The Yin and Yang of Evo/Devo

Leeches of the genus Helobdella as model organisms for Evo-Devo studies

Model organisms are important tools in modern biology and have been used elucidate mechanism underlying processes, such as development, heredity, neuronal signaling, and phototropism, to name but a few. In this context, the use of model organisms is predicated on uncovering evolutionarily conserved features of biological processes in the expectation that the findings will be applicable to organisms that are either inaccessible or intractable for direct experimentation. For the most part, particular species have been adapted as model organisms because they can be easily reared and manipulated in the laboratory. In contrast, a major goal in the field of evolutionary developmental biology (Evo-Devo) is to identify and elucidate the differences in developmental processes among species associated with the dramatic range of body plans among organisms, and how these differences have emerged over time in various branches of phylogeny. At first glance then, it would appear that the concept of model organisms for Evo-Devo is oxymoronic. In fact, however, laboratory-compatible, experimentally tractable species are of great use for Evo-Devo, subject to the condition that the ensemble of models investigated should reflect the range of taxonomic diversity, and for this purpose glossiphoniid leeches are useful. Four decades ago (1975), leeches of the species-rich genus Helobdella (Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae) were collected in Stow Lake, Golden Gate Park, San Francisco, CA (USA). These and other Helobdella species may be taken as Evo-Devo models of leeches, clitellate annelids, and the super-phylum Lophotrochozoa. Here we depict/discuss the biology/taxonomy of these Evo-Devo systems, and the challenges of identifying species within Helobdella. In addition, we document that H. austinensis has been established as a new model organism that can easily be cultivated in the laboratory. Finally, we provide an updated scheme illustrating the unique germ line/soma-differentiation during early development and speculate on the mechanisms of sympatric speciation in this group of aquatic annelids.

Muscle precursor cells in the developing limbs of two isopods (Crustacea, Peracarida): an immunohistochemical study using a novel monoclonal antibody against myosin heavy chain

In the hot debate on arthropod relationships, Crustaceans and the morphology of their appendages play a pivotal role. To gain new insights into how arthropod appendages evolved, developmental biologists recently have begun to examine the expression and function of Drosophila appendage genes in Crustaceans. However, cellular aspects of Crustacean limb development such as myogenesis are poorly understood in Crustaceans so that the interpretative context in which to analyse gene functions is still fragmentary. The goal of the present project was to analyse muscle development in Crustacean appendages, and to that end, monoclonal antibodies against arthropod muscle proteins were generated. One of these antibodies recognises certain isoforms of myosin heavy chain and strongly binds to muscle precursor cells in malacostracan Crustacea. We used this antibody to study myogenesis in two isopods, Porcellio scaber and Idotea balthica (Crustacea, Malacostraca, Peracarida), by immunohistochemistry. In these animals, muscles in the limbs originate from single muscle precursor cells, which subsequently grow to form multinucleated muscle precursors. The pattern of primordial muscles in the thoracic limbs was mapped, and results compared to muscle development in other Crustaceans and in insects.

The leaf index: heteroblasty, natural variation, and the genetic control of polar processes of leaf expansion

The morphology of the leaves of angiosperms exhibits remarkable diversity. One of the factors showing the greatest variability is the leaf index, namely, the ratio of leaf length to leaf width. In some cases, different varieties of a single species or closely related species can be distinguished by differences in leaf index. To some extent, the leaf index reflects the morphological adaptation of leaves to a particular environment. Moreover, physiological conditions or environmental factors can change the leaf index of an individual plant. No good tools have been available for studies of the mechanisms that underlie such biodiversity. However, we have recently obtained some, albeit fragmentary, information about molecular mechanisms of leaf morphogenesis as a result of studies of leaves of the model plant, Arabidopsis thaliana (L.) Heynh. For example, the ANGUSTIFOLIA gene, a homolog of animal CtBP genes, controls leaf width. ANGUSTIFOLIA appears to regulate the polar elongation of leaf cells via control of the arrangement of cortical microtubules. By contrast, the ROTUNDIFOLIA3 gene controls leaf length via the biosynthesis of steroid(s). We provide here an overview of the biodiversity exhibited by the leaf index of angiosperms. In particular, we consider information obtained from studies of mutants and transgenic strains of A. thaliana, from the so-called Evo/devo perspective.

Neurogenesis in the crustacean ventral nerve cord: homology of neuronal stem cells in Malacostraca and Branchiopoda?

In Insecta and malacostracan Crustacea, neurons in the ventral ganglia are generated by the unequal division of neuronal stem cells, the neuroblasts (Nbs), which are arranged in a stereotyped, grid-like pattern. In malacostracans, however, Nbs originate from ectoteloblasts by an invariant lineage, whereas Nbs in insects differentiate without a defined lineage by cell-to-cell interactions within the neuroectoderm. As the ventral ganglia in entomostracan crustaceans were thought to be generated by a general inward proliferation of ectodermal cells, the question arose as to whether neuroblasts in Euarthropoda represent a homologous type of stem cell. In the current project, neurogenesis in metanauplii of the entomostracan crustaceans Triops cancriformis Fabricius, 1780 (Branchiopoda, Phyllopoda) and Artemia salina Linné, 1758 (Branchiopoda, Anostraca) was examined by in vivo incorporation of the mitosis marker bromodeoxyuridine (BrdU) and compared to stem cell proliferation in embryos of the malacostracan Palaemonetes argentinus Nobili, 1901 (Eucarida, Decapoda). The developmental expression of synaptic proteins (synapsins) was studied immunohistochemically. Results indicate that in the ventral neurogenic zone of Branchiopoda, neuronal stem cells with cellular characteristics of malacostracan neuroblasts are present. However, a pattern similar to the lineage-dependent, grid-like arrangement of the malacostracan neuroblasts was not found. Therefore, the homology of entomostracan and malacostracan neuronal stem cells remains uncertain. It is now well established that during arthropod development, identical and most likely homologous structures can emerge, although the initiating steps or the mode of generation of these structures are different. Recent evidence suggests that adult Entomostraca and Malacostraca share corresponding sets of neurons so that the present report provides an example that those homologous neurons may be generated via divergent developmental pathways. In this perspective, it remains difficult at this point to discuss the question of common patterns of stem cell proliferation with regard to the phylogeny and evolution of Atelocerata and Crustacea.

Conservation of glp-1 regulation and function in nematodes

The Caenorhabditis elegans (Ce) glp-1 gene encodes a Notch-like receptor. We have cloned glp-1 from C. briggsae (Cb) and C. remanei (Cr), two Caenorhabditis species that have diverged from C. elegans by roughly 20-40 million years. By sequence analysis, we find that the Cb-GLP-1 and Cr-GLP-1 proteins have retained the same motif architecture as Ce-GLP-1, including number of domains. In addition, two regions (CC-linker and regions flanking the ANK repeats) are as highly conserved as regions previously recognized as essential for signaling (e.g., ANK repeats). Phylogenetic analysis of glp-1 sequences suggests a C. briggsae/C. remanei clade with C. elegans as a sister taxon. Using RNAi to test biological functions, we find that Ce-glp-1, Cb-glp-1, and Cr-glp-1 are all required for proliferation of germline stem cells and for specifying blastomere fates in the embryo. In addition, certain biological roles of Cb-glp-1, e.g., in the vulva, have diverged from those of Ce-glp-1 and Cr-glp-1, suggesting a change in either regulation or function of the Cb-glp-1 gene during evolution. Finally, the regulation of glp-1 mRNA, previously analyzed for Ce-glp-1, is conserved in Cb-glp-1, and we identify conserved 3' UTR sequences that may serve as regulatory elements.

Correlation of diversity of leg morphology in Gryllus bimaculatus (cricket) with divergence in dpp expression pattern during leg development

Insects can be grouped into mainly two categories, holometabolous and hemimetabolous, according to the extent of their morphological change during metamorphosis. The three thoracic legs, for example, are known to develop through two overtly different pathways: holometabolous insects make legs through their imaginal discs, while hemimetabolous legs develop from their leg buds. Thus, how the molecular mechanisms of leg development differ from each other is an intriguing question. In the holometabolous long-germ insect, these mechanisms have been extensively studied using Drosophila melanogaster. However, little is known about the mechanism in the hemimetabolous insect. Thus, we studied leg development of the hemimetabolous short-germ insect, Gryllus bimaculatus (cricket), focusing on expression patterns of the three key signaling molecules, hedgehog (hh), wingless (wg) and decapentaplegic (dpp), which are essential during leg development in Drosophila. In Gryllus embryos, expression of hh is restricted in the posterior half of each leg bud, while dpp and wg are expressed in the dorsal and ventral sides of its anteroposterior (A/P) boundary, respectively. Their expression patterns are essentially comparable with those of the three genes in Drosophila leg imaginal discs, suggesting the existence of the common mechanism for leg pattern formation. However, we found that expression pattern of dpp was significantly divergent among Gryllus, Schistocerca (grasshopper) and Drosophila embryos, while expression patterns of hh and wg are conserved. Furthermore, the divergence was found between the pro/mesothoracic and metathoracic Gryllus leg buds. These observations imply that the divergence in the dpp expression pattern may correlate with diversity of leg morphology.

Serotonin-immunoreactive neurons in the ventral nerve cord of Crustacea: a character to study aspects of arthropod phylogeny

The number of serotonin-expressing neurons in the nervous system of Euarthropoda is small and their neurites have a characteristic branching pattern. They can be identified individually, which provides a character well suited for phylogenetic analyses. In order to gain data that may be useful in the ongoing discussion on insect-crustacean relationships, we documented the pattern of serotonin immunoreactive neurons in the ventral nerve cord of four crustacean species: the phyllocarid malacostracan Nebalia bipes Fabricius, 1780 (Phyllocarida, Leptostraca) and the entomostracans Artemia salina Linnaeus, 1758 (Branchiopoda, Anostraca, Sarsostraca), Triops cancriformis Bosc, 1801 (Branchiopoda, Phyllopoda, Calmanostraca, Notostraca), and Leptestheria dahalacensis Rüppell, 1837 (Branchiopoda, Phyllopoda, Diplostraca, Conchostraca, Spinicaudata). In the entomostracan taxa investigated, the pattern of serotonergic cells in the thoracic hemiganglia comprises an anterior and a posterior bilateral pair of neurons with ipsi- and/or contralateral neurites. Comparing these data to existing information on serotonin-immunoreactivity in the ventral nerve cord of other malacostracan and entomostracan groups enabled us to determine several features of these thoracic neurons being part of the ground pattern of these taxa. Our data demonstrate that studying individually identifiable neurons in Arthropoda can be used to analyse the phylogeny of this taxon.

Morphological analysis of the mammalian postcranium: a developmental perspective

The past two decades have greatly improved our knowledge of vertebrate skeletal morphogenesis. It is now clear that bony morphology lacks individual descriptive specification and instead results from an interplay between positional information assigned during early limb bud deployment and its "execution" by highly conserved cellular response programs of derived connective tissue cells (e.g., chondroblasts and osteoblasts). Selection must therefore act on positional information and its apportionment, rather than on more individuated aspects of presumptive adult morphology. We suggest a trait classification system that can help integrate these findings in both functional and phylogenetic examinations of fossil mammals and provide examples from the human fossil record.

Generation of evolutionary novelty by functional shift

That biological features may change their function during evolution has long been recognized. Particularly, the acquisition of new functions by molecules involved in developmental pathways is suspected to cause important morphologic novelties. However, the current terminology describing functional changes during evolution (co-option or recruitment) fails to recognize important biologic distinctions between diverse evolutionary routes involving functional shifts. The main goal of our work is to stress the importance of an apparently trivial distinction: Whether or not the element that adopts a new function (anything from a morphologic structure to a protein domain) is a single or a duplicated element. We propose that natural selection must act in a radically different way, depending on the historic succession of co-option and duplication events; that is, co-option may provide the selective pressure for a subsequent gene duplication or could be a stabilizing factor that helps maintain redundancy after gene duplication. We review the evidence available on functional changes, focusing whenever possible on developmental molecules, and we propose a conceptual framework for the study of functional shifts during evolution with a level of resolution appropriate to the power of our current methodologies.