How does Drosophila melanogaster overwinter?

In temperate regions low temperatures seem to be the most restrictive factor for survival of Drosophila natural populations, which depends on the capacity of one or more developmental stages to resist unfavourable winter conditions. In this study we have attempted to answer the question of how D. melanogaster overwinters under natural temperature conditions. Only adults overwintered and no diapause was observed in any developmental stage. Thus, developmental duration becomes a decisive component with respect to overwintering potential and, therefore, the preadult stages are unlikely to overwinter. Possible evolutionary steps in adaptation to cold regions are discussed.     

The phylogenetic status of arthropods, as inferred from 18S rRNA sequences

Partial 18S rRNA sequences of five chelicerate arthropods plus a crustacean, myriapod, insect, chordate, echinoderm, annelid, and platyhelminth were compared. The sequence data were used to infer phylogeny by using a maximum-parsimony method, an evolutionary-distance method, and the evolutionary-parsimony method. The phylogenetic inferences generated by maximum-parsimony and distance methods support both monophyly of the Arthropoda and monophyly of the Chelicerata within the Arthropoda. These results are congruent with phylogenies based on rigorous cladistic analyses of morphological characters. Results support the inclusion of the Arthropoda within a spiralian or protostome coelomate clade that is the sister group of a deuterostome clade, refuting the hypothesis that the arthropods represent the "primitive" sister group of a protostome coelomate clade. Bootstrap analyses and consideration of all trees within 1% of the length of the most parsimonious tree suggest that relationships between the nonchelicerate arthropods and relationships within the chelicerate clade cannot be reliably inferred with the partial 18S rRNA sequence data. With the evolutionary-parsimony method, support for monophyly of the Arthropoda is found in the majority of the combinations analyzed if the coelomates are used as "outgroups." Monophyly of the Chelicerata is supported in most combinations assessed. Our analyses also indicate that the evolutionary-parsimony method, like distance and parsimony, may be biased by taxa with long branches. We suggest that a previous study's inference of the Arthropoda as paraphyletic may be the result of (a) having two few arthropod taxa available for analysis and (b) including long-branched taxa.      

Inertial vestibular coding of motion: concepts and evidence

Central processing of inertial sensory information about head attitude and motion in space is crucial for motor control. Vestibular signals are coded relative to a non-inertial system, the head, that is virtually continuously in motion. Evidence for transformation of vestibular signals from head-fixed sensory coordinates to gravity-centered coordinates have been provided by studies of the vestibulo-ocular reflex. The underlying central processing depends on otolith afferent information that needs to be resolved in terms of head translation related inertial forces and head attitude dependent pull of gravity. Theoretical solutions have been suggested, but experimental evidence is still scarce. It appears, along these lines, that gaze control systems are intimately linked to motor control of head attitude and posture.     

TURNOVER OF CELL-RENEWING POPULATIONS UNDERGOING CIRCADIAN RHYTHMS IN CELL PROLIFERATION

Evidence is presented in support of the concept of partial synchrony of cells as the cause for circadian rhythms in DNA synthesis and mitotic activity. The nonuniform age distribution of cells in cycle indicated that equations based on total asynchrony were not applicable for calculation of cytokinetic parameters in cellrenewing populations undergoing circadian rhythms. The integration of the circadian mitotic curve is introduced as a simple and accurate method for determination of proliferation rate and turnover time. An approximately linear increase in the labeling index following repeated injections of ³H-thymidine demonstrated that nearly 100% basal cells in hamster cheek pouch epithelia were in cycle during a turnover time. These experiments suggest that if there is a G₂ phase in cellrenewing tissues, this is short with respect to turnover time and that it may be a specific compartment where the control of cell proliferation operates.     

INCREASED CELL PROLIFERATION WITH PERSISTENCE OF CIRCADIAN RHYTHMS IN HAMSTER CHEEK POUCH NEOPLASMS

Squamous cell neoplasms induced by repeated topical application of 7,12-dimethyl-benz(a)anthracene in Syrian hamster cheek pouch exhibited circadian rhythms of DNA synthesis and mitotic activity. Fluctuations in the fractions of cells in mitosis and DNA synthesis observed in the tumors were approximately in phase with the circadian rhythms from normal precursor epithelium, indicating that some degree of host physiologic modulation persists during neoplastic growth. The labeling (thymidine-³H) and mitotic indices of neoplasms were considerably higher than normal throughout the 24 hr period. The duration of the neoplastic S phase—measured from the PLM curve—was 30% shorter than normal; G₂ did not show detectable variation. The data demonstrated that chemically induced squamous cell neoplasms had markedly increased rates of cell production. It is postulated that applications of a carcinogen upon a cell-renewing population generate a multicompartmental cytokinetic imbalance in which: (1) a higher proportion of G₀ cells is stimulated to enter the cycle; (2) the duration of the cell cycle is shortened; (3) the regulatory mechanisms fail to stimulate an accelerated rate of differentiation to compensate for the overproduction of cells; and (4) the state of proliferative hyperactivity becomes stable. An oncogenic cytokinetic mechanism based solely on a persistent decrease in cell loss (differentiation) is ruled out by the present investigation, at least for squamous cell neoplasms.     

Fine filaments in lymphatic endothelial cells

Several and various types of cells contain fine cytoplasmic filaments closely resembling the myofilaments of muscle cells (2, 18, 23, 24). In many of these cells and especially when cultured, it has been demonstrated that some of these filaments react with heavy meromyosin (HMM) in the same way as do the actin filaments of muscle cells (3, 6 7). This suggests that these filaments may be actinoid and form part of a contractile system. As fine intracytoplasmic filaments do occur in lymphatic endothelial cells (2, 14), we undertook an electron microscope investigation of their fine structure and their reaction on incubation with HMM and EDTA. We postulated that lymphatic endothelial cells possess a contractile filamentous system to which these filaments belong.