Chromosome bands and the subunit structure of Chinese hamster metaphase chromosomes

Isolated Chinese hamster chromosomes dissociate into a series of specific chromatin subunits approximately the size of stainable chromosome bands upon reduction of the divalent ion concentration during or after isolation. At high pH the chromatin in some bands is differentially removable during chromosome isolation, leaving a banded chromosome with a pattern typical of most G-band procedures. This provides an alternate molecular mechanism to explain the production of banded chromosomes by a variety of staining procedures. These results also suggest an approach to chromatin fractionation, using metaphase chromosomes as a starting material.     

Ants on swollen-thorn acacias: species coexistence in a simple system

 On the black cotton soils of the Laikipia ecosystem in Kenya, two swollen-thorn acacia species support nine ant species, four of which are apparently obligate plant-ants. Among the ants, there are five species of Crematogaster, two species of Camponotus, and one each of Tetraponera and Lepisota. Acacia drepanolobium is host to four ant species that are both common and mutually exclusive. These four ant species, and an additional non-exclusive ant species, tend to occur on trees of different sizes, implying a succession of ant occupants. Nonetheless, all four exclusive species occur in substantial proportions on trees of intermediate size. There is direct evidence that an early successional ant species (Tetraponera penzigi) is actively evicted by two late successional ant species in the genus Crematogaster. There was also some evidence of height differentiation among ant species resident on A. seyal. Different acacia-ant species had different direct effects on A. drepanolobium. Extrafloral nectaries were eaten and destroyed only on trees inhabited by Tetraponera. Axillary shoots were eaten only on trees inhabited by C. nigriceps (potentially another early successional ant). This was associated with more new terminal shoots and healthier leaves than other trees, but also the virtual elimination of flowering and fruiting. Different resident acacia-ant species also had characteristic relationships with other insects. Among the four mutually exclusive ant species, only Crematogaster sjostedti was associated with two species of Camponotus, at least one of which (C. rufoglaucus) appears to be a foraging non-resident. A. drepanolobium trees occupied by C. sjostedti were also far more heavily infested with leaf galls than were trees occupied by other ant species. A. drepanolobium trees occupied by C. mimosae and C. sjostedti uniquely had tended adult scale insects. This diversity of ant inhabitants, and their strikingly different relationships with their hosts and other insect species, are examples of coexisting diversity on an apparently uniform resource. Received: 13 November 1995 / Accepted: 16 May 1996     

EMBRYOGENY AND REPRODUCTIVE BIOLOGY OF BOTHRODENDROSTROBUS MUNDUS (LYCOPSIDA)

Permineralized lycopsid megagametophytes and embryos from Upper Carboniferous strata (Westphalian A) at Burnley, England have been found within isolated megaspores assignable to Setosisporites. The specimens illustrate for the first time the reproductive biology and embryogeny of a free-sporing, bisporangiate, Paleozoic lycopod, and permit reinterpretation of the megagametophyte of Bothrodendrostrobus. Megagametophyte development is entirely endosporal. Embryogeny is comparable to that of the extant genus Isoetes, and is fundamentally different from that of the fossil lycopsid, Lepidocarpon—Lepidophloios. This further illustrates the diversity among Paleozoic lycopods and helps to clarify relationships among both fossil and extant lycopsid taxa.     

INCREASE OF DNA POLYMERASE ACTIVITY FIRMLY BOUND TO NUCLEI DURING THE DNA SYNTHETIC PHASE OF THE CELL CYCLE

DNA polymerase activities were measured on nuclear and supernatant fractions obtained from hamster fibroblast cells (the Don-C clone) grown in tissue culture and mitotically synchronized by selective removal of cells arrested in metaphase following a brief exposure to colcemid. A reproducible fraction (5–10%) of the polymerase activity was found to remain bound in the nuclear pellet after repeated cycles of freezing and thawing. The specific activity of this firmly-bound nuclear DNA polymerase was found to increase during S-phase in proportion to DNA synthesis. The bulk of this activity, after extraction in 1 m salt, exhibited an S value of 8·7 on neutral high salt sucrose gradients and was 24 times more active with poly dA. dT₁₀ as template than with heat denatured DNA. The rest of the cellular DNA polymerase activity showed no significant variation correlated with the cell cycle. This activity also had an S value from 8 to 9 but it was only 2·8 times more active with the homopolymer template than with heat denatured DNA. DNA polymerase activity similar to the firmly-bound activity was found in extracts prepared from metaphase chromosomes.     

Simulating flow microfluorometry experiments with the computer language cellsim

CELLSIM is a digital simulation language specifically designed for simulating cell kinetics models. Recently a flow microfluorometry (FMF) command has been added to the language. In the command, the modeler indicates when his simulated FMF analyses are to take place, and what cell states are to be included. He may specify different coefficients of variance and a non-linear DNA synthesis rate for S phase cells. The FMF capability of the language has proved to be a useful tool in several applications.     

Use of mutant cells to localize thymidine kinase in a mammalian cell-free system

The ability of nuclei preparations of Chinese hamster cell lines Don-C and B14I50 (the latter having greatly reduced thymidine kinase activity) to incorporate radioactive thymidine into DNA was measured. By placing the nuclei of one cell line in the cytoplasmic extract of the other cell, we were able to demonstrate that the thymidine kinase activity was largely restricted to the cytoplasm of the Don-C cells. The kinetics of isotope incorporation also suggested that the B14I50 nuclei contained a greatly reduced pool of thymidine triphosphate, compared with the Don-C nuclei.     

Retention of suspended sediment and phosphorus on a freshwater delta, South Lake Tahoe, California

The Upper Truckee River and Trout Creek, two major tributaries inflowing to Lake Tahoe, join to form what was historically the largest wetland in the Sierra Nevada mountain range that separates California and Nevada (USA). In the 1950s the delta floodplain of the Upper Truckee River was greatly reduced in area (38%) by urban development and the diversion of the river into a single excavated channel. Conversely, Trout Creek still flows through a wide marsh system with significant overbank flooding before entering Lake Tahoe. This study hypothesized that river channel reaches that are not incised within the delta floodplain retain more sediment and nutrients as a result of greater floodplain connectivity, compared to more incised and excavated reaches. Suspended sediment (SS) and total phosphorus (TP) load data from the delta formed by the Upper Truckee River and Trout Creek were collected using flow stage sensors, turbidometers and depth-integrated samples. During the spring snowmelt flow events monitored in 2003, SS load was reduced by 13–41% for the Upper Truckee River and by 68–90% for Trout Creek. Similar reductions in TP load were observed: 13–32% for the Upper Truckee River and 61–84% for Trout Creek. Monitoring of Trout Creek indicated a reduction in load per unit volume of 20–34% in a moderately incised reach versus a reduction of 51–77% in a non-incised marsh reach containing lagoons, braided channels and backwater areas created by a beaver dam. Smaller particle sizes, <10 μm, were retained in the lower marsh reach with similar efficiencies as larger particle sizes. If retention rates from the Trout Creek portion of the marsh are applied to the Upper Truckee River, sediment loading to Lake Tahoe for 2003 would have been reduced by 917 tons of SS.