Autumn/winter represented the season with the highest average wind speeds preceding sampling, during which the highest Epi-Chl concentrations were associated with the deepest sites. Principal components analysis highlighted strong associations between Epi-Chl and sites of intermediate depths (2.5–5.5 m) in all seasons except autumn/winter. Epipelic Chl a concentrations (Epi-Chl range: <0.10–6.0 μg Chl a g −1 dw) were highest in winter/spring, a period when water clarity was highest and TS-Chl lowest. Total sediment microalgal Chl a concentrations (TS-Chl range: 5–874 μg Chl a g −1 dw) were highest in winter and in the deepest site (20 m overlying water depth), apparently as a result of phytoplanktonic settling and sediment focussing processes. all viable microalgae in the surface sediments) in a shallow eutrophic loch. We tested the hypothesis that the positioning (across a depth gradient of 2–22 m overlying water depth) and relative biomass (determined using bulk and lens tissue harvested chlorophyll (Chl) a concentrations) of the epipelon community would vary independently with season (12 monthly samples) and across natural gradients of light and habitat disturbance relative to the total benthic algal community (i.e. As such it is important to understand the environmental variables responsible for regulating community structure, positioning and biomass. It is suggested, therefore, that chromosomes at maximum preleptotene condensation cannot enter a meiotic sequence without first undergoing a within-chromosomal reorganization of their chromatin, and that preleptotene chromosome decondensation stage represents such a reorganization.Benthic microalgae are known to perform important ecosystem functions in shallow lakes. Two parallel threads (chromatids) are visible in chromosomes at preleptotene condensation stage while, in chromosomes at leptotene, invariably only a single thread is seen. It is concluded that preleptotene chromosome condensation stage has no effect on normal meiotic chromosome pairing behaviour and probably has no significant function. In 'Black Beauty', however, the control regulating the initiation of meiosis apparently is effective later in the cell cycle and acts only after p.m.cs have entered mitotic prophase. In normal Lilium genotypes meiosis is initiated by p.m.cs at G2 of premeiotic interphase. It is suggested, therefore, that preleptotene condensation and decondensation represent a true mitotic reversion in which metaphase and anaphase are omitted. Similarly, the appearance and behaviour of chromosomes at preleptotene decondensation is indistinguishable from that normally seen at mitotic telophase. The appearance and behaviour of chromosomes at preleptotene condensation stage is the same as that displayed by chromosomes at mitotic prophase. The nature and possible significance of preleptotene chromosome condensation and decondensation are discussed. Thereafter chromosomes underwent decondensation and elongation and eventually passed into leptotene without first entering any other stage. At this stage the appearance of the chromosomes was indistinguishable from that of late prophase or prometaphase chromosomes at mitosis. At maximum chromosome contraction the diploid chromosome number (2n = 24) could often be counted, but no evidence of association of chromosomes was seen. All the p.m.cs within an anther loculus underwent preleptotene chromosome condensation stage with a degree of synchrony not less than that found at early first meiotic prophase. 'Black Beauty' p.m.cs entered preleptotene chromosome condensation stage from G2 of premeiotic interphase having completed DNA synthesis. A brief illustrated description is given of the appearance of p.m.cs at various stages of preleptotene chromosome condensation in both methylene blue stained anther sections, and in Feulgen stained anther squashes. chromosomes during preleptotene condensation and decondensation was essentially the same as that described by Walters (1970, 1972) for the corresponding stages in Lilium longiflorum cv. Moreover, the preleptotene condensation and decondensation stage was apparently shorter in e.m.cs (about 0.7 days) than in p.m.cs. However, the maximum degree of chromosome contraction at the preleptotene condensation stage was much greater in p.m.cs than in e.m.cs. Preleptotene chromosome condensation and contraction also occurred in embryo sac mother cells (e.m.cs) of 'Black Beauty'. The duration of this period was estimated to be about 1.16 days in pollen mother cells (p.m.cs) of plants grown at 20 degrees C. A period of chromosome condensation followed by decondensation occurs between premeiotic interphase and leptotene in Lilium hybrid cv.
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