Parasites and deleterious mutations: interactions influencing the evolutionary maintenance of sex

The restrictive assumptions associated with purely genetic and purely ecological mechanisms suggest that neither of the two forces, in isolation, can offer a general explanation for the evolutionary maintenance of sex. Consequently, attention has turned to pluralistic models (i.e. models that apply both ecological and genetic mechanisms). Existing research has shown that combining mutation accumulation and parasitism allows restrictive assumptions about genetic and parasite parameter values to be relaxed while still predicting the maintenance of sex. However, several empirical studies have shown that deleterious mutations and parasitism can reduce fitness to a greater extent than would be expected if the two acted independently. We show how interactions between these genetic and ecological forces can completely reverse predictions about the evolution of reproductive modes. Moreover, we demonstrate that synergistic interactions between infection and deleterious mutations can render sex evolutionarily stable even when there is antagonistic epistasis among deleterious mutations, thereby widening the conditions for the evolutionary maintenance of sex.     

The evolutionary maintenance of sexual reproduction: The solutions proposed for a longstanding problem

The evolutionary maintenance of mixis is one of the major unsolved problems in modern biology. This paper reviews the phenoraenon of sex, the hypotheses for its maintenance, and recent evidence bearing on the hypotheses. One elegant experiment supports the idea that bacterial transformation, an analogue and possible forerunner of eukaryolic mixis, functions as a repair mechanism. All mechanisms that produce a short-term advantage for sex in eukaryotes and that are supported by experimental results rely on strong genotype by environment interactions for fitness. While many environmental factors are involved, most prominently parasites, disease, and coarse-grained environmental heterogeneity of other sorts, each is effective only insofar as it is involved in a genotype by environment interaction for fitness.     

Plant mimicry: evolutionary constraints

Because plants are sessile and their flowers and fruits are aggregated, plant mimics are less likely to be mistaken for their models than animal mimics which are mobile and dispersed among their models. Therefore, operator species are more likely to be deceived by animal mimics than plant mimics. In addition, the autonomy of plant appendages implies that warning mimicry provides less advantage to plants than to animals because plants suffer less from sampling by naive operators. Therefore, the advantage of warning mimicry is much greater for animals than plants. These reasons may explain why plant mimicry is less common than animal mimicry, based on attraction of rather than avoidance by operator species, and limited to the class of aggressive mimicry.     

Evo-Devo: evolutionary developmental mechanisms

Evolutionary developmental biology (Evo-Devo) as a discipline is concerned, among other things, with discovering and understanding the role of changes in developmental mechanisms in the evolutionary origin of aspects of the phenotype. In a very real sense, Evo-Devo opens the black box between genotype and phenotype, or more properly, phenotypes as multiple life history stages arise in many organisms from a single genotype. Changes in the timing or positioning of an aspect of development in a descendant relative to an ancestor (heterochrony and heterotopy) were two evolutionary developmental mechanisms identified by Ernst Haeckel in the 1870s. Many more have since been identified, in large part because of our enhanced understanding of development and because new mechanisms emerge as development proceeds: the transfer from maternal to zygotic genomic control; cell-to-cell interactions; cell differentiation and cell migration; embryonic inductions; functional interactions at the tissue and organ levels; growth. Within these emergent processes, gene networks and gene cascades (genetic modules) link the genotype with morphogenetic units (cellular modules, namely germ layers, embryonic fields or cellular condensations), while epigenetic processes such as embryonic inductions, tissue interactions and functional integration, link morphogenetic units to the phenotype. Evolutionary developmental mechanisms also include interactions between individuals of the same species, individuals of different species, and species and their biotic and/or abiotic environment. Such interactions link ecological communities. Importantly, there is little to distinguish the causality that underlies these interactions from that which underlies inductive interactions within embryos.     

Fitness trade-offs and the maintenance of alternative male morphs in the bulb mite (Rhizoglyphus robini)

Alternative reproductive phenotypes (ARPs) occur across a wide range of taxa. Most ARPs are conditionally expressed in response to a cue, for example body size, that reliably correlates with the status of the environment: individuals below the (body size) threshold then develop into one morph, and individuals above the threshold develop into the alternative morph. The environmental threshold model provides a theoretical framework to understand the evolution and maintenance of such ARPs, yet no study has examined the underlying fitness functions that are necessary to realize this. Here, we empirically examined fitness functions for the two male morphs of the bulb mite (Rhizoglyphus robini). Fitness functions were derived in relation to male size for solitary males and in relation to female size under competition. In both cases, the fitness functions of the two morphs intersected, and the resulting fitness trade-offs may play a role in the maintenance of this male dimorphism. We furthermore found that competition was strongest between males of the same morph, suggesting that fitness trade-off in relation to male size may persist under competition. Our results are a first step towards unravelling fitness functions of ARPs that are environmentally cued threshold traits, which is essential for understanding their maintenance and in explaining the response to selection against alternative morphs.     

The maintenance of sex as a developmental trap due to sexual selection

The writings of George Williams challenged biologists to think critically about levels of selection, social behavior, and the paradox of sex, whose maintenance by recombination alone has not been convincingly demonstrated in theory or in fact. A solution is suggested by observations of the dependence of females on interaction with males, as a result of sexual selection. This, along with recombination in changing environments and DNA repair during meiosis, may contribute to a pluralistic explanation for the maintenance of sex.     

性的进化:起源和维持

有性繁殖是真核生物生命周期的一个普遍特征。然而,在解释有性繁殖的这种普遍存在时却遇到了许多的困难,包括雄性的双重损失和重组负荷等。那么,有性繁殖是如何起源和维持的？人们提出了许多可能解释的假说,但是到目前为止还没有一个明确的答案。作者就众多的理论做一个简明综述。     

Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes)

Sexual parasitism, a remarkable mode of reproduction unique to some members of the deep-sea anglerfish suborder Ceratioidei, in which males are dwarfed and become permanently attached to much larger females, is described in a number of previously unreported specimens and taxa. Although generally attributed to all ceratioids, and despite more than a sevenfold increase in the number of known parasitized females in collections throughout the world in the past 50 years, the phenomenon is surprisingly confined to few taxa within the suborder. To date, permanently attached males have been found in only 5 of the 11 ceratioid families, 10 of the 35 genera, and 23 of the 160 recognized species. Notes on taxonomic content, available material, occurrence of sexual parasitism, gravid females and ripe males, the development of eyes and nostrils of the males, the ability of males to capture and ingest food independently of the female, occurrences of multiple attachments of males to a single female, and the nature of the fusion between coupled males and females are given for each ceratioid family. This information is then summarized and discussed. Evidence is presented to reaffirm the presence of three reproductive modes: obligatory parasitism, facultative parasitism, and temporary nonparasitic attachment. Additional evidence is provided to reaffirm the hypothesis that sexual parasitism has evolved separately at least three and perhaps five or more times within the suborder.     

Asian colobine social structure: Ecological and evolutionary constraints

Both ecological and phylogenetic factors and their interaction influence primate social structure. Food resources, in particular, affect social structure parameters such as sex/age composition, group size, home range size, and individual density. As a group, the Asian colobines may be partially buffered from the impact of food constraints on social structure, given their specialized digestive physiology. Asian colobines are able to subsist on mature foliage, a relatively abundant and non-patchily distributed resource, when preferred foods are not available. In food limited populations, increases in group size or density should cause increases in home range size. We did not find this relationship for Asian colobines, however, and thus it would appear that the majority of Asian colobine populations are below carrying capacity. Food may be less of a limitation to populations than other factors, such as social stress. The formation of bands (associations between relatively stable groups) has been documented for bothNasalis andRhinopithecus, and based on recent data for severalPresbytis andTrachypithecus species, we predict that band formation is the norm for the Asian colobines, includingPygathrix. Group home ranges forPresbytis andTrachypithecus species typically overlap and both intergroup tolerance and intergroup aggression are observed. This suggests that groups form differentiated relationships, tolerating some groups but not others. Such differentiated relationships are the foundation of band formation.     

Physiological regulatory networks: ecological roles and evolutionary constraints

Ecological and evolutionary physiology has traditionally focused on one aspect of physiology at a time. Here, we discuss the implications of considering physiological regulatory networks (PRNs) as integrated wholes, a perspective that reveals novel roles for physiology in organismal ecology and evolution. For example, evolutionary response to changes in resource abundance might be constrained by the role of dietary micronutrients in immune response regulation, given a particular pathogen environment. Because many physiological components impact more than one process, organismal homeostasis is maintained, individual fitness is determined and evolutionary change is constrained (or facilitated) by interactions within PRNs. We discuss how PRN structure and its system-level properties could determine both individual performance and patterns of physiological evolution.     

Energetics of reproduction and its evolutionary implications

The majority of invertebrates so far studied are more efficient converters of absorbed food energy to gametes than they are, as adults, to somatic tissues. Indeed, reproductive conversion efficiencies sometimes better the best conversion efficiencies associated with somatic production (usually in juveniles). Depending on species, this is achieved either by an increase in absorption relative to respiratory losses, or a reduction in respiratory losses relative to absorption, or by supplementing the energy absorbed from the food with that obtained from somatic stores and tissues, or by a combination of these. The fact that the metabolic processes associated with gamete production are distinct from those associated with the production of somatic tissues has implications for the theory of life-cycle evolution.     

Energetic strategies of shrews: ecological constraints and evolutionary implications

Data on various features influencing resource acquisition and allocation of energy in Soricidae are reviewed. The relationships between these features are examined at the three functional levels that constrain the energetic design of shrews: the maintenance of homeostasis (rate of metabolism and temperature regulation), the balancing of the energy budget (e.g. the influence of body mass, activity rate, various energy saving mechanisms and home range size), and the allocation of energy towards reproduction (e.g. the influence of litter size). Two major contradictory energetic designs may be recognized: one rather expensive design that is observed in cold and temperate climates with relatively predictable fluctuations in resource availability (e.g. in Sorex and Neomys ), and another much less expensive design that is observed mainly in warm or unpredictable environments (e.g. in Crocidura, Suncus and Notiosorex). It is speculated that climate and resource availability impose narrow limits on the evolution of these energetic strategies, mainly because of the small thermal inertia and reduced energetic autonomy of shrews.     

Typology now: homology and developmental constraints explain evolvability

By linking the concepts of homology and morphological organization to evolvability, this paper attempts to (1) bridge the gap between developmental and phylogenetic approaches to homology and to (2) show that developmental constraints and natural selection are compatible and in fact complementary. I conceive of a homologue as a unit of morphological evolvability, i.e., as a part of an organism that can exhibit heritable phenotypic variation independently of the organism’s other homologues. An account of homology therefore consists in explaining how an organism’s developmental constitution results in different homologues/characters as units that can evolve independently of each other. The explanans of an account of homology is developmental, yet the very explanandum is an evolutionary phenomenon: evolvability in a character-by-character fashion, which manifests itself in phylogenetic patterns as recognized by phylogenetic approaches to homology. While developmental constraints and selection have often been viewed as antagonistic forces, I argue that both are complementary as they concern different parts of the evolutionary process. Developmental constraints, conceived of as the presence of the same set of homologues across phenotypic change, pertain to how heritable variation can be generated in the first place (evolvability), while natural selection operates subsequently on the produced variation.

Pattern formation models and developmental constraints

Most schemes for embryonic pattern formation are built around the notion of lateral inhibition. Models of this type arise in many settings, and all share some common characteristics. In this paper we examine a number of pattern formation models and show how the phenomenon of lateral inhibition constrains the possible geometries that can arise.