Does allometry of a sexually selected ornamental trait vary with sexual selection intensity? A multi‐species test in damselflies

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Interspecific variation in the defensive responses of ant mutualists to plant volatiles

In ant–plant mutualist systems, ants patrol their host plants and search for herbivores. Such patrolling can be inefficient, however, because herbivore activity is spatio-temporally unpredictable. It has been proposed that rapid and efficient systems of communication between ants and plants, such as volatile compounds released following herbivory, both elicit defensive responses and direct workers to sites of herbivore activity. We performed bioassays in which we challenged colonies of two Amazonian plant-ants, Azteca sp. and Pheidole minutula , with extracts of leaf tissue from (1) their respective host-plant species ( Tococa bullifera and Maieta guianensis , both Melastomataceae), (2) sympatric ant-plants from the Melastomataceae, and (3) two sympatric but non-myrmecophytic Melastomataceae. We found that ants of both species responded dramatically to host-plant extracts, and that these responses are greater than those to sympatric myrmecophytes. Azteca sp. also responded to non-myrmecophytes with an intensity similar to that of sympatric ant-plants. By contrast, the response of P. minutula to any non-myrmecophytic extracts was limited. These differences may be driven in part by interspecific differences in nesting behaviour; although P. minutula only nests in host plants, Azteca sp. will establish carton satellite nests on nearby plants. We hypothesize that Azteca sp. must therefore recognize and defend a wider array of species than P. minutula .  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 241–249.     

The first record of gynodioecy in a species of Gnidia (Thymelaeaceae) from South Africa

Sexual polymorphism was studied in the shrub Gnidia wikstroemiana (Thunb.) Meisn. from the semiarid Nama Karoo Biome, South Africa. The populations comprised plants bearing either female flowers, or hermaphrodite flowers with variable female function. In two populations, female plants accounted for 36–37% of the flowering plants. Female flowers were smaller and their stamens were reduced to staminodes, but their styles were significantly longer than those of hermaphrodite flowers. Energy investment in flowers and fruits for females and hermaphrodites was measured using bomb calorimetry. Females produce a greater number of less costly flowers than hermaphrodites, and invest less energy per unit in production of flowers and inflorescences. In contrast, females invest more energy per unit in production of fruits and infructescences than hermaphrodites. Females overall invest 7.3% more energy in reproduction than hermaphrodites. Female flowers were obligate out-crossers (xenogamous), with 35% of nonmanipulated, open-pollinated flowers setting fruit, comparable with fruit set among selfed hermaphrodite flowers. The breeding strategy of G. wikstroemiana most closely resembles gynodioecy. This is the first report of sexual dimorphism in Gnidia L. and sub-Saharan Thymelaeaceae.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 152 , 219–233.     

Seed mass and seedling establishment after fire in Ku‐ring‐gai Chase National Park,Sydney, Australia

Abstract Relationships between seed mass and several aspects of plant regeneration ecology were investigated in a post‐fire environment in Ku‐ring‐gai Chase National Park near Sydney, Australia. We found a significant positive relationship between seed mass and time to seedling emergence (P < 0.001) and a strong negative relationship between seed mass and time between emergence and production of the first true leaf (P < 0.001). Surprisingly, we found no relationship between seed mass and seedling establishment (P = 0.21). It seems most likely that this lack of relationship is a result of the many stochastic factors affecting seedling establishment during any given recruitment episode at any given site. A cause of mortality was assigned to 56% of the 781 seedlings that died during the present study. There was no relationship between cause of death and seed mass (P = 0.28). Of the seedlings for which the cause of death was known, 57% were killed by herbivory and 21% were killed by drought. Seedling–seedling competition affected only one species.     

Purification and properties of cassava green mottle, a previously undescribed virus from the Solomon Islands

A sap-transmissible virus obtained from cassava with a green mottle disease occurring at Choiseul, Solomon Islands, was transmitted to 30 species in 12 plant families and was readily seed-borne in Nicotiana clevelandii. In cassava plants infected by inoculation with sap, the first leaves to be infected systemically developed a mottle with some necrosis whereas leaves produced subsequently were symptomless but contained the virus. Most other species developed chlorotic or necrotic local lesions and systemic mottle or necrosis. This was followed, in several species, by production of small symptomless virus-containing leaves. The virus was cultured in N. clevelandii; Chenopodium quinoa was used for local-lesion assays. Leaf extracts from infected N. clevelandii were infective after dilution to 10–⁵ but usually not at 10–⁶, after heating for 10 min at 60°C but not at 65°C, and after storage at 20°C for at least 12 days. The virus has isometric particles of 26 nm diameter which sediment as three components, all containing a protein of mol. wt c. 53000. The two fastest sedimenting components respectively contain single-stranded RNA of mol. wt, estimated after glyoxylation, c. 2.9 × 10⁶ and 2.3 × 10⁶. Both RNA species are needed for infection of plants. In tests with antiserum prepared to purified virus particles, the virus was detected in cassava and N. clevelandii by gel-diffusion precipitin tests, by immunosorbent electron microscopy and by ELISA. Despite its similarity to nepoviruses, the virus did not react with antisera to 18 members of the group. It was named cassava green mottle virus and is considered to be a previously undescribed nepovirus.     

Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)

The members of the genus Sonchus subgenus Dendrosonchus are woody perennials possessing the pachycaul construction which has been proposed as ancestral to that of modern trees and herbs. The endemic flora of Macaronesia, particularly the Canary Islands, is generally considered to be a Tertiary relict, although an opposing view concerning woody plants of island floras has recently been expressed. The anatomy of Dendrosonchus is examined and trends of evolution can be interpreted from the vessel elements of the secondary xylem. With the use of the length to breadth ratios of vessel elements a gradual trend from primitive to derived is illustrated for the members of each of the two sections of Dendrosonchus , which are considered to have evolved in parallel. The wood rays, features of the cypsela and stem anatomy are also shown to be useful in elucidating the evolution of a small, closely related group of plants. Sonchus arboreus in the section Atalanthus and S. brachylobus in the section Dendrosonchus are similar to the two ancestral types proposed for the subgenus. The most derived species are considered to be S. pinnatus subsp. palmensis in the section Atalanthus and S. acaulis in the section Dendrosonchus. The ancestral type for the genus is considered to have been woody and probably of the Dendrosonchus type.     

Rate of Change in Dormancy Level and Light Requirement in Weed Seeds During Stratification

The rate of change in dormancy level and light requirement,induced during stratification at 3.2 °C, was investigatedin seeds ofStellaria media, Cerastium fontanum, Veronica agrestisandTaraxacumofficinale.Two stratification environments, soil and wet filterpaper in petri-dishes, were used. On nine occasions during a6 week stratification period, germination was tested under threelight conditions at 3.5/18.5 °C: (1) darkness; (2) light;and (3) short-term light exposure followed by darkness. Priorto stratification, germination in all species was 89% in lightand 53% in darkness. Within 2 weeks of stratification, germinabilityinV. agrestisandS. mediaseeds decreased in all treatments. InC.fontanumandT. officinaleseeds, germinability also decreasedafter 2 weeks but only in the dark treatment, indicating inductionof a light requirement. After two more weeks of stratification,the induced dormancy inS. mediaseeds stratified in soil becameweaker and the light requirement inT. officinaleseeds was lost.Differences between the two stratification environments and/orinteractions between light conditions, stratification environmentsand time were found for all species. These results suggest that:(1) the dormancy level and the light requirement of seeds maychange dramatically over relatively short-time periods duringstratification; and (2) germinability depends on the stratificationenvironment experienced by the seeds. Predicting the dormancylevel or light requirement in a seed batch is difficult andrequires a thorough knowledge of the effect of the stratificationconditions used.Copyright 1997 Annals of Botany Company Burial; Cerastium fontanumBaumg.; chickweed; cold stratification; common mouse-ear; dandelion; dormancy; light requirement; seed; Stellaria media(L.) Vill.; speedwell; Taraxacum officinalegroup; Veronica agrestisL.; weed