Summary Epigenetics, the transgenerational transfer of phenotypic characters without modification of gene sequence, is a burgeoning area of study in many disciplines of biology. However, the potential impact of this phenomenon on the physiology of animals is not yet broadly appreciated, in part because the phenomenon of epigenetics is not typically part of the design of physiological investigations. Still enigmatic and somewhat ill defined is the relationship between the overarching concept of epigenetics and interesting transgenerational phenomena (e.g. ‘maternal/parental effects’) that alter the physiological phenotype of subsequent generations. The lingering effect on subsequent generations of an initial environmental disturbance in parent animals can be profound, with genes continuing to be variously silenced or expressed without an associated change in gene sequence for many generations. Known epigenetic mechanisms involved in this phenomenon include chromatin remodeling (DNA methylation and histone modification), RNA-mediated modifications (non-coding RNA and microRNA), as well as other less well studied mechanisms such as self-sustaining loops and structural inheritance. In this review we: (1) discuss how the concepts of epigenetics and maternal effects both overlap with, and are distinct from, each other; (2) analyze examples of existing animal physiological studies based on these concepts; and (3) offer a construct by which to integrate these concepts into the design of future investigations in animal physiology.

Barrionuevo, W. R., and W. W. Burggren. O 2 consumption and heart rate in developing zebrafish (Danio rerio): influence of temperature and ambient O 2 . Am. J. Physiol. 276 (Regulatory Integrative Comp. Physiol. 45): R505-R513, 1999.-Body mass, length, oxygen consumption (Ṁ O 2 ) and heart rate (f H ) were measured in ''embryos'' (prior to hatching), ''larvae'' (days 10-20), ''juveniles'' (days 30-70 in 10-day intervals), and ''adults'' (day 100) of the zebrafish Danio rerio. Fish were chronically reared at either 25, 28, or 31°C and then acutely exposed to hypoxia at different developmental stages. We hypothesized that at any given rearing and measurement temperature, D. rerio would maintain Ṁ O 2 at lower ambient PO 2 [i.e., have a lower critical partial pressure (P crit )] as development progressed and that at any given developmental stage individuals reared and measured at higher temperatures would show a more pronounced hypoxic bradycardia. Ṁ O 2 in normoxic fish at 28°C peaked at ϳ40 µmol·g Ϫ1 ·h Ϫ1 at day 10, thereafter falling to 4-5 µmol·g Ϫ1 ·h Ϫ1 at day 100. The Q 10 for Ṁ O 2 was 4-5 in embryos, falling to 2-3 from day 10 to day 60 and rising again to 4-5 at day 100. P crit at 28°C was ϳ80 mmHg in embryos but decreased sharply to 20 mmHg at 100 days, supporting the hypothesis that more mature fish would be better able to oxygen regulate to lower ambient PO 2 levels. P crit increased sharply with measurement temperature. Heart rate (f H ) at 28°C increased from about 125 beats/min in embryos to a peak of ϳ175 beats/min at days 10-30 and then fell to ϳ130 beats/min by day 100. Unlike for Ṁ O 2 , the Q 10 for f H was more constant at 1.2-2.5 throughout development. Hypoxic exposure at any temperature had no effect on f H until ϳday 30, after which time a hypoxic bradycardia was evident. As evident for Ṁ O 2 , the bradycardia in older larvae was more profound at higher temperatures. On the assumption that bradycardia is indicative of hypoxic stress, the increasing prevalence of a hypoxic bradycardia in older, warmer individuals supports the hypothesis that increasing hypoxic susceptibility with development would be exacerbated by increasing temperature. Collectively, these data indicate that the ability to regulate Ṁ O 2 and f H in response to the compounding demands of increased temperature and/or decreased oxygen availability first develops after ϳ20 days in D. rerio and, thereafter, the ability to maintain Ṁ O 2 in the face of ambient hypoxia progressively builds through to adulthood. Additionally, the temperature responses of metabolism and heart rate differ substantially at different phases of development, suggesting a loose coupling between the respiratory and cardiovascular systems, at least early in development. development; hypoxia; embryos THE ZEBRAFISH, Danio rerio (Brachydanio), is a tropical Cyprinid teleost fish that recently has been the focus of increasing numbers of developmental studies. Physiological interest in this species has been spurred, in part, by the relative ease with which ...

Embryonic hemoglobin circulated by the developing heart in the early vertebrate embryo is widely assumed (without substantiation) to perform the same vital role of O2 carriage that it does in fetuses and adults. In order to challenge this assumption, we measured highly O2-dependent physiological variables like O2 consumption, cardiac performance, and initial swim bladder filling in the presence and absence of functional hemoglobin in the embryos and early larvae of the zebra fish, Danio ( = Brachydanio) rerio. Functional ablation of hemoglobin by carbon monoxide or phenylhydrazine did not reduce whole-animal O2 consumption, which was approximately 85 to 90 mumol.g-1.h-1. Similarly, no differences in heart variables like ventricular pressure development or heart rate, which increased from 135 to 175 bpm between stages 36h and 96h (indicating developmental stages 36 and 96 hours after fertilization, respectively), were observed in these experiments. Initial opening of the swim bladder was not influenced in the presence of CO-occupied hemoglobin but was significantly impaired when the embryonic hemoglobin was chemically modified by incubation with phenylhydrazine. That aerobic processes continue without hemoglobin O2 transport indicates the adequacy in the embryo of simple O2 diffusion alone even in developmental stages with extensive convective blood circulation generated by the heart.

The zebrafish embryo is transparent and can tolerate absence of blood flow because its oxygen is delivered by diffusion rather than by the cardiovascular system. It is therefore possible to attribute cardiac failure directly to particular genes by ruling out the possibility that it is due to a secondary effect of hypoxia. We focus here on pickwickm171 (pikm171), a recessive lethal mutation discovered in a large-scale genetic screen. There are three other alleles in the pik complementation group with this phenotype (pikm242, pikm740, pikm186; ref. 3) and one allele (pikmVO62H) with additional skeletal paralysis. The pik heart develops normally but is poorly contractile from the first beat. Aside from the edema that inevitably accompanies cardiac dysfunction, development is normal during the first three days. We show by positional cloning that the 'causative' mutation is in an alternatively-spliced exon of the gene (ttn) encoding Titin. Titin is the biggest known protein and spans the half-sarcomere from Z-disc to M-line in heart and skeletal muscle. It has been proposed to provide a scaffold for the assembly of thick and thin filaments and to provide elastic recoil engendered by stretch during diastole. We found that nascent myofibrils form in pik mutants, but normal sarcomeres are absent. Mutant cells transplanted to wildtype hearts remain thin and bulge outwards as individual cell aneurysms without affecting nearby wildtype cardiomyocytes, indicating that the contractile deficiency is cell-autonomous. Absence of Titin function thus results in blockage of sarcomere assembly and causes a functional disorder resembling human dilated cardiomyopathies, one form of which is described in another paper in this issue.

Summary 1. The exchange of oxygen and carbon dioxide between skin and environment is commonplace in the vertebrates. In many lower vertebrates, the skin is the major or even sole avenue for respiration. 2. As implied by the physical laws governing diffusion of gases, the skin diffusion coefficient, surface area, gas diffusion distance and transcutaneous gas partial pressures may independently or jointly affect cutaneous respiration. In vertebrates, each of these variables has undergone modification that may be related to dependence upon cutaneous gas exchange. 3. Both theoretical models and experimental data suggest that cutaneous gas exchange is limited by the rate of diffusion. However, changes in convection of the respiratory medium and of blood may partially compensate for diffusion limitation, and potentially function in the regulation of cutaneous gas exchange. 4. Typically, the skin is one of several gas exchangers, although many salamanders and some species in other vertebrate groups breathe solely through the skin. The cutaneous contribution to overall gas exchange is often most important in small animals, at cool temperatures, at low levels of activity and in normoxic and normocapnic conditions. Branchial and pulmonary respiration increasingly predominate in other circumstances. 5. Often, the skin figures more prominently in CO2, excretion than in O2, uptake. 6. Cutaneous gas exchange emerges in vertebrates as a process perhaps less effective and more constrained than branchial or pulmonary exchange but also less energetically costly. Its utility is indicated by its wide and successful exploitation in vertebrates occupying a diverse array of habitats.

Epigenetics increasingly occupies a pivotal position in our understanding of inheritance, natural selection and, perhaps, even evolution. A survey of the PubMed database, however, reveals that the great majority (>93%) of epigenetic papers have an intra-, rather than an inter-generational focus, primarily on mechanisms and disease. Approximately ~1% of epigenetic papers even mention the nexus of epigenetics, natural selection and evolution. Yet, when environments are dynamic (e.g., climate change effects), there may be an “epigenetic advantage” to phenotypic switching by epigenetic inheritance, rather than by gene mutation. An epigenetically-inherited trait can arise simultaneously in many individuals, as opposed to a single individual with a gene mutation. Moreover, a transient epigenetically-modified phenotype can be quickly “sunsetted”, with individuals reverting to the original phenotype. Thus, epigenetic phenotype switching is dynamic and temporary and can help bridge periods of environmental stress. Epigenetic inheritance likely contributes to evolution both directly and indirectly. While there is as yet incomplete evidence of direct permanent incorporation of a complex epigenetic phenotype into the genome, doubtlessly, the presence of epigenetic markers and the phenotypes they create (which may sort quite separately from the genotype within a population) will influence natural selection and, so, drive the collective genotype of a population.

First published in 1981, this book presents an original approach to an area of great importance in comparative zoology and physiology and evolutionary biology: the evolution of air breathing in vertebrates from aquatic ancestors. The subject is approached from a functional as well as an anatomical viewpoint, utilising knowledge of the physiology of extant animals to trace probable evolutionary steps. Opening with a brief summary of current views of vertebrate evolution, the authors then go on to deal with problems of oxygen transfer in water and air and the structure and function of gills and lungs. Carbon dioxide transfer in water-breathing forms is seen as being tightly coupled to an ion and acid-base regulation. The evolution of air breathing is seen as a several-stage process, beginning with the evolution of accessory air-breathing structures for oxygen uptake.

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