EFFECTS OF ENVIRONMENTAL VARIATION ON SINKING RATES OF MARINE PHYTOPLANKTON1

The effects of environmental variables, particularly irradiance, on the sinking rates of phytoplankton were investigated using cultures of Chaetoceros gracilis Schütt and C. flexuosum Mangin in laboratory experiments; these data were compared with results from assemblages in the open ocean and marginal ice zone of the Greenland Sea. In culture experiments both the irradiance under which the diatom was grown and culture growth rate were positively correlated with sinking rates. Sinking rates (ψ) in the Greenland Sea were smallest when determined from chlorophyll (mean ψ_chl= 0.14 m · d^?1) and biogenic silica (ψ_si= 0.14 m · d^?1) and greatest when determined from particulate carbon (ψ_c= 0.55 m · d^?1) and nitrogen (ψ_N= 0.64 m · d^?1). Field measurements indicated that variations in sinking may be associated with changes in irradiance and nitrate concentrations. Because these factors do not directly affect water density, they must be inducing physiological changes in the cell which affect buoyancy. Although a direct response to a single environmental variable was not always evident, sinking rates were positively correlated with growth rates in the marginal ice zone, further indicating a connection to physiological processes. Estimats of carbon flux at stations with vertically mixed euphotic zones indicated that approximately 30% of the daily primary production sank from the euphotic zone in the form of small particulates. Calculated carbon flux tended to increase with primary productivity.     

The transfer of pyrethroid resistance resulting from crosses between resistant German cockroaches and susceptible Asian cockroaches

Crosses were made between the Asian cockroach,Blattella asahinai Mizukubo, and resistant strains of the German cockroach,B. germanica (L.), to assess the transfer of pyrethroid resistance to the progeny and to study the inheritance mechanism(s) involved. It was shown that the strain of Asian cockroaches studied was susceptible to four pyrethroids. F₁ progeny were essentially susceptible to the same compounds. Tests with F₂ progeny and those from backcrosses to the resistant parent indicated that the data for each pyrethroid fit an hypothesis of simple, autosomal, nearly completely recessive inheritance. The results are discussed from the standpoint of the impact of the Asian genome on the inheritance mechanism(s).     

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.     

Production of gliotoxin during the pathogenic state in turkey poults byAspergillus fumigatus Fresenius

Turkey poults were given either of two different dosages of two different gliotoxin-producing strains ofAspergillus fumigatus. Infected lung tissue was examined postmortem for the presence of gliotoxin. Gliotoxin was found in lung tissue of ten poults infected with one strain and in seven of ten poults infected with the other strain. Concentrations of gliotoxin in the tissue exceeded 6 ppm in some of the infected tissues. The concentration of gliotoxin found in infected tissue did not appear to be correlated with the dosage of organism given. Considering the pathologic changes observed in turkey poults with aspergillosis and the production of gliotoxin during the pathogenic state in turkey poults, gliotoxin is considered likely to be involved in avian aspergillosis. Disclaimer: Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.     

The transport of morphologically abnormal sperm in the female reproductive tract of mice

Mice of the PL/J strain exhibit a high percentage of morphologically abnormal sperm and provide a model for studying the function of abnormal sperm. The ability of such sperm to reach the site of fertilization within the female reproductive tract has been investigated. We have found a decrease in the percentage of structurally abnormal sperm within the population that reaches the oviduct. This observation suggests either that there is an active selection against abnormal sperm or that they are physiologically disadvantaged in reaching the site of fertilization.