Anatomic variation in the elastic inhomogeneity and anisotropy of human femoral cortical bone tissue is consistent across multiple donors

Numerical models commonly account for elastic inhomogeneity in cortical bone using power-law scaling relationships with various measures of tissue density, but limited experimental data exists for anatomic variation in elastic anisotropy. A recent study revealed anatomic variation in the magnitude and anisotropy of elastic constants along the entire femoral diaphysis of a single human femur (Espinoza Orías et al., 2009). The objective of this study was to confirm these trends across multiple donors while also considering possible confounding effects of the anatomic quadrant, apparent tissue density, donor age, and gender. Cortical bone specimens were sampled from the whole femora of 9 human donors at 20%, 50%, and 80% of the total femur length. Elastic constants from the main diagonal of the reduced fourth-order tensor were measured on hydrated specimens using ultrasonic wave propagation. The tissue exhibited orthotropy overall and at each location along the length of the diaphysis (p < 0.0001). Elastic anisotropy increased from the mid-diaphysis toward the epiphyses (p < 0.05). The increased elastic anisotropy was primarily caused by a decreased radial elastic constant (C(11)) from the mid-diaphysis toward the epiphyses (p < 0.05), since differences in the circumferential (C(22)) and longitudinal (C(33)) elastic constants were not statistically significant (p > 0.29). Anatomic variation in intracortical porosity may account for these trends, but requires further investigation. The apparent tissue density was positively correlated with the magnitude of each elastic constant (p < 0.0001, R(2) > 0.46), as expected, but was only weakly correlated with C(33)/C(11) (p < 0.05, R(2) = 0.04) and not significantly correlated with C(33)/C(22) and C(11)/C(22).     

From process to pattern: how fluctuating predation risk impacts the stress axis of snowshoe hares during the 10-year cycle

Predation is a central organizing process affecting populations and communities. Traditionally, ecologists have focused on the direct effects of predation—the killing of prey. However, predators also have significant sublethal effects on prey populations. We investigated how fluctuating predation risk affected the stress physiology of a cyclic population of snowshoe hares (Lepus americanus) in the Yukon, finding that they are extremely sensitive to the fluctuating risk of predation. In years of high predator numbers, hares had greater plasma cortisol levels at capture, greater fecal cortisol metabolite levels, a greater plasma cortisol response to a hormone challenge, a greater ability to mobilize energy and poorer body condition. These indices of stress had the same pattern within years, during the winter and over the breeding season when the hare:lynx ratio was lowest and the food availability the worst. Previously we have shown that predator-induced maternal stress lowers reproduction and compromises offspring’s stress axis. We propose that predator-induced changes in hare stress physiology affect their demography through negative impacts on reproduction and that the low phase of cyclic populations may be the result of predator-induced maternal stress reducing the fitness of progeny. The hare population cycle has far reaching ramifications on predators, alternate prey, and vegetation. Thus, predation is the predominant organizing process for much of the North American boreal forest community, with its indirect signature—stress in hares—producing a pattern of hormonal changes that provides a sensitive reflection of fluctuating predator pressure that may have long-term demographic consequences.     

Multiple measures elucidate glucocorticoid responses to environmental variation in predation threat

Predator-induced changes in the glucocorticoid responses of prey have been proposed to mediate indirect predator effects on prey demography. Ambiguities exist, however, as to whether differences in predation threat in the environment at large affect the mean glucocorticoid response in wild birds and mammals, and whether this is likely to affect reproduction. Most studies to date that have examined glucocorticoid responses to environmental variation in predation threat have evaluated just one of the several potential measures of the glucocorticoid response, and this may be the source of many ambiguities. We evaluated multiple measures of the glucocorticoid response [plasma total CORTicosterone, corticosteroid binding globulin (CBG) and free CORT] in male and female song sparrows (Melospiza melodia) sampled at locations differing in predation threat in the environment at large, where we have previously reported reproductive differences suggestive of indirect predator effects. Total CORT varied markedly with predation threat in males but not females whereas the opposite was true for CBG, and both sexes demonstrated the same moderately significant free CORT response. Considering all three indices, a glucocorticoid response to environmental variation in predation threat was evident in both sexes, whereas there were ambiguities considering each index singly. We conclude that collecting multiple physiological measures and conducting multivariate analyses may provide a preferable means of assessing glucocorticoid responses to environmental variation in predation threat, and so help clarify whether such glucocorticoid changes affect reproduction in wild birds and mammals.     

Statistical properties of ion channel records. Part II: estimation from the macroscopic current

Macroscopic ion channel current is the summation of the stochastic records of individual channel currents and therefore relates to their statistical properties. As a consequence of this relationship, it may be possible to derive certain statistical properties of single channel records or even generate some estimates of the records themselves from the macroscopic current when the direct measurement of single channel currents is not applicable. We present a procedure for generating the single channel records of an ion channel from its macroscopic current when the stochastic process of channel gating has the following two properties: (I) the open duration is independent of the time of opening event and has a single exponential probability density function (pdf), (II) all the channels have the same probability to open at time t. The application of this procedure is considered for cases where direct measurement of single channel records is difficult or impossible. First, the probability density function (pdf) of opening events, a statistical property of single channel records, is derived from the normalized macroscopic current and mean channel open duration. Second, it is shown that under the conditions (I) and (II), a non-stationary Markov model can represent the stochastic process of channel gating. Third, the non-stationary Markov model is calibrated using the results of the first step. The non-stationary formulation increases the model ability to generate a variety of different single channel records compared to common stationary Markov models. The model is then used to generate single channel records and to obtain other statistical properties of the records. Experimental single channel records of inactivating BK potassium channels are used to evaluate how accurately this procedure reconstructs measured single channel sweeps.     

ABA regulates apoplastic sugar transport and is a potential signal for cold-induced pollen sterility in rice

Cold temperatures cause pollen sterility and large reductions in grain yield in temperate rice growing regions of the world. Induction of pollen sterility by cold involves a disruption of sugar transport in anthers, caused by the cold-induced repression of the apoplastic sugar transport pathway in the tapetum. Here we demonstrate that the phytohormone ABA is a potential signal for cold-induced pollen sterility (CIPS). Cold treatment of the cold-sensitive cultivar Doongara resulted in increased anther ABA levels. Exogenous ABA treatment at the young microspore stage induced pollen sterility and affected cell wall invertase and monosaccharide transporter gene expression in a way similar to cold treatment. In the cold-tolerant cultivar R31, ABA levels were significantly lower under normal circumstances and remained low after cold treatment. The differences in endogenous ABA levels in Doongara and R31 correlated with differences in expression of the ABA biosynthetic genes encoding zeaxanthin epoxidase (OSZEP1) and 9-cis-epoxycarotenoid dioxygenase (OSNCED2, OSNCED3) in anthers. The expression of three ABA-8-hydroxylase genes (ABA8OX1, 2 and 3) in R31 anthers was higher under control conditions and was regulated differently by cold compared with Doongara. Our results indicate that the cold tolerance phenotype of R31 is correlated with lower endogenous ABA levels and a different regulation of ABA metabolism.     

The proximate control of pupal color in swallowtail butterflies: implications for the evolution of environmentally cued pupal color in butterflies (Lepidoptera: Papilionidae)

The environmentally cued production of cryptic green/yellow or brown/melanized pupae is widespread in butterflies, occurring in the Nymphalidae, Pieridae, and the Papilionidae subfamily Papilioninae. The dimorphism is controlled by the hormone pupal melanization reducing factor (PMRF). In the nymphalid Inachis io dibutryl cAMP mimics PMRF, and inhibits pupal melanization. However, in the papilionid Papilio polyxenes PMRF stimulates browning, suggesting that the control of pupal color by PMRF has evolved independently in the swallowtail and nymphalid-pierid lineages. We examined this hypothesis by using ligatures to prevent hormone release in five species representing three Papilioninae tribes. One species, Papilio glaucus, produces only brown pupae. Ligatures resulted in green cuticle posterior to the ligature in all five swallowtail species, including P. glaucus, suggesting that the mode of action of PMRF is the same in the three tribes. We also found that in P. polyxenes injections of dibutryl cAMP into prepupal larvae mimic the effect of PMRF, by causing dose-dependent pupal browning. Our results support the hypothesis that the control of pupal color by PMRF has evolved independently in the two lineages. The observation that green pupal color can be induced in P. glaucus by ligature indicates that environmentally cued pupal color could evolve by facultative inhibition of PMRF release.