Some changes in intertidal sand communities due to thermal pollution

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Abstract
A period of chromosome condensation followed by decondensation occurs between premeiotic interphase and leptotene in Lilium hybrid cv. '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. Preleptotene chromosome condensation and contraction also occurred in embryo sac mother cells (e.m.cs) of 'Black Beauty'. However, the maximum degree of chromosome contraction at the preleptotene condensation stage was much greater in p.m.cs than in e.m.cs. Moreover, the preleptotene condensation and decondensation stage was apparently shorter in e.m.cs (about 0.7 days) than in p.m.cs. The developmental behaviour of p.m.c. 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. 'Croft'. 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. 'Black Beauty' p.m.cs entered preleptotene chromosome condensation stage from G2 of premeiotic interphase having completed DNA synthesis. 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. At maximum chromosome contraction the diploid chromosome number (2n = 24) could often be counted, but no evidence of association of chromosomes was seen. At this stage the appearance of the chromosomes was indistinguishable from that of late prophase or prometaphase chromosomes at mitosis. Thereafter chromosomes underwent decondensation and elongation and eventually passed into leptotene without first entering any other stage. The nature and possible significance of preleptotene chromosome condensation and decondensation are discussed. The appearance and behaviour of chromosomes at preleptotene condensation stage is the same as that displayed by chromosomes at mitotic prophase. Similarly, the appearance and behaviour of chromosomes at preleptotene decondensation is indistinguishable from that normally seen at mitotic telophase. It is suggested, therefore, that preleptotene condensation and decondensation represent a true mitotic reversion in which metaphase and anaphase are omitted. In normal Lilium genotypes meiosis is initiated by p.m.cs at G2 of premeiotic interphase. 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. It is concluded that preleptotene chromosome condensation stage has no effect on normal meiotic chromosome pairing behaviour and probably has no significant function. 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 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.

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