Semelparous Plants Research Articles

The 'queen of the Andes' (Puya raimondii) is genetically fragile and fragmented: a consequence of long generation time and semelparity?

Understanding how life history shapes genetic diversity is a fundamental issue in evolutionary biology, with important consequences for conservation. However, we still have an incomplete picture of the impact of life history on genome-wide patterns of diversity, especially in long-lived semelparous plants. Puya raimondii is a high-altitude semelparous species from the Andes that flowers at 40-100 years of age. We sequenced the whole genome and estimated the nucleotide diversity of 200 individuals sampled from nine populations. Coalescent-based approaches were then used to infer past population dynamics. Finally, these results were compared with results obtained for the iteroparous species, Puya macrura. The nine populations of P. raimondii were highly divergent, highly inbred, and carried an exceptionally high genetic load. They are genetically depauperate, although, locally in the genome, balancing selection contributed to the maintenance of genetic polymorphism. While both P. raimondii and P. macrura went through a severe bottleneck during the Pleistocene, P. raimondii did not recover from it and continuously declined, while P. macrura managed to bounce back. Our results demonstrate the importance of life history, in particular generation time and reproductive strategy, in affecting population dynamics and genomic variation, and illustrate the genetic fragility of long-lived semelparous plants.

In vitro pollen germination of Neelakurunji - a semelparous plant species

The Neelakurunji (Strobilanthes kunthianus) is a semelparous plant species which flowers once in every 12 years in Western Ghats of South India. For the first time, pollen germination medium to germinate kurunji pollen in vitro was developed and reported. The pollen germination medium developed by Brewbaker and Kwack medium was modified, also urea and multivitamin was added to develop a medium which supports more than 91% pollen germination in vitro. Kurunji pollen is elliptical and the pollen tubes peculiarly emerge from the belly of the pollen. This new pollen germination medium (PGM) consisted of 15% sucrose +15% PEG 4000 + 100 mgl–1boric acid + 450 mgl–1 calcium nitrate +75 mgl–1 potassium nitrate+ 150 mgl–1 magnesium nitrate+ 50 mgl–1urea+200 mgl–1Neurobion forte(multivitamin tablet). Urea has been added as one of the constituents to pollen germination medium for the first time. Also the pollen viability and stigma receptivity were tested.

Maguey (Agave salmiana) infructescence morphology and its relationship to yield components

Agaves are long-lived semelparous plants that produce a high number of seeds, in dehiscent capsules, on the apical section of a stalk, up to 5 m long, after 8–25 years. These and other characteristics such as yield and yield components are scarcely evaluated in the plants of the Agave genus. The objective of this study was to quantify the capsules and seeds yield of A. salmiana Otto ex Salm-Dyck plants simultaneously maturing and growing in the same region. Infructescences of three plants simultaneously growing at San Luis Potosi, Mexico, were harvested. On them we evaluated the number of umbels, capsules and seeds (normal and sterile) and their mass per plant. The study was developed on a completely randomized design with each infructescence as an experimental unit. Also, the type distribution of the number and mass of these yield components along the stalk was evaluated with the Chi square test for goodness of fit, the Shapiro–Wilks for normality tests, asymmetry and kurtosis. Data were analyzed with the ANOVA and multiple comparisons by the Tukey test (p ≤ 0.05). The number of umbels per plant (17–25), capsule per umbel (2–179) and per plant (554–1990), normal seed per capsule (0–297), normal seeds per plant (30,610–186,209) and sterile seeds per plant (211,059–619,251) widely and significantly varied among infructescences. Biomass of capsules per plant umbel (3–795 g), biomass of sterile and normal seeds per capsule (0.071–1.449 and 0–3.320 g), per umbel (0.34–97.76 and 0.21–185.26 g) and per plant (182–1052 and 334–2069 g) also varied widely. Seed yield was statistically different between plants simultaneously growing and maturing at the same site.

Characterization of Arabidopsis thaliana regrowth patterns suggests a trade-off between undamaged fitness and damage tolerance.

Herbivory is a fundamental type of plant-animal interaction that presents substantial selection pressure on plants to replace lost tissues and to prevent subsequent losses in fitness. Apical herbivory, which entails removal or damage to the apical meristem, causes a change in plant architecture by disrupting the balance of hormones produced in part by the apical meristem. Therefore, for an annual semelparous plant, the ability to preserve reproductive success following damage (i.e., to tolerate damage) is largely dependent on the plant's pre-damage investment into fitness and its regrowth pattern following damage. Using multiple regression analyses, we assessed the relationship of developmental and architectural traits of experimentally damaged plants relative to undamaged plants of 33 Arabidopsis thaliana genotypes that display a wide range of undamaged fitness and damage tolerance. Our analyses revealed evidence for an evolutionary bet-hedging strategy within a subset of genotypes to presumably maximize fitness under natural herbivory-genotypes with the greatest seed production when undamaged exhibited a significant reduction in seed yield when damaged, while genotypes with low undamaged seed production were the only genotypes whose seed yield increased when damaged. Patterns of endopolyploidy paralleled those of seed production, such that the increase in whole-plant ploidy by genome re-replication during growth/regrowth contributes to undamaged fitness, damage tolerance, and their trade-off. Overall, this study provides the first large-scale characterization of A. thaliana regrowth patterns and suggests that investment into fitness and endopolyploidy when undamaged may come at a cost to tolerance ability once damaged.

Deciphering the complex nature of bolting time regulation in Beta vulgaris.

Only few genetic loci are sufficient to increase the variation of bolting time in Beta vulgaris dramatically, regarding vernalization requirement, seasonal bolting time and reproduction type. Beta species show a wide variation of bolting time regarding the year of first reproduction, seasonal bolting time and the number of reproduction cycles. To elucidate the genetics of bolting time control, we used three F3 mapping populations that were produced by crossing a semelparous, annual sugar beet with iteroparous, vernalization-requiring wild beet genotypes. The semelparous plants died after reproduction, whereas iteroparous plants reproduced at least twice. All populations segregated for vernalization requirement, seasonal bolting time and the number of reproduction cycles. We found that vernalization requirement co-segregated with the bolting locus B on chromosome 2 and was inherited independently from semel- or iteroparous reproduction. Furthermore, we found that seasonal bolting time is a highly heritable trait (h 2>0.84), which is primarily controlled by two major QTL located on chromosome 4 and 9. Late bolting alleles of both loci act in a partially recessive manner and were identified in both iteroparous pollinators. We observed an additive interaction of both loci for bolting delay. The QTL region on chromosome 4 encompasses the floral promoter gene BvFT2, whereas the QTL on chromosome 9 co-localizes with the BR 1 locus, which controls post-winter bolting resistance. Our findings are applicable for marker-assisted sugar beet breeding regarding early bolting to accelerate generation cycles and late bolting to develop bolting-resistant spring and winter beets. Unexpectedly, one population segregated also for dwarf growth that was found to be controlled by a single locus on chromosome 9.

Fast–slow continuum and reproductive strategies structure plant life-history variation worldwide

The identification of patterns in life-history strategies across the tree of life is essential to our prediction of population persistence, extinction, and diversification. Plants exhibit a wide range of patterns of longevity, growth, and reproduction, but the general determinants of this enormous variation in life history are poorly understood. We use demographic data from 418 plant species in the wild, from annual herbs to supercentennial trees, to examine how growth form, habitat, and phylogenetic relationships structure plant life histories and to develop a framework to predict population performance. We show that 55% of the variation in plant life-history strategies is adequately characterized using two independent axes: the fast-slow continuum, including fast-growing, short-lived plant species at one end and slow-growing, long-lived species at the other, and a reproductive strategy axis, with highly reproductive, iteroparous species at one extreme and poorly reproductive, semelparous plants with frequent shrinkage at the other. Our findings remain consistent across major habitats and are minimally affected by plant growth form and phylogenetic ancestry, suggesting that the relative independence of the fast-slow and reproduction strategy axes is general in the plant kingdom. Our findings have similarities with how life-history strategies are structured in mammals, birds, and reptiles. The position of plant species populations in the 2D space produced by both axes predicts their rate of recovery from disturbances and population growth rate. This life-history framework may complement trait-based frameworks on leaf and wood economics; together these frameworks may allow prediction of responses of plants to anthropogenic disturbances and changing environments.

De Novo Sequencing and Characterization of the Floral Transcriptome of Dendrocalamus latiflorus (Poaceae: Bambusoideae)

BackgroundTranscriptome sequencing can be used to determine gene sequences and transcript abundance in non-model species, and the advent of next-generation sequencing (NGS) technologies has greatly decreased the cost and time required for this process. Transcriptome data are especially desirable in bamboo species, as certain members constitute an economically and culturally important group of mostly semelparous plants with remarkable flowering features, yet little bamboo genomic research has been performed. Here we present, for the first time, extensive sequence and transcript abundance data for the floral transcriptome of a key bamboo species, Dendrocalamus latiflorus, obtained using the Illumina GAII sequencing platform. Our further goal was to identify patterns of gene expression during bamboo flower development.ResultsApproximately 96 million sequencing reads were generated and assembled de novo, yielding 146,395 high quality unigenes with an average length of 461 bp. Of these, 80,418 were identified as putative homologs of annotated sequences in the public protein databases, of which 290 were associated with the floral transition and 47 were related to flower development. Digital abundance analysis identified 26,529 transcripts differentially enriched between two developmental stages, young flower buds and older developing flowers. Unigenes found at each stage were categorized according to their putative functional categories. These sequence and putative function data comprise a resource for future investigation of the floral transition and flower development in bamboo species.ConclusionsOur results present the first broad survey of a bamboo floral transcriptome. Although it will be necessary to validate the functions carried out by these genes, these results represent a starting point for future functional research on D. latiflorus and related species.

What is “phenoptosis” and how to fight it?

Phenoptosis is the death of an organism programmed by its genome. Numerous examples of phenoptosis are described in prokaryotes, unicellular eukaryotes, and all kingdoms of multicellular eukaryotes (animals, plants, and fungi). There are very demonstrative cases of acute phenoptosis when actuation of a specific biochemical or behavioral program results in immediate death. Rapid (taking days) senescence of semelparous plants is described as phenoptosis controlled by already known genes and mediated by toxic phytohormones like abscisic acid. In soya, the death signal is transmitted from beans to leaves via xylem, inducing leaf fall and death of the plant. Mutations in two genes of Arabidopsis thaliana, required for the flowering and subsequent formation of seeds, prevent senescence, strongly prolonging the lifespan of this small semelparous grass that becomes a big bush with woody stem, and initiate substitution of vegetative for sexual reproduction. The death of pacific salmon immediately after spawning is surely programmed. In this case, numerous typical traits of aging, including amyloid plaques in the brain, appear on the time scale of days. There are some indications that slow aging of higher animals and humans is also programmed, being the final step of ontogenesis. It is assumed that stepwise decline of many physiological functions during such aging increases pressure of natural selection on organisms stimulating in this way biological evolution. As a working hypothesis, the biochemical mechanism of slow aging is proposed. It is assumed that mitochondria-generated reactive oxygen species (ROS) is a tool to stimulate apoptosis, an effect decreasing with age the cell number (cellularity) of organs and tissues. A group of SkQ-type substances composed of plastoquinone and a penetrating cation were synthesized to target an antioxidant into mitochondria and to prevent the age-linked rise of the mitochondrial ROS level. Such targeting is due to the fact that mitochondria are the only cellular organelles that are negatively charged compared to the cytosol. SkQs are shown to strongly decrease concentration of ROS in mitochondria, prolong lifespan of fungi, invertebrates, fish, and mammals, and retard appearance of numerous traits of aging. Clinical trials of SkQ1 (plastoquinonyl decyltriphenylphosphonium) have been successfully completed so that the Ministry of Health of the Russian Federation recommends drops of very dilute (0.25 µM) solution of this antioxidant as a medicine to treat the syndrome of dry eye, which was previously considered an incurable disease developing with age. These drops are already available in drugstores. Thus, SkQ1 is the first mitochondria-targeted drug employed in medical practice.

The effect of plant architecture on drought resistance: implications for the evolution of semelparity in Erysimum capitatum

Summary1. Constraints of resource allocation between reproduction and adult survival have been implicated in much life‐history variation, yet physiological or functional trade‐offs with juvenile survival may be just as important. Here, we examined selection on a juvenile trait that is a key determinant of semelparous (monocarpic) vs. iteroparous (polycarpic) life‐history expression.2. In Erysimum capitatum, iteroparous plants produce more rosettes at the juvenile stage than do semelparous plants; those rosettes perennate, enabling subsequent reproductive episodes. Thus, the number of rosettes produced before reproduction is a strong determinant of iteroparity. We tested whether increased rosette production compromised juvenile survival under conditions similar to those in which semelparity predominates over iteroparity.3. Using plants from six natural populations, we tested the association between rosette production and juvenile survival under drought conditions typical of the field sites of semelparous E. capitatum populations. We also manipulated rosette number by physically removing rosettes and examined the effect of rosette removal on drought resistance.4. Under drought conditions, plants with fewer rosettes had higher survival, and the physical excision of rosettes improved survival (significantly or marginally) under drought stress in five of six natural populations.5. The lower production of rosettes, typical of semelparous E. capitatum, was associated with increased juvenile survival under drought stress. The results suggest adaptive differentiation of rosette production, at least partially in response to drought stress. Given the role of apical dominance in multiple rosette development, natural selection seems to favour stronger apical dominance under drought conditions. Drought stress is predicted to be more common at high elevation as a result of climate change, and the novel drought stress could increase juvenile mortality of alpine E. capitatum. Because rosette production at the juvenile stage is necessary for iteroparity, these results demonstrate that drought‐induced selection on traits that determine early survival has significant potential to influence the evolution of adult life‐history expression.

Demographic, developmental and life-history variation across altitude in Erysimum capitatum

1. Natural variation in life history provides the opportunity to examine its correlated population performance. Alpine systems are considered to be vulnerable to climate change as high-altitude conditions are predicted to become more similar to those at low altitude. Comparing natural populations that exhibit variation in life history across altitude can provide information on how life history may influence the demography of alpine plants under a range of altitudinally or climatically variable conditions. 2. We compared the abundance, demography, life history and morphological development of Erysimum capitatum populations in alpine and lower-altitude habitats in the Colorado Rocky Mountains. 3. Mortality of all life stages was higher at lower elevation than in alpine sites, and low-elevation plants reproduced more quickly and expressed a semelparous life history more frequently than did alpine plants. Significant variation in post-reproductive survival, i.e. the opportunity for iteroparity, was also found across regions. 4. Several life stages contributed comparably to projected population growth in high-altitude, iteroparous populations, whereas low-elevation semelparous populations depended primarily on seedling recruitment and rapid reproductive maturation for population maintenance. Populations with intermediate levels of iteroparity differed from each other in their demography, suggesting that transitional states between iteroparity and semelparity are unpredictable. 5. Morphological development also differed between iteroparous and semelparous plants, with iteroparous plants producing more vegetative rosettes and maintaining those rosettes in a vegetative state more frequently than semelparous plants. Sites with lower soil water content were more semelparous. 6. Synthesis. These combined results suggest that changes in developmental rates and morphology may promote changes in fundamental life history. Altitudinal variation in morphology and parity is strongly associated with demographic variation across altitude, indicating that plastic or evolutionary changes in these traits would influence population performance across altitudinally or climatically variable conditions.

Density-dependent demographic responses of a semelparous plant to natural variation in seed rain

The link between reproductive and vegetative ecology of flowering plants is rarely explored, despite its importance for understanding population processes and fitness. This link can be studied by using experimental or natural variation in seed input to the soil to assess how reproductive success affects vital rates of offspring. We previously reported for Ipomopsis aggregata that per-seed probability of germinating is insensitive to density of seeds sown into plots, whereas per capita flower production among adults that grow from the seedlings declines in nonlinear fashion with density. Here we describe a parallel non-experimental study. We related seedling emergence to estimated natural seed input (‘seed rain’) in three populations across ten summers and monitored seedlings that emerged in the first two summers throughout their life histories. Seedling emergence in 1996 was linearly related to seed rain from plants that flowered in 1995. This density independent seed-to-seedling transition recurred over the next nine summers, but the slope varied with springtime precipitation. Total numbers of 1996 seedlings that survived to flower and numbers of flowers they produced increased linearly with seed rain in one population, but did not vary detectably in the other two, consistent with negative density dependence. In consequence λ (the dominant eigenvalue of a population projection matrix) decreased from high values at low densities of seed rain to a relatively constant low value with greater seed rain. We also detected density dependence in the 1995 seedling cohort in survival and flower production. The similarity of results from natural and experimental studies supports a conclusion of nonlinear density dependence and shows that characterizing it requires the full life history. For this plant species and others, studies of pollination and fecundity alone may not suffice to draw conclusions about population change or fitness.

Them's the Brakes

Large nearly monoculture stands of Arundinaria, called canebrakes, historically covered millions of acres in the southeastern United States (Platt and Brantley 1997). This ecosystem was vital to wildlife and played a pivotal role in lowland ecosystem functions and the cultural history of native American tribes (e.g., Cherokee and Choctaw). Canebrake decline started around the time of European settlement, mainly due to altered disturbance regimes (fire and flood), overgrazing, clearing for agriculture, and forest canopy closure. Today, canebrakes are regarded as one of United States’ most critically endangered natural plant communities (Noss et al. 1995), covering less than two percent of their former extent. Remnant stands are highly fragmented as well, with expansive canebrakes non-existent. Growing evidence suggests the loss of canebrakes has directly impacted many riparian ecosystems resulting in increased soil erosion, poorer water quality, and decreased flood control (Schoonover and Willard 2003), as well as the extirpation (and perhaps the extinction) of many species (Brantley and Platt 2001). Although interest in canebrake restoration and conservation for wildlife has been expressed by a number of conservation committees and land stewardship agencies, the amount of information about the biology and genetics of canebrakes and resources (e.g., propagation materials) necessary to support large restoration activities is scant. This is partly due to the unusual life cycle of bamboo. Arundinaria is clonal, sending out rhizomes that develop into new culms (ramets); such clones may be expansive in size. Arundinaria is also monocarpic, flowering once every 25–30 yr (hence, a seed source is not available to generate propagules) and subsequently the whole individual (genet) dies. Death may occur over fairly broad areas due to the long period of pre-flowering clonal growth. While small restoration trials and propagation studies have provided some information, further studies are needed if large restoration activities are to be completed successfully. To that end, a symposium was developed in the spring of 2008 at the annual Association of Southeastern Biologists conference to: 1) bring together scientists studying everything from taxonomy to micropropagation, 2) give stateof-the-art knowledge on restoration and propagation techniques, and 3) develop future research needs for Arundinaria species (Franklin et al. 2008). The following papers represent selected works from the speakers at that symposium, and fall under four major themes: 1) phylogeny and population genetics, 2) autecology, 3) ecosystem services, and 4) restoration. The taxonomy of Arundinaria in North America has gone through a variety of changes, but only recently have molecular techniques been used to determine with more certainty the phylogenetic relationships of genus Arundinaria. Triplett and Clark summarize morphological diagnostics as well as molecular results for distinguishing the three species of this North American endemic genus and provide two epitypes for clarification. Long-lived semelparous plants like cane add a further difficulty to understanding population interactions. Knowledge of genetic diversity within and among clonal stands is important for maintaining genetic diversity in restoration efforts and understanding fertilization potential of a known outbreeder. Mathews et al. examine the genetics between and within stands of cane, including flowering stands, to examine the extent of genetic diversity in canebrakes. Due to the extensive loss of canebrakes, it is difficult to know the autecology of the species simply from field studies. A review of historical accounts, together with records of present day distribution and manipulative studies of * email address: Scott.Franklin@unco.edu CASTANEA 74(3): 205–206. SEPTEMBER 2009

Seed banks cause elevated generation times and effective population sizes of Arabidopsis thaliana in northern Europe

It is commonly found that effective population sizes of natural populations are much smaller than census sizes of plants and animals. However, theoretical studies have shown that factors rarely investigated empirically, like seed banks in plants and diapause in animals, may have profound influence on effective sizes. Here we investigate whether the presence of seed banks can explain the relatively high genetic variability observed in northern European Arabidopsis thaliana populations with small census sizes. We have genotyped three above- and below- ground cohorts in 27 Norwegian populations using single nucleotide polymorphism markers. Although the populations varied extensively in levels of variability within and between cohorts, standard genetic population measures were comparable to those obtained in previous studies on above-ground cohorts using microsatellite markers. Estimated effective population sizes are larger for total populations (containing both seed bank and above-ground cohorts for 1 year) compared to each of the cohorts considered separately. Using a conservative approach, we find that the effective sizes are larger than census sizes of local populations, and that the effective generation time is higher than 1 year (3-4 years, on average), making A. thaliana a perennial semelparous plant at many northern European localities.

Reproductive strategies of Strobilanthes kunthianus, an endemic, semelparous species in southern Western Ghats, India

Strobilanthes kunthianus (Acanthaceae) is a semelparous species with synchronized flowering and mast seeding once in 12 years. As semelparous plants have only one chance to reproduce, they are expected to develop effective strategies to prevent reproductive failure. The reproductive strategies of S. kunthianus were investigated by studying the floral traits, pollination biology, and breeding system that are critical for reproductive success. The species exhibits a series of floral traits: (1) gregarious flowering attracts a large number of Apis cerana indica, the major pollinator; (2) the stigma is sensitive to touch by the pollinator; in fresh flowers, the receptive surface faces the entry path of the incoming bee, facilitating pollen deposition; as an immediate response, the stigma curves backwards moving the receptive surface away from the path of the exiting bee, thus preventing autogamy and interference in pollen transfer; (3) flowers remain fresh for 2 days with receptive stigma and nectar and pollen reward. These traits render the species 100% pollination efficient to ensure a high seed set. As the species is self-compatible, the prevailing high degree of geitonogamous pollinations does not interfere with fruit set. The evolution of the adaptive floral traits has facilitated mast seeding in the species. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157, 155–163.

A Tropical, Gregariously Semelparous Bamboo Shows No Seed Dormancy

ABSTRACTSeed dormancy may be disadvantageous for gregariously semelparous plants because it disrupts the high levels of reproductive synchrony necessary for success. Alternately, it may provide a bet‐hedging option for an otherwise ‘all eggs in the one basket’ reproductive strategy. Rapid germination of seeds upon hydration has been demonstrated for a range of tropical, semelparous bamboos, but the fate of seeds that failed to germinate promptly has been inadequately investigated. We demonstrate prompt germination of seeds upon hydration and the absence of a dormant seedbank in a long‐lived, gregariously semelparous bamboo, Bambusa arnhemica, from monsoonal northern Australia. However, we refute the suggestion that seed dormancy is necessarily maladaptive in a gregariously semelparous plant. Rather, caryopsis dormancy may not be possible in a seasonally moist tropical climate. Given an inability to adjust or bet‐hedge their germination, bamboo germinants must cope with the vagaries of the monsoonal climate, a factor that may contribute to the general restriction of bamboos to regions with higher rainfall.

Investment in survival and reproduction along a semelparity–iteroparity gradient in the Beta species complex

Abstract The Beta species complex shows a gradient of life histories from pronounced semelparity (big-bang reproduction) to pronounced iteroparity (repeated reproduction). Models assume a trade-off between investment in reproduction and survival. Reproductive effort is thought to increase with decreasing life span, and to be invariable in semelparous plants and susceptible to environmental conditions in iteroparous plants. These assumptions and hypotheses were verified by a greenhouse experiment testing six different life cycles at three contrasting nutrient levels. This study suggests that reproductive effort is negatively correlated with mean life span along the life-cycle gradient. Unlike semelparous beets, reproductive effort in iteroparous beets is extremely sensitive to nutrient level. Phenotypic correlation between allocation to reproduction and allocation to survival generally appeared significantly negative in the longest-lived iteroparous beets, nonsignificant in intermediate life histories and obviously positive in semelparous beets (no trade-off control).

Pollination ecology of Agave macroacantha (Agavaceae) in a Mexican tropical desert. I. Floral biology and pollination mechanisms

In a study of sexual reproduction in long-lived semelparous plants, we observed Agave macroacantha in the tropical desert of Tehuacán-Cuicatlán, Mexico, describing duration of flowering, flower phenology, and nectar production patterns. We also performed two manipulative experiments evaluating (a) the seed production efficiency of different crossing systems (selfing, cross-pollination, apomixis, and control), and (b) the effect of different pollinators (diurnal exposure to pollinators, nocturnal exposure, exclusion, and control) on the seeds produced. Flowering occurred from early May to late July and had a mean duration of 29 days in the individual rosettes. The flowers were protandrous; anthesis occurred in the afternoon of the third day after floral opening, and the pistils matured in the afternoon of the fifth day. The stigmas remained receptive from dusk to the following morning. Pollination was mostly allogamous. Nectar was produced principally during the night, from the first stages of floral aperture until the stigmas wilted and flowering ceased. The flowers were visited during the day by hymenoptera, butterflies, and hummingbirds and during the night by bats and moths. Only the nocturnal visitors, however, were successful pollinators. Agave macroacantha is extremely dependent on nocturnal pollinators for its reproductive success.

A Model for the Establishment of Polyploidy in Plants: Viable but Infertile Hybrids, Iteroparity, and Demographic Stochasticity

Analysing a mathematical model, a previous study (Rodrı́guez, 1995) found that, when crosses between semelparous plants of different ploidy in deterministic environments produce inviable zygotes, the establishment of polyploids in communities where their deploid ancestors predominate is possible, when fecundity and selfing are high enough and there is a slight niche separation between cytotypes. The present study modifies this model by considering other situations. Results indicate that: (1) when hybrids are viable but infertile the conditions for polyploid establishment are not altered, (2) iteroparity enhances the chances of polyploid invasion, and (3) stochastic effects due to a small number of invaders decrease the chances of invasion, but in biologically reasonable cases there is still a good chance of invasion. The results are discussed in view of patterns reported in empirical studies. It is concluded that the establishment of polyploids in higher plants is not an unlikely event, and this can help to understand its widespread abundance.

Insurance against reproductive failure in a semelparous plant: bulbil formation in Agave macroacantha flowering stalks.

Bulbils are small aerial rosettes that occur on the flowering stalks of semelparous Agave plants and in related families, and that are capable of acting as clones of the parent plant. We hypothesized that bulbil formation was inversely related to fruiting success in the flowering stalk, and we explored this hypothesis in A. macroacantha, a species from South-Central Mexico. Forty randomly chosen plants were divided amongst three treatments: (a) elimination of all floral buds, (b) exclusion of pollinators, and (c) control. We also studied 22 plants in which the flowering stalk had been felled by goat grazing. Between September and November 1991 we kept a record of the numbers of bulbils and capsules produced in each flowering stalk. Significant (P<0.0001) differences between treatments were found in the proportion of plants hearing capsules and bearing bulbils. The control treatment had the highest proportion of plants producing capsules, treatment a had the highest proportion of individuals bearing bulbils, while treatment b showed an intermediate response. In the goat-grazed group, 45% of the plants failed to produce any propagative structure after the stalk was cut, and half of all plants produced bulbils on the remaining stump. A significant inverse relationship between the numbers of capsules and the numbers of bulbils per plant was found for the three randomly assigned treatments. Our results suggest that once the production of the flowering stalk has been triggered and the death of the rosette is irreversible, bulbils may act as an insurance mechanism that increases the probability of successful reproduction of the genet.

On Limits to Seed Production

On Limits to Seed Production