A Test of Neutrality Based on Interlocus Associations

The evolutionary processes governing variability within genomic regions of low recombination have been the focus of many studies. Here, I investigate the statistical properties of a measure of intrlocus genetic associations under the assumption that mutations are selectively neutral and sites are completely linked. This measure, denoted Z(nS), is based on the squared correlation of allelic identity at pairs of polymorphic sites. Upper bounds for Z(nS) are determined by simulations. Various deviations from the neutral model, including several different forms of natural selection, will inflate the value of Z(nS) relative to its neutral theory expectations. Larger than expected values of Z(nS) are observed in genetic samples from the yellow-ac-scute and Adh regions of Drosophila melanogaster.     

Neocentromere-mediated Chromosome Movement in Maize

Neocentromere activity is a classic example of nonkinetochore chromosome movement. In maize, neocentromeres are induced by a gene or genes on Abnormal chromosome 10 (Ab10) which causes heterochromatic knobs to move poleward at meiotic anaphase. Here we describe experiments that test how neocentromere activity affects the function of linked centromere/kinetochores (kinetochores) and whether neocentromeres and kinetochores are mobilized on the spindle by the same mechanism. Using a newly developed system for observing meiotic chromosome congression and segregation in living maize cells, we show that neocentromeres are active from prometaphase through anaphase. During mid-anaphase, normal chromosomes move on the spindle at an average rate of 0.79 μm/min. The presence of Ab10 does not affect the rate of normal chromosome movement but propels neocentromeres poleward at rates as high as 1.4 μm/min. Kinetochore-mediated chromosome movement is only marginally affected by the activity of a linked neocentromere. Combined in situ hybridization/immunocytochemistry is used to demonstrate that unlike kinetochores, neocentromeres associate laterally with microtubules and that neocentromere movement is correlated with knob size. These data suggest that microtubule depolymerization is not required for neocentromere motility. We argue that neocentromeres are mobilized on microtubules by the activity of minus end–directed motor proteins that interact either directly or indirectly with knob DNA sequences. C urrent models suggest that chromosomes move by a combination of forces generated by microtubule disassembly (Inoue and Salmon, 1995; Waters et al., 1996) and the activity of molecular motors (Vernos and Karsenti, 1996; Yen and Schaar, 1996). Microtubule disassembly generates a constant poleward force; while molecular motors can generate force in either poleward or away-from-pole directions, depending on the characteristics of the motor protein. Both plus and minus end–directed microtubule-based motors are localized to kinetochores (Hyman and Mitchison, 1991). Immunolocalization experiments indicate that mammalian kinetochores contain the minus end– directed motor dynein throughout metaphase and anaphase (Pfarr et al., 1990; Steuer et al., 1990). The kinesin-like proteins CENP-E, which has a transient kinetochore localization in animals, and MCAK, which is localized between the kinetochore plates of mammalian chromosomes, are also thought to generate and/or regulate chromosome movement (Yen et al., 1992; Lombillo et al., 1995; Wordeman and Mitchison, 1995).In addition to the molecular motors on kinetochores, several kinesin-like proteins are localized to chromosome arms (Vernos and Karsenti, 1996). Two subfamilies of arm-based motors have been identified in animals: the NOD subfamily (Afshar et al., 1995; Tokai et al., 1996) and the Xklp1/chromokinesin subfamily (Vernos et al., 1995; Wang and Adler, 1995). Both Nod and Xklp1 are required for positioning chromosomes on the metaphase plate, suggesting that they encode plus end–directed motors (Afshar et al., 1995; Vernos et al., 1995). Other evidence suggests that minus end–directed motors interact with chromosome arms. In the plant Haemanthus, a poleward force acts along chromosome arms during metaphase (Khodjakov et al., 1996), and forces propelling chromosome arms poleward have been detected during anaphase in crane fly spermatocytes (Adames and Forer, 1996). Little is known about how poleward arm motility at metaphase–anaphase affects the fidelity or rate of chromosome segregation.The neocentromeres of maize (Rhoades and Vilkomerson, 1942) provide a particularly striking example of poleward chromosome arm motility. In the presence of Abnormal chromosome 10 (Ab10),¹ heterochromatic DNA domains known as knobs are transformed into neocentromeres and mobilized on the spindle (Rhoades and Vilkomerson, 1942; Peacock et al., 1981; Dawe and Cande, 1996). Knobs are primarily composed of a tandem 180-bp repeat (Peacock et al., 1981) which shows homology to a maize B centromere clone (Alfenito and Birchler, 1993). A characteristic feature of neocentromeres is that they arrive at the spindle poles in advance of centromeres; in extreme cases the neocentromere-bearing chromosome arms stretch towards the poles (Rhoades and Vilkomerson, 1942; Rhoades, 1952). A recently identified mutation (smd1) demonstrates that a trans-acting factor(s) encoded on Ab10 is essential for converting the normally quiescent heterochromatic knobs into active neocentromeres (Dawe and Cande, 1996).Here we use neocentromeres as a model for understanding the mechanisms and importance of nonkinetochore chromosome movement. As a part of our analysis, we developed a four-dimensional system for observing chromosome segregation in living meiocytes. Our experiments were designed to determine (a) how poleward arm motility affects the rate and fidelity of chromosome segregation; and (b) whether the mechanism of neocentromere motility is comparable to the mechanism of kinetochore motility.     

Effects of electrically induced fatigue on the twitch and tetanus of paralyzed soleus muscle in humans

Shields, Richard K., Laura Frey Law, Brenda Reiling, KellySass, and Jason Wilwert. Effects of electrically induced fatigueon the twitch and tetanus of paralyzed soleus muscle in humans.J. Appl. Physiol. 82(5):1499-1507, 1997.We analyzed the twitch and summated torque(tetanus) during repetitive activation and recovery of the human soleusmuscle in individuals with spinal cord injury. Thirteen individualswith complete paralysis (9 chronic, 4 acute) had the tibial nerveactivated every 1,500 ms with a 20-Hz train (7 stimuli) for 300 ms anda single pulse at 1,100 ms. The stimulation protocol lasted 3 min andincluded 120 twitches and 120 tetani. Minimal changes were found forthe acute group. The chronic group showed a significant reduction inthe torque and a significant slowing of the contractile speeds of boththe twitch and tetanus. The decrease in the peak twitch torque was significantly greater than the decrease in the peak tetanus torque early during the fatigue protocol for the chronic group. The twitch time to peak and half relaxation time were prolonged during fatigue, which was associated with improved fusion of the tetanus torque. At theend of the fatigue protocol, the decrease in the peak twitch torque wasnot significantly different from the decrease in the peak tetanustorque. After 5 min of rest, the contractile speeds recovered causingthe tetanus to become unfused, but the tetanus torque became lessdepressed than the twitch torque. The differential responses for thetwitch and the tetanus suggest an interplay between optimal fusioncreated from contractile speed slowing and excitation contractioncoupling compromise. These issues make the optimal design of functionalelectrical stimulation systems a formidable task.

     

Evidence for Linkage of Bipolar Disorder to Chromosome 18 with a Parent-of-Origin Effect

A susceptibility gene on chromosome 18 and a parent-of-origin effect have been suggested for bipolar affective disorder (BPAD). We have studied 28 nuclear families selected for apparent unilineal transmission of the BPAD phenotype, by using 31 polymorphic markers spanning chromosome 18. Evidence for linkage was tested with affected-sib-pair and LOD score methods under two definitions of the affected phenotype. The affected-sib-pair analyses indicated excess allele sharing for markers on 18p within the region reported previously. The greatest sharing was at D18S37: 64% in bipolar and recurrent unipolar (RUP) sib pairs (P = .0006). In addition, excess sharing of the paternally, but not maternally, transmitted alleles was observed at three markers on 18q: at D18S41, 51 bipolar and RUP sib pairs were concordant for paternally transmitted alleles, and 21 pairs were discordant (P = .0004). The evidence for linkage to loci on both 18p and 18q was strongest in the 11 paternal pedigrees, i.e., those in which the father or one of the father's sibs is affected. In these pedigrees, the greatest allele sharing (81%; P = .00002) and the highest LOD score (3.51; θ = 0.0) were observed at D18S41. Our results provide further support for linkage of BPAD to chromosome 18 and the first molecular evidence for a parent-of-origin effect operating in this disorder. The number of loci involved, and their precise location, require further study.     

SATELLITE-MONITORED MOVEMENTS AND DIVE BEHAVIOR OF A BOTTLENOSE DOLPHIN (TURSIOPS TRUNCATUS) IN TAMPA BAY, FLORIDA

An adult, female bottlenose dolphin ( Tursiops trucncatus ) was radio tagged and monitored via satellite-based Argos receivers for 25 d from 28 June to 23 July 1990, in Tampa Bay, Florida. A total of 794 transmissions were obtained during 106 satellite passes. A mean of 3.9 (SE = 0.24) locations/day were determined by Service Argos and showed the animal remained in the bay, usually close to the southeastern shore. The dolphin moved at least 581 km at a minimum mean speed of 1.2 (SE = 0.1) km/h. Data from 63, 922 dives were recorded. The animal spent an average of 87.1 (SE = 0.6)% of the time submerged, with a mean dive duration of 25.8 (SE = 0.5) sec. Mean dive duration differed significantly between four periods of the day, as did the mean percent of time spent submerged. During the early morning the animal spent more time at the surface, averaged shorter dives, and was submerged less than other times of day. This is the first study to demonstrate die1 dive cycles in a bottlenose dolphin. Four months after tag loss, the dolphin was photographed with no evidence of necrosis or disfigurement of the dorsal fin. Satellite telemetry was demonstrated as an effective means of documenting the movements and dive behavior of a small inshore cetacean.     

The Trophic Diatom Index: a new index for monitoring eutrophication in rivers

A index for monitoring the trophic status of rivers based on diatom composition (‚trophic diatom index’, TDI) has been developed, in response to the National Rivers Authority (England & Wales)'s needs under the terms of the Urban Wastewater Treatment Directive of the European Community. The index is based on a suite of 86 taxa selected both for their indicator value and ease of identification. When tested on a dataset from 70 sites free of significant organic pollution, this index was more highly correlated with aqueous P concentrations than previous diatom indices. However, where there was heavy organic pollution, it was difficult to separate the effects of eutrophication from other effects. For this reason, the value of TDI is supplemented by an indication of the proportion of the sample that is composed of taxa tolerant to organic pollution. The index was tested on the R. Browney, N-E. England, above and below a major sewage discharge. TDI values indicated that the effect of inorganic nutrients on the river downstream of the discharge was slight as the river was already nutrient-rich, but there was a large increase in the proportion of organic pollution-tolerant taxa. This indicates that the river was already so eutrophic upstream of the discharge that tertiary treatment to remove P would not be effective unless other aspects of the discharge were also improved.     

Effects of cytosolic calcium and limited, possible dual, effects of G protein modulators on guard cell inward potassium channels

The cellular mechanisms that regulate potassium (K⁺) channels in guard cells have been the subject of recent research, as K⁺ channel modulation has been suggested to contribute to stomatal movements. Patch clamp studies have been pursued on guard cell protoplasts of Vicia faba to analyze the effects of physiological cytosolic free Ca²⁺ concentrations, Ca²⁺ buffers and GTP-binding protein modulators on inward-rectifying K⁺ channels. Ca²⁺ inhibition of inward-rectifying K⁺ currents depended strongly on the concentration and effectiveness of the Ca²⁺ buffer used, indicating a large Ca²⁺ buffering capacity and pH increases in guard calls. When the cytosolic Ca²⁺ concentration was buffered to micromolar levels using BAPTA, inward-rectifying K⁺ channels were strongly inhibited. However, when EGTA was used as the Ca²⁺ buffer, much less inhibition was observed, even when pipette solutions contained 1 µM free Ca²⁺. Under the imposed conditions, GTPγS did not significantly inhibit inward-rectifying K⁺ channel currents when cytosolic Ca²⁺ was buffered to low levels or when using EGTA as the Ca²⁺ buffer. Furthermore, GDPβS reduced inward K⁺ currents at low cytosolic Ca²⁺, indicating a novel mode of inward K⁺ channel regulation by G-protein modulators, which is opposite in effect to that from previous reports. On the other hand, when Ca²⁺ was effectively elevated in the cytosol to 1 µM using BAPTA, GTPγS produced an additional inhibition of the inward-rectifying K⁺ channel currents in a population of cells, indicating possible Ca²⁺-dependent action of GTP-binding protein modulators in K⁺ channel inhibition. Assays of stomatal opening show that 90% inhibition of inward K⁺ currents does not prohibit, but slows, stomatal opening and reduces stomatal apertures by only 34% after 2 h light exposure. These data suggest that limited K⁺ channel down-regulation alone may not be rate-limiting, and it is proposed that the concerted action of proton-pump inhibition and additional anion channel activation is likely required for inhibition of stomatal opening. Furthermore, G-protein modulators regulate inward K⁺ channels in a more complex and limited, possibly Ca²⁺-dependent, manner than previously proposed.     

Pre- and post-natal growth and protein turnover in smooth muscle, heart and slow- and fast-twitch skeletal muscles of the rat.

The growth of one smooth and three individual striated muscles was studied from birth to old age (105 weeks), and where possible during the later stages of foetal life also. Developmental changes in protein turnover (measured in vivo) were related to the changing patterns of growth within each muscle, and the body as a whole. Developmental growth (i.e. protein accumulation) in all muscles involved an increasing proportion of protein per unit wet weight, as well as cellular hypertrophy. The contribution of the heart towards whole-body protein and nucleic acid contents progressively decreased from 18 days of gestation to senility. In contrast, post-natal changes in both slow-twitch (soleus) and fast-twitch (tibialis anterior) skeletal muscles remained reasonably constant with respect to whole-body values. Such age-related growth in all four muscle types was accompanied by a progressive decline in both the fractional rates of protein synthesis and breakdown, the changes in synthesis being more pronounced. Age for age, the fractional rates of synthesis were highest in the oesophageal smooth muscle, similar in both cardiac and the slow-twitch muscles, and lowest in the fast-twitch tibialis muscle. Despite these differences, the developmental fall in synthetic rates was remarkably similar in all four muscles, e.g. the rates at 105 weeks were 30-35% of their values at weaning. Such developmental changes in synthesis were largely related to diminishing ribosomal capacities within each muscle. When measured under near-steady-state conditions (i.e. 105 weeks of age), the half-lives of mixed muscle proteins were 5.1, 10.4, 12.1 and 18.3 days for the smooth, cardiac, soleus and tibialis muscles respectively. Old-age atrophy was evident in the senile animals, this being more marked in each of the four muscle types than in the animal as a whole. In each muscle of the senile rats the protein content and composition per unit wet weight, and both the fractional and total rates of synthesis, were significantly lower than in the muscles of younger, mature, animals (i.e. 44 weeks). In the soleus the decreased synthesis rate appeared to be related to a further fall in the ribosomal capacity. In contrast, the changes in synthesis in the three remaining muscles correlated with significant decreases in the synthetic rate per ribosome.(ABSTRACT TRUNCATED AT 400 WORDS)     

Purification and characterization of two essential cytochromes of the thiosulphate-oxidizing multi-enzyme system from Thiobacillus A2 (Thiobacillus versutus)

Four c-type cytochromes were purified by several procedures including chromatography on DEAE-Sepharose CL-6B, Phenyl-Sepharose CL-4B and Sephadex G-75, G-100 and G-200 and chromatofocusing. Cytochrome c-551 had a pI value of 5.2 and an M_r of 260 000 consisting of six non-covalently bound polypeptides each with an M_r of 43 000, and contained four to five haems. Cytochrome c-552.5 had a pI value of 4.8 and an M_r of 56 000 consisting of two polypeptides with the same M_r 29 000, and contained two haems. Cytochromes c-551 and c-552.5 were reduced by ascorbate to about 70 and 60% of the fully dithionite-reduced values, respectively, and both were essential components in the thiosulphate-oxidizing multi-enzyme system (other components of the system were ‘enzyme A’, ‘enzyme B’ and sulphite: cytochrome c oxidoreductase). These two cytochromes functioned as electron carriers and effectors in the oxidation of thiosulphate. Some evidence suggested that cytochrome c-551 might be a specialized electron transfer component for sulphonate-sulphur oxidation. Both cytochromes could be reduced by thiosulphate in the presence of enzymes A and B. Cytochrome c-550 (basic) and cytochrome c-550 (acidic) were small proteins with M_r 15 000 and 14 000 and pI values of over 8 and 5, respectively. Their physiological role is uncertain.