Effects of seed priming on germination and seedling growth of desiccation-sensitive seeds from Mexican tropical rainforest.

Seed priming increases the vigor of seeds and seedlings through metabolic and biochemical processes occurring during controlled hydration, followed by dehydration. In the field, seeds are exposed to hydration-dehydration events in and on the soil after dispersal, as in seed priming. Nevertheless, seed priming has been sparsely tested on desiccation-sensitive seeds, which are vulnerable to climate change effects. We evaluated the effect of two priming methods on seeds from two tropical rainforest species: Cupania glabra and Cymbopetalum baillonii. For hydropriming, the seeds were fully hydrated and then dehydrated to three dehydration levels. For natural priming, the seeds were buried for 12 days in either closed forest or forest gap. Primed seeds were sown in 1% agar medium and placed in an environmental chamber. The growth of the seedlings from the highest germination priming treatments was evaluated for 1 year in the field. Our results showed that for C. glabra and C. baillonii, hydroprimed seeds varied in their germination response, depending on the degree of their dehydration. However, for C. baillonii, hydropriming seems to invigorate seeds, compared to non-imbibed seeds of the same dehydration level. Natural priming increased germination speed in both species without any difference between closed forest and forest gap. Moreover, seeds with natural priming had a higher final germination percentage than seeds with hydropriming. Seedlings from seeds with natural priming showed a higher growth rate than the controls in both species, whereas hydropriming produced a similar effect in C. glabra. Both priming methods could be used for restoration practices with the studied species, natural priming being a novel method. The ecological implications of priming in desiccation sensitive seeds are discussed in this study.

Correlated evolution of diaspore traits and potential frugivore-mediated selection in a fleshy-fruited tropical lineage (Artabotrys, Annonaceae).

Dispersal syndromes are often defined by reference to fruit traits that are associated with distinct frugivore guilds. Studies rarely examine the relationship between seed traits and frugivores or test the alternative hypothesis that traits are shaped by climatic variables. We assess whether the evolution of seed size and physical defense are correlated with dispersal-related traits and climatic variables in Artabotrys, a fleshy-fruited tropical lineage. Diaspore traits and WorldClim bioclimatic variables were compiled for 43 species. Correlated evolution was evaluated using phylogenetic regression and model-fitting approaches. The best-fitting multioptima Ornstein-Uhlenbeck model suggests that lineages with smooth testa and thin pericarp (SP) have evolved toward smaller seeds with a thinner testa, whereas lineages with rough testa and/or thick pericarp have evolved toward larger seeds with a thicker testa. A smooth testa facilitates spitting and/or swallowing of intact seeds while fruits with thin pericarp may be preferentially consumed by frugivores with less destructive oral processing, enabling lower investment in seed physical defense in SP lineages. Moreover, small seeds are more likely to be swallowed intact with a food bolus. The effect of climate on seed size and physical defense is equivocal and warrants further investigation.

Floral evolution by simplification in Monanthotaxis (Annonaceae) and hypotheses for pollination system shifts.

Simplification by reduction has occurred many times independently in the floral evolution of angiosperms. These reductions have often been attributed to changes in reproductive biology. In the angiosperm plant family Annonaceae, most species have flowers with six petals, and many stamens and carpels. In the genus Monanthotaxis several deviations from this pattern have been observed, including flowers that contain three petals and three stamens only. New DNA sequences were generated for 42 specimens of Monanthotaxis. Five chloroplast markers and two nuclear markers for 72 out of 94 species of Monanthotaxis were used to reconstruct a phylogeny of the genus, which revealed several well-supported, morphologically distinct clades. The evolution of four quantitative and two qualitative floral characters was mapped onto this phylogeny, demonstrating a reduction in flower size and number of flower parts in Monanthotaxis. A large variation in stamen forms and numbers, strong correlations between petal size, stamen and carpel number, combined with a non-gradual mode of evolution and the sympatric co-occurrence of Monanthotaxis species from different clades suggest that the high diversity in the African rainforest of this genus is caused by switches in pollination systems.

Stigmatic exudate in the Annonaceae: Pollinator reward, pollen germination medium or extragynoecial compitum?

Although "dry-type" stigmas are widely regarded as ancestral in angiosperms, the early-divergent family Annonaceae has copious stigmatic exudate. We evaluate three putative functions for this exudate: as a nutritive reward for pollinators; as a pollen germination medium; and as an extragynoecial compitum that enables pollen tube growth between carpels. Stigmatic exudate is fructose dominated (72.2%), but with high levels of glucose and sucrose; the dominance of hexose sugars and the diversity of amino acids observed, including many that are essential for insects, support a nutritive role for pollinators. Sugar concentration in pre-receptive flowers is high (28.2%), falling during the peak period of stigmatic receptivity (17.4%), and then rising again toward the end of the pistillate phase (32.9%). Pollen germination was highest in sugar concentrations <20%. Sugar concentrations during the peak pistillate phase therefore provide optimal osmolarity for pollen hydration and germination; subsequent changes in sugar concentration during anthesis reinforce protogyny (in which carpels mature before stamens), enabling the retention of concentrated exudate into the staminate phase as a pollinator food reward without the possibility of pollen germination. Intercarpellary growth of pollen tubes was confirmed: the exudate therefore also functions as a suprastylar extragynoecial compitum, overcoming the limitations of apocarpy.

Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages.

Overhunting in tropical forests reduces populations of vertebrate seed dispersers. If reduced seed dispersal has a negative impact on tree population viability, overhunting could lead to altered forest structure and dynamics, including decreased biodiversity. However, empirical data showing decreased animal-dispersed tree abundance in overhunted forests contradict demographic models which predict minimal sensitivity of tree population growth rate to early life stages. One resolution to this discrepancy is that seed dispersal determines spatial aggregation, which could have demographic consequences for all life stages. We tested the impact of dispersal loss on population viability of a tropical tree species, Miliusa horsfieldii, currently dispersed by an intact community of large mammals in a Thai forest. We evaluated the effect of spatial aggregation for all tree life stages, from seeds to adult trees, and constructed simulation models to compare population viability with and without animal-mediated seed dispersal. In simulated populations, disperser loss increased spatial aggregation by fourfold, leading to increased negative density dependence across the life cycle and a 10-fold increase in the probability of extinction. Given that the majority of tree species in tropical forests are animal-dispersed, overhunting will potentially result in forests that are fundamentally different from those existing now.

The importance of long-distance seed dispersal for the demography and distribution of a canopy tree species.

Long-distance seed dispersal (LDD) is considered a crucial determinant of tree distributions, but its effects depend on demographic processes that enable seeds to establish into adults and that remain poorly understood at large spatial scales. We estimated rates of seed arrival, germination, and survival and growth for a canopy tree species (Miliusa horsfieldii), in a landscape ranging from evergreen forest, where the species' abundance is high, to deciduous forest, where it is extremely low. We then used an individual-based model (IBM) to predict sapling establishment and to compare the relative importance of seed arrival and establishment in explaining the observed distribution of seedlings. Individuals in deciduous forest, far from the source population, experienced multiple benefits (e.g., increased germination rate and seedling survival and growth) from being in a habitat where conspecifics were almost absent. The net effect of these spatial differences in demographic processes was significantly higher estimated sapling establishment probabilities for seeds dispersed long distances into deciduous forest. Despite the high rate of establishment in this habitat, Miliusa is rare in the deciduous forest because the arrival of seeds at long distances from the source population is extremely low. Across the entire landscape, the spatial pattern of seed arrival is much more important than the spatial pattern of establishment for explaining observed seedling distributions. By using dynamic models to link demographic data to spatial patterns, we show that LDD plays a pivotal role in the distribution of this tree in its native habitat.

Functional monoecy due to delayed anther dehiscence: a novel mechanism in Pseuduvaria mulgraveana (Annonaceae).

Unlike most genera in the early-divergent angiosperm family Annonaceae, Pseuduvaria exhibits a diversity of floral sex expression. Most species are structurally andromonoecious (or possibly androdioecious), although the hermaphroditic flowers have been inferred to be functionally pistillate, with sterile staminodes. Pseuduvaria presents an ideal model for investigating the evolution of floral sex in early-divergent angiosperms, although detailed empirical studies are currently lacking. The phenology and pollination ecology of the Australian endemic species Pseuduvaria mulgraveana are studied in detail, including evaluations of floral scent chemistry, pollen viability, and floral visitors. Results showed that the flowers are pollinated by small diurnal nitidulid beetles and are protogynous. Pollen from both hermaphroditic and staminate flowers are shown to be equally viable. The structurally hermaphroditic flowers are nevertheless functionally pistillate as anther dehiscence is delayed until after petal abscission and hence after the departure of pollinators. This mechanism to achieve functional unisexuality of flowers has not previously been reported in angiosperms. It is known that protogyny is widespread amongst early-divergent angiosperms, including the Annonaceae, and is effective in preventing autogamy. Delayed anther dehiscence represents a further elaboration of this, and is effective in preventing geitonogamy since very few sexually mature flowers occur simultaneously in an individual. We highlight the necessity for field-based empirical interpretations of functional floral sex expression prior to evaluations of evolutionary processes.

First fossil record of Alphonsea Hk. f. & T. (Annonaceae) from the Late Oligocene sediments of Assam, India and comments on its phytogeography.

A new fossil leaf impression of Alphonsea Hk. f. & T. of the family Annonaceae is described from the Late Oligocene sediments of Makum Coalfield, Assam, India. This is the first authentic record of the fossil of Alphonsea from the Tertiary rocks of South Asia. The Late Oligocene was the time of the last significant globally warm climate and the fossil locality was at 10°-15°N palaeolatitude. The known palaeoflora and sedimentological studies indicate a fluvio-marine deltaic environment with a mosaic of mangrove, fluvial, mire and lacustrine depositional environments. During the depositional period the suturing between the Indian and Eurasian plates was not complete to facilitate the plant migration. The suturing was over by the end of the Late Oligocene/beginning of Early Miocene resulting in the migration of the genus to Southeast Asia where it is growing profusely at present. The present study is in congruence with the earlier published palaeofloral and molecular phylogenetic data. The study also suggests that the Indian plate was not only a biotic ferry during its northward voyage from Gondwana to Asia but also a place for the origin of several plant taxa.

Neutral dynamics and cluster statistics in a tropical forest.

The neutral theory of biodiversity attributes community structure to the effects of chance alone, assuming that all species and individuals are demographically equivalent. Here we present a spatially explicit version of the neutral theory and test it against the Barro Colorado Island (BCI) data. Monitoring the dynamics of clusters, we show that the effect of local heterogeneities (e.g., microtopography) is weak, making a spatially homogenous model plausible. We then compare the cluster statistics of the three most frequent species with the patterns obtained from neutral dynamics, examining two families of recruitment kernels: one that interpolates between a limited distance and panmictic dispersal (local-global) and one that assumes a scale-free Cauchy kernel. The results rule out the local-global dispersal model and show that the spatial patterns fit very nicely those obtained from the fat-tailed kernel. Our work emphasizes the importance of spatiotemporal cluster dynamics as an instrument for detecting the factors that govern community assembly.

The key role of 4-methyl-5-vinylthiazole in the attraction of scarab beetle pollinators: a unique olfactory floral signal shared by Annonaceae and Araceae.

Cyclocephaline scarabs are specialised scent-driven pollinators, implicated with the reproductive success of several Neotropical plant taxa. Night-blooming flowers pollinated by these beetles are thermogenic and release intense fragrances synchronized to pollinator activity. However, data on floral scent composition within such mutualistic interactions are scarce, and the identity of behaviorally active compounds involved is largely unknown. We performed GC-MS analyses of floral scents of four species of Annona (magnoliids, Annonaceae) and Caladium bicolor (monocots, Araceae), and demonstrated the chemical basis for the attraction of their effective pollinators. 4-Methyl-5-vinylthiazole, a nitrogen and sulphur-containing heterocyclic compound previously unreported in flowers, was found as a prominent constituent in all studied species. Field biotests confirmed that it is highly attractive to both male and female beetles of three species of the genus Cyclocephala, pollinators of the studied plant taxa. The origin of 4-methyl-5-vinylthiazole in plants might be associated with the metabolism of thiamine (vitamin B1), and we hypothesize that the presence of this compound in unrelated lineages of angiosperms is either linked to selective expression of a plesiomorphic biosynthetic pathway or to parallel evolution.

Evolutionary optimization of material properties of a tropical seed.

Here, we show how the mechanical properties of a thick-shelled tropical seed are adapted to permit them to germinate while preventing their predation. The seed has evolved a complex heterogeneous microstructure resulting in hardness, stiffness and fracture toughness values that place the structure at the intersection of these competing selective constraints. Analyses of different damage mechanisms inflicted by beetles, squirrels and orangutans illustrate that cellular shapes and orientations ensure damage resistance to predation forces imposed across a broad range of length scales. This resistance is shown to be around the upper limit that allows cracking the shell via internal turgor pressure (i.e. germination). Thus, the seed appears to strike an exquisitely delicate adaptive balance between multiple selection pressures.

Little ecological divergence associated with speciation in two African rain forest tree genera.

BACKGROUND: The tropical rain forests (TRF) of Africa are the second largest block of this biome after the Amazon and exhibit high levels of plant endemism and diversity. Two main hypotheses have been advanced to explain speciation processes that have led to this high level of biodiversity: allopatric speciation linked to geographic isolation and ecological speciation linked to ecological gradients. Both these hypotheses rely on ecology: in the former conservation of ecological niches through time is implied, while in the latter adaptation via selection to alternative ecological niches would be a prerequisite. Here, we investigate the role of ecology in explaining present day species diversity in African TRF using a species level phylogeny and ecological niche modeling of two predominantly restricted TRF tree genera, Isolona and Monodora (Annonaceae). Both these genera, with 20 and 14 species, respectively, are widely distributed in African TRFs, with a few species occurring in slightly less humid regions such as in East Africa. RESULTS: A total of 11 sister species pairs were identified most of them occurring in allopatry or with little geographical overlap. Our results provide a mixed answer on the role of ecology in speciation. Although no sister species have identical niches, just under half of the tests suggest that sister species do have more similar niches than expected by chance. PCA analyses also support little ecological differences between sister species. Most speciation events within both genera predate the Pleistocene, occurring during the Late Miocene and Pliocene periods. CONCLUSIONS: Ecology is almost always involved in speciation, however, it would seem to have had a little role in species generation within Isolona and Monodora at the scale analyzed here. This is consistent with the geographical speciation model for TRF diversification. These results contrast to other studies for non-TRF plant species where ecological speciation was found to be an important factor of diversification. The Pliocene period appears to be a vital time in the generation of African TRF diversity, whereas Pleistocene climatic fluctuations have had a smaller role on speciation than previously thought.Ecological niche modeling, species level phylogeny, ecological speciation, African tropics, Isolona, Monodora, Annonaceae.

Floral ontogeny of Annonaceae: evidence for high variability in floral form.

BACKGROUND AND AIMS: Annonaceae are one of the largest families of Magnoliales. This study investigates the comparative floral development of 15 species to understand the basis for evolutionary changes in the perianth, androecium and carpels and to provide additional characters for phylogenetic investigation. METHODS: Floral ontogeny of 15 species from 12 genera is examined and described using scanning electron microscopy. KEY RESULTS: Initiation of the three perianth whorls is either helical or unidirectional. Merism is mostly trimerous, occasionally tetramerous and the members of the inner perianth whorl may be missing or are in double position. The androecium and the gynoecium were found to be variable in organ numbers (from highly polymerous to a fixed number, six in the androecium and one or two in the gynoecium). Initiation of the androecium starts invariably with three pairs of stamen primordia along the sides of the hexagonal floral apex. Although inner staminodes were not observed, they were reported in other genera and other families of Magnoliales, except Magnoliaceae and Myristicaceae. Initiation of further organs is centripetal. Androecia with relatively low stamen numbers have a whorled phyllotaxis throughout, while phyllotaxis becomes irregular with higher stamen numbers. The limits between stamens and carpels are unstable and carpels continue the sequence of stamens with a similar variability. CONCLUSIONS: It was found that merism of flowers is often variable in some species with fluctuations between trimery and tetramery. Doubling of inner perianth parts is caused by (unequal) splitting of primordia, contrary to the androecium, and is independent of changes of merism. Derived features, such as a variable merism, absence of the inner perianth and inner staminodes, fixed numbers of stamen and carpels, and capitate or elongate styles are distributed in different clades and evolved independently. The evolution of the androecium is discussed in the context of basal angiosperms: paired outer stamens are the consequence of the transition between the larger perianth parts and much smaller stamens, and not the result of splitting. An increase in stamen number is correlated with their smaller size at initiation, while limits between stamens and carpels are unclear with easy transitions of one organ type into another in some genera, or the complete replacement of carpels by stamens in unisexual flowers.

[Seed germination and key to seedling identification for six native tree species of wetlands from Southeast Mexico]. / Germinación de semillas y clave para la identificación de plántulas de seis especies arbóreas nativas de humedales del sureste de México.

Wetland tree species are of importance for economic and restoration purposes. We describe the germination process and seedling morphology of six arboreal native species typical of Southeastern Mexico: Annona glabra, Ceiba pentandra, Pachira aquatica, Haematoxylum campechianum, Coccoloba barbadensis and Crataeva tapia. A total of 300 seeds per species were planted in a mixture of sand, cocoa plant husk and black soil (1:1:1), and maintained in a tree nursery with 30% artificial shade, from February to November of 2007. We carried out the morphological characterization, and elaborated a key to seedlings based on: 1) germination type 2) seedling axis and 3) leaf elements. P. aquatica has cryptocotylar hypogeal germination, the others have phanerocotylar epigeal germination. Germination rates were high (>86%), except for C. barbadensis (69%).

Germinación de semillas y clave para la identificación de plántulas de seis especies arbóreas nativas de humedales del sureste de México

Seed germination and key to seedling identification for six native tree species of wetlands from Southeast Mexico. Wetland tree species are of importance for economic and restoration purposes. We describe the germination process and seedling morphology of six arboreal native species typical of Southeastern Mexico: Annona glabra, Ceiba pentandra, Pachira aquatica, Haematoxylum campechianum, Coccoloba barbadensis and Crataeva tapia. A total of 300 seeds per species were planted in a mixture of sand, cocoa plant husk and black soil (1: 1: 1), and maintained in a tree nursery with 30% artificial shade, from February to November of 2007. We carried out the morphological characterization, and elaborated a key to seedlings based on: 1) germination type 2) seedling axis and 3) leaf elements. P. aquatica has cryptocotylar hypogeal germination, the others have phanerocotylar epigeal germination. Germination rates were high (>86%), except for C. barbadensis (69%). Rev. Biol. Trop. 58 (2): 717-732. Epub 2010 June 02.

Especies arbóreas de humedales son de importancia económica y para fines de restauración. Nosotros describimos el proceso de germinación y morfología de plántulas de seis especies arbóreas nativas típicas del sureste de México: Annona glabra, Ceiba pentandra, Pachira aquatica, Haematoxylum campechianum, Coccoloba barbadensis y Crataeva tapia. Un total de 300 semillas por especie fueron sembradas en una mezcla de arena, cascarilla de cacao y tierra negra (1: 1: 1), y mantenidas en invernadero a 30% de sombra artificial, de febrero a noviembre de 2007. Se realizó la caracterización morfológica, y elaboró una clave de plántulas con base en: 1) tipo de germinación 2) eje de la plántula y 3) elementos foliares. P. aquatica presenta germinación criptocotilar hipogea, las otras tienen germinación fanerocotilar epigea. Las tasas de germinación fueron altas (>86%), a excepción de C. barbadensis (69%).

Floral evolution in the Annonaceae: hypotheses of homeotic mutations and functional convergence.

The recent publication of hypotheses explaining the homeotic control of floral organ identity together with the availability of increasingly comprehensive and well-resolved molecular phylogenies presents an ideal opportunity for reassessing current knowledge of floral diversity and evolution in the Annonaceae. This review summarizes currently available information on selected aspects of floral structure and function, including: changes in the number of perianth whorls and the number of perianth parts per whorl; the evolution of sympetaly; the diversity and evolution of pollination chambers (with a novel classification of seven main structural forms of floral chamber based on the different arrangement, size and shape of petals); the evolution of perianth glands; floral unisexuality and hypotheses explaining the unexpectedly high frequency of occurrence of androdioecy; the origin and possible function of inner and outer staminodes; the evolution of stamen connective diversity and theca septation; and the origin of 'true' syncarpy and functionally equivalent extragynoecial compita. In each case, current ideas on the origin, evolution and function are discussed. The information presented in this review enables two main conclusions to be drawn. The first is that changes in the homeotic control of floral organ identity may have had a profound impact on floral structure in several disparate lineages in the family. This is most obvious in Fenerivia, in which a centrifugal shift of floral organ identity has occurred, and in Dasymaschalon, in which a reverse (centripetal) shift has occurred. Other genera that have gained or lost entire perianth whorls are likely to have undergone similar homeotic changes. Attention is also drawn to the extensive functional convergence in Annonaceae flowers, with widespread homoplasy in many characters that have previously been emphasized in higher-level classifications.

The olfactory component of floral display in Asimina and Deeringothamnus (Annonaceae).

Floral scent is a key component of floral display, and probably one of the first floral attractants linking insect pollinators to the radiation of Angiosperms. In this article, we investigate floral scent in two extra-tropical genera of Annonaceae. We discuss floral scent in the context of differing pollination strategies in these genera, and compare their scent to that of a close tropical relative. Floral volatiles were collected for Annona glabra, Asimina and Deeringothamnus whole flowers and dissected floral organs, using a standardized static-headspace solid phase microextraction method. Scents were analyzed using gas chromatography-mass spectrometry, and identified using known standards. The floral scents of these species are highly dynamic, varying between floral organs, sexual stages and species. Maroon-flowered species of Asimina produce 'yeasty' odors, dominated by fermentation volatiles and occasionally containing sulfurous or nitrogenous compounds. White-flowered species of Asimina and Deeringothamnus produce pleasant odors characterized by lilac compounds, benzenoids and hydrocarbons. Annona glabra produces a strong, fruity-acetonic scent dominated by 3-pentanyl acetate and 1,8-cineole. The fermented/decaying scents of maroon-flowered species of Asimina suggest mimicry-based pollination strategies similar to aroids and stapeliads, whereas the pleasant scents of white-flowered species of Asimina suggest honest, reward-based pollination strategies. The scent of Annona glabra is typical of specialized beetle pollination systems common to tropical Annonaceae.

Perfume-collecting male euglossine bees as pollinators of a basal angiosperm: the case of Unonopsis stipitata (Annonaceae).

Pollination of Unonopsis stipitata (Annonaceae) by males of two perfume-collecting bees, Euglossa imperialis and Eulaema bombiformis (Euglossini) is described. This is the first detailed account of this pollination mode in a member of a basal angiosperm family. Pollinator behaviour, identification of the odour bouquet and electrophysiological reaction of one of the two pollinators to the odour bouquet were determined. The collected odour is produced by 'osmophores' located adaxially on the petals. Starch and polysaccharides accumulated in petals are metabolized during odour emission. Mainly monoterpenes were detected in the scent samples, among them trans-carvone oxide. This molecule is thought by several authors to be the key attractant for male Eulaema bees and may be pivotal for convergent evolution of the perfume-collecting syndrome among dicotyledonous and monocotyledonous plants. It is speculated that Unonopsis, which on the basis of molecular age dating is considered a relatively recent genus of the Annonaceae (being 15-30 million years old), has diversified in relation to male euglossine bee pollinators.