Growth and reproduction of Arabidopsis thaliana in relation to storage of starch and nitrate in the wild-type and in starch-deficient and nitrate-uptake-deficient mutants

We have investigated the interactions between resource assimilation and storage in rosette leaves, and their impact on the growth and reproduction of the annual species Arabidopsis thaliana. The resource balance was experimentally perturbed by changing (i) the external nutrition, by varying the nitrogen supply; (ii) the assimilation and reallocation of resources from rosette leaves to reproductive organs, by cutting or covering rosette leaves at the time of early flower bud formation, and (iii) the internal carbon and nitrogen balance of the plants, by using isogenic mutants either lacking starch formation (PGM mutant) or with reduced nitrate uptake (NU mutant). When plants were grown on high nitrogen, they had higher concentrations of carbohydrates and nitrate in their leaves during the rosette phase than during flowering. However, these storage pools did not significantly contribute to the bulk flow of resources to seeds. The pool size of stored resources in rosette leaves at the onset of seed filling was very low compared to the total amount of carbon and nitrogen needed for seed formation. Instead, the rosette leaves had an important function in the continued assimilation of resources during seed ripening, as shown by the low seed yield of plants whose leaves were covered or cut off. When a key resource became limiting, such as nitrogen in the NU mutants and in plants grown on a low nitrogen supply, stored resources in the rosette leaves (e.g. nitrogen) were remobilized, and made a larger contribution to seed biomass. A change in nutrition resulted in a complete reversal of the plant response: plants shifted from high to low nutrition exhibited a seed yield similar to that of plants grown continuously on a low nitrogen supply, and vice versa. This demonstrates that resource assimilation during the reproductive phase determines seed production. The PGM mutant had a reduced growth rate and a smaller biomass during the rosette phase as a result of changes in respiration caused by a high turnover of soluble sugars ( Caspar et al. 1986 ; W. Schulze et al. 1991 ). During flowering, however, the vegetative growth rate in the PGM mutant increased, and exceeded that of the wild-type. By the end of the flowering stage, the biomass of the PGM mutant did not differ from that of the wild-type. However, in contrast to the wild-type, the PGM mutant maintained a high vegetative growth rate during seed formation, but had a low rate of seed production. These differences in allocation in the PGM mutant result in a significantly lower seed yield in the starchless mutants. This indicates that starch formation is not only an important factor during growth in the rosette phase, but is also important for whole plant allocation during seed formation. The NU mutant resembled the wild-type grown on a low nitrogen supply, except that it unexpectedly showed symptoms of carbohydrate shortage as well as nitrogen deficiency. In all genotypes and treatments, there was a striking correlation between the concentrations of nitrate and organic nitrogen and shoot growth on the one hand, and sucrose concentration and root growth on the other. In addition, nitrate reductase activity (NRA) was correlated with the total carbohydrate concentration: low carbohydrate levels in starchless mutants led to low NRA even at high nitrate supply. Thus the concentrations of stored carbohydrates and nitrate are directly or indirectly involved in regulating allocation.     

Weed invasion in East Africa: insights from herbarium records

The invasion process was documented from data on the labels of specimens stored in the East African Herbarium at the National Museums of Kenya. We analysed data from seven abundant alien plant species: Ageratum conyzoides L. (Asteraceae), Capsella bursa‐pastoris (L.) Med . (Brassicaceae ) , Datura stramonium L. (Solanaceae), Galinsoga parviflora Cav. (Asteraceae), Tagetes minuta L. (Asteraceae), Spergula arvensis L. (Caryophyllaceae) and Stellaria media (L.) Vill. (Caryophyllaceae) and compared these data with the spread of two native weeds: Cynoglossum coeruleum A.DC. (Boraginaceae) and Senecio discifolius Oliv. (Asteraceae). Although all investigated species had been already recorded before the Second World War, most specimens were collected between 1960 and 1980. This regional spread is correlated with a change in the agricultural systems of Kenya. The early records of alien weeds were restricted to higher altitudes. With the increasing human population and the associated increase of agricultural activities (e.g. irrigation in arid areas) the weed species from South America were able to spread to lower altitudes.     

Flies on thistles: support for synchronous speciation?

Synchronous speciation of hosts and herbivorous insects predicts a congruent topology of host and insect phylogenies and similar evolutionary ages of host and insect taxa. To test these predictions for the specialized herbivorous fly genus Urophora (Diptera: Tephritidae), we used three different approaches. (i) We generated a phylogenetic tree of 11 European Urophora species from allozyme data and constructed a phylogeny of their hosts from published sources. Superimposing the Urophora tree on the host-plant tree we found no evidence for general congruence. (ii) We correlated genetic distances (Nei distances) of the host plants vs. the genetic distances of associated Urophora species. Overall, the relationship was not positive. Nevertheless, for some pairs of Urophora species and host plants genetic distances were in the same order of magnitude. (iii) We collected allozyme data for pairs of thistle taxa and pairs of herbivores on thistles together with independent time estimates. With these data we calibrated a molecular clock. There was a non-linear relationship between phylogenetic age and genetic distance, rendering the dating of deep events in thistle–insect evolution difficult. Nevertheless the derived molecular clock showed that the split of insect taxa lagged behind the split of hosts.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 775–783.     

Extreme habitats are not refuges: poeciliids suffer from increased aerial predation risk in sulphidic southern Mexican habitats

Extreme environments are often considered a predation refuge for organisms living in them. In southern Mexico several species of poeciliid fishes are undergoing incipient speciation in a variety of extreme (i.e. permanently dark and/or sulphidic) freshwater systems, and previous research has demonstrated reproductive isolation between populations from sulphidic and adjacent benign habitats. In the present study, we investigated bird predation rates (measured as successful captures per minute) in two sulphidic surface and several benign surface habitats, to test the hypothesis that extreme habitats are predation refuges. We found capture rates to be approximately 20 times higher in sulphidic environments: probably facilitated by extremophile poeciliids spending most of their time at the water surface, where they engage in aquatic surface respiration as a direct response to hypoxia. Even birds that are usually not considered major fish predators regularly engage in fish predation in the toxic habitats of southern Mexico. Our results demonstrate that extreme environments do not necessarily represent a refuge from predation, and we discuss the general importance of predation in driving incipient speciation in these systems. Finally, we hypothesize that natural selection via avian predation may play an important role in maintaining reproductive isolation between divergent poeciliid populations. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 417–426.     

EVIDENCE FOR A HYDRODYNAMIC MECHANISM OF SOUND PRODUCTION BY COURTING MALES OF THE NOTCHTONGUE GOBY,BATHYGOBIUS CURACAO (METZELAAR)

ABSTRACT

Male gobies of the genus Bathygobius are soniferous during courtship. The mechanism by which the sounds are produced is, however, unknown. Early studies on sound production by males of B. soporator suggested that these sounds are hydrodynamic in nature, being produced by the forcible ejection of water through the gill opening. The mechanism of sound production by the closely related species B. curacao was investigated and three lines of evidence are presented which support the hydrodynamic hypothesis. First, similarities between the sounds produced by courting males and by ejecting water through a pipette demonstrated that hydrodynamic forces can readily produce such sounds. Second, the behavioural motor patterns occurring during sound production are consistent with the hypothesis that water is being ejected through the gill openings. Finally, morphological examination revealed an apparent lack of specialised features associated with sound production, effectively eliminating stridulatory and swim bladder mechanisms. These results represent the strongest evidence to date in support of any proposed mechanism of sound production in the gobiids.     

Colchicine-binding Proteins in Chromatin and Membranes

THE cytoplasm of several cell types contains a protein which is characterized by its specific capability to bind colchicine^1–4 and which may be in dynamic equilibria between monomers and a distinct form of polymer structure, the microtubules. Studies of the character of this protein have hitherto usually proceeded from the cellular homogenate to the 100,000g supernatant which, at least in brain tissue, provides the largest enrichment and recovery. Two lines of observations prompted us to examine the possibility that colchicine-binding proteins exist in association with structures other than microtubules. First, considerable amounts of colchicine-binding protein are recovered in the 100,000g sediments. This is especially noticeable with mammalian liver but has also been noted during vertebrate brain fractionation^4–7. Second, certain structures exhibit frequent and conspicuous associations with microtubules, in particular membranes^8,9 and chromosomal structures. Membrane preparations too have been reported to share some protein properties in common with tubulin^10,11.     

Carrot fly cultivar preferences: some influencing factors

Abstract. 1. Factors affecting cultivar preferences by the carrot fly Psila rosae F. were examined in choice tests using foliage from carrot lines representing extremes of resistance and susceptibility to attack.
2. Flies were offered a choice to oviposit on foliage, artificial substrates impregnated with leaf surface extracts or enclosed in vapours of different cultivars to establish the importance of contact and olfactory stimuli. Susceptible cv. Danvers Half Long 126 foliage was preferred over that of resistant cvs. Clause's Sytan Original and Tip-Top. But, whereas the Sytan leaf surface extract was as effective as that of Danvers, the surface extract of Tip-Top was very much inferior as an oviposition stimulant. By contrast, Sytan and Tip-Top foliar vapours were both more attractive to the fly than that from Danvers.
3. Headspace vapours over Sytan foliage evoked significantly higher electroantennogram responses than those from Danvers.
4. Cold-trapped foliar volatiles of Sytan and Danvers were analysed by gas chromatography linked with antennographic detection. Foliage of Sytan released, among other chemo-stimulants, higher levels of host plant attractants, i.e. green leaf aldehydes and the phenylpropanoids, trans-asarone and trans-methylisoeugenol.
5. These findings indicate that olfactory and contact chemostimuli may be involved in preferences shown by the carrot fly for carrot cultivars, but these factors alone do not fully account for the preferences observed.