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.     

Parasites delay worker reproduction in bumblebees: consequences for eusociality

Workers in eusocial insects usually tend the brood of the queenand so achieve representation in the next generation throughaiding relatives to reproduce. However, workers of some eusocialspecies, such as bumblebees, are capable of reproductive activityeven in the presence of the queen (in queen-right colonies),and worker reproduction is associated with aggressive behaviorsand egg cannibalism, both of which reduce colony efficiency.Thus, factors that affect worker ovariandevelopment, a preconditionfor reproduction, can influence social harmony and colony productivity.Parasites are a ubiquitous and important part of the bioticenvironment of all organisms. Here we show that parasites playan important role in the reproductive physiology of worker bumblebeesin queen-right colonies of Bombus terrestris, affecting thepattern and timing of ovarian development and oviposition. Workersfrom colonies parasitized with the intestinal trypanosome Crithidiabombi had less developed ovaries than workers of the same agefrom unparasitized colonies. In addition, parasitized colonieswere smaller than unparasitized colonies for about the firsthalf of colony development. This generated further demographiceffects such that workers were on average younger in parasitizedthan in unparasitized colonies around the time of the onsetof worker oviposition, and worker oviposition occurred significantlylater in parasitized colonies. Workers in parasitized coloniestherefore had lower individual reproductive potential and werecooperative for a larger proportion of the colony cycle thanthose in unparasitized colonies. In this system, where transmissionof the parasite between years probably occurs only in infested,young queens, this effect may represent an adaptation on thepart of the parasite to ensure its successful passage throughthe winter. Parasites, by reducing the cost of worker cooperation,may facilitate queen control over her worker force and playan important role in moderating the social organization of eusocialinsect colonies.     

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.     

Physiological consequences of an altered flow regime on Alabama bass (Micropterus henshalli)

There are numerous studies on the effects of dams on aquatic biota, yet relatively little is known about whether hydropeaking activities cause physiological change in fish. Using Alabama bass (Micropterus henshalli) as a model, we evaluated whether hydropeaking in a regulated river altered glucocorticoid stress responsiveness relative to fish from an unregulated tributary. Blood samples were collected at the time of capture (baseline) and then collected again after a 1‐hr period of confinement (response). Leukocyte profiles (blood smears) were created and plasma was extracted to assess plasma cortisol levels and neutrophils and lymphocyte (N:L) ratios, between sites and times to evaluate differences between sites and the two sampling periods. Baseline cortisol levels were higher in fish collected from the regulated river compared to those from unregulated site, but response levels of cortisol were similar between sites. Baseline and response level N:L ratios did not differ between sites. High baseline levels of cortisol suggested that fish exposed to regulated flows expressed an altered stress response and were likely in an allostatic state, i.e., attempting to acclimate. Further research is needed to understand how altered stress responses due to hydropeaking flows may be affecting fish.     

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 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.     

Demographic consequences of greater clonal than sexual reproduction in Dicentra canadensis

Clonality is a widespread life history trait in flowering plants that may be essential for population persistence, especially in environments where sexual reproduction is unpredictable. Frequent clonal reproduction, however, could hinder sexual reproduction by spatially aggregating ramets that compete with seedlings and reduce inter‐genet pollination. Nevertheless, the role of clonality in relation to variable sexual reproduction in population dynamics is often overlooked. We combined population matrix models and pollination experiments to compare the demographic contributions of clonal and sexual reproduction in three Dicentra canadensis populations, one in a well‐forested landscape and two in isolated forest remnants. We constructed stage‐based transition matrices from 3 years of census data to evaluate annual population growth rates, λ. We used loop analysis to evaluate the relative contribution of different reproductive pathways to λ. Despite strong temporal and spatial variation in seed set, populations generally showed stable growth rates. Although we detected some pollen limitation of seed set, manipulative pollination treatments did not affect population growth rates. Clonal reproduction contributed significantly more than sexual reproduction to population growth in the forest remnants. Only at the well‐forested site did sexual reproduction contribute as much as clonal reproduction to population growth. Flowering plants were more likely to transition to a smaller size class with reduced reproductive potential in the following year than similarly sized nonflowering plants, suggesting energy trade‐offs between sexual and clonal reproduction at the individual level. Seed production had negligible effects on growth and tuber production of individual plants. Our results demonstrate that clonal reproduction is vital for population persistence in a system where sexual reproduction is unpredictable. The bias toward clonality may be driven by low fitness returns for resource investment in sexual reproduction at the individual level. However, chronic failure in sexual reproduction may exacerbate the imbalance between sexual and clonal reproduction and eventually lead to irreversible loss of sex in the population.     

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.