Plasticity, not genetic variation, drives infection success of a fungal parasite.

Hosts strongly influence parasite fitness. However, it is challenging to disentangle host effects on genetic vs plasticity-driven traits of parasites, since parasites can evolve quickly. It remains especially difficult to determine the causes and magnitude of parasite plasticity. In successive generations, parasites may respond plastically to better infect their current type of host, or hosts may produce generally 'good' or 'bad' quality parasites. Here, we characterized parasite plasticity by taking advantage of a system in which the parasite (the yeast Metschnikowia bicuspidata, which infects Daphnia) has no detectable heritable variation, preventing rapid evolution. In experimental infection assays, we found an effect of rearing host genotype on parasite infectivity, where host genotypes produced overall high or low quality parasite spores. Additionally, these plastically induced differences were gained or lost in just a single host generation. Together, these results demonstrate phenotypic plasticity in infectivity driven by the within-host rearing environment. Such plasticity is rarely investigated in parasites, but could shape epidemiologically important traits.

Differential host susceptibility to Batrachochytrium dendrobatidis, an emerging amphibian pathogen.

The amphibian fungal pathogen Batrachochytrium dendrobatidis (Bd) has received considerable attention due to its role in amphibian population declines worldwide. Although many amphibian species appear to be affected by Bd, there is little information on species-specific differences in susceptibility to this pathogen. We used a comparative experimental approach to examine Bd susceptibility in 6 amphibian species from the United States. We exposed postmetamorphic animals to Bd for 30 days and monitored mortality, feeding rates, and infection levels. In all species tested, Bd-exposed animals had higher rates of mortality than unexposed (control) animals. However, we found differences in mortality rates among species even though the amount of Bd detected on the different species' bodies did not differ. Of the species tested, southern toads (Anaxyrus terrestris) and wood frogs (Lithobates sylvaticus) had the highest rates of Bd-related mortality. Within species, we detected lower levels of Bd on individuals that survived longer and found that the relationship between body size and infection levels differed among species. Our results indicate that, even under identical conditions, amphibian species differ in susceptibility to Bd. This study represents a step toward identifying and understanding species variation in disease susceptibility, which can be used to optimize conservation strategies.

Time-domain analysis of auditory-nerve-fiber firing rates.

Time-domain analysis of firing-rate data from over 200 fibers from the auditory nerve of cat has been used to estimate the formants of the synthetic-syllable stimuli. Distinct groups of fibers are identified based on intervals between peaks in the fiber firing rates. The large extent of some of these groups--over an octave in terms of characteristic frequency--and the lack of short intervals in the longer-interval groups suggest that the behavior of the nonlinear cochlear filters for these signals is effectively wideband with steep high-frequency cutoffs. The measured intervals within each group are very similar, and correspond to the period of the formant that dominates the group's response. These intervals are used to estimate the dynamic speech formants. The overall formant estimates are better than those of the previous spectral analyses of the neural data, and the details of lower-formant dynamics are tracked more precisely. The direct temporal representation of the formant in contrasted with the diffuse spectral representation, the dependence of spectral peaks on nonformant parameters, and the distortion of the spectrum by rectification. It is concluded that a time-domain analysis of the responses to complex stimuli can be an important addition to frequency-domain analysis for neural data, cochlear models, and machine processing of speech.

Proportionality of intracellular resistance changes and receptor potentials in hair-cell models.

Russell and Sellick (Russell, I.J. and Sellick, P.M. (1978): J. Physiol. (London) 284, 261-290) reported measurements in guinea pig inner hair cells of d.c. resistance and d.c. potential changes in response to tones delivered to the tympanic membrane. They reported that d.c. potential changes were proportional to d.c. resistance changes for a broad range of tone levels and frequencies, and concluded that this result is inconsistent with the variable-resistance model proposed by Davis (Davis, H. (1958): Ann. Otol, Rhinol, Laryngol. 67, 789-801). We show that this proportionality is consistent with the model. Furthermore, we show that such proportionality is a special case of a general property of a model that is a generalization of the Davis model. Namely, we show that the time-varying receptor potential is proportional to the time-varying resistance change.

Stop consonant discrimination based on human audition.

A system for discrimination of stop consonants has been designed on the basis of studies of auditory physiology and psychophysics. The system consists of a one-third octave filter bank as an approximation to auditory tuning curves, a bank of high speed, wide dynamic range envelope detectors, a logarithmic amplifier, and a digital computer for analysis and display. Features, chosen on the basis of psychophysical experiments, are then abstracted, and fed to a discriminant analysis program which decides on the most probable phomene. Discrimination accuracy of about 77% for stop consonants in initial position has been achieved, with a 15-speaker data set.

Two determinants of localization acuity in the horizontal plane.

An attempt was made to extend the auditory localization model of Searle, Braida, Davis, and Colburn [J. Acoust. Soc. Am. 60, 1164-1175 (1976)] to regions beyond the fontal horizontal plane. On the basis of two experiments, it appears that the assumption of acuity independent of angle is not valid for these more extensive regions, although the distribution of error magnitude can be accounted for by a span-specific unitary acuity. However, the assertion by the same authors that acuity is partially determined by the angular size of the speaker span was substantiated by direct experimental test. The results confirm the initial suggestion of Searle et al. that a complete account of the sound localization process must include both span and angular dependencies of acuity, and indicate average error to be a promising measurement technique in resolving this particular problem.