Showing papers in "Proceedings of The Royal Society B: Biological Sciences in 1996"

Evaluating Loci for Use in the Genetic Analysis of Population Structure

Abstract: Loci that show unusually low or high levels of genetic differentiation are often assumed to be subject to natural selection. We propose a method for the identification of loci showing such disparities. The differentiation can be quantified using the statistic F ST . For a range of population structures and demographic histories, the distribution of F ST is strongly related to the heterozygosity at a locus. Outlying values of F ST can be identified in a plot of F ST vs . heterozygosity using a null distribution generated by a simple genetic model. We use published data-sets to illustrate the importance of the relationship with heterozygosity. We investigate a number of models of population structure, and demonstrate that the null distribution is robust to a wide range of conditions. In particular, the distribution is robust to differing mutation rates, and therefore different molecular markers, such as allozymes, restriction fragment length polymorphisms (RFLPS) and single strand conformation polymorphisms (SSCPS) can be compared together. We suggest that genetic variation at a discrepant locus, Identified under these conditions, is likely to have been influenced by natural selection, either acting on the locus itself or at a closely linked locus.

The lek paradox and the capture of genetic variance by condition dependent traits

Abstract: Recent evidence suggests that sexually selected traits have unexpectedly high genetic variance. In this paper, we offer a simple and general mechanism to explain this observation. Our explanation offers a resolution to the lek paradox and rests on only two assumptions; condition dependence of sexually selected traits and high genetic variance in condition. The former assumption is well supported by empirical evidence. We discuss the evidence for the latter assumption. These two assumptions lead inevitably to the capture of genetic variance into sexually selected traits concomitantly with the evolution of condition dependence. We present a simple genetic model to illustrate this view. We then explore some implications of genic capture for the coevolution of female preference and male traits. Our exposition of this problem incidentally leads to new insights into the similarities between sexually selected traits and life history traits, and therefore into the maintenance of high genetic variance in the latter. Finally, we discuss some shortcomings of a recently proposed alternative solution to the lek paradox; selection on variance.

Synaesthesia in Phantom Limbs Induced with Mirrors

Abstract: Although there is a vast clinical literature on phantom limbs, there have been no experimental studies on the effects of visual input on phantom sensations. We introduce an inexpensive new device--a 'virtual reality box'--to resurrect the phantom visually to study inter-sensory effects. A mirror is placed vertically on the table so that the mirror reflection of the patient's intact had is 'superimposed' on the felt position of the phantom. We used this procedure on ten patients and found the following results. 1. In six patients, when the normal hand was moved, so that the phantom was perceived to move in the mirror, it was also felt to move; i.e. kinesthetic sensations emerged in the phantom. In D.S. this effect occurred even though he had never experienced any movements in the phantom for ten years before we tested him. He found the return of sensations very enjoyable. 2. Repeated practice led to a permanent 'disappearance' of the phantom arm in patient D.S. and the hand became telescoped into the stump near the shoulder. 3. Using an optical trick, impossible postures--e.g. extreme hyperextension of the fingers--could be induced visually in the phantom. In one case this was felt as a transient 'painful tug' in the phantom. 4. Five patients experienced involuntary painful 'clenching spasms' in the phantom hand and in four of them the spasms were relieved when the mirror was used to facilitate 'opening' of the phantom hand; opening was not possible without the mirror. 5. In three patients, touching the normal hand evoked precisely localized touch sensations in the phantom. Interestingly, the referral was especially pronounced when the patients actually 'saw' their phantom being touched in the mirror. Indeed, in a fourth patient (R.L.) the referral occurred only if he saw his phantom being touched: a curious form of synaesthesia. These experiments lend themselves readily to imaging studies using PET and fMRI. Taken collectively, they suggest that there is a considerable amount of latent plasticity even in the adult human brain. For example, precisely organized new pathways, bridging the two cerebral hemispheres, can emerge in less than three weeks. Furthermore, there must be a great deal of back and forth interaction between vision and touch, so that the strictly modular, hierarchical model of the brain that is currently in vogue needs to be replaced with a more dynamic, interactive model, in which 're-entrant' signalling plays the main role.

The evolution of polyandry I : intragenomic conflict and genetic incompatibility

Abstract: Why do females across a wide range of taxa mate with more than one male? We suggest that a better understanding of polyandry may be gained by considering the implications of intragenomic conflict for female reproductive success. Here, we revisit the literature on cellular endosymbionts, transposable elements, segregation distorters, maternal-effect lethals and genomically imprinted genes to show that each of these selfish genetic elements can modify maternal and paternal haplotypes in ways that render them incompatible within the developing embryo. We propose that the cumulative threat to female reproductive success of genetic incompatibility arising from intragenomic conflict may be an important force driving the evolution of polyandry. By mating with more than one male, females can potentially exploit post-copulatory mechanisms for minimizing the risk and/or cost of fertilization by genetically incompatible sperm. This hypothesis differs fundamentally from other genetic benefit models of polyandry in that the fitness consequences of intragenomic conflict depend on an interaction between parental genomes and are thus non-additive. Reciprocal evolutionary change between selfish genetic elements and their suppressors, combined with the capacity of these elements for horizontal transfer between species, is likely to ensure the persistence of genetic incompatibility as a threat to female reproductive success.

Local Adaptation and Gene-For-Gene Coevolution in a Metapopulation Model

Abstract: In several reciprocal cross-infection experiments parasites were found to be significantly more adapted to their local host populations than to hosts from distant populations. We developed a metapopulation model, taking explicit account of both population densities and gene frequencies, to determine the influence of ecological and genetical parameters on the local adaptation of the parasites and on the spatial distribution of resistance and virulence genes. Our results point to the predominant effect of ecological parameters such as parasite growth rate and host and parasite migration rates on coevolutionary outcomes. In particular, the parasites are more likely to be adapted to their local host population than to allopatric hosts when the parasite migration rate is larger than the host migration rate. The opposite should be observed whenever hosts migrate more than parasites.

Sex identification in birds using two CHD genes

Abstract: In theory, birds should control the sex ratio of the offspring they produce. In practice, we have very limited evidence to support this idea because of our difficulty in sexing nestling birds. In addition, extinction is facing an increasing number of birds. Our ability to help includes captive breeding which, again, is difficult if male and female adults cannot be recognized. Here we describe the discovery of a W-linked gene in the Great tit (Parus major). It is named CHD-W (chromodomain-helicase-DNA-binding protein W-linked), it is highly conserved and it is W-chromosome linked in a range of bird species. These birds also possess a second, non-W-linked CHD gene (CHD-NW). A single, simple polymerase chain reaction technique based on both genes can be used to identify the sex in a wide variety of birds.

Female fitness in Drosophila melanogaster: an interaction between the effect of nutrition and of encounter rate with males.

Abstract: Female Drosophila melanogaster were maintained at five levels of nutrition, with either continuous or intermittent exposure to males. Remating frequency increased with nutrition and was higher with continuous exposure to males. Age-specific and lifetime egg production increased with increasing nutrition, but lifespan peaked at intermediate nutrition. Females on the three highest nutritional levels showed a cost of mating in reduced survival, but only at the highest food level did this reduced lifespan lead to a significant cost of mating for lifetime egg production. The data suggest that remating frequencies in laboratory cultures may evolve to a low enough level for the cost of mating to be only weakly expressed, if at all. Further data are required to assess the importance of the cost of mating in natural populations, where the evolution of low remating frequencies might be expected to be opposed by other costs.

First Gene on the Avian W Chromosome (CHD) Provides a Tag for Universal Sexing of Non-Ratite Birds

Abstract: The avian W chromosome shares many features with the mammalian Y chromosome: it is small, mostly heterochromatic, and filled with large repetitive arrays. No gene so far been assigned to the W chromosome in any bird species and, as a practical consequence, a general tag for avian gender identification on the molecular level is lacking. Here I describe the isolation of a chicken homologue to the mouse chromo-helicase-DNA binding (CHD) gene which encodes a protein involved in global regulation of transcriptional activation on the chromatin level. The avian CHD gene exists in two genomic copies, one of which termed CHD-W) was located on the W chromosome in all non-ratio species investigated. The gene displays extreme levels of sequence conservation since chicken CHD-W and mouse CHD are 82.9% and 95.6% identical at the nucleotide and amino acid level respectively. Molecular sexing can be accomplished in probably all non-ratite birds by hybridizing Southern blots with CHD probes, PCR-based gender identification is also demonstrated. A general system for avian sexing should facilitate many studies of behaviour, evolutionary ecology, genetics, and evolution.

Colour Vision as an Adaptation to Frugivory in Primates

Abstract: Most mammals possess two classes of cone, sensitive to short and to long wavelengths of light, but Old World primates (Catarrhini) have distinct medium and long wavelength sensitive classes. The sensitivities of these cones photopigments are alike in all catarrhines with peaks at about 440 nm (\`blue'), 533 nm (\`green') and 565 nm (`red'). One possible reason for the evolution and conservatism of catarrhine trichromacy is that colour vision is a specialization for finding food. A model of retinal coding of natural spectra, based on discrimination thresholds, is used to examine the usefulness of dichromatic and trichromatic vision for finding fruit, and for identifying fruit and leaves by colour. For identification tasks the dichromat's eye is almost as good as a trichromat's, but the trichromat has an advantage for detecting fruit against a background of leaves.

The African rain forest : main characteristics of changes in vegetation and climate from the Upper Cretaceous to the Quaternary

Abstract: This chapter sets out to give a historical overview of the African rain forest from its origins, towards the end of the Cretaceous period. The areas around the Gulf of Guinea, in particular from Ivory Coast to Nigeria and especially Cameroon, Gabon and Congo, appear to have been already occupied at this time by wet tropical forest formations mainly composed of Angiosperms which were then becoming established. In the course of the Tertiary period the combined effect of the equator being situated further north than now and the development of the Antarctic ice cap favoured the development of wet tropical conditions over a large part of North Africa which in turn led to the extension of tropical forest to various sites on the shores of the Tethys Sea. There were probably at this time common taxa and similar vegetation patterns stretching from the Gulf of Guinea to the Tethys Sea.Towards the end of the Tertiary, the equator reached its present position and the northern hemisphere ice caps appeared, and these phenomena resulted in the disappearance of the forest formations spread across the north of Africa, and the concentration of these formations near the equatorial zone around the Gulf of Guinea and in the Congo–Zaire basin. From 800 000 years ago onwards the marked glacial variations at middle and high latitudes in both hemispheres, with a periodicity of about 100 000 years determined by the orbital variations of the earth around the sun, lowered temperatures in equatorial areas and brought arid climates at times of maximum glacial extension. The most arid periods resulted in the fragmentation of the forest cover, and the forest biotopes and their biodiversity were preserved in a series of refugia. The lowering of temperatures also resulted in the extension of montane flora to low altitudes, with migration of montane flora and fauna between main mountain ranges. These compounded phenomena of isolation and migration, probably involving genie exchange, must have resulted in numerous speciation phenomena. Subsequently, such montane flora or fauna became isolated on mountain areas during periods of maximum warming, in the last instance in the course of the Holocene, when a vast forest cover became re-established from Guinea westwards, and to the East as far as the Lake Victoria area. The phases of maximum fragmentation, which appear to have been connected with only the coldest periods – in the last instance during the second part of isotopic stages 6 (from c. 160 to 130 000 years) and 2 (from c. 24 to 12000 years BP) – relate to less than 10% of the last 800 000 years, and the phases of maximum forest extension would likewise appear to be less than 10% of the period. The remaining 80–90% of the time relates to ‘intermediate situations’ which varied from period to period, and these intermediate extension situations seem to have been the norm over the larger part of the Quaternary, rather than the present situation which is closer to a situation of maximum extension.

The cost of reproduction: a new link between current reproductive effort and future reproductive success

Abstract: Although a negative trade-off between current reproductive effort and future reproductive output has for long been hypothesized and in some cases empirically shown, no mechanism with the potential to mediate such a reproductive cost from one season to another has been demonstrated. Here we suggest such a mechanism: the time and/or energy constraints during moult. We manipulated current reproduction of blue tits (Parus caeruleus) by delaying their breeding attempts. This resulted in reproductive costs both in terms of decreased survival and lower subsequent reproductive success in the form of delayed start of breeding (females) or a smaller clutch (males). We found that delayed birds expended about 15% more energy in thermoregulation during the subsequent winter than did control birds. Thus, more time and/or energy directed to reproduction, may constrain moult which results in the production of feathers with low insulating capacity. These feathers are retained during the following winter and breeding season. Thus, this mechanism has the potential to link reproductive activities in one season to future reproductive success.

Generating 'Tiger' as an Animal Name or a Word Beginning with T: Differences in Brain Activation

Abstract: Positron emission tomography was used to investigate differences in regional cerebral activity during word retrieval in response to different prompts. The contrast of semantic category fluency and initial letter fluency resulted in selective activation of left temporal regions; the reverse contrast yielded activation in left frontal regions (BA44/6). A further comparison between types of category fluency demonstrated a more anterior temporal response for natural kinds and more posterior activation for manipulable manmade objects. These results support behavioural data suggesting that category fluency is relatively more dependent on temporal-lobe regions, and initial letter fluency on frontal structures; and that categorical word retrieval is not a uniformly distributed function within the brain. This is compatible with the category-specific deficits observed after some focal lesions.

Quantitative Study of the Fixed Threshold Model for the Regulation of Division of Labour in Insect Societies

Abstract: A simple model of regulation of division of labour in insect societies is introduced and studied. Individuals are assumed to respond to task-related stimuli with response thresholds. When the intensity of a particular stimulus exceeds an individual9s response threshold, the individual engages in task performance with high probability, and successful task performance reduces the intensity of the stimulus. If individuals belonging to different (physical or behavioural) castes have different response thresholds, and if thresholds are assumed to remain fixed over the timescales of experiments, this model can account for some observations on ant species of Pheidole (Wilson 1984).

Sperm Competition Games: Individual Assessment of Sperm Competition Intensity by Group Spawners

Abstract: A distinction is made between sperm competition risk (where there is typically a low probability of competition between two ejaculates) and sperm competition intensity (where typically two or more ejaculates compete). The relation between sperm competition intensity and sperm expenditure can be radically different across species from that within a species. Across species, the average ejaculate expenditure will increase with the average intensity of sperm competition. But within a species, the reverse trend is generally predicted for greater than two males competing for the same set of eggs. These effects are demonstrated with three sperm competition game models. They are devised mainly for externally fertilizing group-spawning species such as many fish, in which males group around a female and ejaculate when the female sheds her eggs. Fertilization is assumed to be instantaneous and each male gains a proportion of the eggs equal to his sperm number divided by the total sperm. In the first model, males cannot assess the number of competitors, and their ejaculate effort is shaped by the average number of males for the species or locally isolated deme. The proportion of reproductive effort expended on the ejaculate is predicted to increase as (N-1)/N, where N = the mean number of competing males present at a spawning. Thus if N is large, ejaculate expenditure dominates reproductive effort. In the second model, males can estimate whether there are more or less than average numbers of competitors present at a spawning, and in the third model, males can assess the number of competitors exactly. As in the first model, these models confirm that the mean ejaculate effort should increase with the mean number of competitors for the species. However, they predict that males should decrease their sperm expenditure as the estimated number of competitors present at a given spawning increases above two. These conclusions do not apply to sperm competition risk: there is thus no conflict with earlier models based on risk.

Recognition Errors and Probability of Parasitism Determine Whether Reed Warblers Should Accept or Reject Mimetic Cuckoo Eggs

Abstract: Reed warblers sometimes make recognition errors when faced with a mimetic cuckoo egg in their nest and reject one or more of their own eggs rather than the foreign egg. Using the framework of signal detection theory, we analyse responses to model eggs to quantify the costs and benefits of acceptance versus rejection in parasitized and unparasitized nests. We show that below a threshold of 19-41% parasitism, the warblers should accept mimetic cuckoo eggs because the costs of rejection outweigh the benefits, whereas above this threshold they should reject. The warblers behaved as predicted; when they saw a cuckoo at their nest they usually showed rejection, but without the sight of the cuckoo they behaved appropriately for the average parasitism rate in Britain (6%) and tended to accept.

Frequency-dependent maintenance of left handedness in humans

Abstract: The percentage (10-13%) of left handedness in humans has apparently not changed since the Neolithic. Left handedness is heritable and appears to be repeatedly associated with some reduced fitness components; the persistence of left handedness implies that left handers have a fitness advantage in some situations. We propose that left handers have a frequency-dependent advantage in fights and for that reason a fitness advantage. To test this hypothesis, left handedness frequencies in the general population and in sporting individuals (both students and the sporting elite) have been compared, as sporting performance is likely to be a good indicator of fighting abilities. The higher proportion of left-handed individuals in interactive sports (reflecting some fighting elements), reaching 50% in some sports categories, but not in noninteractive sports, is consistent with the fighting hypothesis. The greater frequency of left handedness in males than in females is also consistent with this hypothesis, as male-male fights are universally more frequent than other combinations. The frequency-dependent advantage in fights of left handers might explain the stability of left handedness.

Auditory temporal coding in dyslexia

Abstract: Developmental dyslexia is generally believed to result from impaired linguistic processing rather than from deficits in low-level sensory function. Challenging this view, we studied the perception of non-verbal acoustic stimuli and low-level auditory evoked potentials in dyslexic adults. Compared with matched controls, dyslexics were selectively impaired in tasks (frequency discrimination and binaural unmasking) which rely on decoding neural discharges phase-locked to the fine structure of the stimulus. Furthermore, this ability to use phase-locking was related to reading ability. In addition, the evoked potential reflecting phase-locked discharges was significantly smaller in dyslexics. These results demonstrate a low-level auditory impairment in dyslexia traceable to the brainstem nuclei.

Neocortex size and behavioural ecology in primates

Abstract: The neocortex is widely held to have been the focus of mammalian brain evolution, but what selection pressures explain the observed diversity in its size and structure? Among primates, comparative studies suggest that neocortical evolution is related to the cognitive demands of sociality, and here I confirm that neocortex size and social group size are positively correlated once phylogenetic associations and overall brain size are taken into account. This association holds within haplorhine but not strepsirhine primates. In addition, the neocortex is larger in diurnal than in nocturnal primates, and among diurnal haplorhines its size is positively correlated with the degree of frugivory. These ecological correlates reflect the diverse sensory-cognitive functions of the neocortex.

Evolutionary Conservation of Microsatellite Flanking Regions and their Use in Resolving the Phylogeny of Cichlid Fishes (Pisces: Perciformes)

Abstract: A phylogeny of the principal lineages of cichlid fishes and two other fish families of the suborder Labroidei was based on phylogenetic information from DNA sequences of the flanking region of a (CA)n microsatellite locus. Microsatellite (CA)n containing clones from a genomic library of an African cichlid fish from Lake Tanganyika, Tropheus moorii, were sequenced and primers for the polymerase chain reaction designed. All primers amplified the homologous microsatellite loci in many more than the source species and one microsatellite flanking locus (TmoM27) was particularly conserved and amplified in several lineages of perciform fishes that diverged more than 80-100 million years ago. Despite the extensive level of evolutionary conservation of this microsatellite flanking region (MFR), this nuclear region contained reliable phylogenetic information in the form of both point and length mutations. A phylogeny of cichlids based on this MFR agrees with other phylogenetic hypotheses based on morphological, mitochondrial, and anonymous nuclear DNA. Madagascan and Indian cichlids are found to be paraphyletic and the most basal group in the family Cichlidae. African and Neotropical cichlids are both monophyletic and sistergroups. Within African lineages, the East African cichlids are most likely to be monophyletic and the West African cichlids are probably paraphyletic and basal to all African species. The focal microsatellite locus contained much variation in (CA)n repeats in African cichlids and in surfperches (up to 64 repeats), but was short (with only 2-4 repeats) and almost invariant in Neotropical cichlids. The design of phylogenetically highly versatile MFR-primers will be of use not only for phylogeny reconstruction among families of perciform fishes, but also for population-level work in the thousands of species belonging to this highly species-rich suborder of fishes.

Impaired Predator Evasion in Fat Blackcaps (Sylvia atricapilla)

Abstract: When birds are attacked by predators, take-off ability is crucial for the chance of survival. Recently, theoretical studies have predicted that predation risk in terms of reduced flight performance increases with body mass. However, empirical data are scarce. Because migratory birds sometimes double their body mass, mass dependent predation risk may be especially important during migratory fattening. Here we present the first study of take-off ability in relation to migratory fat load. Alarmed take-off flights of caged blackcaps (Sylvia atricapilla) induced by a simulated predator attack were analysed in terms of velocity and angle of ascent. Fat loads (percentage of fat-free body mass) of the birds ranged from 1% to 59%. An increase in fat load was found to influence both velocity and angle of ascent. From our results we calculated that blackcaps carrying 60% fat loads would have 32% lower angle of ascent and 17% lower velocity than blackcaps carrying no fat load. Even though the effect of fat load on the blackcaps was less than indicated in previous experimental studies of other species, our results suggest that the large fat loads needed for migration probably place them at increased risk of predation.

Noise Colour and the Risk of Population Extinctions

Abstract: A recurrent problem in ecology and conservation biology is to estimate the risk of population extinctions. Extinction probabilities are not only imperative for conservation and management, but may also elucidate basic mechanisms of the regulation of natural populations (Burgman et al. 1993; Pimm 1994). The usual way of modelling stochastic influence on population dynamics has been to assume that the external noise is uncorrelated. This means that each and every randomly drawn noise value is totally independent on previous ones. This is what is generally called \`white' noise. However, the noise itself can be temporally autocorrelated. That is, the values of the random numbers used in the noise process will depend on previous ones. Here we show that the autocorrelation, or colour, of the external noise assumed to influence population dynamics strongly modifies estimated extinction probabilities. For positively autocorrelated (\`red') noise, the risk of extinction clearly decreases the stronger the autocorrelation is. Negatively autocorrelated (`blue') noise is more ambiguously related to extinction probability. Thus, the commonly assumed white noise in population modelling will severely bias population extinction risk estimates. Moreover, the extinction probability estimates are also significantly dependent on model structure which calls for a cautious use of traditional discrete-time models.

Energetic Constraints and Male Mate-Securing Tactics in the Damselfly Calopteryx splendens xanthostoma (Charpentier)

Abstract: Males of the damselfly Calopteryx splendens xanthostoma (Charpentier) demonstrate territorial and non-territorial mate securing tactics. Non-territorial males obtain a territory in one of two ways: they either wait for a territory to become vacant, or they fight with and displace a territory holder. The estimated reproductive success of territorial males was a thousand times greater than that of non-territorial males, suggesting that males should fight hard to become, and remain, territorial. Our results show that the ability to win fights, and therefore be territorial, is dependent on energy reserves (fat). Energy reserves were correlated with the age of the individual. Young, pre-territorial, males had excess fat; territorial males that had gained their territory by fighting had intermediate fat reserves; older males that had been displaced from a territory had very low levels of fat. Previous studies of calopterygid territoriality have suggested that resident-intruder or other uncorrelated asymmetries are important in determining the outcome of a conflict. We provide an alternative explanation centring around Grafen9s (1987) `desperado9 effect and the energy constraint on a male9s ability to obtain a territory.

Host-symbiont conflict over the mixing of symbiotic lineages.

Abstract: Host and symbiont often conflict over patterns of symbiont transmission. Symbionts favour dispersal out of the host to avoid competition with close relatives. Migration leads to competition among different symbiotic lineages, with potentially virulent side-effects on the host. The hosts are favoured to restrict symbiont migration and reduce the virulent tendencies of the symbionts. Reduced mixing of symbionts would, in many cases, lower symbiont virulence and increase the mean fitness of the host population. But a host modifier allele that reduced symbiont mixing increases only when directly associated with reduced virulence. The association between modifiers and reduced virulence depends on the particular details of symbiont biology. The importance of this direct association between modifier and virulence was first noted by Hoekstra (1987) when studying the evolution of uniparental inheritance of cytoplasmic elements. I apply Hoekstra's insight to a wide range of host-symbiont life histories, expanding the scope beyond cytoplasmic inheritance and genomic conflict. My comparison of differing symbiont life histories leads to a careful analysis of the conditions under which hosts are favoured to control mixing of their symbionts.

Learning to find your way: a role for the human hippocampal formation

Abstract: The importance of the hippocampal formation of the brain for allocentric spatial mapping of the environment has been suggested by animal lesion and electrophysiological work. Here we describe a positron emission tomography (PET) study designed to investigate the regional cerebral blood flow changes associated with topographical memory formation in humans, i.e. the formation of representations of large-scale environments necessary for way-finding. Topographical learning of an urban environment from viewing of film footage depicting navigation was associated with activation of the right parahippocampal gyrus and hippocampus, with activation also of the left parahippocampal gyrus. In addition, there was activity in the pretuneus. In contrast, the encoding of non-navigation episodic memory in a similar realworld context was not associated with activity in the hippocampal formation. Our results shed light on the neural basis of the human representation of large-scale space pinpointing a particular role for the human hippocampal formation in learning to find one's way.

Extreme Divergence of Mitochondrial DNA within Species of Pulmonate Land Snails

Abstract: Mitochondrial DNA, inherited predominantly through the female line, has been exceptionally useful for reconstructing phylogenies (Avise, in Molecular markers, natural history and evolution. New York: Chapman and Hall (1994)). However, at the lowest taxonomic level, if there are polymorphisms within species the lineages of mitochondria need not correspond to the lineages of the species (Avise, in Phil. Trans. R. Soc. Lond. B 312, 325-342 (1986)). We find that a classic organism in ecological genetics, Cepaea nemoralis, has the most extreme intraspecific variation and polymorphism so far recorded, and that at least one other pulmonate land mollusc also has very high levels of mitochondrial diversity. Making the simplest assumptions, the data suggest times of divergence as long ago as 20 million years between haplotypes now coexisting within a single population. There are four overlapping explanations of the diversity: (i) that mitochondrial evolution in pulmonates is exceptionally fast; (ii) that the morphs have differentiated in isolated `refuges' and then come together; (iii) that natural selection has acted to preserve the variation; and (iv) that the population structure of pulmonates favours the persistence of ancient haplotypes. We argue for the importance of the last explanation.

Spatial Evolutionary Game Theory: Hawks and Doves Revisited

Abstract: We consider a spatial generalization of evolutionary game theory in which strategies are distributed over a spatial array of sites. We assume that the strategy corresponding to a given site has local interactions with the strategies sitting on neighbouring sites, and that the strategies change if neighbouring strategies are doing better. After briefly setting the stage with a formal definition of spatial evolutionary game theory, we consider the spatial extension of the Hawk-Dove game, and we show that the results are qualitatively different from those obtained from classical evolutionary game theory. For example, the proportion of Hawks in the population is in general lower in the spatial game than in the classical one. We also consider spatial generalizations of the extensions of the Hawk-Dove game obtained by including strategies such as Retaliator and Bully. Here, too, the results from the spatial game are very different from the classical results. In particular, with space Retaliator is a much more successful strategy than one would expect from classical considerations. This suggests that, in general, spatial structure may facilitate the evolution of strategies such as Retaliator, which do not necessarily prosper classically, and which are reminiscent of the \`nice', \`provokable' and `forgiving' strategies which seem to play a central role in the evolution of cooperation. The results indicate that including spatial structure in evolutionary game theory is a fruitful extension.

470 million years of conservation of microsatellite loci among fish species.

Abstract: Primers for 18 microsatellite loci originally isolated from whiting (Merlangius merlangus, n = 6), stickleback (Gasterosteus aculeatus, n = 5) and cod (Gadus morhua, n = 7) were tested across a panel of diverse fish species, representing the three principal superclasses and most principal superorders of fish, to examine conservation of microsatellite regions across distantly related taxa. Three methods were used. First, amplified fragments were analysed by Southern blotting using the relevant microsatellite motif probes. A total of 17 of the tested primer pairs gave a product in the expected size range in at least four of 11 tested species. Second, for two study loci the amplified polymerase chain reaction products were cloned and sequenced in five fish species to reveal a high level of conservation of the flanking and microsatellite sequences. Finally, the 17 loci successfully amplified in non-source species were tested for polymorphism in groups of unrelated individuals from nine species, in several cases revealing extensive polymorphism. Levels of polymorphism were generally high in species from which the loci were derived or among closely related species. The conservation of flanking sequences for particular microsatellite motifs over the span of fish evolution represented in the test species (470 million years) far exceeds that hitherto reported and lends support to the suggestion (derived from studies of whales and marine turtles) that the rate of base substitution in nuclear and mitochondrial sequences is lower in aquatic than terrestrial organisms. A further explanation could be that these sequences, although generally considered neutral, may play an important role in eukaryotic genomes, and may be under strong selective constraints. The study suggests that heterologous primers will be a ready source of polymorphic markers among fish species, but also indicates that caution should be used in cross-species comparisons of variability.

Evolution of reproductive systems in the genus Silene

Abstract: The genus Silene contains both hermaphrodite, gynodioecious and dioecious species, dioecy being represented in three sections of the genus. To locate the events of change of reproductive systems, we compared ITS sequences of 22 species of Silene chosen throughout the whole genus, and four putative outgroup species. Gynodioecy, which is the most common reproductive system within the genus Silene and in closely related genera such as Saponaria and Dianthus , is proposed to be ancestral in the genus. Dioecy has evolved at least twice: once in the section containing S. latifolia , and once in the clade containing S. otites and S. acaulis ssp. bryoides . Evolution towards hermaphroditism, associated with evolution of selfing, has also occurred at least twice, in S. gallica and S. conica .

Parasite-Host Coevolution and Geographic Patterns of Parasite Infectivity and Host Susceptibility

Abstract: Ebert (1994) has proposed the rule that parasites are, with few exceptions, more infective to sympatric hosts than to allopatric hosts. We test this rule using field data for schistosome infections of planorbid snails and find that, although sympatric parasite-host combinations do tend to be more compatible, there are exceptions where particular allopatric parasite-host populations are significantly more compatible. We develop a mathematical model of the dynamics of the parasite-host interaction where parasite infectivity and host susceptibility are defined by the matching of genotypes in a diploid system, The model predicts dynamic polymorphisms where parasite allele frequencies track host allele frequencies but with a lag. Because of this lag, it is possible for allopatric combinations to be more compatible than sympatric combinations. Any 'rule' that precludes this possibility is unlikely to prove robust.

Mutations in S-cone pigment genes and the absence of colour vision in two species of nocturnal primate

Abstract: Most primates have short-wavelength sensitive (S) cones and one or more types of cone maximally sensitive in the middle to long wavelengths (M/L cones) These multiple cone types provide the basis for colour vision Earlier experiments established that two species of nocturnal primate, the owl monkey (Aotus trivirgatus) and the bushbaby (Otolemur crassicaudatus), lack a viable population of S cones Because the retinas of these species have only a single type of M/L cone, they lack colour vision Both of these species have an S-cone pigment gene that is highly homologous to the human S-cone pigment gene Examination of the nucleotide sequences of the S-cone pigment genes reveals that each species has deleterious mutational changes: in comparison to the sequence for the corresponding region of the human gene, exon 4 of the bushbaby S-cone pigment gene has a two nucleotide deletion and a single nucleotide insertion that produces a frame shift and results in the introduction of a stop codon Exon 1 of the owl monkey S-cone pigment gene likewise contains deletions and insertions that produce a stop codon The absence of colour vision in both of these nocturnal primates can thus be traced to defects in their S-cone pigment genes