The consequences of earlier reproduction in declining populations

Conflicting claims have been published concerning the consequences of earlier reproduction. In particular, it has been claimed that earlier reproduction will increase the rate of decline in a declining population, have no effect in a stable population, and enhance the rate of growth in an increasing population. However, if fecundity and mortality are conceptually separated so that earlier reproduction is defined as moving the fecundity column of the life table forward in time but not changing the mortality column, then earlier reproduction will nearly always cause the rate of change of a population to become more positive. This definition is believed to be more consistent with what biologists think of as earlier reproduction. Also, this concept requires that the cost of reproducing earlier be explicitly addressed rather than hiding it in assumptions implicit in a definition of earlier reproduction. This paper reconciles published mathematical demography models with published biological claims concerning earlier reproduction.     

The consequences for human reproduction of a robust inflammatory response

Innate and adaptive immune responsiveness is variable within the population. Since robust immune reactions are critical to the survival of humans, the existence of immune variability in the population suggests the existence of competing, alternative phenotypes. Although women with powerful immune responsiveness may be more likely to survive to reproduce, their reproductive experiences may be less successful than women who are not as responsive. Normal pregnancy elicits a maternal inflammatory reaction. This can be understood on the basis of maternal-fetal conflict theory: inflammation is a component of the maternal attempt to limit excessive fetal demands. However, an overly aggressive inflammatory reaction has been shown to relate to a variety of adverse reproductive outcomes. Reviewed here are several examples, including the fallopian tube damage that results from pelvic inflammatory disease, the upregulated inflammatory response among women who develop preeclampsia, an association between immune hyperresponsiveness and premature delivery, and the relationship between autoimmune diseases and multiple adverse pregnancy outcomes. The hypothesis that immune hyperresponsiveness limits reproductive capacity suggests many avenues for research.     

The biological consequences of altered MHC class I expression in tumours.

The identification of different mechanisms by which tumours escape from the immune system has helped to evaluate the clinical relevance of a variety of phenotypic changes that occur during tumour development. Among them, changes in HLA class I expression play a leading role in the tumour-host environment since HLA class I molecules interact with T lymphocytes for antigen presentation and with NK cells for inhibition/activation of these immune effector cells. Our laboratory has proposed a classification of the altered HLA class I phenotypes frequently found in human tumours, into five major groups. This review focuses on the tumour phenotypes found in primary and metastatic lesions, the molecular mechanisms that give rise to each phenotype and the clinical implications of these findings.     

Evolutionary consequences of altered atmospheric oxygen in Drosophila melanogaster

Twelve replicate populations of Drosophila melanogaster, all derived from a common ancestor, were independently evolved for 34+ generations in one of three treatment environments of varying PO(2): hypoxia (5.0-10.1 kPa), normoxia (21.3 kPa), and hyperoxia (40.5 kPa). Several traits related to whole animal performance and metabolism were assayed at various stages via "common garden" and reciprocal transplant assays to directly compare evolved and acclimatory differences among treatments. Results clearly demonstrate the evolution of a greater tolerance to acute hypoxia in the hypoxia-evolved populations, consistent with adaptation to this environment. Greater hypoxia tolerance was associated with an increase in citrate synthase activity in fly homogenate when compared to normoxic (control) populations, suggesting an increase in mitochondrial volume density in these populations. In contrast, no direct evidence of increased performance of the hyperoxia-evolved populations was detected, although a significant decrease in the tolerance of these populations to acute hypoxia suggests a cost to adaptation to hyperoxia. Hyperoxia-evolved populations had lower productivity overall (i.e., across treatment environments) and there was no evidence that hypoxia or hyperoxia-evolved populations had greatest productivity or longevity in their respective treatment environments, suggesting that these assays failed to capture the components of fitness relevant to adaptation.     

Elimination of altered karyotypes by sexual reproduction preserves species identity.

Resolving the persistence of sexual reproduction despite its overwhelming costs (known as the paradox of sex) is one of the most persistent challenges of evolutionary biology. In thinking about this paradox, the focus has traditionally been on the evolutionary benefits of genetic recombination in generating offspring diversity and purging deleterious mutations. The similarity of pattern between evolution of organisms and evolution among cancer cells suggests that the asexual process generates more diverse genomes owing to less controlled reproduction systems, while sexual reproduction generates more stable genomes because the sexual process can serve as a mechanism to "filter out" aberrations at the chromosome level. Our reinterpretation of data from the literature strongly supports this hypothesis. Thus, the principal consequence of sexual reproduction is the reduction of drastic genetic diversity at the genome or chromosome level, resulting in the preservation of species identity rather than the provision of evolutionary diversity for future environmental challenges. Genetic recombination does contribute to genetic diversity, but it does so secondarily and within the framework of the chromosomally defined genome.     

Maternal investment in reproduction and its consequences in leatherback turtles

Maternal investment in reproduction by oviparous non-avian reptiles is usually limited to pre-ovipositional allocations to the number and size of eggs and clutches, thus making these species good subjects for testing hypotheses of reproductive optimality models. Because leatherback turtles (Dermochelys coriacea) stand out among oviparous amniotes by having the highest clutch frequency and producing the largest mass of eggs per reproductive season, we quantified maternal investment of 146 female leatherbacks over four nesting seasons (2001–2004) and found high inter- and intra-female variation in several reproductive characteristics. Estimated clutch frequency [coefficient of variation (CV) = 31%] and clutch size (CV = 26%) varied more among females than did egg mass (CV = 9%) and hatchling mass (CV = 7%). Moreover, clutch size had an approximately threefold higher effect on clutch mass than did egg mass. These results generally support predictions of reproductive optimality models in which species that lay several, large clutches per reproductive season should exhibit low variation in egg size and instead maximize egg number (clutch frequency and/or size). The number of hatchlings emerging per nest was positively correlated with clutch size, but fraction of eggs in a clutch yielding hatchlings (emergence success) was not correlated with clutch size and varied highly among females. In addition, seasonal fecundity and seasonal hatchling production increased with the frequency and the size of clutches (in order of effect size). Our results demonstrate that female leatherbacks exhibit high phenotypic variation in reproductive traits, possibly in response to environmental variability and/or resulting from genotypic variability within the population. Furthermore, high seasonal and lifetime fecundity of leatherbacks probably reflect compensation for high and unpredictable mortality during early life history stages in this species.     

The consequences of rare sexual reproduction by means of selfing in an otherwise clonally reproducing species

Clonal reproduction of diploids leads to an increase in heterozygosity over time. A single round of selfing will then create new homozygotic genotypes. Given the same allele frequencies, heritable genetic variation is larger when there are more extreme, i.e. homozygotic genotypes. So after a long clonal expansion, one round of selfing increases heritable genetic variation, but any fully or partially recessive deleterious alleles simultaneously impose a fitness cost. Here we calculate that the cost of selfing in the yeast Saccharomyces is experienced only by a minority of zygotes. This allows a round of selfing to act as an evolutionary capacitor to unlock genetic variation previously found in a cryptic heterozygous form. We calculate the evolutionary consequences rather than the evolutionary causes of sex. We explore a range of parameter values describing sexual frequencies, focusing especially on the parameter values known for wild Saccharomyces. Our results are largely robust to many other parameter value choices, so long as meiosis is rare relative to the strength of selection on heterozygotes. Results may also be limited to organisms with a small number of genes. We therefore expect the same phenomenon in some other species with similar reproductive strategies.     

The circadian timing system and reproduction in mammals.

Circadian systems in a wide variety of organisms all appear to include three basic components: 1) biological oscillators that maintain a self-sustained circadian periodicity in the absence of environmental time cues; 2) input pathways that convey environmental information, especially light cues, that can entrain the circadian oscillations to local time; and 3) output pathways that drive overt circadian rhythms, such as the rhythms of locomotor activity and a variety of endocrine rhythms. In mammals, the circadian system is employed in the regulation of reproductive physiology and behavior in two very important ways. 1) In some species, there is a strong circadian component in the timing of ovulation and reproductive behavior, ensuring that these events will occur at a time when the animal is most likely to encounter a potential mate. 2) Many mammals exhibit seasonal reproductive rhythms that are largely under photoperiod regulation; in these species, the circadian system and the pineal gland are crucial components of the mechanism that is used to measure day length. The rhythm of pineal melatonin secretion is driven by a neural pathway that includes the circadian oscillator(s) in the suprachiasmatic nuclei. Melatonin is secreted at night in all mammals, and the duration of each nocturnal episode of melatonin secretion is inversely related to day length. The pineal melatonin rhythm appears to serve as an internal signal that represents day length and that is capable of regulating a variety of seasonal variations in physiology and behavior.     

The mirror neuron system and the consequences of its dysfunction

The discovery of premotor and parietal cells known as mirror neurons in the macaque brain that fire not only when the animal is in action, but also when it observes others carrying out the same actions provides a plausible neurophysiological mechanism for a variety of important social behaviours, from imitation to empathy. Recent data also show that dysfunction of the mirror neuron system in humans might be a core deficit in autism, a socially isolating condition. Here, we review the neurophysiology of the mirror neuron system and its role in social cognition and discuss the clinical implications of mirror neuron dysfunction.     

Seasonal regulation of reproduction: altered role of melatonin under naturalistic conditions in hamsters

The seasonal reproductive cycle of photoperiodic rodents is conceptualized as a series of discrete melatonin-dependent neuroendocrine transitions. Least understood is the springtime restoration of responsiveness to winter-like melatonin signals (breaking of refractoriness) that enables animals to once again respond appropriately to winter photoperiods the following year. This has been posited to require many weeks of long days based on studies employing static photoperiods instead of the annual pattern of continually changing photoperiods under which these mechanisms evolved. Maintaining Siberian hamsters under simulated natural photoperiods, we demonstrate that winter refractoriness is broken within six weeks after the spring equinox. We then test whether a history of natural photoperiod exposure can eliminate the requirement for long-day melatonin signalling. Hamsters pinealectomized at the spring equinox and challenged 10 weeks later with winter melatonin infusions exhibited gonadal regression, indicating that refractoriness was broken. A photostimulatory effect on body weight is first observed in the last four weeks of winter. Thus, the seasonal transition to the summer photosensitive phenotype is triggered prior to the equinox without exposure to long days and is thereafter melatonin-independent. Distinctions between photoperiodic and circannual seasonal organization erode with the incorporation in the laboratory of ecologically relevant day length conditions.     

Quelques consequences physiologiques de mutations affectant la reproduction du Penicillium baarnense van Beyma

Résumé La formation des deux appareils de reproduction, sexuel et conidien, duPenicillium baarnense dépend de gènes détectés par mutation. L'expression de ces gènes peut être affectée par'l environnement dans lequel le Champignon se développe. En particulier, il est possible de faire apparaître des structures conidiennes chez des mutants génétiquement aconidiens en modifiant la composition du milieu de culture. L'étude physiologique des mutants induits et de la souche sauvage montre que les déficiences géniques se traduisent par des diminutions du taux d'ATP mycélien ainsi que par des modifications de l'activité des transaminases et de la glutamate-deshydrogénase liée au NAD.

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The emergence of penicilli and cleistothecia inPenicillium baarnense is governed by several genes which have been detected by mutagenesis. The morphological aspect of this fungus, as it results of the function of these genes, can be transformed by acting on physicochemical environment. Particularly, condidiogenesis occurs in genetically non-conidial strains, when the medium is modified. With conidial strains, it can be suppressed when some chemicals are added to the culture medium. Compared physiological studies of mutant and wild-type strains show that mutations affect the level of cytoplasmic ATP and the specific activities of transaminases and glutamic-acid dehydrogenase (NAD dependant).

     

Postnatal developmental consequences of altered insulin sensitivity in female sheep treated prenatally with testosterone

Prenatally testosterone (T)-treated female sheep exhibit ovarian and endocrinological features that resemble those of women with polycystic ovarian syndrome (PCOS), which include luteinizing hormone excess, polyfollicular ovaries, functional hyperandrogenism, and anovulation. In this study, we determined the developmental impact of prenatal T treatment on insulin sensitivity indexes (ISI), a variable that is affected in a majority of PCOS women. Pregnant ewes were treated with 60 mg testosterone propionate intramuscularly in cottonseed oil two times a week or vehicle [control (C)] from days 30 to 90 of gestation. T-females weighed less than C-females or males (P < 0.05) at birth and at 5 wk of age. T-females had an increased anogenital ratio. An intravenous glucose tolerance test followed by an insulin tolerance test conducted after an overnight fast at 5, 20, and 30 wk of age (n = 7-8/group) revealed that ISI were higher at 5 than 30 wk of age in C-females, reflective of a developing insulin resistance associated with puberty. T-females had higher basal insulin levels, higher fasting insulin-to-glucose ratio, and higher incremental area under the insulin curve to the glucose challenge. The ISI of T-females was similar to that of males. No differences in ISI were evident between groups at 20 and 30 wk of age. Mean basal plasma glucose concentrations and glucose disappearance and uptake did not differ between groups at any age. Our findings suggest that prenatal T treatment leads to offspring with reduced birth weight and impaired insulin sensitivity in early postnatal life.