Phenotypic and Developmental Dissection of an Instance of the Island Rule

Organismal body weight correlates with morphology, life history, physiology, and behavior, making it perhaps the most telling single indicator of an organism's evolutionary and ecological profile. Island populations provide an exceptional opportunity to study body weight evolution. In accord with the "island rule," insular small-bodied vertebrates often evolve larger sizes, whereas insular large-bodied vertebrates evolve smaller sizes. To understand how island populations evolve extreme sizes, we adopted a developmental perspective and compared a suite of traits with established connections to body size in the world's largest wild house mice from Gough Island and mice from a smaller-bodied mainland strain. We pinpoint 24-hour periods during the third and fifth week of age in which Gough mice gain exceptionally more weight than mainland mice. We show that Gough mice accumulate more visceral fat beginning early in postnatal development. During a burst of weight gain, Gough mice shift toward carbohydrates and away from fat as fuel, despite being more active than and consuming equivalent amounts of food as mainland mice. Our findings showcase the value of developmental phenotypic characterization for discovering how body weight evolves in the context of broader patterns of trait evolution.

Population Genomics of Giant Mice from the Faroe Islands: Hybridization, Colonization, and a Novel Challenge to Identifying Genomic Targets of Selection

Populations that colonize islands provide unique insights into demography, adaptation, and the spread of invasive species. House mice on the Faroe Islands evolved exceptionally large bodies after colonization, generating interest from biologists since Darwin. To reconstruct the evolutionary history of these mice, we sequenced genomes of population samples from three Faroe Islands (Sandoy, Nólsoy, and Mykines) and Norway as a mainland comparison. Mice from the Faroe Islands are hybrids between the subspecies Mus musculus domesticus and M. m. musculus, with ancestry alternating along the genome. Analyses based on the site frequency spectrum of single nucleotide polymorphisms and the ancestral recombination graph (ARG) indicate that mice arrived on the Faroe Islands on a timescale consistent with transport by Norwegian Vikings, with colonization of Sandoy likely preceding colonization of Nólsoy. Substantial reductions in nucleotide diversity and effective population size associated with colonization suggest that mice on the Faroe Islands evolved large body size during periods of heightened genetic drift. Genomic scans for positive selection uncover windows with unusual site frequency spectra, but this pattern is mostly generated by clusters of singletons in individual mice. Variants showing evidence of selection in both Nólsoy and Sandoy based on the ARG are enriched for genes with neurological functions. Our findings reveal a dynamic evolutionary history for the enigmatic mice from Faroe Island and emphasize the challenges that accompany population genomic inferences in island populations.

Size matters: micro-evolution in Polynesian rats highlights body size changes as initial stage in evolution

Microevolutionary patterns in populations of introduced rodent species have often been the focus of analytic studies for their potential relevance to understanding vertebrate evolution. The Polynesian rat (Rattus exulans) is an excellent proxy species because of its wide geographic and temporal distribution: its native and introduced combined range spans half the globe and it has been living for at least seven centuries wherever it was introduced. The objective of this study was to assess the effects of long-term isolation (insularity; up to 4,000 years) and geographic variables on skull shape variation using geometric morphometrics. A sample of 513 specimens from 103 islands and four mainland areas was analysed. This study, to my knowledge the first to extensively sample introduced rats, analysed 59 two-dimensional landmarks on the skull. Landmarks were obtained in three separate aspects (dorsal, lateral, ventral skull view). The coordinate data were then subjected to a multivariate ordination analysis (principal components analysis, or PCA), multivariate regressions, and a canonical variates analysis (CVA). Three measures of disparity were evaluated for each view. The results show that introduced Polynesian rats evolve skull shapes that conform to the general mammalian interspecific pattern of cranial evolutionary allometry (CREA), with proportionally longer snouts in larger specimens. In addition, larger skulls are more tubular in shape than the smaller skulls, which are more balloon-shaped with a rounder and wider braincase relative to those of large skulls. This difference is also observed between the sexes (sexual dimorphism), due to the slightly larger average male size. Large, tubular skulls with long snouts are typical for Polynesia and Remote Oceania, where no native mammals occur. The greater disparity of Polynesian rats on mammal species-poor islands ('exulans-only' region) provides further insight into how diversity may affect diversification through ecological release from predators and competitors.

Of mice and the 'Age of Discovery': the complex history of colonization of the Azorean archipelago by the house mouse (Mus musculus) as revealed by mitochondrial DNA variation

Humans have introduced many species onto remote oceanic islands. The house mouse (Mus musculus) is a human commensal and has consequently been transported to oceanic islands around the globe as an accidental stowaway. The history of these introductions can tell us not only about the mice themselves but also about the people that transported them. Following a phylogeographic approach, we used mitochondrial D-loop sequence variation (within an 849- to 864-bp fragment) to study house mouse colonization of the Azores. A total of 239 sequences were obtained from all nine islands, and interpretation was helped by previously published Iberian sequences and 66 newly generated Spanish sequences. A Bayesian analysis revealed presence in the Azores of most of the D-loop clades previously described in the domesticus subspecies of the house mouse, suggesting a complex colonization history of the archipelago as a whole from multiple geographical origins, but much less heterogeneity (often single colonization?) within islands. The expected historical link with mainland Portugal was reflected in the pattern of D-loop variation of some of the islands but not all. A more unexpected association with a distant North European source area was also detected in three islands, possibly reflecting human contact with the Azores prior to the 15th century discovery by Portuguese mariners. Widening the scope to colonization of the Macaronesian islands as a whole, human linkages between the Azores, Madeira, the Canaries, Portugal and Spain were revealed through the sharing of mouse sequences between these areas. From these and other data, we suggest mouse studies may help resolve historical uncertainties relating to the 'Age of Discovery'.

Invasive house mice facing a changing environment on the Sub-Antarctic Guillou Island (Kerguelen Archipelago)

Adaptation to new environments is a key feature in evolution promoting divergence in morphological structures under selection. The house mouse (Mus musculus domesticus) introduced on the Sub-Antarctic Guillou Island (Kerguelen Archipelago) had and still has to face environmental conditions that likely shaped the pattern and pace of its insular evolution. Since mouse arrival on the island, probably not more than two centuries ago, ecological conditions dramatically differed from those available to their Western European commensal source populations. In addition, over the last two decades, the plant and animal communities of Guillou Island were considerably modified by the eradication of rabbits, the effects of climate change and the spread of invasive species detrimental to native communities. Under such a changing habitat, the mouse response was investigated using a morphometric quantification of mandible and molar tooth, two morphological structures related to food processing. A marked differentiation of the insular mice compared with their relatives from Western Europe was documented for both mandibles and molar shapes. Moreover, these shapes changed through the 16 years of the record, in agreement with expectations of drift for the molar, but more than expected by chance for the mandible. These results suggest that mice responded to the recent changes in food resources, possibly with a part of plastic variation for the mandible prone to bone remodelling. This pattern exemplifies the intricate interplay of evolution, ecology and plasticity that is a probable key of the success of such an invasive rodent facing pronounced shifts in food resources exploitation under a changing environment.

House mouse colonization patterns on the sub-Antarctic Kerguelen Archipelago suggest singular primary invasions and resilience against re-invasion

BACKGROUND

Starting from Western Europe, the house mouse (Mus musculus domesticus) has spread across the globe in historic times. However, most oceanic islands were colonized by mice only within the past 300 years. This makes them an excellent model for studying the evolutionary processes during early stages of new colonization. We have focused here on the Kerguelen Archipelago, located within the sub-Antarctic area and compare the patterns with samples from other Southern Ocean islands.

RESULTS

We have typed 18 autosomal and six Y-chromosomal microsatellite loci and obtained mitochondrial D-loop sequences for a total of 534 samples, mainly from the Kerguelen Archipelago, but also from the Falkland Islands, Marion Island, Amsterdam Island, Antipodes Island, Macquarie Island, Auckland Islands and one sample from South Georgia. We find that most of the mice on the Kerguelen Archipelago have the same mitochondrial haplotype and all share the same major Y-chromosomal haplotype. Two small islands (Cochons Island and Cimetière Island) within the archipelago show a different mitochondrial haplotype, are genetically distinct for autosomal loci, but share the major Y-chromosomal haplotype. In the mitochondrial D-loop sequences, we find several single step mutational derivatives of one of the major mitochondrial haplotypes, suggesting an unusually high mutation rate, or the occurrence of selective sweeps in mitochondria.

CONCLUSIONS

Although there was heavy ship traffic for over a hundred years to the Kerguelen Archipelago, it appears that the mice that have arrived first have colonized the main island (Grande Terre) and most of the associated small islands. The second invasion that we see in our data has occurred on islands that are detached from Grande Terre and were likely to have had no resident mice prior to their arrival. The genetic data suggest that the mice of both primary invasions originated from related source populations. Our data suggest that an area colonized by mice is refractory to further introgression, possibly due to fast adaptations of the resident mice to local conditions.

Evolutionary and functional properties of a two-locus beta-globin polymorphism in Indian house mice

Electrophoretic surveys of hemoglobin (Hb) polymorphism in house mice from South Asia and the Middle East have revealed that two alternative beta-globin haplotypes, Hbb(d) and Hbb(p), are often present at intermediate frequencies in geographically disparate populations. Both haplotypes harbor two functionally distinct beta-globin paralogs, HBB-T1 (which encodes the beta-chain subunits of the major Hb isoform) and HBB-T2 (which encodes the beta-chains of the minor Hb isoform). The Hbb(d) and Hbb(p) haplotypes share identical HBB-T1 alleles, but products of the alternative HBB-T2 alleles (d(minor) and p(minor)) are distinguished by two amino acid substitutions. To investigate the possible adaptive significance of the Hbb(d)/Hbb(p) polymorphism we conducted a population genetic analysis of the duplicated beta-globin genes of Indian house mice (Mus castaneus) in conjunction with experimental studies of Hb function in inbred strains of mice that carry the alternative Hbb(d) and Hbb(p) haplotypes. The main objectives of this study were (i) to characterize patterns of nucleotide polymorphism and linkage disequilibrium in the duplicated beta-globin genes of M. castaneus, (ii) to test the hypothesis that the Hbb(d) and Hbb(p) haplotypes are maintained as a balanced polymorphism, and (iii) to assess whether allelic differences in the alternative minor Hb isoforms (d(minor) and p(minor)) are associated with different O(2)-binding properties. A multilocus analysis of polymorphism and divergence revealed that levels of diversity at the HBB-T2 gene exceeded neutral expectations, and reconstructed haplotype networks for both beta-globin paralogs revealed extensive allele sharing with several other closely related species of Mus. However, despite this suggestive evidence for balancing selection, O(2)-equilibrium curves revealed no discernible functional differences between red cell lysates containing the d(minor) and p(minor) Hb isoforms. If the d(minor) and p(minor) alleles are maintained as a balanced polymorphism, our results indicate that the associated fitness variance is not directly related to respiratory functions of Hb.

Genetics and origin of a Drosophila melanogaster population recently introduced to the Seychelles

During an extensive survey of drosophilid fauna in 1977, D. melanogaster was not collected in the Seychelles. However, a population was found in 1981 in Victoria city, suggesting a recent introduction of this species. With respect to allozyme frequencies or ethanol tolerance, this population is almost identical to European ones and very different from those living under a similar equatorial climate on the African continent. The frequencies of rare biochemical alleles perhaps suggested that this population was founded by a small number of flies, less than ten inseminated females. For various biometrical traits, the situation was not so clear: according to the trait considered, Seychellian flies are either intermediate between European and African populations or closer to the latter. These data suggest that a few flies, recently introduced from a temperate (European?) country, built up a big population which is now on the way to adapting itself to new tropical conditions. Such an involuntary experiment should afford a unique opportunity to distinguish the respective roles of drift and adaptation in the evolution of D. melanogaster geographic races.

Complex signatures of selection and gene conversion in the duplicated globin genes of house mice

Results of electrophoretic surveys have suggested that hemoglobin polymorphism may be maintained by balancing selection in natural populations of house mice, Mus musculus. Here we report a survey of nucleotide variation in the adult globin genes of house mice from South America. We surveyed nucleotide polymorphism in two closely linked alpha-globin paralogs and two closely linked beta-globin paralogs to test whether patterns of variation are consistent with a model of long-term balancing selection. Surprisingly high levels of nucleotide polymorphism at the two beta-globin paralogs were attributable to the segregation of two highly divergent haplotypes, Hbbs (which carries two identical beta-globin paralogs) and Hbbd (which carries two functionally divergent beta-globin paralogs). Interparalog gene conversion on the Hbbs haplotype has produced a highly unusual situation in which the two paralogs are more similar to one another than either one is to its allelic counterpart on the Hbbd haplotype. Levels of nucleotide polymorphism and linkage disequilibrium at the two beta-globin paralogs suggest a complex history of diversity-enhancing selection that may be responsible for long-term maintenance of alternative protein alleles. The alternative two-locus beta-globin haplotypes are associated with pronounced differences in intraerythrocyte glutathione and nitric oxide metabolism, suggesting a possible mechanism for selection on hemoglobin function.

Mouse (Mus musculus) stocks derived from tropical islands: new models for genetic analysis of life-history traits

Founder effects, together with access to unoccupied ecological niches, may allow rodent populations on isolated islands to evolve constellations of life‐history traits that distinguish them from their mainland relatives, for example in body size, litter size, and longevity. In particular, low intrinsic mortality risks on islands with reduced predator numbers and not subject to harsh winter climates may in principle support the development of stocks with extended longevity. Conversely, the conditions under which laboratory rodents are typically bred are thought to select for genotypes that produce large, rapidly maturing races with high early reproductive rates but diminished longevity. To test these ideas, and to generate new mouse stocks suitable for genetic and molecular analysis of the processes that time life‐history events, we have developed specific pathogen‐free stocks from mice trapped from three distinct populations: the U.S. mainland (Idaho) and the tropical Pacific islands Majuro and Pohnpei. Mice from all three locations were found to be shorter and lighter, to have smaller litters, and to have higher faecal corticosterone levels than mice of a genetically heterogeneous stock derived from four common laboratory inbred strains. Among the wild‐derived stocks, mice from Pohnpei and Majuro were significantly lighter and shorter than Idaho‐derived animals, even in populations kept from birth under identical housing conditions. Litter size and reproductive success rates did not differ significantly among the three wild‐derived stocks. Although further work will be needed to see if, as predicted, the wild‐derived stocks differ from one another and from laboratory mice in longevity, these stocks provide useful tools for genetic dissection of factors that regulate body size and reproductive success.

Energy balance and reproduction

The physiological mechanisms that control energy balance are reciprocally linked to those that control reproduction, and together, these mechanisms optimize reproductive success under fluctuating metabolic conditions. Thus, it is difficult to understand the physiology of energy balance without understanding its link to reproductive success. The metabolic sensory stimuli, hormonal mediators and modulators, and central neuropeptides that control reproduction also influence energy balance. In general, those that increase ingestive behavior inhibit reproductive processes, with a few exceptions. Reproductive processes, including the hypothalamic-pituitary-gonadal (HPG) system and the mechanisms that control sex behavior are most proximally sensitive to the availability of oxidizable metabolic fuels. The role of hormones, such as insulin and leptin, are not understood, but there are two possible ways they might control food intake and reproduction. They either mediate the effects of energy metabolism on reproduction or they modulate the availability of metabolic fuels in the brain or periphery. This review examines the neural pathways from fuel detectors to the central effector system emphasizing the following points: first, metabolic stimuli can directly influence the effector systems independently from the hormones that bind to these central effector systems. For example, in some cases, excess energy storage in adipose tissue causes deficits in the pool of oxidizable fuels available for the reproductive system. Thus, in such cases, reproduction is inhibited despite a high body fat content and high plasma concentrations of hormones that are thought to stimulate reproductive processes. The deficit in fuels creates a primary sensory stimulus that is inhibitory to the reproductive system, despite high concentrations of hormones, such as insulin and leptin. Second, hormones might influence the central effector systems [including gonadotropin-releasing hormone (GnRH) secretion and sex behavior] indirectly by modulating the metabolic stimulus. Third, the critical neural circuitry involves extrahypothalamic sites, such as the caudal brain stem, and projections from the brain stem to the forebrain. Catecholamines, neuropeptide Y (NPY) and corticotropin-releasing hormone (CRH) are probably involved. Fourth, the metabolic stimuli and chemical messengers affect the motivation to engage in ingestive and sex behaviors instead of, or in addition to, affecting the ability to perform these behaviors. Finally, it is important to study these metabolic events and chemical messengers in a wider variety of species under natural or seminatural circumstances.

Leptin and metabolic control of reproduction

Leptin treatment prevents the effects of fasting on reproductive processes in a variety of species. The mechanisms that underlie these effects have not been elucidated. Progress in this area of research might be facilitated by viewing reproductive processes in relation to mechanisms that maintain fuel homeostasis. Reproduction, food intake, and fuel partitioning can be viewed as homeostatic responses controlled by a sensory system that monitors metabolic signals. These signals are generated by changes in intracellular metabolic fuel availability and oxidation rather than by changes in the amount of body fat or by changes in any aspect of body composition. Leptin might be viewed as either a mediator or as a modulator of the intracellular metabolic signal. Consistent with its purported action as a mediator of the metabolic signal, leptin synthesis and secretion are influenced acutely by changes in metabolic fuel availability, and these changes might lead to changes in reproductive function. The effects of leptin treatment on reproduction are blocked by treatments that inhibit intracellular fuel oxidation. Metabolic signals that inhibit reproduction in leptin-treated animals might act via neural pathways that are independent of leptin's action. Alternatively, both leptin and metabolic inhibitors might interact at the level of intracellular fuel oxidation. In keeping with the possibility that leptin modulates the metabolic signal, leptin treatment increases fuel availability, uptake, and oxidation in particular tissues. Leptin might affect reproduction indirectly by altering fuel oxidation or other peripheral processes such as gastric emptying. Reproductive processes are among the most energetically expensive in the female repertoire. Because leptin increases energy expenditure while simultaneously inhibiting energy intake, it may have limited use as a long-term treatment for infertility.

The uses of intraspecific variation in aging research

The artificial creation of genetically long-lived populations of several invertebrate species has illustrated how researchers may take advantage of genetic variation within a species to investigate the nature and mechanisms of aging. The advantage of studying intraspecific variation is that populations will be generally similar except for the relevant demographic differences. Also, there are reasons to suspect that genetic mechanisms of aging may differ from mechanisms associated with life extension via environmental manipulations such as caloric restriction. However creating a long-lived mammalian aging model will be expensive and time consuming. An alternative approach is to seek to identify naturally occurring slowly aging populations to contrast mechanistically with a reference population. Ecologists have already noted that demographic alterations of the appropriate type are frequently associated with populations from differing latitudes, differing altitudes, or from islands. Therefore, it is likely that genetically longer- (and shorter)-lived mammal populations of the same species already exist in nature, and could potentially be exploited to inquire into the genetics and mechanisms of aging and longevity. Of particular interest is the indication that some island populations of house mice may exhibit extended longevity compared with laboratory strains.

Metabolic fuels and reproduction in female mammals

A complete reproductive cycle of ovulation, conception, pregnancy, and lactation is one of the most energetically expensive activities that a female mammal can undertake. A reproductive attempt at a time when calories are not sufficiently available can result in a reduced return on the maternal energetic investment or even in the death of the mother and her offspring. Numerous physiological and behavioral mechanisms link reproduction and energy metabolism. Reproductive attempts may be interrupted or deferred when food is scarce or when other physiological processes, such as thermoregulation or fattening, make extraordinary energetic demands. Food deprivation suppresses both ovulation and estrous behavior. The neural mechanisms controlling pulsatile release of gonadotropin-releasing hormone (GnRH) and, consequently, luteinizing hormone secretion and ovarian function appear to respond to minute-to-minute changes in the availability of metabolic fuels. It is not clear whether GnRH-secreting neurons are able to detect the availability of metabolic fuels directly or whether this information is relayed from detectors elsewhere in the brain. Although pregnancy is less affected by fuel availability, both lactational performance and maternal behaviors are highly responsive to the energy supply. When a reproductive attempt is made, changes in hormone secretion have dramatic effects on the partitioning and utilization of metabolic fuels. During ovulatory cycles and pregnancy, the ovarian steroids, estradiol and progesterone, induce coordinated changes in the procurement, ingestion, metabolism, storage, and expenditure of metabolic fuels. Estradiol can act in the brain to alter regulatory behaviors, such as food intake and voluntary exercise, as well as adenohypophyseal and autonomic outputs. At the same time, ovarian hormones act on peripheral tissues such as adipose tissue, muscle, and liver to influence the metabolism, partitioning and storage of metabolic fuels. During lactation, the peptide hormones, prolactin and growth hormone, rather than estradiol and progesterone, are the principal hormones controlling partitioning and utilization of metabolic fuels. The interactions between metabolic fuels and reproduction are reciprocal, redundant, and ubiquitous; both behaviors and physiological processes play vital roles. Although there are species differences in the particular physiological and behavioral mechanisms mediating nutrition-reproduction interactions, two findings are consistent across species: 1) Reproductive physiology and behaviors are sensitive to the availability of oxidizable metabolic fuels. 2) When reproductive attempts are made, ovarian hormones play a major role in the changes in ingestion, partitioning, and utilization of metabolic fuels.

Wild mice in the cold: some findings on adaptation

The house mouse, Mus domesticus, can thrive in natural environments much below its optimum temperature. Thermogenesis is then above that at more usual temperatures. In addition, body weight, and the weights of brown adipose tissue and the kidneys, may be higher than usual. In free populations of house mice cold lowers fertility and may prevent breeding. Other possible limiting factors on breeding are food supply, shelter for nesting and social interactions. In captivity, wild-type house mice exposed to severe cold (around 0 degrees C) at first adapt ontogenetically by shivering and reduced activity. But raised thermogenesis is soon achieved without shivering; nest-building improves; and readiness to explore may be enhanced. Endocrine changes probably include, at least initially, a rise in adrenal cortical activity and in catecholamine secretion. Some females become barren, but many remain fertile. The maturity of fertile females is, however, delayed and intervals between births are lengthened; nestling mortality rises. A limiting factor during lactation may be the capacity of the gut. Similar adaptive changes are observed during winter in some species of small mammals that do not hibernate. But neither the house mouse nor other species present a single, universal pattern of cold-adaptation. Wild-type mice bred for about 10 generations in a warm laboratory environment (20-23 degrees C) change little over generations. In cold they become progressively heavier and fatter at all ages; they mature earlier, and nestling mortality declines. The milk of such 'Eskimo' females is more concentrated than that of controls. If 'Eskimo' mice are returned to a warm environment, they are more fertile, and rear heavier young, than controls that remained in the warm. Despite the heavier young, litter size is not reduced: it may be increased, probably as a result of a higher ovulation rate. Parental effects have been analyzed by cross-fostering and hybridizing. Survival, growth and fertility are all favourably influenced by the intra-uterine and nest environments provided by 'Eskimo' females. 'Eskimo' males are also better fathers. Hence after ten generations the phenotype of cold-adapted house mice shows the combined effects of (a) an ontogenetic response to cold, (b) a superior parental environment and (c) a change genotype. The secular changes in the cold that lead to this phenotype give the appearance of evolution in miniature; but it is equally possible that they represent a genetical versatility that allows rapid, reversible shifts in response to environmental demands.(ABSTRACT TRUNCATED AT 400 WORDS)

Restricted water intake influences male reproduction in two strains of house mice (Mus musculus)

Males from two strains of house mice (Mus musculus) were subjected to ten weeks of simulated drought in the laboratory. Water availability was reduced to 50% of ad lib intake for 5 weeks, then further restricted to 25% of ad lib consumption for an additional 5 weeks. Individuals of the highly inbred (CF1) strain were generally unaffected by water restriction. Testicular and epididymal mass of restricted CF1 animals did not change relative to control mice with ad lib access to water. Seminal vesicle mass decrease in water restricted CF1 males, but spermatogenesis was not significantly influenced. Body mass was reduced 25.4% after water restriction. In contrast, male F4 progeny of wild-caught Mus substantially reduced reproductive organ mass after limited water intake. Spermatogenesis was significantly diminished, but no animals became completely aspermatic. Body mass declined 12.6% in water restricted wild-strain Mus as compared to animals with ad lib water availability. These results are discussed in terms of their possible ecological significance.

Genetic association between nest building and brown adipose tissue thermogenesis in female house mice

Mice selectively bred for either high or low levels of thermoregulatory nest building were cold-acclimated (5 degrees C) for 3 weeks without nesting material; then body weight and food intake were measured. The mice selected for low nest building (Lows) of both sexes showed lower feed efficiencies than the high nest-building mice (Highs), although their body weights were not significantly different (Table 1). This adds to a large body of evidence which suggests that nest building and feed efficiency were influenced by a common mechanism (Lacy et al. 1978; Sulzbach and Lynch 1984; Lynch et al. 1981; Lynch and Roberts 1984). Brown adipose tissue mitochondrial GDP binding and cytochrome c oxidase activity were measured in the above mice. In females, the Lows had 100% higher levels of total GDP binding than the Highs, while no difference between the lines was seen in males. Thus in the High females, lower energy expenditure through brown fat thermogenesis may account for their greater feed efficiency. In males, the genetic differences in feed efficiency must be due to differences in either thermogenesis in tissues other than brown fat, or mechanisms which reduce heat loss.

Physiological variation of mouse haemoglobins

Polymorphism at Hbb (haemoglobin beta-chain) is widespread in natural populations of the house mouse, Mus musculus, and appears to be maintained by natural selection. This report is an attempt to correlate genotypic fluctuations at Hbb with a most important physiological attribute of haemoglobin, its oxygen carrying capacity. Oxygen affinity has been studied and P50 values have been measured in 12 inbred strains as well as wild-caught mice from Skokholm island. The mean P50 of each inbred strain is a constant characteristic, although there is high within-strain variation and the oxygen affinity of the blood of an individual can fluctuate considerably from week to week. The causes of this variation remain obscure but neither within-strain nor between-strain differences are correlated with known modulators of oxygen binding. In general, the blood of mice of inbred strains as well as wild-caught mice that are homozygous for Hbbd tends to have a higher oxygen affinity than that from comparable animals homozygous for Hbbs, but it seems likely that the oxygen dissociation properties of haemoglobin are not the only ones important in determining differential survival of a particular Hbb type under varying environmental stress.

Nonspecific esterases of Mus musculus

Seventeen genes controlling the expression of carboxylic ester hydrolases, commonly known as esterases, have been identified in the mouse Mus musculus. Seven esterase loci are found on chromosome 8, where two clusters of esterase loci occur. It seems probable that the genes within these clusters have arisen from a common ancestral gene by tandem duplication. Close linkage of esterase genes is also found in the rat, rabbit, and prairie vole. Some mouse esterases appear to be homologous with certain human esterases. The function of these nonspecific enzymes is still unknown.