of pollination timing and pollinator guild

? Premise of the study: It has been proposed that species of columnar cacti from dry tropical areas depend on bats for their reproduction, whereas species from dry subtropical areas are also pollinated by other species. To test this hypothesis, we examined the effects of pollinator guild and of variation in time and space on the reproductive success of a widespread species. ? Methods: Changes in fruit set, seed set, and pollinator activity through time were recorded in three widely separated populations of Stenocereus thurberi. Breeding system and sources of pollination limitation were determined by controlled pollinator exclusions in each population. ? Key results: Significant differences were found in the timing of activity and in the effectiveness of pollinators among sites. In the northern and central populations, reproductive success depends on bats, whereas in the southern population a combination of pollinators was more effective. No difference between open and hand cross-pollination treatments was found in the northern and central populations, which suggests no pollen limitation. However, significant differences were detected in the southern population, which indicates temporal differences in pollinator abundance or arrival time. ? Conclusions: Local variation in pollinator assemblages and reproductive success could greatly affect the evolution of pollination systems. The pattern of generalist pollination in the southernmost populations and specialized pollination in the central and northern populations contradicts the hypothesis of latitudinal variation. In the absence of nocturnal pollinators, the accumulated nectar can sustain visits by diurnal pollinators, a bet-hedging strategy that increases the chances of fruit set in some populations.     

Phenotypic selection on flowering phenology and size in two dioecious plant species with different pollen vectors

Dioecious plants may be pollinated biotically by animals or abiotically via wind or water currents. It has been hypothesized that these two types of pollen vectors might impose contrasting selective pressures on plant flowering phenology. In the present study we describe the flowering phenology of two sympatric dioecious species with contrasting pollination modes: Mercurialis perennis (wind‐pollinated) and Tamus communis (insect‐pollinated). We estimated selection differentials and gradients for flowering time and flowering synchrony. As flowering time might depend on the accumulation of enough internal resources, we also estimated direct and indirect selection on plant size. Both species have male‐biased sexual ratios, and males are bigger and produce larger flower displays than females, but only in T. communis do males bloom earlier and for longer than females. Selection gradients suggest that selection tends to favor early‐flowering females of T. communis. There is no evidence of direct current selection on the flowering phenology of M. perennis. Intersexual differences in phenology fit with sex allocation and sexual selection theories. As we hypothesized, phenology of the animal‐pollinated species is under stronger selection than that of the wind‐pollinated species and we discuss the potential role of pollen vectors in shaping the flowering phenologies of the study species.     

Recurring weather extremes alter the flowering phenology of two common temperate shrubs

The aim of this study is to explore the effects of heavy rain and drought on the flowering phenology of two shrub species Genista tinctoria and Calluna vulgaris. We conducted a field experiment over five consecutive years in Central Europe, applying annually recurring extreme drought and heavy rain events on constructed shrubland communities and recorded the flowering status. Further, we correlated spring temperature and precipitation with the onset of flowering. Both species showed a response to extreme weather events: drought delayed the mid flowering date of Genista tinctoria in 3 of 5 years by about 1 month and in 1 year advanced the mid flowering date by 10 days, but did not affect the length of flowering. Mid flowering date of Calluna vulgaris was not affected by drought, but the length of flowering was extended in 2 years by 6 and 10 days. For C. vulgaris the closer the drought occurred to the time of flowering, the larger the impact on the flowering length. Heavy rainfall advanced mid flowering date and reduced the length of flowering of Genista tinctoria by about 2 months in 1 year. Mid flowering date of Calluna vulgaris was not affected by heavy rain, but the length of flowering was reduced in 1 year by 4 days. Our data suggest that extreme weather events, including alterations to the precipitation regime, induce phenological shifts of plant species of a substantial magnitude. Thus, the impacts of climate extremes on plant life cycles may be as influential as gradual warming. Particularly, the variability in the timing of precipitation events appears to have a greater influence on flowering dynamics than the magnitude of the precipitation.     

Effects of experimental shifts in flowering phenology on plant-pollinator interactions

Climate change has led to phenological shifts in flowering plants and insect pollinators, causing concern that these shifts will disrupt plant-pollinator mutualisms. We experimentally investigated how shifts in flowering onset affect pollinator visitation for 14 native perennial plant species, six of which have exhibited shifts to earlier flowering over the last 70 years and eight of which have not. We manipulated flowering onset in greenhouses and then observed pollinator visitation in the field. Five of six species with historically advanced flowering received more visits when flowering was experimentally advanced, whereas seven of eight species with historically unchanged flowering received fewer visits when flowering earlier. This pattern suggests that species unconstrained by pollinators have advanced their flowering, whereas species constrained by pollinators have not. In contrast to current concern about phenological mismatches disrupting plant-pollinator mutualisms, mismatches at the onset of flowering are not occurring for most of our study species.     

Environmental influences on the phenology and abundance of flowering by Androsace septentrionalis (Primulaceae)

We studied the timing and abundance of flowering by Androsace septentrionalis L. (Primulaceae), an indeterminate winter annual or short-lived perennial, in 2 × 2 m plots at the Rocky Mountain Biological Laboratory in Colorado, USA, from 1982 to 2000. Flowers were counted every other day for most or all of the growing season in seven plots in a rocky meadow habitat and nine plots in a wet meadow habitat. The phenology and abundance of flowering were both highly variable, with mean dates of first flowering ranging from 16 May to 12 July and maximum daily counts of flowers ranging from 1 to 1187. Snowmelt date was the primary determinant of timing of flowering. For rocky meadow plots, the previous year's summer precipitation and the current year's average minimum temperature in May had significant effects on maximum number of flowers produced, but no environmental variable we considered was significantly correlated with flower abundance in the wet meadow plots. Length of flowering in individual plots ranged from 2 to 85 d, and many plot-years had both primary (about 1 mo) and secondary (about 10-12 d) flowering periods. The predicted increase in variability of precipitation accompanying climate change will affect negatively the long-term abundance and persistence of this species at our study site.     

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.     

Constraints on flowering phenology in a phryganic (East Mediterranean shrub) community

The phenological pattern of flowering at the community level was studied in a Greek phryganic ecosystem near Athens for 4 consecutive years. Flowering is strongly seasonal: 80% of the insect-pollinated flora, which consists of 133 species, blooms between February and June. There is a variably expressed secondary flowering period in autumn. The pollinating fauna follows a strongly correlated pattern of abundance. Two types of plants were distinguished: pauciflorous species bearing <10 flowers that are large compared to the plant body, and multiflorous species with many small flowers. Pauciflorous species flower in the winter half of the year, while multiflorous species flower mainly in the summer half. The mean flower life spans are 9 and 3 days, respectively. The duration of flowering (DF) for each species is 55 days on average, which is long compared to other communities. The DF shows year-to-year variations, concomitant with the vicissitudes of the climate. The start of flowering of a species is statistically correlated with the temperature in the previous month, not with rainfall; its end date of flowering only partly compensates for the time gained or lost. DF is maximal in winter. The average flower life span of species flowering at any given date varies strongly and independently of the average DF. We tested the hypothesis that flowering phenology is set by phylogenetic and life form constraints. This could not be corroborated for phylogeny, evidently because of the overriding influence of the mediterranean climate, and probably also for biogeographical reasons. In contrast, life forms and multiflorous and pauciflorous species show strong differences. Many (51) of the species are therophytes; we tested the hypothesis that because of their annual habit they would be more dependent on pollination than perennials. Thus we anticipated that therophytic species would be differentiated from perennials in their flowering phenologies. This is not corroborated. We therefore conclude that the seed bank plays a role that is analogous to that of a perennial plant body.     

Differential effects of weather,plant phenology and predators on the seasonal variation of aphids on cabbage

The aphids Lipaphis erysimi pseudobrassicae (Davis) and Myzus persicae (Sulzer) pose serious threats to the production of cruciferous crops in the tropics. Understanding their population dynamics is important for developing integrated pest management programmes to minimize their damage to crops. This study investigated the effects of climatic factors, natural enemies and plant age on the population dynamics of these pests. The population density of aphids and their natural enemies in 20 cabbage plants, and weather conditions were monitored for five cropping seasons from 2019 to 2021 in two agroecological zones of Ghana (Coastal Savannah and Deciduous Forest zones). The highest population density of L. e pseudobrassicae was recorded in January (dry season) in both agroecological zones, while the highest population density for M. persicae occurred in September (minor rainy season) and August (dry spell) in the Coastal Savannah and Deciduous Forest zones, respectively. The highest aphid densities were noted to occur during periods with low relative humidity and low rainfall. The population density of L. e. pseudobrassicae was significantly negatively related to plant age, air temperature and relative humidity, and positively related to syrphids (Paragus borbonicus) and spiders in the Coastal Savannah zone, while in the Deciduous Forest zone, it was significantly positively related to coccinellids. On the other hand, M. persicae population density was significantly positively related to syrphids and coccinellids in the Deciduous Forest zone. Rainfall negatively affected syrphids in the Coastal Savannah zone, while air temperature positively affected syrphids and negatively affected spiders in the Deciduous Forest zone. Coccinellids had a significant positive relationship with relative humidity in the Deciduous Forest zone. This study provides important insights into the key factors that regulate aphid population densities on cabbage and will support development of timely interventions to manage these pests.     

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.     

Effect of flower number on the pollinator attractiveness and the threshold plant size for flowering in Pertya triloba (Asteraceae)

Pertya triloba (Asteraceae) is a perennial herb growing in the understory of deciduous broad-leaved forests in central Japan. Its capitulum consists of a single floret that differs from those of most other Asteraceae species. A bagging experiment clearly showed that breeding of P. triloba required cross-pollination. By using female fertility as a measure of pollination success, the effect of an attractive floral display on the threshold plant size for flowering in P. triloba was surveyed. The number of florets per plant in a P. triloba population was artificially adjusted between one and 30. In both control plants and experimental plants with adjusted floret number, the seed set increased with an increase in the number of florets per plant and became constant (60%) when the floret number per plant was more than 10. Thus, the number of florets per plant has an important role in attracting pollinators, and more than 10 florets per plant must be the effective floral display size. In the P. triloba population, 98.5% of the flowering plants had 10 or more flowers. The effective floral display size might be determined through the mutual relationship between plants and pollinators. The number of florets per plant increased with an increase in the plant leaf area. Therefore, the threshold plant size for flowering might be determined by the productivity needed to support an attractive floral display.     

Polyploidy,flowering phenology and climatic adaptation in Heteropogon contortus (Gramineae)

Chromosome numbers of 2n = 39, 40, 50, 60, 69, 70, 80 and 90 are reported for Heteropogon contortus from a world collection. The numbers 2n = 39, 69, 70 and 90 are new to the literature. The cytogeographic distribution indicates that the tropical latitudes are almost exclusively occupied by tetraploids while the sub-tropical latitudes are characterized by a wide range of ploidy levels from tetraploid to nonaphid. Observations on the time of flowering of these accessions using uniform grass garden techniques indicate that pan-tropical populations are made up almost exclusively of late flowering lines and those from subtropical areas are of mixed lines showing great diversity from early to late flowering types. The association of polyploidy and flowering time are discussed in relation to the possible origin of H. contortus, to its adaptation and migration. It seems reasonable to suggest that polyploidy and the development of an earlier flowering response are both adoptively advantageous and have been selected for the species’migration into sub-tropical latitudes.     

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.