Cladocerans possess traits such as resistant diapausing eggs and rapid parthenogenetic reproduction that make them efficient invaders of new habitats. Nearly all known invasions have been successful, perhaps because failed invasions are difficult to detect. It is possible, however, to identify past failed invasions, by studying the diapausing egg bank. Daphnia ephippia were found in the sediments of Onondaga Lake, New York that could neither be hatched nor identified using egg‐case morphology. Instead, we used sequences of the 12S ribosomal ribonucleic acid (rRNA) gene of mitochondrial deoxyribonucleic acid (mtDNA) extracted from diapausing eggs to identify the unknown Daphnia. We compared these DNA sequences with those generated from morphologically identified Daphnia species collected in Onondaga Lake, and with published sequences for other North American Daphnia species. The invader was identified as Daphnia curvirostris, a Eurasian species that has been only reported once before from North America, in extreme northwestern Canada. The discovery of it in Onondaga Lake signifies a greater than 4,500‐km range extension for this species. On the basis of the sediment ephippial data, D. curvirostris first appeared in the lake about 1952, reached maximum abundance during the period of peak pollution (1950s–1980s), and then essentially disappeared after 1983 when lake water quality improved. As with the finding of another exotic cladoceran, D. exilis, in Onondaga Lake (Hairston et al. 1999a), it is likely that chemical industry activities on the lakeshores were the original source of invading D. curvirostris, that pollution allowed this species to become established in the lake, and that the reduction in pollution ultimately led to its disappearance from the water column.
We have shown that quiescent cells as well as those in the GI phase of the cell cycle synthesize histones at a reduced but significant rate. Now, we show that the histones synthesized during GO and GI are stably incorporated into nuclei soon after synthesis. Micrococcal nuclease digestion of nuclei isolated from cells in GO and GI revealed that the specific histone variants synthesi~d in these different physiologi~l states are found associated with DNA as nucleosomes. Nucleosomes were separated by ~lyacryl~ide gel electrophoresis in a reducing buffer so that histone spot morphology, particularty that of the H3s was improved.Histone Chromatin
We have demonstrated that Herr's 4 1/2 clearing fluid, developed for use with plant tissues, can be successfully used for the microscopic examination of thick sections of normal and neoplastic mammalian tissues. Rat Novikoff hepatoma, rat liver, and human colon and skin samples were fixed in Bouin's, stained with iron hematoxylin, treated with Herr's 4 1/2 clearing fluid and examined by phase contrast microscopy. Tissue architecture and cytological detail were easily observed by focusing through tissue sections as thick as 70 mu. The method permits rapid microscopic examination of mammalian tissues and enables the investigator to detect readily morphological abnormalities within a tissue.
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