An ancient push-pull pollination mechanism in cycads

Changes in the Carotenoids of Zamia dressleri Leaves during Development

It has been observed that the leaves of some Zamia species undergo a kind of "reverse ripening"; that is, they change from their original brown color to green during development. We assumed that this strange color change was due to the change in carotenoid composition, so we followed the changes for several weeks. The detailed carotenoid composition and content at different stages of development of the leaves was determined with HPLC-DAD focusing on the changes in red and yellow carotenoids. The total and relative amounts of red and yellow carotenoids were determined simultaneously from one measurement from a saponified and/or unsaponified extract. At the beginning of development, the concentration of red carotenoids was higher than that of the yellow ones; it decreased drastically until 22 days and continued to decrease slowly until they completely disappeared. The concentration of yellow carotenoids decreased at the beginning as well, but after 22 days it started to increase. The amount of red carotenoids started to decrease when the leaflet stopped growing. Lutein is the main component in old leaflets, which is not a red carotenoid precursor. Red carotenoids can always be found in their esterified form in the leaves. These findings support the hypothesis that red and yellow carotenoid accumulation are independent and probably have different functions in the leaflet. The strange color change was explained based on the compartmentalization of red and yellow carotenoids and on the changing activity of the enzyme capsanthin-capsorubin synthase responsible for the synthesis of red carotenoids capsorubin and capsanthin.

Reconciling fossils with phylogenies reveals the origin and macroevolutionary processes explaining the global cycad biodiversity

The determinants of biodiversity patterns can be understood using macroevolutionary analyses. The integration of fossils into phylogenies offers a deeper understanding of processes underlying biodiversity patterns in deep time. Cycadales are considered a relict of a once more diverse and globally distributed group but are restricted to low latitudes today. We still know little about their origin and geographic range evolution. Combining molecular data for extant species and leaf morphological data for extant and fossil species, we study the origin of cycad global biodiversity patterns through Bayesian total-evidence dating analyses. We assess the ancestral geographic origin and trace the historical biogeography of cycads with a time-stratified process-based model. Cycads originated in the Carboniferous on the Laurasian landmass and expanded in Gondwana in the Jurassic. Through now-vanished continental connections, Antarctica and Greenland were crucial biogeographic crossroads for cycad biogeography. Vicariance is an essential speciation mode in the deep and recent past. Their latitudinal span increased in the Jurassic and restrained toward subtropical latitudes in the Neogene in line with biogeographic inferences of high-latitude extirpations. We show the benefits of integrating fossils into phylogenies to estimate ancestral areas of origin and to study evolutionary processes explaining the global distribution of present-day relict groups.

Phylogenomics illuminates the phylogeny of flower weevils (Curculioninae) and reveals ten independent origins of brood-site pollination mutualism in true weevils

Weevils are an unusually species-rich group of phytophagous insects for which there is increasing evidence of frequent involvement in brood-site pollination. This study examines phylogenetic patterns in the emergence of brood-site pollination mutualism among one of the most speciose beetle groups, the flower weevils (subfamily Curculioninae). We analysed a novel phylogenomic dataset consisting of 214 nuclear loci for 202 weevil species, with a sampling that mainly includes flower weevils as well as representatives of all major lineages of true weevils (Curculionidae). Our phylogenomic analyses establish a uniquely comprehensive phylogenetic framework for Curculioninae and provide new insights into the relationships among lineages of true weevils. Based on this phylogeny, statistical reconstruction of ancestral character states revealed at least 10 independent origins of brood-site pollination in higher weevils through transitions from ancestral associations with reproductive structures in the larval stage. Broadly, our results illuminate the unexpected frequency with which true weevils-typically specialized phytophages and hence antagonists of plants-have evolved mutualistic interactions of ecological significance that are key to both weevil and plant evolutionary fitness and thus a component of their deeply intertwined macroevolutionary success.

Pollination: Multimodal signaling in a co-dependent thermogenic gymnosperm system

Plant-pollinator interaction research commonly focuses on more apparent traits, floral morphology and odor. A new study of a thermogenic gymnosperm brood-site pollination system via a specialist beetle reveals intricate long- and short-range multimodal signals are involved in host finding.

Cone humidity is a strong attractant in an obligate cycad pollination system

Studies of pollination biology often focus on visual and olfactory aspects of attraction, with few studies addressing behavioral responses and morphological adaptation to primary metabolic attributes. As part of an in-depth study of obligate nursery pollination of cycads, we find that Rhopalotria furfuracea weevils show a strong physiological response and behavioral orientation to the cone humidity of the host plant Zamia furfuracea in an equally sensitive manner to their responses to Z. furfuracea-produced cone volatiles. Our results demonstrate that weevils can perceive fine-scale differences in relative humidity (RH) and that individuals exhibit a strong behavioral preference for higher RH in binary choice assays. Host plant Z. furfuracea produces a localized cloud of higher than ambient humidity around both pollen and ovulate cones, and R. furfuracea weevils preferentially land at the zone of maximum humidity on ovulate cones, i.e., the cracks between rows of megasporophylls that provide access to the ovules. Moreover, R. furfuracea weevils exhibit striking antennal morphological traits associated with RH perception, suggesting the importance of humidity sensing in the evolution of this insect lineage. Results from this study suggest that humidity functions in a signal-like fashion in this highly specialized pollination system and help to characterize a key pollination-mediating trait in an ancient plant lineage.

Plant ecology: New insights into the adaptive significance of rapid floral movements

Rapid movements by plants have been a subject of interest since Linnaeus in the 18th century. Largely lacking, however, has been a modern understanding of the adaptive significance of such movements. A recent study shows that rapid stamen movements can increase male fitness by scaring off pollinators once they have pollen placed on them.

Establishing an efficient protoplast transient expression system for investigation of floral thermogenesis in aroids

Floral thermogenesis is an important reproductive strategy for attracting pollinators. We developed essential biological tools for studying floral thermogenesis using two species of thermogenic aroids, Symplocarpus renifolius and Alocasia odora. Aroids contain many species with intense heat-producing abilities in their inflorescences. Several genes have been proposed to be involved in thermogenesis of these species, but biological tools for gene functional analyses are lacking. In this study, we aimed to develop a protoplast-based transient expression (PTE) system for the study of thermogenic aroids. Initially, we focused on skunk cabbage (Symplocarpus renifolius) because of its ability to produce intense as well as durable heat. In this plant, leaf protoplasts were isolated from potted and shoot tip-cultured plants with high efficiency (ca. 1.0 × 10⁵/g fresh weight), and more than half of these protoplasts were successfully transfected. Using this PTE system, we determined the protein localization of three mitochondrial energy-dissipating proteins, SrAOX, SrUCPA, and SrNDA1, fused to green fluorescent protein (GFP). These three GFP-fused proteins were localized in MitoTracker-stained mitochondria in leaf protoplasts, although the green fluorescent particles in protoplasts expressing SrUCPA-GFP were significantly enlarged. Finally, to assess whether the PTE system established in the leaves of S. renifolius is applicable for floral tissues of thermogenic aroids, inflorescences of S. renifolius and another thermogenic aroid (Alocasia odora) were used. Although protoplasts were successfully isolated from several tissues of the inflorescences, PTE systems worked well only for the protoplasts isolated from the female parts (slightly thermogenic or nonthermogenic) of A. odora inflorescences. Our developed system has a potential to be widely used in inflorescences as well as leaves in thermogenic aroids and therefore may be a useful biological tool for investigating floral thermogenesis.

Unique chemistry associated with diversification in a tightly coupled cycad-thrips obligate pollination mutualism

Cycad cone thermogenesis and its associated volatiles are intimately involved in mediating the behavior of their obligate specialist pollinators. In eastern Australia, thrips in the Cycadothrips chadwicki species complex are the sole pollinators of many Macrozamia cycads. Further, they feed and reproduce entirely in the pollen cones. M. miquelii, found only in the northern range of this genus, is pollinated only by a C. chadwicki cryptic species that is the most distantly related to others in the complex. We examined the volatile profile from M. miquelii pollen and ovulate (receptive and non-receptive) cones to determine how this mediates pollination mechanistically, using GC-MS (gas chromatography-mass spectrometry) and behavioral tests. Monoterpenes comprise the bulk of M. miquelii volatile emissions, as in other Macrozamia species, but we also identified compounds not reported previously in any cycad, including three aliphatic esters (prenyl acetate and two of uncertain identity) and two aliphatic alcohols. The two unknown esters were confirmed as prenyl (3-methylbut-2-enyl) esters of butyric and crotonic ((E))-but-2-enoic) acids after chemical synthesis. Prenyl crotonate is a major component in emissions from pollen and receptive ovulate cones, is essentially absent from non-receptive cones, and has not been reported from any other natural source. In field bioassays, Cycadothrips were attracted only to those volatile treatments containing prenyl crotonate. We discuss M. miquelii cone odorants relative to those of other cycads, especially with respect to prenyl crotonate being a species-specific signal to this northern C. chadwicki cryptic species, and how this system may have diversified.

Cycad-Weevil Pollination Symbiosis Is Characterized by Rapidly Evolving and Highly Specific Plant-Insect Chemical Communication

Coevolution between plants and insects is thought to be responsible for generating biodiversity. Extensive research has focused largely on antagonistic herbivorous relationships, but mutualistic pollination systems also likely contribute to diversification. Here we describe an example of chemically-mediated mutualistic species interactions affecting trait evolution and lineage diversification. We show that volatile compounds produced by closely related species of Zamia cycads are more strikingly different from each other than are other phenotypic characters, and that two distantly related pollinating weevil species have specialized responses only to volatiles from their specific host Zamia species. Plant transcriptomes show that approximately a fifth of genes related to volatile production are evolving under positive selection, but we find no differences in the relative proportion of genes under positive selection in different categories. The importance of phenotypic divergence coupled with chemical communication for the maintenance of this obligate mutualism highlights chemical signaling as a key mechanism of coevolution between cycads and their weevil pollinators.

Ecology and evolution of cycad-feeding Lepidoptera

Cycads are an ancient group of tropical gymnosperms that are toxic to most animals - including humans - though the larvae of many moths and butterflies (order: Lepidoptera) feed on cycads with apparent immunity. These insects belong to distinct lineages with varying degrees of specialisation and diverse feeding ecologies, presenting numerous opportunities for comparative studies of chemically mediated eco-evolutionary dynamics. This review presents the first evolutionary evaluation of cycad-feeding among Lepidoptera along with a comprehensive review of their ecology. Our analysis suggests that multiple lineages have independently colonised cycads from angiosperm hosts, yet only a few clades appear to have radiated following their transitions to cycads. Defensive traits are likely important for diversification, as many cycad specialists are warningly coloured and sequester cycad toxins. The butterfly family Lycaenidae appears to be particularly predisposed to cycad-feeding and several cycadivorous lycaenids are warningly coloured and chemically defended. Cycad-herbivore interactions provide a promising but underutilised study system for investigating plant-insect coevolution, convergent and divergent adaptations, and the multi-trophic significance of defensive traits; therefore the review ends by suggesting specific research gaps that would be fruitfully addressed in Lepidoptera and other cycad-feeding insects.