Showing papers in "The Quarterly Review of Biology in 1996"

Models of parasite virulence

Abstract: Several evolutionary processes influence virulence, the amount of damage a parasite causes to its host. For example, parasites are favored to exploit their hosts prudently to prolong infection and avoid killing the host. Parasites also need to use some host resources to reproduce and transmit infections to new hosts. Thus parasites face a tradeoff between prudent exploitation and rapid reproduction-a life history tradeoff between longevity and fecundity. Other tradeoffs among components of parasite fitness also influence virulence. For example, competition among parasite genotypes favors rapid growth to achieve greater relative success within the host. Rapid growth may, however, lower the total productivity of the local group by overexploiting the host, which is a potentially renewable food supply. This is a problem of kin selection and group selection. I summarize models of parasite virulence with the theoretical tools of life history analysis, kin selection, and epidemiology. I then apply the theory to recent empirical studies and models of virulence. These applications, to nematodes, to the extreme virulence of hospital epidemics, and to bacterial meningitis, show the power of simple life history theory to highlight interesting questions and to provide a rich array of hypotheses. These examples also show the kinds of conceptual mistakes that commonly arise when only a few components of parasite fitness are analysed in isolation. The last part of the article connects standard models of parasite virulence to diverse topics, such as the virulence of bacterial plasmids, the evolution of genomes, and the processes that influenced conflict and cooperation among the earliest replicators near the origin of life.

Seasonal changes in immune function.

Abstract: Winter is energetically demanding Physiological and behavioral adaptations have evolved among nontropical animals to cope with winter because thermoregulatory demands increase when food availability decreases Seasonal breeding is central within the suite of winter adaptations among small animals Presumably, reproductive inhibition during winter conserves energy at a time when the adds of producing viable young are low In addition to the well-studied seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among many populations of mammals and birds in the field Challenging winter conditions, such as low ambient temperatures and decreased food availability, can directly induce death via hypothermia, starvation or shock In some cases, survival in demanding winter conditions puts individuals under great physiological stress, defined here as an adaptive process that results in elevated blood levels of glucocorticoids The stress of coping with energetically demanding conditions can also indirectly cause illness and death by compromising immune function Presumably, the increased blood concentrations of adrenocortical steroids in response to winter stressors compromise immune function and accelerate catabolic mechanisms in the field, although the physiological effects of elevated glucocorticoids induced by artificial stressors have been investigated primarily in the laboratory However, recurrent environmental stressors could reduce survival if they evoke persistent glucocorticoid secretion The working hypothesis of this article is that mechanisms have evolved in some animals to combat seasonal stress-induced immunocompromise as a temporal adaptation to promote survival Furthermore, we hypothesize that mechanisms have evolved that allow individuals to anticipate periods of immunologically challenging conditions, and to cope with these seasonal health-threatening conditions The primary environmental cue that permits physiological anticipation of season is the daily photoperiod; however, other environmental factors may interact with photoperiod to affect immune function and disease processes The evidence for seasonal fluctuations in lymphatic organ size, structure, immune function, and disease processes, and their possible interactions with recurrent environmental stressors, is reviewed Seasonal peaks of lymphatic organ size and structure generally occur in late autumn or early winter and seasonal minima are observed prior to the onset of breeding Although many of the field data suggest that immune function and disease processes are also enhanced during the winter, the opposite seasonal pattern is also observed in some studies We propose that compromised immune function may be observed in some populations during particularly harsh winters when stressors override the enhancement of immune function evoked by short day lengths Because so many factors covary in field studies, assessment of our proposal that photoperiod mediates seasonal changes in immune function requires laboratory studies in which only photoperiod is varied A review of the effects of photoperiod on immune function in laboratory studies reveals that exposure to short day lengths enhances immune function in every species examined Short day exposure in small mammals causes reproductive inhibition and concomitant reduction in plasma levels of prolactin and steroid hormones, as well as alterations in the temporal pattern of pineal melatonin secretion These hormones affect immune function, and influence the development of opportunistic diseases, including cancer: however, it appears that either prolactin or melatonin secretion is responsible for mediating the effects of photoperiod on immune function Taken together, day length appears to affect immune function in many species, including animals that typically do not exhibit reproductive responsiveness to day length

The Evolution of Threshold Traits in Animals

Abstract: Within a population there are frequently several discrete morphs. While in some cases, particularly color polymorphisms, this variation can be explained by simple Mendelian modes of inheritance, in many cases the evidence suggests a polygenic pattern of inheritance. The threshold model of quantitative genetics, in which discrete morphs are determined by some underlying continuously distributed trait and a threshold(s) of expression, is applied appropriately in these cases. The discrete morphs exhibited in cyclomorphosis, pedomorphosis, pedogenesis, "protective" dimorphisms, trophic dimorphisms, wing dimorphism, and mating strategies can all be analyzed by using this model. Analyses of a wide range of different types of threshold traits show that there is typically a large additive genetic component, but that there is also strong environmental induction. A review of studies shows that no morph has a universally higher fitness, but that there is a tradeoff, with the relative fitnesses of two morphs being co...

Biodiversity at the microbial level: The number of free-living ciliates in the biosphere

Abstract: For more than 200 years, ciliated protozoa have been identified and allocated Species names largely on the basis of the rich morphological variety they present. We have examined the species richness of all free-living ciliate genera, described historical trends in the descriptions of new species, and estimated the number of species currently known. We have quantified the value of taxonomic revisions, and conclude that the number of known, extant free-living species is close to 3000. We have investigated the concept of "species" and the meaning of "biodiversity" in relation to ciliates, and conclude that the bilogical species concept is neither appropriate nor practicable. Insofar as ciliate morphology is closely correlated with the function of the organism in nature, the morphospecies concept is as valid as any, and probably more pragmatic than any other. Thus, when speaking of "species diversity," or the "biodiversity" of ciliates, we refer to diversity of form and function. The majority of ciliate speci...

The evolution of endometrial cycles and menstruation.

Abstract: According to a recent hypothesis, menstruation evolved to protect the uterus and oviducts from sperm-borne pathogens by dislodging infected endometrial tissue and delivering immune cells to the uterine cavity. This hypothesis predicts the following: (1) uterine pathogens should be more prevalent before menses than after menses, (2) in the life histories of females, the timing of menstruation should track pathogen burden, and (3) in primates, the copiousness of menstruation should increase with the promiscuity of the breeding system. I tested these predictions and they were not upheld by the evidence. I propose the alternative hypothesis that the uterine endometrium is shed/resorbed whenever implantation fails because cyclical regression and renewal is energetically less costly than maintaining the endometrium in the metabolically active state required for implantation. In the regressed state, oxygen consumption (per mg protein/h) in human endometria declines nearly sevenfold. The cyclicity in endometrial oxygen consumption is one component of the whole body cyclicity in metabolic rate caused by the action of the ovarian steroids on both endometrial and nonendometrial tissue. Metabolic rate is at least 7% lower, on average, during the follicular phase than during the luteal phase in women, which signifies an estimated energy savings of 53 MJ over four cycles, or nearly six days worth of food. Thus the menstrual cycle revs up and revs down, economizing on the energy costs of reproduction. This economy is greatest during the nonbreeding season and other periods of amenorrhea when the endometrium remains in a regressed state and ovarian cycling is absent for a prolonged period of time. Twelve months of amenorrhea save an estimated 130 MJ, or the energy required by one woman for nearly half a month. By helping females to maintain body mass, energy economy will promote female fitness in any environment in which fecundity and survivorship is constrained by the food supply. Endometrial economy may be of ancient evolutionary origin because similar reproductive structures, such as the oviducts of lizards, also regress when a fertilized egg is unlikely to be present. Regression of the endometrium is usually accompanied by reabsorption, but in some species as much as one third of the endometrial and vascular tissue is shed as the menses. Rather than having an adaptive basis in ecology or behavior, variation in the degree of menstrual bleeding in primates shows a striking correlation with phylogeny. The endometrial microvasculature is designed to provide the blood supply to the endometrium and the placenta, and external bleeding appears to be a side effect of endometerial regression that arises when there is too much blood and other tissue for complete reabsorption. The copious bleeding of humans and chimps can be attributed to the large size of the uterus relative to adult female body size and to the design of the microvasculature in catarrhines.

Sex Chromatin in Lepidoptera

Abstract: Like mammals, Lepidoptera possess female-specific sex chromatin. In a compilation of new and published data, 81 % of the 238 investigated Lepidoptera species display one or more heterochromatin bodies infemale somatic interphase cells, but not in male cells. In contrast with the similar phenomenon in mammals, this sex-specific heterochromatin does notfunction as a dosage compensation mechanism. Most Lepidoptera have a WZ/ZZ sex chromosome mechanism, and the sex chromatin is derived from the univalent W sex chromosome. Sex chromatin is regarded as an indicator of an advanced stage of Wchromosome evolution. In species with a Z/ZZ sex chromosome mechanism, loss of the Wchromosome is accompanied by loss ofthefemale-specific heterochromatin. Since sex chromatin can be discerned easily in interphase nuclei, and especially so in the highly polyploid somatic cells, it is a useful marker for diagnosing chromosomal sex of embryos and larvae, and of identifying sex chromosome aberrations in mutagenesis screens. All species with sex chromatin belong to theDitrysia, the main clade of Lepidoptera that contains more than 98 % of all extant species. Sex chromatin has not been reportedfor clades that branched off earlier. The nonditrysian clades share this character with Trichoptera, a sister group of the Lepidoptera. We propose that Lepidoptera originally had a Z/ZZ sex chromosome mechanism like Trichoptera; the WZ/ZZ sex chromosome mechanism evolved later in the ditrysian branch of Lepidoptera. Secondary losses of the W chromosome account for the sporadically occurring Z/ZZ sex chromosome systems in ditrysianfamilies. The lepidopteran sex chromatin, therefore, appears to mirror thefull evolutionary life cycle of a univalent sex chromosomefrom its birth through heterochromatinization to sporadic loss.

The Autonomy of Biology: The Position of Biology Among the Sciences

Abstract: Philosophers of science have claimed that the position of biology among the sciences is the most prominent and controversial issue of the philosophy of biology. Some authors consider biology merely a "province" of physics and reducible to physics, others uphold the autonomy of biology, while still others have decided that biology lacks the rigor to justify being considered a genuine science. In my own analysis of this problem. I have concluded that the science of biology has all the attributes of a genuine science, as well as a number of other characteristics restricted to biology. These characteristics are listed and discussed. They justify ranking biology as an autonomous science, just as autonomous as physics and many other sciences.

On the origin of elementary hexoses.

Abstract: A possible scenario on the origin of elementary hexoses (fructose, glucose, mannose, and galactose) is proposed: (1) Formol reaction, an autocatalytic polymerization reaction, occurred on the primitive earth under weakly alkaline conditions to generate various small molecular compounds. (2) Among them, glyceraldehyde and dihydroxyacetone undergo aldol condensation to produce stable ketohexoses, fructose and sorbose. (3) Subsequently, Lobry de Bruyn rearrangement converts fructose into stable aldohexoses, glucose, and mannose. (4) Thus, prebiotically synthesized ?first triplet? hexoses would have been available for utilization by ancestral primitive microorganisms. (5) After the development of biochemical pathways, various saccharides, including galactose, were biosynthesized from glucose and mannose as ?bricolage products?; the utilization of galactose as a key recognition molecule, based on its distinctive axial 4-OH and its outermost location in glycoconjugates, owing to its late arrival, may have evolved concomitantly with the evolution of multicellular organisms.

A History of Studies of Visual Accommodation in Birds

Abstract: Since 1813, when Crampton first described the ciliary muscle of the avian eye, there has been little agreement on how birds are able to change the focus of their eyes. Numerous later studies on the eyes of a variety of bird species contradicted earlier findings or proposed new accommodative mechanisms. The resulting confusion persists today, and a number of significant works on the avian eye perpetuate many of the myths developed during the 1800s. There is little consensus on avian accommodation; the early literature contains many accurate descriptions of the mechanisms, along with elegant experimental evidence to support them. Much of the early literature, however, is in German and has remained obscure. Further, among the mechanistic descriptions of avian accommodation are many that are incorrect. The current confusion can be attributed in part to the fact that some birds have both corneal and lenticular accommodation. It is unclear to what extent differential bird species employ both mechanisms, or depe...

Nuclear Waste: The Academy and Million-Year Estimates

Abstract: P REPARING TO BUILD the world's first permanent repository for high-level nuclear waste at Yucca Mountain, Nevada, the U.S. government has spent nearly $5 billion on site studies. In its 1992 Energy Policy Act, Congress directed the Environmental Protection Agency (EPA) to develop standards for the proposed Yucca Mountain Repository. Congress also asked the U.S. National Academy of Sciences to advise the agency on the technical bases for such standards. In a report published last August, the Academy's Board on Radioactive Waste Management gave its advice to the EPA. This advice is supposed to assist the government in achieving what no one has ever accomplished: to secure nuclear waste in perpetuity, so that it presents no threat to the biosphere. The Academy report, Technical Basesfor Yucca Mountain Standards (NRC 1995), is a landmark document that significantly advances our understandings of both the science and the policy that are relevant to radioactive waste disposal. It has many positive features, notably its recommendation that compliance with the risk standard for radioactive waste be measured at the time of peak risk, whenever it occurs (NRC 1995:2, 55-56, 67); its conclusion that there is no scientific basis for limiting safety concerns to merely 10,000 years (NRC 1995: 56); and its important stance in favor of intergenerational equity. The study also does an excellentjob of emphasizing the fact that it is not possible to assess the frequency of intrusion into a permanent repository for a million years into the future (NRC 1995:2, 73). It explains that there is no system (based on active institutional controls) for postclosure oversight of the repository that is able to prevent an unreasonable risk of breaching the engineered barriers (NRC 1995:11). The document is straightforward about many important uncertainties in its recommendations about radioactive waste disposal, site modeling, and performance assessment generally (NRC 1995:19-20). It also stresses that there is no sharp dividing line between science and policy (NRC 1995:viii); that there is a limited scientific basis for choosing one waste policy option over another (NRC 1995: viii); and that the committee ought not recommend what levels of risk are acceptable because this is a policy decision (NRC 1995:20, 49). Perhaps, most importantly, the Academy report wisely recommends choosing future Yucca Mountain exposure scenarios on the basis of rulemaking with full public participation (NRC 1995:99, 127). Such conclusions are both balanced and defensible. Despite its strengths, the Academy report has at least three controversial aspects that