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ScienceWeek
ANIMAL BIOLOGY: ON NEWTS AND EFTS
The following points are made by J. Brockes and A. Kumar (Current Biology 2005 15:R42):
1) Newts are aquatic, tailed urodele amphibians with a complex life history. All urodeles are salamanders, and newts are salamanders which live in the water as adults. Three species of newt that are used in experimental biology are Notophthalmus viridescens, the red-spotted newt from the United States, Pleurodeles waltl, the Iberian newt, and Cynops pyrroghaster, the Japanese newt. Another urodele used in the laboratory is the axolotl, Ambystoma mexicanum, which is a neotenic larval salamander and not a newt.
2) Newt is derived from "eft", the original word for these animals. Eft is now used exclusively for the terrestrial juvenile form. During their life, newts go through three distinct stages: aquatic larval, terrestrial juvenile, and aquatic adult. After larval metamorphosis is completed and the gills are lost, the juveniles leave the water. During their terrestrial life, Notophthalmus juveniles are 3-5 cm in size with reddish coloration and are called "red efts". During their land life, which lasts between one to three years, the efts become sexually mature. After the second metamorphosis the adults return to the water and the coloration changes to a greenish olive with red spots along the dorsal side.
3) News have been attractive for research several reasons. First, they provided the major focus for amphibian embryology before the era of Xenopus. Second, their nuclei have a large haploid DNA content, in the range of 20-40 pg. Therefore, they offer large cells, nuclei, and chromosomes, including the particularly fine lampbrush chromosomes in growing oocytes. Lampbrush chromosomes are elongated diplotene bivalents which can be a millimeter or more in length. Third, they have extensive regenerative ability as adults, including regeneration in the central nervous system.
4) Newts have contributed a great deal to biology in different areas. Hans Spemann and Hilde Mangold's experiments that led to the discovery of the embryonic organizer were performed on newt embryos. Roger Sperry's influential experiment on neural specificity was first reported on newts. Sperry cut the optic nerve, rotated the retina by 180 degrees, challenged the regenerated animal with a lure in the upper visual field and found that the animal moved to the lower field. Newts have also contributed to cytogenetics and cell biology. For example, Oscar Miller obtained remarkable images of ribosomal DNA transcription in newt oocyte nuclei, and Joe Gall demonstrated from the kinetics of DNAse digestion of newt lampbrush chromosomes that one chromatid is maintained by a single DNA double helix, and that the axis (two chromatids) is maintained by two helices. Conly Rieder's work on microtubule and spindle dynamics has exploited the large flat newt lung cells. In regeneration research, Goro Eguchi and Tokindo Okada demonstrated that during lens regeneration pigment epithelial cells of the iris can convert to lens cells, a process they termed "transdifferentiation". Finally, Marcus Singer demonstrated that regeneration of the limb and division of the limb stem cells is dependent on an intact nerve supply.[1]
References:
1. Brockes, J.P. and Kumar, A. (2002). Plasticity and reprogramming of differentiated cells in amphibian regeneration. Nat. Rev. Mol. Cell Biol. 3, 566-574.
Current Biology http://www.current-biology.com
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ECOLOGY: ON THE DECLINE OF AMPHIBIANS
The following points are made by Nigel Williams (Current Biology 2004 14:R986):
1) Concern about the apparent decline of many amphibian species around the globe over recent decades, often in habitats considered to be of high quality, has prompted fresh worries about the extent of environmental factors that may be playing a part. A new first-ever global study estimates that one third of the world's amphibian species are in danger of extinction. Researchers think that the mysterious collapse in numbers might be a warning that our environment may be in a worse state than we think -- amphibians are known to be the most vulnerable of all animals to subtle changes in their ecosystems.
2) Researchers have known since the 1980s that many of the world's amphibian species are vanishing but the scale of the decline revealed by the new survey has created great concern. More than 520 scientists from more than 60 countries took part in the three-year Global Amphibian Assessment, a study of the world's 5743 known amphibian species. One in three -- a total of 1856 species -- are apparently threatened with extinction. The latest count shows 122 amphibian species have become extinct in the past 20 years. The problem is particularly acute amongst the New World species. This region is home to more than half of the world's known species of amphibians -- its 3046 described species comprise 53 per cent of the world's total.
3) Brazil and Columbia have the greatest diversity with 731 and 698 species respectively. The top five countries for amphibians (including Ecuador, Peru and Mexico) are all in the New World and Venezuela and the US are also in the top ten. At the other end of the diversity scale, a number of Caribbean islands have just one native species each. Nearly 40 per cent of New World amphibians are threatened with extinction, including 337 species that are critically endangered -- on the brink of extinction. Nine species in the region have gone extinct in the past 100 years and another 117 species are "possibly extinct", meaning that researchers are unaware of any existing populations but have not performed the extensive searching required to place these species in the extinct category.
4) Many of these declines are recent. Since approximately 1980, four species have gone extinct, and 109 species have become possibly extinct in this region. Within the New World, Caribbean species are the most threatened -- 84 per cent of the region's 171 species, followed by central America with 52 per cent of its 685 species threatened. The risk facing New World amphibians is considerably higher than for either birds -- 10 per cent -- or mammals -- 16 per cent in the same region. Amphibian species occurring at high elevations with restricted distributions and characterized by terrestrial life cycles, rather than those using a mix of aquatic and terrestrial habitats, are more likely to be threatened than species with other characteristics, the survey reveals.
Current Biology http://www.current-biology.com
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ECOLOGY: AMPHIBIAN DECLINE AND EMERGING DISEASES
The following points are made by J.M. Kiesecker et al (American Scientist 2004 92:138):
1) Across the globe hundreds of species of frogs, toads, salamanders and newts are in dramatic decline. At the same time, new and often serious infectious diseases seem to be sickening people. Might these unhappy developments be connected? Might they indeed share a root cause: the changes in our world brought about by a growing human population? From several lines of inquiry, evidence is accumulating to support such a conclusion -- that environmental degradation wrought by people is contributing to both trends.
2) The human species now numbers 6.3 billion, and collectively we have altered between one-third and one-half of the Earth's land surface. After being stable for millennia, atmospheric carbon dioxide has increased by 30 percent in the last two centuries. Our actions fix more nitrogen than all natural terrestrial sources, and we utilize over half of all accessible surface freshwater. These are dramatic changes, even though we cannot always determine their long-term significance. The immediate consequences seem to be disproportionately borne by frogs, which have suffered massive mortality in recent years.
3) The authors are engaged in studies of this surge in amphibian deaths. While many cases can be linked directly to single, proximate factors such as habitat loss, numerous populations have declined in protected parks and nature reserves, even in remote wilderness areas -- places that are removed from our modern effluvium and that ought to be insulated from human influence. Yet across the globe, many amphibian species have experienced increased disease- and parasite-prevalence, causing massive mortality. Developmental malformations associated with parasitic infection are also frequent: In some groups 90 percent are severely deformed, with extra or missing limbs.
4) The origins of these catastrophic losses are complex. Several agents can act synergistically to endanger a population. Depending on the specific locale, forces such as climate change, habitat destruction, environmental chemicals, fertilizer runoff and the introduction of exotic species have all been implicated in the threat.
5) So how is the global decline of amphibians related to increased disease prevalence among humans and wild-life? The link is suggestive, not proven, but there are compelling similarities between recent disease outbreaks in many animals. Amphibians have been hit particularly hard because of their life cycle and physiology: Frogs and salamanders are exquisitely sensitive to environmental changes. This property casts them in the role of biological Cassandras, prophesying a pessimistic message of environmental degradation that we don't want to hear. Like Homer's Trojans, we've mostly ignored their warnings.(1-5)
References (abridged):
1. Blaustein, A. R., and J. M. Kiesecker. 2002. Complexity in conservation: Lessons from the global decline of amphibian populations. Ecology Letters 5:597-608
2. Croon, B. 1996. Frog data and observations. Earth Focus, Winter.
3. Daszak, P., A. A. Cunningham and A. D. Hyatt. 2000. Emerging infectious diseases of wildlife: threats to biodiversity and human health. Science 287:443-449.
4. Fauci, A. S. 2001. Infectious diseases: Considerations for the 21st century. Clinical Infectious Diseases 32:675-685
5. Hero, J.-M., and L. Shoo. 2003. Conservation of amphibians in the Old World tropics: Defining unique problems associated with regional fauna. In Amphibian Conservation, ed. R. D. Semlitsch. Washington, D.C.: Smithsonian Institution Press.
American Scientist http://www.americanscientist.org
ScienceWeek http://scienceweek.com
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