A phylogenetic perspective on P transposable element evolution in Drosophila

The P element, originally described in Drosophila melanogaster, is one of the best-studied eukaryotic transposable elements. In an attempt to understand the evolutionary dynamics of the P element family, an extensive phylogenetic analysis of 239 partial P element sequences has been completed. These sequences were obtained from 40 species in the Drosophila subgenus Sophophora. The phylogeny of the P element family is examined in the context of a phylogeny of the species in which these elements are found. An interesting feature of many of the species examined is the coexistence in the same genome of P sequences belonging to two or more divergent subfamilies. In general, P elements in Drosophila have been transmitted vertically from generation to generation over evolutionary time. However, four unequivocal cases of horizontal transfer, in which the element was transferred between species, have been identified. In addition, the P element phylogeny is best explained in numerous instances by horizontal transfer at various times in the past. These observations suggest that, as with some other transposable elements, horizontal transfer may play an important role in the maintenance of P elements in natural populations.

Pitch perception: A dynamical-systems perspective

Two and a half millennia ago Pythagoras initiated the scientific study of the pitch of sounds; yet our understanding of the mechanisms of pitch perception remains incomplete. Physical models of pitch perception try to explain from elementary principles why certain physical characteristics of the stimulus lead to particular pitch sensations. There are two broad categories of pitch-perception models: place or spectral models consider that pitch is mainly related to the Fourier spectrum of the stimulus, whereas for periodicity or temporal models its characteristics in the time domain are more important. Current models from either class are usually computationally intensive, implementing a series of steps more or less supported by auditory physiology. However, the brain has to analyze and react in real time to an enormous amount of information from the ear and other senses. How is all this information efficiently represented and processed in the nervous system? A proposal of nonlinear and complex systems research is that dynamical attractors may form the basis of neural information processing. Because the auditory system is a complex and highly nonlinear dynamical system, it is natural to suppose that dynamical attractors may carry perceptual and functional meaning. Here we show that this idea, scarcely developed in current pitch models, can be successfully applied to pitch perception.

Planetary exploration in the time of astrobiology: Protecting against biological contamination

These are intriguing times in the exploration of other solar-system bodies. Continuing discoveries about life on Earth and the return of data suggesting the presence of liquid water environments on or under the surfaces of other planets and moons have combined to suggest the significant possibility that extraterrestrial life may exist in this solar system. Similarly, not since the Viking missions of the mid-1970s has there been as great an appreciation for the potential for Earth life to contaminate other worlds. Current plans for the exploration of the solar system include constraints intended to prevent biological contamination from being spread by solar-system exploration missions.

Atmospheric oxygen over Phanerozoic time

It is quite possible that the level of atmospheric oxygen has varied (roughly between 15 and 30% O2) over the past 550 million years. This variation is suggested by modeling of the carbon and sulfur cycles, by the excessive sediment burial of organic matter that accompanied the advent of large vascular land plants, and by recent physiological studies that relate to biological evolution.