Ectoderm removal prevents cutaneous nerve formation and perturbs sensory axon growth in the chick hindlimb

Target tissues are thought to provide important cues for growing axons, yet there is little direct evidence that they are essential for axonal pathfinding. Here we examined whether target ectoderm is necessary for the formation of cutaneous nerves, and for the normal growth and guidance of cutaneous axons as they first enter the limb plexus. To do this, we removed a patch of ectoderm from the chick hindlimb at various times during early axon outgrowth. We find there is a critical period when cutaneous nerve formation requires target ectoderm. When the ectoderm is absent during this time, axons progress into the limb more slowly and, although a few sensory axons occasionally diverge a short distance from the plexus, they do not form a discrete nerve that travels to the skin. A few days later, when the nerve pattern is mature, axons normally destined for the 'deprived' cutaneous nerve are not segregated appropriately within the plexus. Some cutaneous axons are instead misdirected along an inappropriate cutaneous nerve, while others have seemingly failed to reach their correct target, or a suitable alternative, and died. These results demonstrate that the target ectoderm is necessary for normal sensory axon growth and guidance in the hindlimb.     

Olfactory-mediated parasite recognition and avoidance: linking genes to behavior

A major cost of social behavior is the increased risk of exposure to parasites and infection. Animals utilize social information, including chemical signals, to recognize and avoid conspecifics infected with either endoparasites or ectoparasites. Here, we briefly discuss the relations among odors, parasite recognition, and avoidance, and consider some of the associated hormonal, neural, and genomic mechanisms. In rodents, odor cues mediate sexual and competitive interactions and are of major importance in individual recognition and mate detection and choice. Female mice distinguish between infected and uninfected males by urinary odors, displaying aversive response to, and avoidance of, the odors of infected individuals. This reduces both the likelihood of the transmission of parasites to themselves and allows females to select for parasite-free males. This set of olfactory and mate choice responses can be further modulated by social factors such as previous experience and exposure to infected males and the mate choices of other females. Male mice, who also face the threat of infection, similarly distinguish and avoid parasitized individuals by odor, thus reducing their likelihood of infection. This recognition and avoidance of the odors of infected individuals involves genes for the neuropeptide, oxytocin (OT), and estrogenic mechanisms. Mice with deletions of the oxytocin gene [OT knockout mice (OTKO)] and mice whose genes for estrogen receptor (ER)-alpha or ER-beta have been disrupted [ER knockout mice (ERKO), alpha-ERKO and beta-ERKO] are specifically impaired in their recognition of, aversion to, and memory of the odors of infected individuals. These findings reveal some of the genes involved in the mediation of social recognition in the ecologically relevant context of parasite recognition and avoidance.     

Physiological and microfluorometric studies of reduction and clearance of retinal in bleached rod photoreceptors

The visual cycle comprises a sequence of reactions that regenerate the visual pigment in photoreceptors during dark adaptation, starting with the reduction of all-trans retinal to all-trans retinol and its clearance from photoreceptors. We have followed the reduction of retinal and clearance of retinol within bleached outer segments of red rods isolated from salamander retina by measuring its intrinsic fluorescence. Following exposure to a bright light (bleach), increasing fluorescence intensity was observed to propagate along the outer segments in a direction from the proximal region adjacent to the inner segment toward the distal tip. Peak retinol fluorescence was achieved after approximately 30 min, after which it declined very slowly. Clearance of retinol fluorescence is considerably accelerated by the presence of the exogenous lipophilic substances IRBP (interphotoreceptor retinoid binding protein) and serum albumin. We have used simultaneous fluorometric and electrophysiological measurements to compare the rate of reduction of all-trans retinal to all-trans retinol to the rate of recovery of flash response amplitude in these cells in the presence and absence of IRBP. We find that flash response recovery in rods is modestly accelerated in the presence of extracellular IRBP. These results suggest such substances may participate in the clearance of retinoids from rod photoreceptors, and that this clearance, at least in rods, may facilitate dark adaptation by accelerating the clearance of photoproducts of bleaching.     

Reproductive development according to elevation in a seasonally breeding male songbird

Seasonal temperate zone breeders respond to increasing day length to anticipate the approach of spring breeding conditions. Other (supplementary) environmental cues, such as temperature and precipitation, were historically thought to play unimportant roles in reproductive timing. We demonstrate variation in reproductive timing across small geographic distances by examining the vernal testicular recrudescence of adult song sparrows (Melospiza melodia morphna) breeding in coastal (0–10 m elevation) and montane (280–1220 m elevation) habitats. Each year, these birds experienced the same photoperiod, but were exposed to different supplementary cues that varied with altitude. Coastal birds experienced warmer and more stable temperatures during late winter and early spring than did montane birds. We measured bud opening, emergence of new green shoots, and arthropod biomass to monitor the pace of springs approach. New spring shoots emerged 2 months earlier on the coast than in the mountains and buds on flowering trees and shrubs also tended to open earlier at the coast. Arthropod biomass was similar in both the mountains and the coast during early spring, and began to increase in early summer. Reproductive morphology (i.e. testis volume and cloacal protuberance length) developed up to 2 months earlier on the coast than in the mountains. Testicular recrudescence occurred earlier on the coast in most years and proceeded at a faster rate in 1 year. Circulating levels of luteinizing hormone, follicle stimulating hormone and prolactin increased through the season, but did not correlate with differences between sites. Both populations responded similarly when exposed to identical photoperiodic cues in the laboratory. Therefore, we suggest that an integrated response to cues characteristic of location and elevation account for differences in patterns measured in the field.     

Plumage brightness predicts male mating success in the lekking golden-collared manakin, Manacus vitellinus

The evolution of colorful plumage has been dramatic in lekkingspecies. Several studies show that the size of colorful traitsinfluence female choice in leks; however, relatively littleis known about the specific function of color, in particularits spectral properties, in lekking taxa. To determine the importanceof color in a lekking species, we monitored the mating successof male golden-collared manakins, Manacus vitellinus, and relatedthis to spectral measures of their colorful plumage, as wellas other morphological and behavioral traits shown to be importantin other lekking species. We found that lek centrality, malebody size, and plumage brightness were associated with malemating success. Only plumage brightness, however, entered amultiple regression model, indicating that plumage is the overallbest predictor of mating success. These results provide evidencethat the spectral properties of colorful plumage predict malemating success in a lekking species and provide important insightinto why many lekking birds are dichromatic and elaborate incoloration.     

The antifungal drug voriconazole is an efficient inhibitor of brain cholesterol 24S-hydroxylase in vitro and in vivo

Cholesterol 24S-hydroxylase (CYP46A1) is of key importance for cholesterol homeostasis in the brain. This enzyme seems to be resistant toward most regulatory factors and at present no drug effects on its activity have been described. The crystal structures of the substrate-free and substrate-bound CYP46A1 were recently determined (Mast et al., Crystal structures of substrate-bound and substrate-free cytochrome P450 46A1, the principal cholesterol hydroxylase in the brain. Proc. Natl. Acad. Sci. USA. 2008. 105: 9546–9551). These structural studies suggested that ligands other than sterols can bind to CYP46A1. We show here that the antifungal drug voriconazole binds to the enzyme in vitro and inhibits CYP46A1-mediated cholesterol 24-hydroxylation with a Ki of 11 nM. Mice treated with daily intraperitoneal injections of voriconazole for 5 days had high levels of voriconazole in the brain and significantly reduced brain levels of 24S-hydroxycholesterol. The levels of squalene, lathosterol, and HMG-CoA reductase mRNA were reduced in the brain of the voriconazole-treated animals as well, indicating a reduced cholesterol synthesis. Most of this effect may be due to a reduced utilization of cholesterol by CYP46A1. One of the side-effects of voriconazole is visual disturbances. Because CYP46A1 is also expressed in the neural retina, we discuss the possibility that the inhibition of CYP46A1 by voriconazole contributes to these visual disturbances.     

Site fidelity and homing in tropical coral reef cardinalfish: are they using olfactory cues?

A number of tropical coral reef fish hold station and display restricted home ranges. If artificially displaced, they will return to their home site. We questioned if marine fish are using the same mechanisms for home site detection as many freshwater fish, that is, by olfactory sensing of chemical signals deposited on the substrate by conspecific fish. Behavioral experiments were conducted on Lizard Island Research Station, Queensland, Australia, in 2001 and 2002. Five-lined cardinalfish (Cheilodipterus quinquelineatus) were tested in groups with split-branded cardinalfish (Apogon compressus) as a reference species and individually against Apogon leptacanthus as well as conspecifics of another reef site. The group tests showed that both species preferred artificial reef sites that had previously been occupied by conspecifics. Individual C. quinquelineatus preferred scent of conspecifics from their own reef site to that from another site. They also preferred the scent released by artificial reefs previously occupied by conspecifics of their reef site to that of similar reefs previously occupied by conspecifics of another reef site. No discrimination between species from the same reef site was obtained in experiments with individual fish. Our data suggest that cardinalfish are keeping station and are homing by use of conspecific olfactory signals.     

Detection and avoidance of an entomopathogenic fungus by a generalist insect predator

Abstract.  1. Increasing evidence suggests that insects can assess their environment based on cues related to mortality risks to themselves or their offspring. Limited knowledge is available on such abilities in relation to entomopathogenic fungi, which can cause significant mortality in insect populations. In laboratory bioassays, the ability of the generalist predator Anthocoris nemorum L. (Heteroptera: Anthocoridae) to detect the presence of its natural enemy, the fungal pathogen Beauveria bassiana (Balsamo) Vuillemin (Ascomycota: Hypocreales) was investigated.
2. Behavioural observations were conducted on adults of A. nemorum foraging in choice and non-choice arenas treated with conidia suspensions of B. bassiana or just the carrier (control). The arenas consisted either of nettle leaves or soil. Additionally, behaviours in response to sporulating nettle aphid cadavers compared with uninfected aphids or paper balls were evaluated on nettle leaves. An oviposition experiment was also conducted in choice arenas on conidia-treated and untreated nettle leaves.
3. Males and females detected and avoided contact with leaf surfaces inoculated with B. bassiana . Females that were forced to enter fungus-treated leaf surfaces were very reluctant to do so. When females encountered cadavers sporulating with B. bassiana they rapidly withdrew compared with harmless paper ball dummies. Soil inoculated with B. bassiana did not affect A. nemorum behaviour or residence time compared with control soil. Females inserted significantly more eggs in control leaf areas compared with areas treated with B. bassiana conidia.
4. All results suggest that A. nemorum detects and avoids the pathogen B. bassiana when it forages on host plants with which it is adapted but not on soil surfaces. The adaptive significance of detection of entomopathogenic fungi is discussed.     

Auditory lateralization in bushcrickets: a new dichotic paradigm

Pair formation in the bushcricket Gampsocleis gratiosa is achieved through acoustic signalling by the male and phonotactic approaches of the female towards the calling song. On a walking belt in the free sound field, females tracked the position of the speaker broadcasting the male calling song with a remarkable precision, deviating by no more than 10 cm in either direction from the ideal course. Starting with stimulus angles of 6–10° the females significantly turned to the correct side, and with stimulus angles greater than 25° no incorrect turns were made. Using neurophysiological data on the directionality of the ear we calculated that with such stimulus angles the available binaural intensity difference is in the order of 1–2 dB. We developed a dichotic ear stimulation device for freely moving females with a cross-talk barrier of about 50 dB, which allowed to precisely apply small binaural intensity differences. In such a dichotic stimulation paradigm, females on average turned to the tronger stimulated side starting with a 1 dB difference between both ears. The significance of such a reliable lateralization behaviour with small interaural intensity differences for phonotactic behaviour under natural conditions is discussed.     

Habitat-specific sensory-exploitative signals in birds: propensity of dipteran prey to cause evolution of plumage variation in flush-pursuit insectivores

Sensory exploitation occurs when signals trigger behavioral reactions that diminish the receiver's fitness. Research in this area focuses on the match between the signal's form and the receiver's sensitivity, but the effect of habitat on interspecific sensory exploitation is rarely addressed. Myioborus redstarts use conspicuous wing and tail displays of contrasting black-and-white plumage patches to flush dipteran insects, which are then pursued and captured in flight. Previous studies have shown that by increasing the distance at which insects perform an escape response, conspicuous visual displays improve the birds' foraging performance. We tested the hypothesis that selection for a visual signal that maximizes prey escape distance under local habitat conditions can lead to the evolution of geographic variation in plumage pattern among Myioborus redstarts. Using models of foraging birds, we recorded the escape responses of Dipterous insects to a range of plumage patterns and background tones (from light to dark) to determine whether the plumage pattern that maximizes prey flushing is dependent upon that habitat (background) against which birds are viewed by their prey. Our results indicate that the effectiveness of a particular plumage pattern in flushing dipteran prey depends strongly on the background against which that plumage pattern is displayed, and darker habitat (background) conditions generally favor plumages with more extensive patches of white in the tail. However, the addition of white wing patches that imitate the plumage of the painted redstart (Myioborus pictus) generally increases insect escape responses but reduces the effect that tail pattern variation and background tone have on escape behavior. These experiments support the hypothesis that habitat-specific natural selection to enhance sensory exploitation of prey escape responses could produce geographic variation in plumage patterns of flush-pursuers.