Search PubMed⌕ Search

Biomedical subjects

A Veerman

Publications and source records attributed to A Veerman.

13 recordsLinked to original sources
PubMed2001

Photoperiodic time measurement in insects and mites: a critical evaluation of the oscillator-clock hypothesis.

The validity of the oscillator-clock hypothesis for photoperiodic time measurement in insects and mites is questioned on the basis of a re-interpretation of available experimental evidence. The possible role of the circadian system in photoperiodism in arthropods is critically reviewed. Apart from the outcome of kinetic experiments, based on diel and non-diel light/dark cycles, evidence from various genetic and physiological experiments is discussed in relation to the oscillator-clock hypothesis. The conclusion is that photoperiodic time measurement in insects and mites is performed by a non-circadian 'hourglass' clock. Experimental evidence suggests a non-clock role for the circadian system in the photoperiodic mechanism of insects and mites.

Journal Article↗
PubMed1998

Response to photoperiod during diapause development in the spider mite Tetranychus urticae.

To study the question whether photoperiodic time measurement in the spider mite Tetranychus urticae is based on a qualitative or quantitative principle, the duration of diapause development was determined in individual females at various constant photoperiods at 19 degrees C. Diapause duration at all four long-night treatments fluctuated around 64.5 days, varying from 62.2 at LD 12:12h to 66.4 at LD 10:14h. The within-treatment variation in diapause duration of the long-night groups appeared to be significantly correlated to the nightlength of the photoperiods used; the longer the nightlength, the higher the within-treatment variation. Frequency distributions of females completing diapause under the two regimes with nightlengths near the critical nightlength were skewed to the right. Mean diapause durations at these regimes, LD 13:11h and LD 14:10h, were 25.4 and 11.9 days, respectively. Mites completed diapause rapidly and synchronously under the three short-night photoperiods tested; within two weeks after transfer from cold storage at 4 degrees C to the diapause terminating regimes at 19 degrees C all females started reproduction. Mean diapause durations were 8.1, 6.4 and 6.5 days for the short-night treatments LD 15:9h, LD 17:7h and LD 19:5h, respectively. The coefficients of variation of diapause duration (variability within groups relative to the mean) of the short-night and the long-night groups varied from 18 to 42%; the coefficients of the two intermediate groups were 69and 81%. There was a clear difference in diapause duration between long-night and short-night groups, but no significant difference was present in this characteristic between different long-night groups on the one hand and only a small difference between different short-night groups on the other. These results support the hypothesis that photoperiodic time measurement in the spider mite is based on a qualitative principle; photoperiods are classified as either 'long' or 'short' in relation to a 'critical' photoperiod. However, around the critical nightlength, intermediate responses were observed which might hint at the quantitative nature of the underlying mechanism. Therefore, although most results are in agreement with the hypothesis of a qualitative mechanism, it cannot be excluded that photoperiodic time measurement in the spider mite is based on a quantitative principle.

Journal Article↗
PubMed1993

The same photoperiodic clock may control induction and maintenance of diapause in the spider mite Tetranchus urticae.

In the spider mite Tetranychus urticae, both diapause induction (which takes place during the larval and nymphal stages) and diapause maintenance (in the adult female) are under photoperiodic control. The question of whether or not the same photoperiodic clock is involved in both photoperiodic reactions was investigated in eight strains of the spider mite, originating from different localities in Europe. The methods employed consisted of (1) determination of the relative importance of the photophase and scotophase in the two photoperiodic reactions; (2) comparison of photoperiodic response curves for diapause induction and diapause maintenance; and (3) determination of the effect of light breaks on the capacity of long nights to maintain diapause, and comparison with the effect of light breaks in diapause induction experiments. The scotophase appeared to be much more important than the photophase for both diapause induction and diapause maintenance. In all strains the critical daylength for diapause maintenance, measured at the moment of saturation of the response to long daylengths, was identical to the critical daylength for diapause induction. However, the critical daylength for diapause maintenance appeared to be labile; it shifted gradually to shorter values as the mites were kept in the cold for a longer period of time, or were kept at a higher temperature for a progressively longer period of time after their stay in the cold room. This seems to reflect a gradual loss of photoperiodic control of diapause maintenance as diapause development proceeds. Photoperiods close to the critical daylength appeared to be less strong with regard to diapause maintenance than shorter daylengths. Quantitative differences in the "strength" of different daylengths were found in all strains investigated. Interruption of the night by short pulses of light revealed either one or two peaks of sensitivity in the night, or one broad "trough" where the two peaks had merged. However, in each case maximal sensitivity to the light breaks occurred at the same position in the night for diapause induction and diapause maintenance. The many similarities found lead to the conclusion that most probably the same photoperiodic clock mechanism is involved in both diapause induction and diapause maintenance in T. urticae.

Animals↗
PubMed1991

Site-specific deletions of the mitochondrial genome in the Pearson marrow-pancreas syndrome.

The Pearson marrow-pancreas syndrome is a fatal disorder involving the hematopoietic system and the exocrine pancreas in early infancy. We have previously shown that this disease results from a widespread defect of oxidative phosphorylation. Here, we describe deletions of the mitochondrial (mt) genome between repeated 8- to 13-bp sequences as consistent features of the disease. Studying a series of nine unrelated children, including the patient originally reported by H. Pearson, we found five different types of direct repeats at the boundaries of the mtDNA deletions and we provided evidence for conservation of the 3'-repeated sequence in the deletions. In addition, we found a certain degree of homology between the nucleotide composition of the direct repeats and several structures normally involved in mtDNA replication and mtRNA processing. These results are consistent either with the recognition and cleavage of a particular DNA sequence with a factor of still unknown origin or with a homologous recombination between direct-repeat mtDNA sequences in the Pearson syndrome.

Base Sequence↗
PubMed1990

Geographical variation in photoperiodic induction of diapause in the spider mite (Tetranychus urticae): a causal relation between critical nightlength and circadian period?

The photoperiodic response of 10 strains of the two-spotted spider mite (Tetranychus urticae), originating between 40.5 degrees and 60 degrees N in Western and Central Europe, was found to be highly variable. The critical nightlength for photoperiodic induction of diapause was strongly correlated with latitude for the lowland populations and varied from 7.75 hr in the north to 13.25 hr in the south. The length of the circadian period, taken as the peak-to-peak interval in response curves of resonance experiments done with T. urticae, varied between 17.75 and 21.5 hr and appeared weakly correlated with latitude. Only a very weak correlation was observed between critical nightlength and circadian period. These results do not provide evidence in favor of a circadian-based photoperiodic clock in T. urticae. On the other hand, they also do not refute this possibility, as there may be other circadian or noncircadian factors affecting the critical nightlength, which could mask the influence of circadian period.

Animals↗