Time of origin of Mauthner's neuron in Xenopus laevis embryos

The time of the last DNA replication of the Mauthner's neuron precursor cell has been investigated using radioautography. Embryos of Xenopus laevis were labeled at different stages of early development by single microinjections of tritiated thymidine. Labeling times were designed to cover the entire period of development between gastrula and hatching stages. The embryos were fixed at later stages (41 to 44, according to Nieuwkoop and Faber, 1967), when the Mauthner neuron can be readily distinguished by its characteristically large size and large nucleolus.Mauthner neurons of embryos which received tritiated thymidine from stage 10 (beginning of gastrulation) to stage 12 (advanced gastrula, medium yolk plug) were always labeled. Those embryos which received the isotope at or after stage $\text{12}\text{1}\text{2}$ (advanced gastrula, small yolk plug) were never found labeled. These results imply that the last DNA replication of the cell destined to give rise to the Mauthner neuron occurs during the last gastrula stages. This last DNA replication immediately proceeds the time of the so-called “histogenetic determination” of the Mauthner neuron proposed to correspond to stage 13 (slit blastopore) by Stefanelli (1951).Therefore it appears that the developmental program of the Mauthner neuron involves a remarkably early cessation of DNA replication closely followed by histogenetic determination. This is the earliest known event of this type for a specific, well characterized neuron in the amphibian embryo.     

Primate evolution of a human chromosome 1 hypervariable repetitive element

Summary The clone designated hMF #1 represents a clustered DNA family, located on chromosome 1, consisting of tandem arrays displaying a monomeric length of 40 bp and a repetition frequency of approximately 7×10³ copies per haploid genome. The sequence hMF #1 reveals multiple restriction fragment length polymorphisms (RFLPs) when human genomic DNA is digested with a variety of 4–6-bp recognition sequence restriction enzymes (i.e., Taq I, Eco RI, Pst I, etc.). When hamster and mouse genomic DNA was digested and analyzed, no cross-species homology could be observed. Further investigation revealed considerable hybridization in the higher primates (chimpanzee, gorilla, and orangutan) as well as some monkey species.The evolutionary relationship of this repetitive DNA sequence, found in humans, to that of other primates was explored using two hybridization methods: DNA dot blot to establish copy number and Southern DNA analysis to examine the complexity of the RFLPs. Homology to the hMF #1 sequence was found throughout the suborder Anthropoidea in 14 ape and New and Old World monkey species. However the sequence was absent in one species of the suborder Prosimii. Several discrepancies between established evolutionary relationships and those predicted by hMF #1 exist, which suggests that repetitive elements of this type are not reliable indicators of phylogenetic branching patterns. The phenomenon of marked diversity between sequence homologies and copy numbers of dispersed repetitive DNA of closely related species has been observed inDrosophila mice,Galago, and higher primates. We report here a similar phenomenon for a clustered repeat that may have originated at an early stage of primate evolution.     

Enhancement of habitat heterogeneity and species richness on rocky shores inundated by sand

Summary Many rocky shores are subject to periodic inundation by sand, which is often thought to reduce species richness by eliminating organisms intolerant of sand scour or sand smothering. However, regular disturbance (e.g. inundation) should promote richness by preventing the development of low diversity climax communities. A study of faunal richness on 10 regularly inundated shores showed that inundation does promote richness, but by increasing habitat heterogeneity. Some species are excluded from parts of the shore by sand, but because of the patchiness of sand deposits they are rarely excluded from the entire shore. Other species are found only on rocks associated with sand, while typically sandy shore animals occur in the sand deposits themselves. Total richness (281 species) was greater than for local noninundated shores and sandy beaches combined.     

Immunodetection of cytoskeletal structures and the Eg5 motor protein on deep-etch replicas of Xenopus egg cortices isolated during the cortical rotation

Summary— We have developed a new method for immunogold detection on deep-etch replicas of isolated Xenopus egg cortices in order to examine the interactions of different cortical elements in three dimensions at high resolution. We have applied this technique to vegetal cortices isolated during the second half of the first cell cycle. The vegetal cortical region at this time is the site of cellular machinery responsible for the ‘cortical rotation’. The entire cortex translocates with respect to the inner cytoplasm, relocating dorsalising determinants to the future dorsal side of the egg. The aligned microtubules in the shear zone between cytoplasm and cortex, implicated in the cortical rotation, were found to be organised as interweaving loose bundles. Interleaved amongst these aligned microtubules were extensive sheets of ER lying in layers parallel to the egg surface. Cytokeratin filaments were found to associate closely with the microtubules over short stretches. Putative actin filaments were present in the shear zone and in the cortex. Eg5, an abundant kinesin-related microtubule motor protein, and candidate for a role in generating cortical rotation movement, showed an almost exclusive localisation to microtubules. Immunofluorescence studies of cortices treated with detergent to disrupt ER or cold to depolymerise microtubules confirmed that Eg5 associates primarily with microtubules. We propose revised models for the mechanism of cortical rotation based on these observations and conclude that Eg5 is unlikely to move ER relative to microtubules during the cortical rotation.     

Fetal ultrasonography: Normal biometric ranges in the baboon (Papio hamadryas)

Normal biometric ranges for fetal growth in a captive breeding baboon (Papio hamadryas) colony are described. Measurements include crown-rump length, biparietal diameter, binocular distance, head circumference, abdominal circumference, femur length and amniotic fluid index. The pattern of fetal growth is compared with other baboon subspecies and man. The uses and limitations of such data for breeding colony management and optimum utilisation of experimentally derived data are discussed.     

Four-million-year-Old hominids from East Lake Turkana,Kenya

A piece of mandible and several isolated teeth are reported from fluviatile sediments older than 4 million years at East Lake Turkana. They most closely resemble hominids from Laetoli, Tanzania and Hadar, Ethiopia which have been assigned to Australopithecus afarensis. © 1994 Wiley-Liss, Inc.     

Genetic differentiation between Carex lasiocarpa and C. Pellita (Cyperaceae) in north america

Carex lasiocarpa and C. pellita (sect. Carex) share a very similar morphology and have overlapping ranges in North America, but are found in different habitats characterized by contrasting soil types and pH. We studied allozyme variation and chromosome numbers to assess genetic differentiation between the two taxa. Both principal components analysis on the allele frequencies from 12 putative enzyme-coding loci and cluster analysis of genetic identities separated 51 sampled populations into two groups that were consistent with recognized structural differences between C. lasiocarpa and C. pellita. Mean within-group genetic identities were 0.95 for C. lasiocarpa and 0.93 for C. pellita; mean between-group genetic identity was 0.81. With the exception of two rare alleles, the alleles of C. pellita were a subset of those found in C. lasiocarpa. Principal components analysis of measurements of structural characters from voucher specimens representing 46 populations also separated the two species with minimal overlap. Meiotic squashes of microsporocytes revealed haploid chromosome numbers of 38 and 38 + 1 for C. lasiocarpa and 41 and 40 + 1 for C. pellita. These data support the continued recognition of the two taxa as distinct species, and suggest that C. pellita may be a daughter species still in the process of divergence from C. lasiocarpa.     

Costs of reproduction in the pink lady's slipper orchid (Cypripedium acaule): defoliation,increased fruit production,and fire

An earlier experiment with the pink lady's slipper orchid demonstrated that plant leaf area was lowered only after successive years of increased fruit production. This result suggested that the cost of reproduction was small in relation to the energy budget of the plant. To test this idea, plants were subjected to experimental hand-pollination treatments to increase fruit set as well as leaf removal treatments to decrease the energy budget of plants. Changes in plant size in years 2 and 3 and, to some extent, rate of flowering, were determined by a combination of initial plant size, leaf removal treatments in year 1, fruit production in year 1, and damage from an unplanned fire in year 2. Plants that had both leaves removed and produced a fruit in 1987 decreased in size in the following 2 years in comparison with other treatment groups. The cost of fruit production was not apparent in plants that had only one or no leaves removed. Plants apparently have to be put into severe physiological stress in order for a cost of reproduction to appear in the following year. The cost of producing one fruit was a decline of plant size in the following year of 30 cm², which is very similar to our previous experiment using a different design. An additional experiment failed to find evidence that these plants increase their photosynthetic rate to compensate for the loss of leaves or the cost of maturing fruit. Published experiments in both the greenhouse and the field that failed to find a cost of reproduction should be reevaluated in terms of the intensity of treatment imposed and the overall energy budget of the plant in field situations.     

Activation of TNF-R1 receptor in the presence of copper kills TNF resistant CEM leukemic T cells

The cytotoxic effects of TNF on malignant cells are known to be mediated through high affinity surface receptors. The precise mechanism by which transformed cells are selectively killed by the activation of these receptors is yet unknown, but several intracellular signaling pathways are known to be involved. Phospholipase A2 activation by TNF-α has been shown to be important in the transduction of signals leading to cell death. We have used monitoring of extracellular acidification rate as a measure of cellular metabolism to follow the early time course of TNF effects on a human leukemic T cell line (CEM-SS cells). CEM-SS cells were relatively resistant to TNF cell killing but TNF caused an early stimulation of metabolism within 2-4 hr, followed by a suppression of metabolic activity occurring over 20 hr. In contrast, a TNF sensitive subclone of CEM cells (C1Ca) showed a rapid and dramatic decrease in metabolic activity corresponding to cytotoxicity within 18 hr. It was discovered that cupric o-phenanthroline markedly potentiated the effects of TNF on the resistant CEM-SS cells leading to cell death. This observation was specific for copper because ferric o-phenanthroline was without effect at the same concentration. The copper cytotoxic effect was shown to be mediated through the TNF-R1 receptor and independent of phospholipase A2 signaling. © 1994 Wiley-Liss, Inc.