The evolution of regeneration: adaptive or inherent?

If regeneration were adaptive, it would have arisen autonomously by natural selection from non-regenerative antecedents. Unless each episode coincidentally reinvented the same method of regeneration independently, one would expect the various lineages to differ basically from each other, which they do not. On the other hand, if regeneration were inherent to metazoan life, a derivative of embryogenesis, its various expressions should be as much like each other as they resemble the development of embryonic appendage buds, which they do. It follows that the uneven distribution of regeneration must have been due to its extinction here and there, not as a negative adaptation by natural selection but as a pleiotropic epiphenomenon linked to more useful adaptations with which it was incompatible. In vertebrate evolution, these adaptations have included the transition from aquatic to terrestrial habitats and the modification of poikilothermic to homeothermic metabolism. The former advance rendered the regeneration of weight-bearing limbs impractical; the latter favored rapid wound healing and scar formation which effectively precluded blastema formation. If the latent capacity for regeneration persists in non-regenerative appendages, as would seem to be the case, then the restoration of its overt expression should be possible if the mechanisms of its inhibition could be discovered and eventually rendered ineffectual.     

The phylogeny and evolution of thePterygodium—Corycium complex (Coryciinae,Orchidaceae)

The leaf-anatomy, palynology, seed-morphology, vegetative morphology and especially the highly complicated floral morphology of theCoryciinae s. str. (Diseae: Orchidoideae: Orchidaceae) are described and illustrated in detail. On the basis of these characters the presumed phylogeny, based on a rigorous cladistic analysis, is presented. The cladistic biogeographical analysis of theCoryciinae s. str. shows that it is a member of the Afrotemperate Track, with a pattern of vicariance events typical of the members of this track. An analysis of the patterns of speciation shows that allopatric speciation appears to be rare, and that parapatric speciation across edaphic boundaries may be the most important factor. Proceeding from the information presented, a new classification of the group is proposed in which we recognize the four generaCeratandra, Evotella, Pterygodium andCorycium. The new monotypic genusEvotella comprises a species originally described asPterygodium rubiginosum. The three species of the genusAnochilus are transferred toCorycium andPterygodium. P. magnum, which was originally described asPterygodium but was transferred toCorycium lately, is placed in a monotypic section ofPterygodium.     

The antigenic evolution of influenza: drift or thrift?

It is commonly assumed that antibody responses against the influenza virus are polarized in the following manner: strong antibody responses are directed at highly variable antigenic epitopes, which consequently undergo ‘antigenic drift’, while weak antibody responses develop against conserved epitopes. As the highly variable epitopes are in a constant state of flux, current antibody-based vaccine strategies are focused on the conserved epitopes in the expectation that they will provide some level of clinical protection after appropriate boosting. Here, we use a theoretical model to suggest the existence of epitopes of low variability, which elicit a high degree of both clinical and transmission-blocking immunity. We show that several epidemiological features of influenza and its serological and molecular profiles are consistent with this model of ‘antigenic thrift’, and that identifying the protective epitopes of low variability predicted by this model could offer a more viable alternative to regularly update the influenza vaccine than exploiting responses to weakly immunogenic conserved regions.     

Pseudopollen in Dendrobium unicum Seidenf. (Orchidaceae): Reward or Deception?

BACKGROUND AND AIMS: In 1987, Kjellsson and Rasmussen described the labellar trichomes of Dendrobium unicum Seidenf. and proposed that these hairs function as pseudopollen. Pseudopollen is a mealy material that superficially resembles pollen, is usually laden with food substances and is formed when labellar hairs either fragment into individual cells or become detached from the labellum. However, the trichomes of D. unicum are very different from pseudopollen-forming hairs found in other orchid genera such as Maxillaria and Polystachya. Moreover, Kjellsson and Rasmussen were unable to demonstrate the presence of food substances within these trichomes and argued that even in the absence of food substances, the hairs, in that they superficially resemble pollen, can still attract insects by deceit. The aim of this paper is to investigate whether the labellar trichomes of D. unicum contain food reserves and thus reward potential pollinators or whether they are devoid of foods and attract insects solely by mimicry. METHODS: Light microscopy, histochemistry and transmission electron microscopy. KEY RESULTS: Dendrobium unicum produces pseudopollen. Pseudopollen here, however, differs from that previously described for other orchid genera in that the pseudopollen-forming trichomes consist of a stalk cell and a 'head' of component cells that separate at maturity, in contrast to Maxillaria and some Polystachya spp. where pseudopollen is formed by the fragmentation of moniliform hairs. Moreover, the pseudopollen of Maxillaria and Polystachya largely contains protein, whereas in D. unicum the main food substance is starch. CONCLUSIONS: Flowers of D. unicum, rather than attracting insects solely by deceit may also reward potential pollinators.     

The evolution of sexual dimorphism in parasitic cuckoos: sexual selection or coevolution?

Sexual dimorphism is ubiquitous in animals and can result from selection pressure on one or both sexes. Sexual selection has become the predominant explanation for the evolution of sexual dimorphism, with strong selection on size-related mating success in males being the most common situation. The cuckoos (family Cuculidae) provide an exceptional case in which both sexes of many species are freed from the burden of parental care but where coevolution between parasitic cuckoos and their hosts also results in intense selection. Here, we show that size and plumage differences between the sexes in parasitic cuckoos are more likely the result of coevolution than sexual selection. While both sexes changed in size as brood parasitism evolved, we find no evidence for selection on males to become larger. Rather, our analysis indicates stronger selection on parasitic females to become smaller, resulting in a shift from dimorphism with larger females in cuckoos with parental care to dimorphism with larger males in parasitic species. In addition, the evolution of brood parasitism was associated with more cryptic plumage in both sexes, but especially in females, a result that contrasts with the strong plumage dimorphism seen in some other parasitic birds. Examination of the three independent origins of brood parasitism suggests that different parasitic cuckoo lineages followed divergent evolutionary pathways to successful brood parasitism. These results argue for the powerful role of parasite-host coevolution in shaping cuckoo life histories in general and sexual dimorphism in particular.     

The origin of RNA interference: Adaptive or neutral evolution?

The origin of RNA interference (RNAi) is usually explained by a defense-based hypothesis, in which RNAi evolved as a defense against transposable elements (TEs) and RNA viruses and was already present in the last eukaryotic common ancestor (LECA). However, since RNA antisense regulation and double-stranded RNAs (dsRNAs) are ancient and widespread phenomena, the origin of defensive RNAi should have occurred in parallel with its regulative functions to avoid imbalances in gene regulation. Thus, we propose a neutral evolutionary hypothesis for the origin of RNAi in which qualitative system drift from a prokaryotic antisense RNA gene regulation mechanism leads to the formation of RNAi through constructive neutral evolution (CNE). We argue that RNAi was already present in the ancestor of LECA before the need for a new defense system arose and that its presence helped to shape eukaryotic genomic architecture and stability.

Where does RNA interference come from? This Essay describes a new step-by-step evolutionary model of how RNA interference might have originated in early eukaryotes through neutral events from the molecular machinery present in prokaryotes.     

The evolution of early animal embryos: conservation or divergence?

There is a remarkable similarity in the appearance of groups of animal species during periods of their embryonic development. This classic observation has long been viewed as an emphatic realization of the principle of common descent. Despite the importance of embryonic conservation as a unifying concept, models seeking to predict and explain different patterns of conservation have remained in contention. Here, we focus on early embryonic development and discuss several lines of evidence, from recent molecular data, through developmental networks to life-history strategies, that indicate that early animal embryos are not highly conserved. Bringing this evidence together, we argue that the nature of early development often reflects adaptation to diverse ecological niches. Finally, we synthesize old and new ideas to propose a model that accounts for the evolutionary process by which embryos have come to be conserved.     

The flower biology ofCephalanthera longifolia (Orchidaceae)—pollen imitation and facultative floral mimicry

Solitary bees (Halictus sp.) were found to be the effective pollinators ofCephalanthera longifolia. In the same foraging flight the bees also visit flowers ofCistus salviifolius which has a similar colour pattern.Cephalanthera offers no reward to its pollinators, but orange papillae on its labellum successfully imitate pollen ofCistus. AsCephalanthera also attracts pollinators in the absence ofCistus, this is regarded as facultative floral mimicry.     

The evolution of life cycle complexity in aphids: Ecological optimization or historical constraint?

For decades, biologists have debated why many parasites have obligate multihost life cycles. Here, we use comparative phylogenetic analyses of aphids to evaluate the roles of ecological optimization and historical constraint in the evolution of life cycle complexity. If life cycle complexity is adaptive, it should be evolutionarily labile, that is, change in response to selection. We provide evidence that this is true in some aphids (aphidines), but not others (nonaphidines)—groups that differ in the intensity of their relationships with primary hosts. Next, we test specific mechanisms by which life cycle complexity could be adaptive or a constraint. We find that among aphidines there is a strong association between complex life cycles and polyphagy but only a weak correlation between life cycle complexity and reproductive mode. In contrast, among nonaphidines the relationship between life cycle complexity and host breadth is weak but the association between complex life cycles and sexual reproduction is strong. Thus, although the adaptiveness of life cycle complexity appears to be lineage specific, across aphids, life cycle evolution appears to be tightly linked with the evolution of other important natural history traits.     

Pollinator attraction in Anacamptis papilionacea (Orchidaceae): a food or a sex promise?

The Mediterranean orchid Anacamptis papilionacea , despite showing a typical food-deceptive floral display, has also been reported to frequently attract male pollinators, suggesting a potentiality for sexual attraction. In a survey from a southern Italian population of A. papilionacea and their hybrids with Anacamptis morio , we collected 37 pollinators belonging to five bee species carrying 126 orchid pollinia. The main pollinator of A. papilionacea was Anthophora crinipes male (48.6%), but the number of females was not negligible (22.9%). We also found pollinator sharing between the hybrid and the parental species. Our findings confirm that, contrary to other food-deceptive species, A. papilionacea mainly attracts male insects, but also that, in contrast to sexually deceptive species, this attraction is not specific. We suggest that A. papilionacea adopts a complex mix of food and sexually deceptive pollination and could represent a helpful model for studying the transition between different pollination strategies.     

Exine Micromorphology of Orchidinae (Orchidoideae,Orchidaceae): Phylogenetic Constraints or Ecological Influences?

BACKGROUND AND AIMS: Pollen characters have been widely used in defining evolutionary trends in orchids. In recent years, information on pollination biology and phylogenetic patterns within Orchidinae has become available. Hence, the aim of the presented work is to re-evaluate exine micromorphology of Orchidinae in light of recent phylogenetic studies and to test whether pollen micromorphology strictly depends on phylogenetic relationships among species or whether it is influenced by the marked differences in pollination ecology also reported among closely related species. METHODS: Pollen sculpturing of 45 species of Orchidinae and related taxa was investigated using scanning electron microscopy. To cover potential intraspecific variation, several accessions of the same species were examined. KEY RESULTS: Orchidinae show remarkable variation in exine sculpturing, with a different level of variation within species groups. In some genera, such as Serapias (rugulate) and Ophrys (psilate to verrucate), intrageneric uniformity corresponds well to a common pollination strategy and close relationships among species. However, little exine variability (psilate-scabrate and scabrate-rugulate) was also found in the genus Anacamptis in spite of striking differences in floral architecture and pollination strategies. A larger variety of exine conditions was found in genera Dactylorhiza (psilate, psilate-scabrate and reticulate) and Orchis s.s. (psilate, reticulate, perforate-rugulate and baculate) where no unequivocal correspondence can be found to either phylogenetic patterns or pollination strategies. CONCLUSIONS: Changes in pollen characteristics do not consistently reflect shifts in pollination strategy. A unique trend of exine evolution within Orchidinae is difficult to trace. However, the clades comprising Anacamptis, Neotinea, Ophrys and Serapias show psilate to rugulate or scabrate pollen, while that of the clade comprising Chamorchis, Dactylorhiza, Gymnadenia, Orchis s.s., Platanthera, Pseudorchis and Traunsteinera ranges from psilate to reticulate. Comparison of the data with exine micromorphology from members of the tribe Orchidieae and related tribes suggests a possible general trend from reticulate to psilate.     

The emergence of technology: A cultural step or long-term evolution?

There are two hypotheses to explain the emergence of the technical behaviour: (1) a sudden and fast appearance across the African continent; (2) a long period of emergence and consolidation before it began to spread. We defend this second hypothesis and suggest that the hominid project started around 3 Myr ago, with a ‘biofunctional’ stage. Later than 2.5 Myr, the ‘biomorphotechnical’ stage began (Mode 1), and, at around 1.7 Myr, another stage emerged and developed: the ‘biopotential’ stage (Mode 2). All these stages consist of a two-part process: innovation/emergence, and socialization/generalization of a technical mode.     

Why do paralogs persist? Molecular evolution of CYCLOIDEA and related floral symmetry genes in Antirrhineae (Veronicaceae)

CYCLOIDEA (CYC) and DICHOTOMA (DICH) are paralogous genes that determine adaxial (dorsal) flower identity in the bilaterally symmetric flowers of Antirrhinum majus (snapdragon). We show here that the duplication leading to the existence of both CYC and DICH in Antirrhinum occurred before the radiation of the Antirrhineae (the tribe to which snapdragon belongs). We find no additional gene duplications within Antirrhineae. Using explicit codon-based models of evolution in a likelihood framework, we show that patterns of molecular evolution after the duplication that gave rise to CYC and DICH are consistent with purifying selection acting at both loci, despite their known functional redundancy in snapdragon. However, for specific gene regions, purifying selection is significantly relaxed across DICH lineages, relative to CYC lineages. In addition, we find evidence for relaxed purifying selection along the lineage leading to snapdragon in one of two putative functional domains of DICH. A model of selection accounting for the persistence of paralogous genes in the absence of diversifying selection is presented. This model takes into account differences in the degree of purifying selection acting at the two loci and is consistent with subfunctionalization models of paralogous gene evolution.     

The evolution of endothermy in terrestrial vertebrates: Who? When? Why?

Avian and mammalian endothermy results from elevated rates of resting, or routine, metabolism and enables these animals to maintain high and stable body temperatures in the face of variable ambient temperatures. Endothermy is also associated with enhanced stamina and elevated capacity for aerobic metabolism during periods of prolonged activity. These attributes of birds and mammals have greatly contributed to their widespread distribution and ecological success. Unfortunately, since few anatomical/physiological attributes linked to endothermy are preserved in fossils, the origin of endothermy among the ancestors of mammals and birds has long remained obscure. Two recent approaches provide new insight into the metabolic physiology of extinct forms. One addresses chronic (resting) metabolic rates and emphasizes the presence of nasal respiratory turbinates in virtually all extant endotherms. These structures are associated with recovery of respiratory heat and moisture in animals with high resting metabolic rates. The fossil record of nonmammalian synapsids suggests that at least two Late Permian lineages possessed incipient respiratory turbinates. In contrast, these structures appear to have been absent in dinosaurs and nonornithurine birds. Instead, nasal morphology suggests that in the avian lineage, respiratory turbinates first appeared in Cretaceous ornithurines. The other approach addresses the capacity for maximal aerobic activity and examines lung structure and ventilatory mechanisms. There is no positive evidence to support the reconstruction of a derived, avian-like parabronchial lung/air sac system in dinosaurs or nonornithurine birds. Dinosaur lungs were likely heterogenous, multicameral septate lungs with conventional, tidal ventilation, although evidence from some theropods suggests that at least this group may have had a hepatic piston mechanism of supplementary lung ventilation. This suggests that dinosaurs and nonornithurine birds generally lacked the capacity for high, avian-like levels of sustained activity, although the aerobic capacity of theropods may have exceeded that of extant ectotherms. The avian parabronchial lung/air sac system appears to be an attribute limited to ornithurine birds.     

Diversity and evolution of a trait mediating ant–plant interactions: insights from extrafloral nectaries in Senna (Leguminosae)

Background and Aims

Plants display a wide range of traits that allow them to use animals for vital tasks. To attract and reward aggressive ants that protect developing leaves and flowers from consumers, many plants bear extrafloral nectaries (EFNs). EFNs are exceptionally diverse in morphology and locations on a plant. In this study the evolution of EFN diversity is explored by focusing on the legume genus Senna, in which EFNs underwent remarkable morphological diversification and occur in over 80 % of the approx. 350 species.

Methods

EFN diversity in location, morphology and plant ontogeny was characterized in wild and cultivated plants, using scanning electron microscopy and microtome sectioning. From these data EFN evolution was reconstructed in a phylogenetic framework comprising 83 Senna species.

Key Results

Two distinct kinds of EFNs exist in two unrelated clades within Senna. ‘Individualized’ EFNs (iEFNs), located on the compound leaves and sometimes at the base of pedicels, display a conspicuous, gland-like nectary structure, are highly diverse in shape and characterize the species-rich EFN clade. Previously overlooked ‘non-individualized’ EFNs (non-iEFNs) embedded within stipules, bracts, and sepals are cryptic and may represent a new synapomorphy for clade II. Leaves bear EFNs consistently throughout plant ontogeny. In one species, however, early seedlings develop iEFNs between the first pair of leaflets, but later leaves produce them at the leaf base. This ontogenetic shift reflects our inferred diversification history of iEFN location: ancestral leaves bore EFNs between the first pair of leaflets, while leaves derived from them bore EFNs either between multiple pairs of leaflets or at the leaf base.

Conclusions

EFNs are more diverse than previously thought. EFN-bearing plant parts provide different opportunities for EFN presentation (i.e. location) and individualization (i.e. morphology), with implications for EFN morphological evolution, EFN–ant protective mutualisms and the evolutionary role of EFNs in plant diversification.     

Seed viability in declining populations of Caladenia rigida (Orchidaceae): are small populations doomed?

Despite comparatively good rates of pollination and seed production, some populations of the endangered terrestrial orchid Caladenia rigida continue to decline. To determine whether seed quality may be limiting reproductive potential, we assessed seed viability among declining populations of C.?rigida (in the southern part of its distribution) and among populations that are regarded as stable (in the northern part of its distribution). We also compared differences in seed viability to plant traits, population size and habitat characteristics (soil properties, canopy cover, presence of proximate vegetation). Seed capsules from southern populations were significantly smaller, with only 9% of seeds being viable, compared to 36% in capsules from northern populations. Soil phosphorus concentrations differed between regions, but other habitat characteristics did not correlate with seed viability. Using calculations based on seedling recruitment data from other Caladenia species, we predict that seed output is insufficient to ensure the long-term persistence of the smallest C.?rigida populations.