Sensillar types of the ovipositor of Dasineura brassicae: structure and relation to oviposition behaviour

ABSTRACT. The distal part of the ovipositor of Dasineura brassicae Winn. (Diptera; Cecidomyiidae) possesses forty to forty-five sensilla of three morphological types. Most are provided with a cuticular bristle, which projects from the surface of the ovipositor; fifteen have a taste/tactile function based on fine structural characteristics; about twenty-five are innervated by a single sensory cell, specialized for mechanoreception. Scolopidial sensory receptors are anchored to the cuticle inside the distal part of the ovipositor, they probably respond to changes in length of the ovipositor. Different sensory systems are involved in the choice of oviposition site; compound eyes and antennae are probably active in the earlier stages, whereas the receptors of the ovipositor appear well suited to govern the last steps in this behaviour.     

Electrical penetration graphs from Cicadulina mbila on maize, the fine structure of its stylet pathways and consequences for virus transmission efficiency

Five distinct electrical penetration graph waveforms characterising the feeding behaviour of the leafhopper Cicadulina mbila Naudé (Homoptera: Cicadellidae) on maize (Zea mays L.) were obtained using a DC based system. The waveforms were distinguished by spectral features and by statistical analysis of their median voltages, durations and time to first waveform recording. By changing the polarity of the system voltage and the level of the input resistor it was shown that the waveforms are mainly determined by the electromotive force (emf) component. Based on the correlation between waveforms and the fine structure of the stylet pathways observed by transmission electron microscopy, insect's activities have been associated with five waveforms: stylet pathway formation (waveform 1), active ingestion (waveform 2), putative stylet work (waveform 3), salivation (waveform 4) and passive ingestion (waveform 5). Like waveform E1 and E2 of aphids, waveforms 4 and 5 of C. mbila correspond to feeding activities in sieve tubes. However, unlike aphids which probe briefly in non-vascular cells, waveform 2 corresponds to active ingestion in cells, where the cell content is partially ingested and hence the organelles' integrity severely affected. These observations suggest that this specific feeding feature, typical of leafhoppers, determines their ability to acquire geminivirus virions located in the plant cell nucleus.     

Ultrastructural responses of the desiccation tolerant plants Xerophyta viscosa and X. retinervis to dehydration and rehydration

This paper compares the changes in water content, chlorophyll a fluorescence and leaf ultrastructure during dehydration and rehydration in two desiccation tolerant plants Xerophyta viscosa and X. retinervis. Both species showed decreasing quantum efficiency of photosystem 2 (F_v/F_m) with decreasing water content. Extreme water loss observed after 25 d of dehydration resulted in considerable damage of leaf tissue ultrastructure. After rehydration, both species need several days to reconstitute their photosynthetic machinery.     

Similarities between a predicted secondary structure for the M1 RNA ribozyme and the tRNA binding center of 16 S rRNA from E. coli

We propose a new model for the secondary structure of the M1 RNA component of E. coli RNase P which is based on significant sequence homologies with parts of the E. coli 16 S rRNA. A large domain of the new model resembles closely the secondary structure of the tRNA binding center of 16 S rRNA. We suggest that this domain of M1 RNA when functioning as a ribozyme binds the mature part of the precursor tRNA.