Evolution of nectar spur length in a clade of Linaria reflects changes in cell division rather than in cell expansion.

Background and Aims: Nectar spurs (tubular outgrowths of a floral organ which contain, or give the appearance of containing, nectar) are hypothesized to be a 'key innovation' which can lead to rapid speciation within a lineage, because they are involved in pollinator specificity. Despite the ecological importance of nectar spurs, relatively little is known about their development. We used a comparative approach to investigate variation in nectar spur length in a clade of eight Iberian toadflaxes. Methods: Spur growth was measured at the macroscopic level over time in all eight species, and growth rate and growth duration compared. Evolution of growth rate was reconstructed across the phylogeny. Within the clade we then focused on Linaria becerrae and Linaria clementei, a pair of sister species which have extremely long and short spurs, respectively. Characterization at a micromorphological level was performed across a range of key developmental stages to determine whether the difference in spur length is due to differential cell expansion or cell division. Key Results: We detected a significant difference in the evolved growth rates, while developmental timing of both the initiation and the end of spur growth remained similar. Cell number is three times higher in the long spurred L. becerrae compared with L. clementei, whereas cell length is only 1.3 times greater. In addition, overall anisotropy of mature cells is not significantly different between the two species. Conclusions: We found that changes in cell number and therefore in cell division largely explain evolution of spur length. This contrasts with previous studies in Aquilegia which have found that variation in nectar spur length is due to directed cell expansion (anisotropy) over variable time frames. Our study adds to knowledge about nectar spur development in a comparative context and indicates that different systems may have evolved nectar spurs using disparate mechanisms.

Differentiation in plant epidermal cells.

The plant epidermis is a multifunctional tissue playing important roles in water relations, defence and pollinator attraction. This range of function is performed by a number of different types of specialized cells, which differentiate from the early undifferentiated epidermis in adaptively significant patterns and frequencies. These various cells show different degrees of morphological specialization, but there is evidence to suggest that even the less specialized cell types may require certain signals to ensure their correct differentiation and patterning. Epidermal cells may potentially adopt certain fates through a cell lineage based mechanism or a cell interaction mechanism. Work on stomatal development has focused on the cell lineage mechanism and work on trichome differentiation has focused on the cell interaction model. Recent work on the Arabidopsis trichome suggests that interactions between neighbouring cells reinforce initial differences, possibly in levels of gene expression or cell cycle stage, to commit cells to different developmental programmes. In this review these mechanisms are explored in a number of specialized cell types and the further interactions between different developmental programmes are analysed. It is in these interactions between differentiating cells adopting different cell fates that the key to the patterning of a multifunctional tissue must lie.

Development of several epidermal cell types can be specified by the same MYB-related plant transcription factor.

A MYB-related transcription factor (MIXTA) that controls development of conical cell form is expressed only in the inner epidermis of Antirrhinum petals. Expression of this gene throughout transgenic tobacco plants leads to excess numbers of multicellular trichomes on leaves and floral organs as well as the novel production of conical cells on leaves. These data indicate that conical cells and trichomes are produced by a common developmental pathway. The timing of MIXTA expression suggests that the choice between the cell types depends on the competence for cell division at the time at which the controlling gene is expressed. Duplication of genes and their association with different cis-regulatory regions may therefore result in the specification of novel plant cell types.

Cellular differentiation in the shoot epidermis.

The recent advances in defining genes involved in shoot epidermal cell differentiation are impressive, especially the characterisation of genes involved in cellular patterning. The additional influences of environment and hormones on cellular patterning have recently been emphasised, and important connections have been made to changes in vegetative and reproductive growth phases. Despite these advances the cellular basis for differentiation remains less well defined, but now genetic and cell biological analysis from yeast may provide important models on which to develop further understanding.

Cocaine-induced increases in EEG alpha and beta activity: evidence for reduced cortical processing.

To understand the effects of cocaine on the cerebral cortex, 14 male polydrug abusers were enrolled in a study on the effects of cocaine on the electroencephalogram (EEG). The experimental treatments were placebo, 20 mg cocaine or 40 mg cocaine i.v. administered in a double-blind, pseudorandom design. The EEG was recorded from 13 electrode positions over the left hemisphere during a 3-minute baseline recording and for 30 minutes after initiation of the i.v. injection. The spectral power for delta, theta, alpha and beta EEG bands was calculated from data collected in each 3-minute interval. Cocaine significantly increased beta in frontal and central areas and enhanced alpha in frontal and temporal regions. Cocaine-induced increases in EEG beta power had a cortical distribution similar to those produced by barbiturates and benzodiazepines. As all of these drugs reduce cortical glucose metabolism, the increases in beta power may reflect a reduction in cortical neural activity.

Flower colour intensity depends on specialized cell shape controlled by a Myb-related transcription factor.

Flower colour is determined primarily by the production of pigments, usually anthocyanins or carotenoids, but the shade and intensity of the colour are often changed by other factors such as vacuolar compounds, pH and metal ions. Pigmentation can also be affected by the shape of epidermal cells, especially those facing prospective pollinators. A conical shape is believed to increase the proportion of incident light that enters the epidermal cells, enhancing light absorption by the floral pigments, and thus the intensity of their colour. We have identified a gene (mixta) that affects the intensity of pigmentation of epidermal cells in Antirrhinum majus petals. The cells of the corolla lobes fail to differentiate into their normal conical form in mixta mutants. We have cloned the mixta gene by transposon tagging; its sequence reveals that it encodes a Myb-related protein that probably participates in the transcriptional control of epidermal cell shape.

Effects of cocaine on P3B in cocaine abusers.

Little is known about the effects of cocaine on cognitive tasks. Event-related potentials (ERP) were recorded in 7 cocaine abusers during the performance of the auditory oddball task before and after the intravenous injections of saline and cocaine (60-80 mg). The P3B and slow wave components of the ERP were significantly larger 60-210 min after the cocaine than after the placebo injection. The results suggest that cocaine abusers have difficulty in maintaining optimal stimulus processing during extended testing. Cocaine blocks this decrement in stimulus processing.