Biomedical subjects

Darcy B Kelley

Publications and source records attributed to Darcy B Kelley.

8 recordsLinked to original sources

PubMed2005

Historical perspective: Hormonal regulation of behaviors in amphibians.

This review focuses on research into the hormonal control of behaviors in amphibians that was conducted prior to the 21st century. Most advances in this field come from studies of a limited number of species and investigations into the hormonal mechanisms that regulate reproductive behaviors in male frogs and salamanders. From this earlier research, we highlight five main generalizations or conclusions. (1) Based on studies of vocalization behaviors in anurans, testicular androgens induce developmental changes in cartilage and muscles fibers in the larynx and thereby masculinize peripheral structures that influence the properties of advertisement calls by males. (2) Gonadal steroid hormones act to enhance reproductive behaviors in adult amphibians, but causal relationships are not as well established in amphibians as in birds and mammals. Research into the relationships between testicular androgens and male behaviors, mainly using castration/steroid treatment studies, generally supports the conclusion that androgens are necessary but not sufficient to enhance male behaviors. (3) Prolactin acts synergistically with androgens and induces reproductive development, sexual behaviors, and pheromone production. This interaction between prolactin and gonadal steroids helps to explain why androgens alone sometimes fail to stimulate amphibian behaviors. (4) Vasotocin also plays an important role and enhances specific types of behaviors in amphibians (frog calling, receptivity in female frogs, amplectic clasping in newts, and non-clasping courtship behaviors). Gonadal steroids typically act to maintain behavioral responses to vasotocin. Vasotocin modulates behavioral responses, at least in part, by acting within the brain on sensory pathways that detect sexual stimuli and on motor pathways that control behavioral responses. (5) Corticosterone acts as a potent and rapid suppressor of reproductive behaviors during periods of acute stress. These rapid stress-induced changes in behaviors use non-genomic mechanisms and membrane-associated corticosterone receptors.

Amphibians↗

PubMed2005

Evolution of RAG-1 in polyploid clawed frogs.

Possible genetic fates of a gene duplicate are silencing, redundancy, subfunctionalization, or novel function. These different fates can be realized at the DNA, RNA, or protein level, and their genetic determinants are poorly understood. We explored molecular evolution of duplicated RAG-1 genes in African clawed frogs (Xenopus and Silurana) (1) to examine the fate of paralogs of this gene at the DNA level in terms of recombination, positive selection, and gene degeneration and in the absence of extensive recombination among alleles at different paralogs, (2) to test phylogenetic hypotheses about the origins of polyploid species. We found that recombination between different RAG-1 paralogs is infrequent, that degeneration of some paralogs has occurred via stop codons and frameshift mutations, and that this degeneration occurred in paralogs inherited from only one diploid progenitor species. Simulations and phylogenetic analyses of RAG-1 and mitochondrial DNA support one origin of extant tetraploids in Xenopus and at least one origin in Silurana, five allopolyploid origins of extant octoploids, and two allopolyploid origins of extant dodecaploids. In allopolyploid species, which inherit a complete genome from two different ancestors, genes inherited from the same ancestor have a longer period of coevolution than genes inherited from different ancestors. Because of this, gene ancestry could potentially influence gene fate: interacting paralogs derived from the same lower ploidy ancestor might have similar genetic destinies.

Animals↗

PubMed2004

Vocal communication in frogs.

The robust nature of vocal communication in frogs has long attracted the attention of natural philosophers and their biologically inclined successors. Each frog species produces distinctive calls that facilitate pre-mating reproductive isolation and thus speciation. In many terrestrial species, a chorus of simultaneously calling males attracts females to breeding sites; reproductive females then choose and locate one male, using distinctive acoustic cues. Males compete with each other vocally and sometimes physically as well. Anuran acoustic signaling systems are thus subject to the strong pressures of sexual selection. We are beginning to understand the ways in which vocal signals are produced and decoded by the nervous system and the roles of neurally active hormones in both processes.

Animal Communication↗

PubMed2004

A mitochondrial DNA phylogeny of African clawed frogs: phylogeography and implications for polyploid evolution.

The African clawed frogs (Silurana and Xenopus), model organisms for scientific inquiry, are unusual in that allopolyploidization has occurred on multiple occasions, giving rise to tetraploid, octoploid, and dodecaploid species. To better understand their evolution, here we estimate a mitochondrial DNA phylogeny from all described and some undescribed species. We examine the timing and location of diversification, and test hypotheses concerning the frequency of polyploid speciation and taxonomy. Using a relaxed molecular clock, we estimate that extant clawed frog lineages originated well after the breakup of Gondwana, about 63.7 million years ago, with a 95% confidence interval from 50.4 to 81.3 million years ago. Silurana and two major lineages of Xenopus have overlapping distributions in sub-Saharan Africa, and dispersal-vicariance analysis suggests that clawed frogs originated in central and/or eastern equatorial Africa. Most or all extant species originated before the Pleistocene; recent rainforest refugia probably acted as "lifeboats" that preserved existing species, rather than "species pumps" where many new successful lineages originated. We estimate that polyploidization occurred at least six times in clawed frogs.

Africa↗

PubMed2003

Functional specialization of male and female vocal motoneurons.

Vocal behaviors of African clawed frogs (Xenopus laevis) are produced by a single pair of muscles. This simplification, relative to other vertebrates, allows us to more easily determine how CNS motor pathways function to produce sex-specific songs. We describe here certain sexually differentiated properties of vocal motoneurons that are matched to male and female vocal demands. Both active and passive membrane properties differ between the sexes. Male motoneurons have lower input resistances and larger membrane capacitances than female motoneurons. Two distinct firing patterns are found, in different proportions, in males and females. The strongly adapting neurons that predominate in males initiate spikes at short, reliable latencies, whereas the weakly adapting motoneurons characteristic of females translate graded levels of depolarization into different firing rates. Low-threshold potassium currents (IKL) predominate in males. Hyperpolarization-activated cationic currents (IH) are found almost exclusively in males. Modeling results indicate that sex-typical active and passive properties can account for the occurrence of strongly and weakly adapting spike trains in the sexes. In particular, IKL seem to play an important role in determining the firing patterns of neurons. We suggest that these physiological differences facilitate transformation of synaptic inputs into male- and female-specific outputs that generate sexually distinct songs in vivo.

Action Potentials↗

PubMed2003

Vocal circuitry in Xenopus laevis: telencephalon to laryngeal motor neurons.

Sexually differentiated calling patterns of Xenopus laevis are conveyed to the vocal organ by a dedicated neuromuscular system. Here, we define afferents to vocal motor neurons and determine whether the connectivity of the vocal pathway is sexually differentiated. The use of fluorescent dextran amines and the isolated brain preparation readily permitted identification of anterograde and retrograde connectivity patterns. The whole-mount preparation allowed us to observe projections in their entirety, including cells of origin of a projection (for retrograde projections), terminal fields (for anterograde connections), and fiber tracts. Major findings are the confirmation of a robust and reciprocal connection between cranial nucleus (n.) IX-X and the pretrigeminal nucleus of the dorsal tegmental area of the medulla (DTAM) as well as between DTAM and the ventral striatum (VS). Newly revealed is the extensive connectivity between the rostral subdivision of the dorsal nucleus raphe (rRpd) and candidate vocal nuclei. In contrast to previous results using peroxidase, we did not observe dramatic sex differences in connectivity, although some connections were less robust in female than in male brains. Some retrograde connections previously observed (e.g., anterior preoptic area to DTAM) were not confirmed. Plausible hypotheses are that a set of rhombencephalic neurons located in DTAM, the inferior reticular formation and n.IX-X are responsible for generating patterned vocal activity, that activity is modulated by neurons in rRpd, and that activity in VS (particularly that evoked by conspecific calls), together with effects of steroid hormones at many sites in the vocal circuit, contribute to the initiation of calling.

Afferent Pathways↗

PubMed2003

Estrogen receptors in Xenopus: duplicate genes, splice variants, and tissue-specific expression.

The estrogenic steroid hormones, acting primarily through the nuclear estrogen receptors ERalpha and ERbeta, regulate sexual differentiation in a wide variety of vertebrates. In the frog Xenopus laevis, estrogen regulates the strength of vocal neuromuscular synapses and contributes to the physiological basis of sexually differentiated songs. To understand the mechanisms by which estrogen produces these effects, we have characterized the ERs of X. laevis and their expression in laryngeal muscle and other tissues. We found a remarkable molecular diversity in the estrogen receptor population within individuals. First, we have identified two distinct ERalpha genes, xlERalpha1 and xlERalpha2, which represent, to our knowledge, the first discovery of retained duplicates of the ERalpha gene in any species. These two genes are highly conserved at the amino acid level but have distinct nucleotide sequences; moreover, ERalpha2 has no N-terminal domain. Cloning of ERalpha and ERbeta in the related species Xenopus tropicalis and phylogenetic analysis indicate that the two xlERalpha loci were generated by a duplication specific to the X. laevis lineage-most likely the genome duplication that led to a doubling of the X. laevis chromosome number about 30 million years ago. The primary ER expressed in X. laevis laryngeal muscle is the novel gene xlERalpha2; ERalpha1 is primarily expressed in liver, forebrain, and oviduct. Alternatively spliced transcripts of both xlERalpha1 and xlERalpha2 are also expressed in a tissue-specific manner. We propose that complementary spatial expression of these two genes and their alternatively spliced transcripts contributes to their conservation over such a long period of time, consistent with the subfunctionalization model for evolution after gene duplication.

Amino Acid Sequence↗

PubMed2003

Estrogen receptor expression in laryngeal muscle in relation to estrogen-dependent increases in synapse strength.

In Xenopus laevis, the laryngeal neuromuscular synapse is the final effector for sexually differentiated song production. Females have stronger laryngeal synapses than males, and synapse strength is estrogen dependent. Estrogen-induced increases in synaptic strength require at least 3 weeks of exposure, suggesting that the hormone acts via a classical genomic mechanism involving the estrogen receptor (ER). The locus of the sex difference in synapse strength, determined using quantal analysis, is presynaptic, leading to the prediction that estrogen acts directly on vocal motor neurons. However, laryngeal motor neurons do not accumulate estrogen. Estrogen might instead affect motor neuron transmitter release via a retrograde signal from its target muscle. To test this hypothesis, we determined whether laryngeal muscle expresses ER. With RT-PCR using primers that recognize highly conserved domains of the ERalpha, mRNA products of the predicted size were amplified from laryngeal muscle as well as from other classical target tissues (forebrain and oviduct). Northern blots using a portion of the PCR product as primer revealed the same-sized band in oviduct and laryngeal muscle. Immunocytochemistry and Western blots confirmed the presence of ER protein in laryngeal muscle fibers and revealed several proteins in laryngeal muscle, brain and liver; among these was an approximately 66-kD protein - presumed to be full-length ER - that was the only one found in oviduct. Estrogen treatment of juveniles resulted in an upregulation of the 66-kD ER protein concomitant with an increase in quantal content. Taken together, these experiments strongly suggest that the ER is expressed by laryngeal muscle; this receptor could mediate estrogen-dependent changes in synaptic strength via retrograde signaling.

Animals↗