Saltational evolution of trunk segment number in centipedes

Two-way exchanges between animal and plant biology, with focus on evo-devo

By definition, biology is the science of all living beings. However, horizons restricted to either plants or animals have characterized the development of life sciences well beyond the emergence of unified perspectives applying to all forms of life, such as the cell theory or the theory of evolution. Separation between botanical and zoological traditions is not destined to go extinct easily, or shortly. Disciplinary isolation is emphasized by institutional contexts such as scientific societies and their congresses, specialist journals, disciplines recognized as teaching subjects and legitimate and fundable research fields. By shaping the personal agendas of individual scientists, this has a strong impact on the development of biology. In some fields, botanical and zoological contributions have long being effectively intertwined, but in many others plant and animal biology have failed to progress beyond a marginal dialogue. Characteristically, the so-called “general biology” and the philosophy of biology are still zoocentric (and often vertebrato- or even anthropocentric). In this article, I discuss legitimacy and fruitfulness of some old lexical and conceptual exchanges between the two traditions (cell, tissue, and embryo). Finally, moving to recent developments, I compare the contributions of plant vs. animal biology to the establishment of evolutionary developmental biology. We cannot expect that stronger integration between the different strands of life sciences will soon emerge by self-organization, but highlighting this persisting imbalance between plant and animal biology will arguably foster progress.

The Continuity Principle and the Evolution of Replication Fidelity

Evolution in modern life requires high replication fidelity to allow for natural selection. A simulation model utilizing simulated phenotype data on cellular probability of survival was developed to determine how self-replication fidelity could evolve in early life. The results indicate that initial survivability and replication fidelity both contribute to overall fitness as measured by growth rates of the cell population. Survival probability was the more dominant feature, and evolution was possible even with zero replication fidelity. A derived formula for the relationship of survival probability and replication fidelity with growth rate was consistent with the simulated empirical data. Quantitative assessment of continuity and other evidence was obtained for a saltation (non-continuous) evolutionary process starting from low to moderate levels of survival probability and self-replication fidelity to reach the high levels seen in modern life forms.

Growth zone segmentation in the milkweed bug Oncopeltus fasciatus sheds light on the evolution of insect segmentation

Background

One of the best studied developmental processes is the Drosophila segmentation cascade. However, this cascade is generally considered to be highly derived and unusual, with segments being patterned simultaneously, rather than the ancestral sequential segmentation mode. We present a detailed analysis of the segmentation cascade of the milkweed bug Oncopletus fasciatus, an insect with a more primitive segmentation mode, as a comparison to Drosophila, with the aim of reconstructing the evolution of insect segmentation modes.

Results

We document the expression of 12 genes, representing different phases in the segmentation process. Using double staining we reconstruct the spatio-temporal relationships among these genes. We then show knock-down phenotypes of representative genes in order to uncover their roles and position in the cascade.

Conclusions

We conclude that sequential segmentation in the Oncopeltus germband includes three slightly overlapping phases: Primary pair-rule genes generate the first segmental gene expression in the anterior growth zone. This pattern is carried anteriorly by a series of secondary pair-rule genes, expressed in the transition between the growth zone and the segmented germband. Segment polarity genes are expressed in the segmented germband with conserved relationships. Unlike most holometabolous insects, this process generates a single-segment periodicity, and does not have a double-segment pattern at any stage. We suggest that the evolutionary transition to double-segment patterning lies in mutually exclusive expression patterns of secondary pair-rule genes. The fact that many aspects of the putative Oncopeltus segmentation network are similar to those of Drosophila, is consistent with a simple transition between sequential and simultaneous segmentation.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1293-z) contains supplementary material, which is available to authorized users.

Mandibulate convergence in an armoured Cambrian stem chelicerate

BACKGROUND

Chelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile "panchelicerate" body plan.

RESULTS

In this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates.

CONCLUSIONS

Although habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheirans Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods-Chelicerata and Mandibulata-and implies that this diversification was facilitated by the expansion of durophagous niches.

How reversible is development? Contrast between developmentally plastic gain and loss of segments in barnacle feeding legs

Segmented organisms and structures have fascinated biologists since William Bateson first described homeotic transformation and recognized the fundamental evolutionary significance of segmental organization. On evolutionary time scales, segments may be lost or gained during major morphological transitions. But how segment loss compares to gain on developmental time scales remains mysterious. Here, we examine the ease of reverse development (opposite to normal growth) by comparing developmentally plastic leg segment loss versus gain in individual barnacles transplanted between different water flow conditions. Plastic segment addition occurred rapidly (one to two molts) and exclusively near the limb base. In contrast, developmentally plastic segment loss-the first observation in any arthropod-took much longer (>10 molts) and, remarkably, occurred throughout the leg (23% of losses occurred mid-limb). Segment loss was not a simple reversal of segment addition. Intersegmental membranes fused first, followed by elimination of duplicate tendons and gradual shortening (but not loss) of duplicate setae. Setal loss, in particular, may impose a severe developmental constraint on arthropod segment fusion. This asymmetric developmental potential (time lag of phenotypic response)-plastic segment addition (amplified normal development) is faster and more orderly than segment loss (reverse development)-adds a new dimension to models of developmental plasticity because the cost of making a developmental mistake in one direction will be greater than in the other.

Species diversity vs. morphological disparity in the light of evolutionary developmental biology

BACKGROUND

Two indicators of a clade's success are its diversity (number of included species) and its disparity (extent of morphospace occupied by its members). Many large genera show high diversity with low disparity, while others such as Euphorbia and Drosophila are highly diverse but also exhibit high disparity. The largest genera are often characterized by key innovations that often, but not necessarily, coincide with their diagnostic apomorphies. In terms of their contribution to speciation, apomorphies are either permissive (e.g. flightlessness) or generative (e.g. nectariferous spurs).

SCOPE

Except for Drosophila, virtually no genus among those with the highest diversity or disparity includes species currently studied as model species in developmental genetics or evolutionary developmental biology (evo-devo). An evo-devo approach is, however, potentially important to understand how diversity and disparity could rapidly increase in the largest genera currently accepted by taxonomists. The most promising directions for future research and a set of key questions to be addressed are presented in this review.

CONCLUSIONS

From an evo-devo perspective, the evolution of clades with high diversity and/or disparity can be addressed from three main perspectives: (1) evolvability, in terms of release from previous constraints and of the presence of genetic or developmental conditions favouring multiple parallel occurrences of a given evolutionary transition and its reversal; (2) phenotypic plasticity as a facilitator of speciation; and (3) modularity, heterochrony and a coupling between the complexity of the life cycle and the evolution of diversity and disparity in a clade. This simple preliminary analysis suggests a set of topics that deserve priority for scrutiny, including the possible role of saltational evolution in the origination of high diversity and/or disparity, the predictability of morphological evolution following release from a former constraint, and the extent and the possible causes of a positive correlation between diversity and disparity and the complexity of the life cycle.

Genome evolution: groping in the soil interstices

Centipedes are a very old lineage of terrestrial animals. The first completely sequenced myriapod genome reveals that the blind centipede Strigamia maritima has no gene for light-sensory proteins, lacks the canonical circadian clock and possesses unusual features related to chemosensory perception.

A soul of truth in things erroneous: Popper's "amateurish" evolutionary philosophy in light of contemporary biology

This paper will critically assess Popper's evolutionary philosophy. There exists a rich literature on the topic with which we have many reservations. We believe that Popper's evolutionary philosophy should be assessed in light of the intriguing theoretical insights offered, during the last 10 years or so, by the philosophy of biology, evolutionary biology and molecular biology. We will argue that, when analysed in this manner, Popper's ideas concerning the nature of selection, Lamarckism and the theoretical limits of neo-Darwinism can be appreciated in their full biological and philosophical value.

Segmentation of the millipede trunk as suggested by a homeotic mutant with six extra pairs of gonopods

BACKGROUND

The mismatch between dorsal and ventral trunk features along the millipede trunk was long a subject of controversy, largely resting on alternative interpretations of segmentation. Most models of arthropod segmentation presuppose a strict sequential antero-posterior specification of trunk segments, whereas alternative models involve the early delineation of a limited number of 'primary segments' followed by their sequential stereotypic subdivision into 2n definitive segments. The 'primary segments' should be intended as units identified by molecular markers, rather than as overt morphological entities. Two predictions were suggested to test the plausibility of multiple-duplication models of segmentation: first, a specific pattern of evolvability of segment number in those arthropod clades in which segment number is not fixed (e.g., epimorphic centipedes and millipedes); second, the occurrence of discrete multisegmental patterns due to early, initially contiguous positional markers.

RESULTS

We describe a unique case of a homeotic millipede with 6 extra pairs of ectopic gonopods replacing walking legs on rings 8 (leg-pairs 10-11), 15 (leg-pairs 24-25) and 16 (leg-pairs 26-27); we discuss the segmental distribution of these appendages in the framework of alternative models of segmentation and present an interpretation of the origin of the distribution of the additional gonopods.The anterior set of contiguous gonopods (those normally occurring on ring 7 plus the first set of ectopic ones on ring 8) is reiterated by the posterior set (on rings 15-16) after exactly 16 leg positions along the AP body axis. This suggests that a body section including 16 leg pairs could be a module deriving from 4 cycles of regular binary splitting of an embryonic 'primary segment'.

CONCLUSIONS

A very likely early determination of the sites of the future metamorphosis of walking legs into gonopods and a segmentation process according to the multiplicative model may provide a detailed explanation for the distribution of the extra gonopods in the homeotic specimen. The hypothesized steps of segmentation are similar in both a normal and the studied homeotic specimen. The difference between them would consist in the size of the embryonic trunk region endowed with a positional marker whose presence will later determine the replacement of walking legs by gonopods.

A spectrum of pleiotropic consequences in development due to changes in a regulatory pathway

Regulatory evolution has frequently been proposed as the primary mechanism driving morphological evolution. This is because regulatory changes may be less likely to cause deleterious pleiotropic effects than changes in protein structure, and consequently have a higher likelihood to be beneficial. We examined the potential for mutations in trans acting regulatory elements to drive phenotypic change, and the predictability of such change. We approach these questions by the study of the phenotypic scope and size of controlled alteration in the developmental network of the bacterium Myxococcus xanthus. We perturbed the expression of a key regulatory gene (fruA) by constructing independent in-frame deletions of four trans acting regulatory loci that modify its expression. While mutants retained developmental capability, the deletions caused changes in the expression of fruA and a dramatic shortening of time required for completion of development. We found phenotypic changes in the majority of traits measured, indicating pleiotropic effects of changes in regulation. The magnitude of the change for different traits was variable but the extent of differences between the mutants and parental type were consistent with changes in fruA expression. We conclude that changes in the expression of essential regulatory regions of developmental networks may simultaneously lead to modest as well as dramatic morphological changes upon which selection may subsequently act.

Multi-scale relationships between numbers and size in the evolution of arthropod body features

Size-related changes of form in animals with periodically patterned body axes and post-embryonic growth discontinuously obtained throughout a series of moulting episodes cannot be accounted for by allometry alone. We address here the relationships between body size and number and size of appropriately selected structural units (e.g., segments), which may more or less closely approximate independent developmental units, or unitary targets of selection, or both. Distinguishing between units fundamentally involving one cell only or a small and fixed number of cells (e.g., the ommatidia in a compound eye), and units made of an indeterminate number of cells (e.g., trunk segments), we analyze and discuss a selection of body features of either kind, both in ontogeny and in phylogeny, through a review of current literature and meta-analyses of published and unpublished data. While size/number relationships are too diverse to allow easy generalizations, they provide conspicuous examples of the complex interplay of selective forces and developmental constraints that characterizes the evolution of arthropod body patterning.

Genotype-phenotype mapping and the end of the 'genes as blueprint' metaphor

In a now classic paper published in 1991, Alberch introduced the concept of genotype-phenotype (G-->P) mapping to provide a framework for a more sophisticated discussion of the integration between genetics and developmental biology that was then available. The advent of evo-devo first and of the genomic era later would seem to have superseded talk of transitions in phenotypic space and the like, central to Alberch's approach. On the contrary, this paper shows that recent empirical and theoretical advances have only sharpened the need for a different conceptual treatment of how phenotypes are produced. Old-fashioned metaphors like genetic blueprint and genetic programme are not only woefully inadequate but positively misleading about the nature of G-->P, and are being replaced by an algorithmic approach emerging from the study of a variety of actual G-->P maps. These include RNA folding, protein function and the study of evolvable software. Some generalities are emerging from these disparate fields of analysis, and I suggest that the concept of 'developmental encoding' (as opposed to the classical one of genetic encoding) provides a promising computational-theoretical underpinning to coherently integrate ideas on evolvability, modularity and robustness and foster a fruitful framing of the G-->P mapping problem.

Phylo-evo-devo: combining phylogenetics with evolutionary developmental biology

As a result of the integration of molecular and morphological approaches for the reconstruction of phylogenies, and of the intertwining of developmental and evolutionary biology, further prospects are open for a fruitful interaction between these two fields in what we may call a phylo-evo-devo approach.Wiegmann et al.'s molecular phylogeny of the holometabolous insect orders, recently published in BMC Biology, offers a good opportunity to revisit the inverted positions of wings and halteres in the Diptera and the Strepsiptera in terms of a putative homeotic mutation in the Hox gene Ultrabithorax. The main finding of this paper is that Strepsiptera are closely related to the Coleoptera rather than Diptera, as recently claimed. Through this exemplary case, the paper demonstrates the value of the reciprocal illumination we can expect from the integration of a good phylogeny and a sound knowledge of the evolvability of developmental mechanisms.