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The discipline of functional morphology grew out of a comparative anatomical tradition, its transformation into a modern experimental science facilitated largely by technological advances. Early morphologists, such as Cuvier, felt that function was predictable from organismal form, to the extent that animals and plants represented perfect adaptations to their habits. However, anatomy alone could not reveal how organisms actually performed their activities. Recording techniques capable of capturing fast motion were first required to begin to understand animal movement. Muybridge is most famous for his pioneering work in fast photography in the late 19th century, enabling him to “freeze” images of even the fastest horse at a full gallop. In fact, contemporary kinematic analysis grew directly out of the techniques Muybridge developed. Marey made perhaps an even greater contribution to experimental science through his invention of automatic apparati for recording events of animal motion. Over the first half of the 20th century, scientists developed practical methods to record activity patterns from muscles of a living, behaving human or animal. The technique of electromyography, initially used in clinical applications, was co-opted as a tool of organismal biologists in the late 1960s. Comparative anatomy, kinematic analysis and electromyography have for many years been the mainstay of vertebrate functional morphology; however, those interested in animal form and function have recently begun branching out to incorporate approaches from experimental biomechanics and other disciplines (see accompanying symposium papers), and functional morphology now stands at the threshold of becoming a truly integrative, central field in organismal biology.
Salmonids undergo a developmental transition from parr to smolt that involves a number of physiological and morphological changes. In recent years, my laboratory has studied shifts in red muscle function at this parr-smolt transformation (PST) in rainbow trout, Oncorhynchus mykiss. Parr red muscle has faster contraction kinetics than smolts, including faster rates of activation and relaxation and a faster maximum shortening velocity. At PST, a transition in swimming behavior is also observed, with lower tailbeat frequencies and longer EMG duty cycles in the older smolts. Lastly, there is molecular correlate to changes in kinetics and behavior. During PST, there is a developmental reduction in the number of myosin heavy chain (MHC) isoforms in the red muscle of rainbow trout. Since MHC composition of muscle can determine contractile properties, these molecular results suggest a mechanism for the transition in red muscle kinetics and steady swimming. The red muscle of parr is more likely to contain the fast-twitch or white isoform of MHC, resulting in faster contractile properties of that muscle and higher tailbeat frequencies during steady swimming. Lastly, experimental work supports the conclusion that the shift in kinetics causes the observed shift in swimming behavior.
Comparative physiologists and ecologists have searched for a specific morphological, physiological or biochemical parameter that could be easily measured in a captive, frozen, or preserved animal, and that would accurately predict the routine behavior or performance of that species in the wild. Many investigators have measured the activity of specific enzymes in the locomotor musculature of marine fishes, generally assuming that high specific activities of enzymes involved in aerobic metabolism are indicators of high levels of sustained swimming performance and that high activities of anaerobic metabolic enzymes indicate high levels of burst swimming performance. We review the data that support this hypothesis and describe two recent studies we have conducted that specifically test the hypothesis that biochemical indices of anaerobic or aerobic capacity in fish myotomal muscle correlate with direct measures of swimming performance. First, we determined that the maximum speed during escapes (C-starts) for individual larval and juvenile California halibut did not correlate with the activity of the enzyme lactate dehydrogenase, an index of anaerobic capacity, in the myotomal muscle, when the effects of fish size are factored out using residuals analysis. Second, we found that none of three aerobic capacity indices (citrate synthase activity, 3-hydroxy-o-acylCoA dehydrogenase activity, and myoglobin concentration) measured in the slow, oxidative muscle of juvenile scombrid fishes correlated significantly with maximum sustained speed. Thus, there was little correspondence between specific biochemical characteristics of the locomotor muscle of individual fish and whole animal swimming performance. However, it may be possible to identify biochemical indices that are accurate predictors of animal performance in phylogenetically based studies designed to separate out the effects of body size, temperature, and ontogenetic stage.
The functional significance of many features of the reptilian cardiopulmonary system remains unknown; particularly the importance of cardiac shunts. One hypothesis for a physiological function for shunts is that they play a role in myocardial oxygenation and are therefore important when cardiac work is elevated. In this study we examined cardiac function by monitoring electrocardiograms in red-eared slider turtles (Trachemys scripta) with a reduced myocardial oxygen supply. Exposing the animals to a hypoxic gas mixture reduced oxygen levels in the pulmonary venous return. When cardiac work was elevated during hypoxia, the electrocardiogram changed in a manner consistent with myocardial hypoxia, suggesting enrichment of the luminal blood with oxygen by the intracardiac shunt facilitates cardiac performance.
Inflatable penises have evolved independently at least four times in amniotes, specifically in mammals, turtles, squamates, and the archosaurs. Males in these lineages therefore share the functional problem of building a penis out of soft and flexible tissues that can increase its flexural stiffness and resist bending during copulation. Research on penile erectile tissues in mammals and turtles shows that these two taxa have convergently evolved an axial orthogonal array of collagen fibers to reinforce the penis during erection and copulation; in both lineages, the collagen fibers in the array are crimped and folded in the flaccid penis. Collagen fiber straightening during erection increases the stiffness of the tissue and allows changes in penile radius that increase its second moment of area: both of these changes increase the flexural stiffness of the penis as a whole. And once erect, axial orthogonal arrays have the highest flexural stiffness of any fiber arrangement. The high degree of anatomical convergence (to the level of microanatomical features) within mammals and turtles suggests that the stiffness requirements for copulation produce an extremely restrictive selective regime in organisms that evolve inflatable penises.
Functional morphology has benefited greatly from the input of techniques and thinking from other disciplines. This has been especially productive in situations where each discipline has made significant contributions to a particular research topic. A combination of methodologies from functional morphology and developmental biology has allowed us to characterize feeding mechanics of first-feeding larval zebrafish (Danio rerio). Contrary to kinematic patterns commonly seen in adult teleosts, larval zebrafish showed no lateral abduction during the expansive phase of a suction-feeding event. Instead, dorsoventral expansion of the buccal chamber, more typical of patterns seen in primitive fishes, characterized the expansive phase. Moreover, a pronounced preparatory phase during which the buccal chamber is constricted by the protractor hyoideus was consistently seen in first-feeding larval kinematics. Key kinematic variables associated with first feeding correlated significantly with the hydrodynamic regime as measured by the Reynolds number. Using the tools of both functional morphology and developmental biology we have not only determined which cranial muscles are important for successful feeding but also uncovered important physiological differences in muscle structure. Muscles necessary for the rapid dorsoventral expansion of the head are composed primarily of fast-twitch fibers while those involved in more tonic contractions such as hyoid protraction have more slow-twitch muscle fibers. While most evolutionary developmental studies have examined mechanisms responsible for large evolutionary changes in morphology, we propose that the type of data uncovered in functional studies can lead to the generation of hypotheses concerning the developmental mechanisms responsible for smaller intra- and/or interspecific changes.
Why do some animals swim by rowing appendages back and forth while others fly by flapping them up and down? One hypothesis suggests the answer lies in the sharply divergent physical environments encountered by small, slow animals, and large, fast animals. Flapping appendages allow large animals to move through a fluid environment quickly and efficiently. As size and speed decrease, however, viscous drag increasingly dominates the force balance, with negative consequences for both rowing and flapping appendages. Nevertheless, comparative data suggest that flapping does not occur in animals at Reynolds numbers (Re) less than about 15. I used a computer simulation experiment to address the question, “Below what Re is rowing more effective than flapping?” The simulation, which employed a simple quasi-steady, blade-element model of virtual oscillating appendages, has several important results. First, the mechanical efficiency of both rowing and flapping decrease dramatically with scale. Second, the performance of rowing can increase substantially by taking advantage of several dynamic shape modifications, including area and span reduction during the recovery stroke. Finally, the relative performance of rowing and flapping is dependent on the advance ratio, which is a function of the travel speed relative to the oscillation frequency. The model predicts that rowing is more efficient than flapping at Re < 20 for animals moving throughout the range of typically observed advance ratios.
Despite enormous progress during the last twenty years in understanding the mechanistic basis of aquatic animal propulsion—a task involving the construction of a substantial data base on patterns of fin and body kinematics and locomotor muscle function—there remains a key area in which biologists have little information: the relationship between propulsor activity and water movement in the wake. How is internal muscular force translated into external force exerted on the water? What is the pattern of fluid force production by different fish fins (e.g., pectoral, caudal, dorsal) and how does swimming force vary with speed and among species? These types of questions have received considerable attention in analyses of terrestrial locomotion where force output by limbs can be measured directly with force plates. But how can forces exerted by animals moving through fluid be measured? The advent of digital particle image velocimetry (DPIV) has provided an experimental hydrodynamic approach for quantifying the locomotor forces of freely moving animals in fluids, and has resulted in significant new insights into the mechanisms of fish propulsion. In this paper we present ten “lessons learned” from the application of DPIV to problems of fish locomotion over the last five years. (1) Three-dimensional DPIV analysis is critical for reconstructing wake geometry. (2) DPIV analysis reveals the orientation of locomotor reaction forces. (3) DPIV analysis allows calculation of the magnitude of locomotor forces. (4) Swimming speed can have a major impact on wake structure. (5) DPIV can reveal interspecific differences in vortex wake morphology. (6) DPIV analysis can provide new insights into the limits to locomotor performance. (7) DPIV demonstrates the functional versatility of fish fins. (8) DPIV reveals hydrodynamic force partitioning among fins. (9) DPIV shows that wake interaction among fins may enhance thrust production. (10) Experimental hydrodynamic analysis can provide insight into the functional significance of evolutionary variation in fin design.
Like many marine crustaceans, mantis shrimp rely on their sense of smell to find food, mates, and habitat. In order for olfaction to function, odorant molecules in the surrounding fluid must gain access to the animal's chemosensors. Thus fluid motion is important for olfaction, both in terms of the large scale fluid movements (currents, waves, etc.) that advect the odorants to the vicinity of the sensors, and the small-scale viscosity dominated flows that determine odorant access to the surface of the sensor. In order to understand how stomatopods interpret their chemical environment, I investigated how stomatopod chemosensory morphology and the movement of the structures bearing the chemosensors affect fluid access to the sensor surface in Gonodactylaceus mutatus. Preliminary results from new directions are presented, including mathematical modeling of molecular flux at the sensor surface, field studies of the effects of ambient flow on odor sampling behavior, and flume experiments testing the ability of stomatopods to trace odor plumes. Finally, I show how the use of multiple techniques from several disciplines leads to new ideas about the functional morphology of stomatopod antennules.
Researchers strive to understand what makes species different, and what allows them to survive in the time and space that they do. Many models have been advanced which encompass an array of ecological, evolutionary, mathematical, and logical principles. The goal has been to develop ecological theories that can, among other things, make specific and robust predictions about how and where organisms should live and what organisms should utilize. The role of functional morphology is often an under-appreciated parameter of these models. A more complete understanding of how anatomical features work to allow the organism to accomplish certain tasks has allowed us to revisit some of these ideas with a new perspective. We illustrate our view of this role for functional morphology in ecology by considering the issue of specialization: we attempt to align several definitions of specialization based upon shared ecological and evolutionary principles, and we summarize theoretical predictions regarding why an organism might specialize. Kinematic studies of prey capture in several types of fishes are explored with regard to the potential ecological and evolutionary consequences of specialization, most notably in the area of trade-offs. We suggest that a functional morphological perspective can increase our understanding of the ecological concepts of specialization and it consequences. The kinds of data that functional morphologists collect can help us to quantify organismal performance associated with specialization and the union of functional morphology with ecology can help us to better understand not just how but why organisms interact in the manner that they do.
Performance studies have long been a cornerstone of evolutionary studies of adaptation because of their purported importance for fitness. Nevertheless, for most systems, the mechanistic link among habitat use, morphology and performance is poorly understood. Further, few studies consider how behavior affects the relationship between morphology and performance. Here, I highlight the utility of considering both of these neglected areas by discussing studies in two systems: (1) the evolution of habitat use in Caribbean Anolis lizards, and (2) the evolution of limb function in desert lizards. Caribbean Anolis lizards partition the habitat via selection of different perch diameters, and surface diameter also exerts a strong effect on locomotor performance. Phylogenetic analyses show that Anolis species tend to avoid using perches in which their performance is submaximal, and also show that species with large performance breadths use a greater range of habitats. The underlying basis of this performance to habitat use link is a trade-off between the ability to sprint quickly on broad surfaces and the ability to move effectively on narrow surfaces. Studies of the kinematics of high-speed locomotion in five morphologically distinct lizard species reveal that some species exhibited behaviors that greatly enhanced their performance abilities relative to other species, suggesting that behavior can play a key role in the link between morphology and performance. Overall, these findings underscore the value of using a mechanistic approach for studying the links between habitat use, morphology and behavior.
Research on symbiosis (including antagonistic and mutualistic associations) wrestles, directly or indirectly, with the paradox: why are symbiotic associations so prevalent in the biosphere in the face of ubiquitous immune or antibiotic defenses among organisms? The symposium “Living Together: the Dynamics of Symbiotic Interactions” considered several questions: 1. How do symbiotic species partners come together? Do symbioses share similar patterns of signal recognition and response? 2. What roles do nutrients and metabolites play in symbiotic interactions, and how are metabolic exchanges affected by environmental changes? 3. In what ways do the dynamics of multispecies symbioses differ from two-species associations? 4. How do antagonistic (parasitic, pathogenic) symbioses differ from mutualistic ones? In what ways do changes in the biotic and physical environment affect the evolutionary balance of symbiotic associations? 5. What are the coevolutionary patterns of symbiotic associations? 6. Which research techniques, and strategies of experimental design, might be useful across a broad range of symbiotic associations?
Two themes emerged from the symposium. First, all the participants have incorporated multiple techniques and perspectives into their work, approaches which facilitate the understanding of symbiotic dynamics at several levels of biological organization. Secondly, many of the papers addressed genetic and environmental variation in symbiotic interactions. Such approaches are useful tools for analysis of the mechanics of interspecies interactions and for characterization of the most important factors which influence them. They provide us with the tools to evaluate symbioses in a world of complexity, variation and change.
The nitrogen-fixing symbiosis between Rhizobiaceae and legumes is one of the best-studied interactions established between prokaryotes and eukaryotes. The plant develops root nodules in which the bacteria are housed, and atmospheric nitrogen is fixed into ammonia by the rhizobia and made available to the plant in exchange for carbon compounds. It has been hypothesized that this symbiosis evolved from the more ancient arbuscular mycorrhizal (AM) symbiosis, in which the fungus associates with roots and aids the plant in the absorption of mineral nutrients, particularly phosphate. Support comes from several fronts: 1) legume mutants where Nod− and Myc− co-segregate, and 2) the fact that various early nodulin (ENOD) genes are expressed in legume AM. Both strongly argue for the idea that the signal transduction pathways between the two symbioses are conserved. We have analyzed the responses of four classes of non-nodulating Melilotus alba (white sweetclover) mutants to Glomus intraradices (the mycorrhizal symbiont) to investigate how Nod− mutations affect the establishment of this symbiosis. We also re-examined the root hair responses of the non-nodulating mutants to Sinorhizobium meliloti (the nitrogen-fixing symbiont). Of the four classes, several sweetclover sym mutants are both Nod− and Myc−. In an attempt to decipher the relationship between nodulation and mycorrhiza formation, we also performed co-inoculation experiments with mutant rhizobia and Glomus intraradices on Medicago sativa, a close relative of M. alba. Even though sulfated Nod factor was supplied by some of the bacterial mutants, the fungus did not complement symbiotically defective rhizobia for nodulation.
Traditionally, the field of parasitology has dealt with eukaryotic animals, to the exclusion of viruses, bacteria, fungi, etc., which is the way it will be approached here. The focus of the present paper will be on certain ecological aspects of the life cycles and life-history strategies employed by the Digenea, a diverse group of platyhelminths that includes some 25,000 species. More specifically, the review will consider the nature of host/parasite interactions within molluscan intermediate hosts and the manner in which these interactions, or lack thereof, function in structuring trematode infracommunities within these molluscan intermediate hosts. Literature in this area suggests that predation/competition may be a significant structuring force for infracommunities in certain marine prosobranchs, but not others, and that temporal/spatial factors may be involved as structuring mechanisms in at least some freshwater pulmonates.
The hindgut microbiota of termites includes an abundant and morphologically diverse population of spirochetes. However, our understanding of these symbionts has remained meager since their first observation in termite guts by Leidy over a century ago, in part because none had ever been isolated in culture. Recently, this situation has changed dramatically with the application of cultivation-independent molecular methods to determine their phylogeny, and with the isolation of the first pure cultures. The emerging picture is that earth's termites constitute an enormous reservoir of novel spirochetes, which possess metabolic properties (H2/CO2-acetogenesis and N2 fixation) hitherto unrecognized in spirochetes and which contribute to the carbon, nitrogen and energy requirements of their termite host. These discoveries help to explain the enigmatic dominance of CO2-reductive acetogenesis over methanogenesis in the hindgut of many termites, as well as the old observation that elimination of spirochetes from the gut results in decreased termite survival.
All animals, including humans, are adapted to life in a microbial world. Anaerobic habitats have existed continuously throughout the history of the earth, the gastrointestinal tract being a contemporary microniche. Since microorganisms colonize and grow rapidly under the favorable conditions in the gut they could compete for nutrients with the host. This microbial challenge has modified the course of evolution in animals, resulting in selection of complex animal-microbe relationships that vary tremendously, ranging from competition to cooperation. The ecological and evolutionary interactions between herbivorous dinosaurs and the first mammalian herbivores and their food plants are reconstructed using knowledge gained during the study of modern living vertebrates, especially foregut and hindgut fermenting mammals. The ruminant is well adapted to achieve maximal digestion of roughage using the physiological mechanism at the reticulo-omasal orifice which selectively retains large particles in the reticulo-rumen. However, the most obvious feature of all ruminants is the regurgitation, rechewing and reswallowing of foregut digesta termed rumination. Foregut fermenting mammals also share interesting and unique features in two enzymes, stomach lysozyme and pancreatic ribonuclease which accompany and are adaptations to this mode of digestion. The microbial community inhabiting the gastrointestinal tract is represented by all major groups of microbes (bacteria, archaea, ciliate protozoa, anaerobic fungi and bacteriophage) and characterized by its high population density, wide diversity and complexity of interactions. The development and application of molecular ecology techniques promises to link distribution and identity of gastrointestinal microbes in their natural environment with their genetic potential and in situ activities.
Molecular tools based on small subunit (SSU) rDNA gene sequences offer a powerful and rapid tool for the analysis of complex microbial communities found in the gastrointestinal tracts (GIT) of food animal species. Extensive comparative sequence analysis of SSU rRNA molecules representing a wide diversity of organisms shows that different regions of the molecule vary in sequence conservation. Oligonucleotides complementing regions of universally conversed SSU rRNA sequences are used as universal probes, while those complementing more variable regions of sequence are useful as selective probes targeting species, genus, or phylogenetic groups. Different approaches derive different information and this is highly dependent on the type of target nucleic acid employed and the conceptual and technical basis used for nucleic acid probe design. Generally these approaches can be divided into DNA-based methods employing empirically characterized probes and rRNA-based methods based on comparative sequence analysis for design and interpretation of “rational” probes. Polymerase chain reaction (PCR) based techniques can also be applied to the analysis of microbial communities in the GIT. Direct cloning of SSU rDNA genes amplified from these complex communities can be used to determine the extent of diversity in these GIT communities. Denaturing gradient gel electrophoresis (DGGE) is another powerful tool for profiling microbial diversity of microbial communities in GI tracts. Sequence analysis of the excised DGGE amplicons can then be used to presumptively identify predominant bacterial species. Examples of how these molecular approaches are being used to study the microbial diversity of communities from steers fed different diets, swine fed probiotics, and Atlantic salmon fed aquaculture diets are presented.
A PCR based quantitative assay was used to determine Wolbachia infection levels in three different Drosophila strains. In addition, confocal microscopy was used to confirm and calibrate these results. Wolbachia infection levels ranged from 2,600 to 18,500 per egg. Single ovaries and testes from each of the three strains were also assayed using the calibrated quantitative PCR assay. A general correlation was found between bacterial levels in eggs and those found in ovaries and testis. These infection levels were consistent with the expression of cytoplasmic incompatibility (CI). In two strains of D. simulans, although the overall bacterial numbers were not significantly different, they exhibited different levels of CI. A direct correlation between the number of infected developing sperm cysts in these strains and CI levels was observed. This calibrated assay should provide a useful baseline for future comparative work, particularly between laboratories.
Cytoplasmic incompatibility (CI) induced by intracellular bacteria is a possible mechanism for speciation. Growing empirical evidence suggests that bacteria of the group Wolbachia may indeed act as isolating factors in recent insect speciation. Wolbachia are cytoplasmically transmitted and can cause uni- or bidirectional CI. We present a mainland-island model to investigate how much impact Wolbachia can have on genetic divergence between populations. In the first scenario we assume that the island population has diverged at a selected locus and ask whether genetic divergence will be maintained after introduction of migration from the mainland. In the second we explore whether divergence will originate under migration. For simplicity, the host organisms are modeled as haploid sexuals. Simulations show that if each population is initially infected with a different strain of Wolbachia, then higher levels of divergence occur at the locally selected locus than in the absence of Wolbachia. A weaker effect is seen when there is only unidirectional CI caused by a single strain of Wolbachia on the island. CI increases divergence because it reduces effective migration between mainland and island. Migrants suffer from being confronted with the wrong CI system and this also applies to their matrilineal descendants. Moreover, there is a strong linkage disequilibrium between host genotype and infection state, which helps to maintain Wolbachia differences between the populations in the face of migration A sex bias in migration can either increase or decrease the effect of Wolbachia on divergence. Results support the view that Wolbachia has the potential for increasing divergence between populations and thus could enhance probabilities of speciation.
Classic ectomycorrhizal symbioses are mutualisms that involve the exchange of fixed carbon for mineral nutrients between plant roots and fungi. They are unique in the way they contain features of both intimate and diffuse symbioses. The degree of host specificity varies, particularly among the fungi. Here we examine two exceptional cases of specificity to see what they tell us about the advantages of specificity, how it is initiated, and the potential role that it plays in complex ecosystems. The first case involves non-photosynthetic epiparasitic plants, which contrary to virtually all other plants, exhibit high levels of specificity toward their fungal hosts. The second case involves suilloid fungi; this is the largest monophyletic group of ectomycorrhizal fungi that is essentially restricted to associations with a single plant family. In both cases, new symbioses are initiated by dormant propagules that are stimulated to germinate by chemical cues from the host. This reduces the cost of wasting propagules on non-hosts. The advantages of specificity remain unclear in both cases, but we argue that increased benefit to the specialist may result from specialized physiological adaptations. We reexamine the idea that specialist fungi may help their hosts compete in complex ecosystems by reducing facultative epiparasitism by other plants, and suggest an alternative hypothesis for the observed pattern.
Fungal endophytes are extremely common and highly diverse microorganisms that live within plant tissues, but usually remain asymptomatic. Endophytes traditionally have been considered plant mutualists, mainly by reducing herbivory via production of mycotoxins, such as alkaloids. However, the vast majority of endophytes, especially horizontally-transmitted ones commonly found in woody plants, apparently have little or no effect on herbivores. For the systemic, vertically-transmitted endophytes of grasses, mutualistic interactions via increased resistance to herbivores and pathogens are more common, as predicted by evolutionary theory. However, even in these obligate symbioses, endophytes are often neutral or even pathogenic to the host grass, depending on endophyte and plant genotype and environmental conditions.
We present a graphical model based upon variation in nitrogen flux in the host plant. Nitrogen is a common currency in endophyte/host and plant/herbivore interactions in terms of limitations to host plant growth, enhanced uptake by endophytes, demand for synthesis of nitrogen-rich alkaloids, and herbivore preference and performance. Our graphical model predicts that low alkaloid-producing endophytes should persist in populations when soil nutrients and herbivory are low. Alternatively, high alkaloid endophytes are favored under increasing herbivory and increasing soil nitrogen, at least to some point. At very high soil nitrogen levels, uninfected plants may be favored over either type of infected plants. These predictions are supported by patterns of infection and alkaloid production in nature, as well by a manipulative field experiment. However, plant genotype and other environmental factors, such as available water, interact with the presence of the endophyte to influence host plant performance.
Mutualistic interactions are widespread and obligatory for many organisms, yet their evolutionary persistence in the face of cheating is theoretically puzzling. Nutrient-acquisition symbioses between plants and soil microbes are critically important to plant evolution and ecosystem function, yet we know almost nothing about the evolutionary dynamics and mechanisms of persistence of these ancient mutualisms. Partner-choice and partner-fidelity are mechanisms for dealing with cheaters, and can theoretically allow mutualisms to persist despite cheaters.
Many models of cooperative behavior assume pairwise interactions, while most plant-microbe nutrient-acquisition symbioses involve a single plant interacting with numerous microbes. Market models, in contrast, are well suited to mutualisms in which single plants attempt to conduct mutually beneficial resource exchange with multiple individuals. Market models assume that one partner chooses to trade with a subset of individuals selected from a market of potential partners. Hence, determining whether partner-choice occurs in plant-microbe mutualisms is critical to understanding the evolutionary persistence and dynamics of these symbioses. The nitrogen-fixation/carbon-fixation mutualism between leguminous plants and rhizobial bacteria is widespread, ancient, and important for ecosystem function and human nutrition. It also involves single plants interacting simultaneously with several to many bacterial partners, including ineffective (“cheating”) strains. We review the existing literature and find that this mutualism displays several elements of partner-choice, and may match the requirements of the market paradigm. We conclude by identifying profitable questions for future research.
Many of the most commonly cited examples of exquisite adaptation are of coevolved symbioses. As we learn more about the coevolutionary process, however, it is becoming increasingly evident that coevolution may also keep populations moderately maladapted much of the time. As a result, coevolving populations may only rarely occupy adaptive peaks, because the selective landscape is under continual change through reciprocal selection on the species themselves. These shifting patterns of coadaptation are further shaped by the geographic structure of most species. Selection mosaics across landscapes and coevolutionary hotspots can favor different evolutionary trajectories in different populations. The combined action of gene flow, random genetic drift, and local extinction of populations may then continually remold these local patterns, creating a geographic mosaic in the degrees of maladaptation found within local interactions. Recent mathematical models of the geographic mosaic of coevolution suggest that complex mosaics of maladaptation are a likely consequence of spatially structured species interactions. These models indicate that the spatial structure of maladaptation may depend upon the type of coevolutionary interaction, the underlying selection mosaic, and patterns of gene flow across landscapes. By maintaining local polymorphisms and driving the divergence of populations, coevolution may produce spatial patterns of maladaptation that are a source of ongoing innovation and diversification in species interactions.
Advances in molecular genetic techniques have provided new approaches for addressing evolutionary questions about brood parasitic birds. We review recent studies that apply genetic data to the systematics, population biology, and social systems of avian brood parasites and suggest directions for future research. Recent molecular systematics studies indicate that obligate brood parasitism has evolved independently in seven different avian lineages, a tally that has increased by one in cuckoos (Cuculiformes) and decreased by one in passeriforms (Passeriformes) as compared to conventional taxonomy. Genetic parentage analyses suggest that brood parasitic birds are less promiscuous than might be expected given their lack of nesting and parental care behavior. Host-specificity in brood parasites, which has important implications for host-parasite coevolution, has been evaluated using both population genetic and parentage analyses. Female lineages are faithful to particular host species over evolutionarily significant time scales in both common cuckoos (Cuculus canorus) and indigobirds (Vidua spp.), but differences in the host-specificity of male parasites has resulted in different patterns of diversification in these two lineages. Future research on brood parasitism will benefit from the availability of comprehensive molecular phylogenies for brood parasites and their hosts and from advances in functional genomics.
Malaria has been invoked, perhaps more than any other infectious disease, as a force for the selection of human genetic polymorphisms. Evidence for genome-shaping interactions can be found in the geographic and ethnic distributions of the hemoglobins, blood group antigens, thalassemias, red cell membrane molecules, human lymphocyte antigen (HLA) classes, and cytokines. Human immune responses and genetic variations can correspondingly influence the structure and polymorphisms of Plasmodium populations, notably in genes that affect the success and virulence of infection. In Africa, where the burden from Plasmodium falciparum predominates, disease severity and manifestations vary in prevalence among human populations. The evolutionary history and spread of Plasmodium species inform our assessment of malaria as a selective force. Longstanding host-pathogen relationships, as well as recent changes in this dynamic, illustrate the selective pressures human and Plasmodium species place on one another. Investigations of malaria protection determinants and virulence factors that contribute to the complexity of the disease should advance our understanding of malaria pathogenesis.