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T Lamb

Publications and source records attributed to T Lamb.

8 recordsLinked to original sources
PubMed1992

Mitochondrial DNA evolution at a turtle's pace: evidence for low genetic variability and reduced microevolutionary rate in the Testudines.

Evidence is compiled suggesting a slowdown in mean microevolutionary rate for turtle mitochondrial DNA (mtDNA). Within each of six species or species complexes of Testudines, representing six genera and three taxonomic families, sequence divergence estimates derived from restriction assays are consistently lower than expectations based on either (a) the dates of particular geographic barriers with which significant mtDNA genetic clades appear associated or (b) the magnitudes of sequence divergence between mtDNA clades in nonturtle species that otherwise exhibit striking phylogeographic concordance with the genetic partitions in turtles. Magnitudes of the inferred rate slowdowns average eightfold relative to the "conventional" mtDNA clock calibration of 2%/Myr sequence divergence between higher animal lineages. Reasons for the postulated deceleration remain unknown, but two intriguing correlates are (a) the exceptionally long generation length most turtles and (b) turtles' low metabolic rate. Both factors have been suspected of influencing evolutionary rates in the DNA sequences of some other vertebrate groups. Uncertainities about the dates of cladogenetic events in these Testudines leave room for alternatives to the slowdown interpretation, but consistency in the direction of the inferred pattern, across several turtle species and evolutionary settings, suggests the need for caution in acceptance of a universal mtDNA-clock calibration for higher animals.

Animals↗
PubMed1992

The application of bioassays in risk assessment of environmental pollution.

Increased contamination of the environment by toxic chemicals has resulted in the need for sensitive assays to be used in risk assessment of polluted sites. Traditional tests are useful to detect and measure concentrations of chemicals in the environment and in tissues. However, physicochemical assays possess deficiencies that impair their use in evaluating complex environmental contamination. We have developed cytogenetic procedures, including chromosomal, micronucleus, and flow cytometric assays, to assess the mutagenic damage of petrochemicals and low-level radioactivity on indigenous terrestrial and aquatic wildlife populations. These procedures are sensitive to the perturbation of DNA that results from exposure to mutagenic contaminants in both field and laboratory studies. The use of natural populations of animals in biomonitoring, combined with traditional chemical assays, will ultimately provide sufficient information to estimate the risk to human health and environmental quality from anthropogenic pollution.

Animals↗
PubMed1991

Genetic damage in a population of slider turtles (Trachemys scripta) inhabiting a radioactive reservoir.

Turtles inhabiting a radioactive reservoir appear to experience genetic damage due to environmental exposure to low concentrations of long-lived radionuclides. Total body burdens for the 50 reservoir turtles examined in the survey ranged from 164.7-4679.3 Bq for cesium-137 and from 462.6-5098.3 Bq for strontium-90. Flow cytometric (FCM) assays of red blood cell nuclei demonstrated significantly greater variation in DNA content for the reservoir turtles than for turtles from a nearby, non-radioactive site. Furthermore, two of the reservoir turtles possessed FCM profiles that are indicative of aneuploid mosaicism. These data strongly suggest that exposure to low-level radiation may involve a sensitive genetic response in a natural population.

Animals↗
PubMed1991

The role of steroids in reproduction in female elasmobranchs and reptiles.

Adequate evidence exists to suggest the importance of temporal changes in steroid hormone ratios in the normal reproductive/vitellogenin cycle in oviparous and viviparous elasmobranchs and reptiles. In oviparous species, where the cycle is relatively short, secretion of gonadal hormones is synchronous; thus inhibitory actions of progesterone (P) on hepatic or reproductive tract functions would be offset by stimulatory actions of estradiol (E), resulting in appropriate vitellogenin secretion and reproductive tract development. In viviparous species, temporal asynchrony of E and P secretion occurs, and the actions of the individual hormones can be more easily dissected out. Thus, during gestation, where P is the dominant hormone, antagonistic or stimulatory actions of E may be prevented, and the inhibitory action of P on vitellogenesis dominant. Hence vitellogenesis is limited to the follicular phase and eggs are retained. Although the elasmobranch and reptilian species discussed here do not form a continuum through phylogenesis, but rather are extant forms of a particular line of evolution, it is possible to extrapolate from these observations to the probable endocrine interactions in a species as viviparity evolves from oviparity. The theoretical intermediate stage would involve; (a) egg retention, (b) extension of the luteal phase and increased P secretion and (c) resulting in E/P asynchrony and potential expression of "independent" P action, egg retention and yolk suppression.

Animals↗
PubMed1989

DNA fingerprints from hypervariable mitochondrial genotypes.

Conventional surveys of restriction-fragment polymorphisms in mitochondrial DNA of menhaden fish (Brevoortia tyrannus/patronus complex) and chuckwalla lizards (Sauromalus obesus) revealed exceptionally high levels of genetic variation, attributable to differences in mtDNA size as well as in restriction sites. The observed probabilities that any two randomly drawn individuals differed detectably in mtDNA genotype were 0.998 and 0.983 in the two species, respectively. Thus, the variable gel profiles provided unique mtDNA "fingerprints" for most conspecific animals assayed. mtDNA fingerprints differ from nuclear DNA fingerprints in several empirical respects and should find special application in the genetic assessment of maternity.

Animals↗
PubMedDate not supplied

Size polymorphism and heteroplasmy in the mitochondrial DNA of lower vertebrates.

The mitochondrial DNA of the bowfin fish and each of two species of treefrogs displays large-scale size variation. Within each species, mitochondrial genomes span more than a 700 base pair range, and the size polymorphism is localized to one portion of the genome. In addition, about 5 percent of the total 357 individuals surveyed were observed to carry two size classes of mtDNA. These findings are among the few documented instances of extensive within-species mtDNA size polymorphism and individual heteroplasmy, and constitute exceptions to previously reached generalizations about the molecular basis of mtDNA variation.

Animals↗