Ambystoma

We used pitfall trapping and surveys of down wood to sample amphibian populations on clearcuts, young, mature, and oldgrowth forests on Forest Service or Bureau of Land Management land in the Coast Range of Oregon in 1984 and 1985. We attempted to identify species or unique groups of species associated with old-growth forests and structural components of the habitat associated with abundance of amphibians. Pitfall traps captured 10 species and 1878 individual amphibians in 45 forested stands and three clearcuts in 80 nights in 1984 and 1985. Western redback salamanders, ensatinas, and roughskin newts were the species most commonly captured by pitfall traps in both forested stands and clearcuts. Significantly more Olympic salamanders were captured in oldgrowth compared to mature and young forest stands. On a moisture gradient in old growth, tailed frogs were more abundant in wet stands. Western redback salamanders were most abundant on steep, rocky slopes, but few strong correlations...

Non‐technical summary  The retina can process over 10 log‐units of changes in light intensity allowing us to see in dim starlight to bright daylight. These changes in luminosity are detected first by photoreceptors, rods and cones, and then relayed to second‐order neurons, ON and OFF bipolar cells (BCs), before finally being encoded as spikes by ganglion cells (GCs). We found that ON BCs process light signals differently depending on the luminosity of the signal. In response to bright light, ON BCs undergo Ca2+‐dependent depression, referred to as desensitization, producing a transient response to a sustained light stimulus; however, no such transformation occurred in dim light. Furthermore, every ON BC was capable of producing desensitization. This desensitization was passed to GCs, resulting in transient spike trains. In conclusion, ON BCs play an important role in retinal processing by converting a sustained light signal into a transient one, in order to prevent saturation.

The pattern of migration and motile activity of developing pigment cells of the Mexican axolotl, Ambystoma mexicanum, were analyzed by time‐lapse cinemicrography in vivo and in culture. In vivo, melanocytes of dark (D/–) larvae migrate from dorsal to ventral in a highly directional manner. They are elongated and aligned parallel to the direction of migration. Nearly all protrusive activity occurs at their ventral, leading edges. Translocation occurs at a mean rate of 0.7 μm/min and involves alternate or simultaneous advance of the leading and trailing edges of the cell. Indirect evidence suggests that cytoplasmic flow is common. Directional migration occurs in apparent absence of contact between melanocytes. In white (d/d) larvae, protrusive activity is infrequent and the melanocytes move slowly or not at all. Explanted neural crest cells of dark and white larvae attach, spread, and differentiate into melanophores and xanthophores in culture. Individual cultured cells are unbiased i...

Nonrandom associations of alleles or haplotypes with geographical location can arise from restricted gene flow, historical events (fragmentation, range expansion, colonization), or any mixture of these factors. In this paper, we show how a nested cladistic analysis of geographical distances can be used to test the null hypothesis of no geographical association of haplotypes, test the hypothesis that significant associations are due to restricted gene flow, and identify patterns of significant association that are due to historical events. In this last case, criteria are given to discriminate among contiguous range expansion, long-distance colonization, and population fragmentation. The ability to make these discriminations depends critically upon an adequate geographical sampling design. These points are illustrated with a worked example: mitochondrial DNA haplotypes in the salamander Ambystoma tigrinum. For this example, prior information exists about restricted gene flow and likely historical events, and the nested cladistic analyses were completely concordant with this prior information. This concordance establishes the plausibility of this nested cladistic approach, but much future work will be necessary to demonstrate robustness and to explore the power and accuracy of this procedure.

La tesis versa sobre la contribución de Rafael Montes de Oca a la ornitología mexicana a través del Museo Nacional y desde la Sociedad Mexicana de Historia Natural durante la segunda mitad del siglo XIX. Se abordan cuestiones el discurso científico, el uso de la imagen en los estudios de colibríes y la relación naturalista-territorio.

SUMMARYAquatic amphibian eggs frequently encounter hypoxic conditions that have the potential to limit oxygen uptake and thereby slow embryonic development and hatching. Oxygen limitation might be avoided if egg capsule surface area and oxygen conductance increased in response to hypoxia. We investigated this possibility in two salamander species, Ambystoma annulatum and Ambystoma talpoideum. The effective surface area of egg capsules increased in response to hypoxia, which increased the conductance for oxygen and enhanced oxygen transport. The ability of amphibian eggs to adjust their conductance in response to oxygen availability may increase survival in hypoxic environments.

Establishing genomic resources for closely related species will provide comparative insights that are crucial for understanding diversity and variability at multiple levels of biological organization. We developed ESTs for Mexican axolotl (Ambystoma mexicanum) and Eastern tiger salamander (A. tigrinum tigrinum), species with deep and diverse research histories. Approximately 40,000 quality cDNA sequences were isolated for these species from various tissues, including regenerating limb and tail. These sequences and an existing set of 16,030 cDNA sequences for A. mexicanum were processed to yield 35,413 and 20,599 high quality ESTs for A. mexicanum and A. t. tigrinum, respectively. Because the A. t. tigrinum ESTs were obtained primarily from a normalized library, an approximately equal number of contigs were obtained for each species, with 21,091 unique contigs identified overall. The 10,592 contigs that showed significant similarity to sequences from the human RefSeq database reflect...

When introduced or cultivated plants or animals hybridize with their native relatives, the spread of invasive genes into native populations might have biological, aesthetic, and legal implications. Models suggest that the rate of displacement of native by invasive alleles can be rapid and inevitable if they are favored by natural selection. We document the spread of a few introduced genes 90 km into a threatened native species (the California Tiger Salamander) in 60 years. Meanwhile, a majority of genetic markers (65 of 68) show little evidence of spread beyond the region where introductions occurred. Using computer simulations, we found that such a pattern is unlikely to emerge by chance among selectively neutral markers. Therefore, our results imply that natural selection has favored both the movement and fixation of these exceptional invasive alleles. The legal status of introgressed populations (native populations that are slightly genetically modified) is unresolved by the US E...

The development of ectoderm isolated from the animal pole of axolotl gastrulae is monitored by light microscopy, electron microscopy and analysis of newly synthesized proteins, glycoproteins and glycolipids. When control embryos are undergoing neurulation it is shown that the explants autonomously begin to express epidermal markers and do not express mesodermal markers. However the results suggest that not all the cells become epidermal and electron microscope examination shows that only the outer layer does so, the inner cells remaining undifferentiated.

A search has been made for regional differences in macromolecular synthesis in the axolotl embryo at a stage when the principal regions have become determined but terminal differentiation has not yet begun. The epidermis of the neurula makes a number of abundant proteins which are not made elsewhere. Some of these are identified by immunoprecipitation as cytokeratins (relative molecular masses (Mr) 62,59,54,51 and 46×103). At the same stage a network of tonofilaments becomes visible by electron microscopy and is also confined to the epidermis. There is no significant incorporation of 3H-sugars into glycoproteins before neurulation. During neurulation specific species begin to be made by the notochord (Mr 265 and 185 ×103), the mesoderm (315×103) and the epidermis (170×103). The latter species is water soluble, has a native Mr of 470×103, is somewhat resistant to trypsin and has a high sugar content. Since these characteristics do not relate to any known glycoprotein it has been give...

The smallmouth salamander (Ambystoma texanum) is a widely distributed member of the family Ambystomatidae found throughout much of the central United States. We developed primers targeting a 147 base pair fragment of the mitochondrial cytochrome b region of the smallmouth salamander and tested these primers in silico, in vitro, and in vivo in laboratorybased studies. We believe these validated tools will be widely useful in the detection of A. texanum in breeding ponds, particularly in habitats where multiple species of Ambystoma occur.

This introduction to new patterning theories for the vertebrate dentition outlines the historical concepts to explain graded sequences in tooth shape in mammals (incisors, canines, premolars, molars) which change in evolution in a linked manner, constant for each region. The classic developmental models for shape regulation, known as the ‘regional field’ and ‘dental clone’ models, were inspired by the human dentition, where it is known that the last tooth in each series is the one commonly absent. The mouse, as a valuable experimental model, has provided data to test these models and more recently, based on spatial‐temporal gene expression data, the ‘dental homeobox code’ was proposed to specify regions and regulate tooth shape. We have attempted to combine these hypotheses in a new model of the combinatorial homeobox gene expression pattern with the clone and field theories in one of ‘co‐operative genetic interaction’. This also explains the genetic absence of teeth in humans ascri...