A successful assimilation of heterochrony cast in terms of rate genes and the ability to ignore anything associated with recapitulation i. The exclusion of practical features of comparative embryology like the focus on marine invertebrate larvae offers another route for illustrating that the Modern Synthesis emerged with development playing a small role at best. But de Beer was conversant with a third option—larval forms as indicators of developmental variability relevant to substantial evolutionary change Brigandt The significance of this line of reasoning was not lost on other embryological researchers Berrill But since Mayr and others were not engaged in embryological practices where this was salient, the diversity and variation of marine invertebrate larvae was of no abiding interest.
Epistemic Values and the Consequences of Exclusion 4. Smocovitis emphasized the issue of confirmation how particular model organisms confirm or disconfirm evolutionary theories , but we can also scrutinize discovery, concept formation, and theory formulation among others. This is where historiographic theses about the Modern Synthesis link up with ongoing theory and practice in biology.
One specific connection between historiography and biological reasoning is through epistemic values. Epistemic values play a role in deciding between empirically equivalent hypotheses, determining whether one should favor a theory in the absence of new evidence, evaluating which methodologies to employ, and ascertaining whether an explanatory approach is adequate. They are values because they operate when evidence is evaluated, come into conflict with each other, and their merit must be adjudicated separately in each particular situation. The first of these is theoretical generality.
An adequate theory of evolution should be generally applicable to different species. Although it is impossible to study every organism with respect to all features of the evolutionary process, evolutionary theorists should attempt to produce an account that is not biased toward some subset of species e. The second epistemic value is explanatory completeness. Despite the inability to exhaustively describe phenomena, researchers aim to lay bare all relevant aspects of phenomena under scrutiny within a specific domain Schlegel This value operates at multiple levels in biological research.
For any particular problem, such as speciation, explanatory completeness pertains to comprehending all involved elements, such as population structure, geographic variation, and genetic architecture. If the problem is more restricted e. The complex relationship between the multitude of epistemic and non-epistemic values operating in science is discussed extensively elsewhere e. Similarly, if the goal is to account for biological phenomena that involve speciation, adaptation, co-evolution, and biogeography, then the requirement of explanatory completeness will be expanded accordingly.
Thus, the epistemic value of explanatory completeness is relative to the aim of an investigation, which determines the relevant aspects that must be incorporated. More abstractly, explanatory completeness at the level of the theory of evolution is related to the major problem agendas of evolutionary biology, such as adaptation, inheritance, speciation, evolvability, novelty, and co-evolution Love forthcoming. To ignore one or more of these, even if we have secured generality or completeness for others, leads to an incomplete account of evolutionary processes.
To acknowledge this incompleteness is not to admit that a past or present theory of evolution is explanatorily impotent. In fact, we can heartily affirm that the Modern Synthesis was more complete than any alternatives on offer at the time by focusing on the different levels at which explanatory completeness obtains. The acknowledgement only signals that there are outstanding targets of explanation that require attention.
Explanatory completeness is always tied to these targets, which can be specified in the case where the overarching research goal is to produce a synthetic theory of evolution. The relations between generality and completeness are complicated. Theoretical generality is not a guarantee of explanatory completeness, and completeness with respect to a model species does not license generality.
No strict logical relations hold among these epistemic values but they can be empirically correlated. And, the pursuit of other epistemic values e. Despite this complexity, there is an enduring commitment among researchers to produce an evolutionary theory that exhibits theoretical generality and explanatory completeness. The node that brings these epistemic values together here is the issue of model organisms and the scientific reasoning activity of theory formulation rather than confirmation. Model organisms are premised on some assumption of generality often accompanied by the opportunity to explore biological processes in greater detail.
Focused scrutiny of one particular system can facilitate deeper understanding than the superficial study of many systems. The premise is that this understanding will generalize to the many unstudied or lesser-studied systems; sometimes the premise is supported and sometimes it is not Hanken ; Bolker ; Metscher and Ahlberg But model organisms are not often chosen with completeness across multiple problems or all of evolutionary theory as the epistemic value.
Model organisms are attractive for their appropriateness to particular problems. A great model species for understanding fertilization may be lousy for investigating gastrulation, but understanding both processes is necessary to have a complete account of ontogeny. Practical utility has theoretical consequences—generality might be purchased at the expense of completeness or vice versa.
One needs the right tool for the job Burian . In answer to the first question, marine invertebrates are relevant to theoretical generality in one straightforward way. They are the predominant grade across the metazoan tree of life, found in species from all but one phylum Figure 1. Their exclusion from the Modern Synthesis means that for any particular area considered, whether geographic variation, chromosome behavior, population structure, or developmental processes, theoretical generality is not met.
How can we be confident that the relevant aspects of evolutionary mechanisms described in terrestrial vertebrates or invertebrates can be extrapolated? Despite this clear relevance of marine invertebrate exclusion for theoretical generality, the more important issues surround explanatory completeness. The epistemic value of completeness for the Modern Synthesis or current evolutionary theory is directly related to how many processes are understood to be involved in evolution.
If we understand completeness in terms of standard topics in evolutionary textbooks e. How are marine invertebrates relevant for theorizing about biogeography or evolvability? If they are relevant to a problem but absent from studies concerned with it, then the value of generality returns under a more precise guise. And if marine invertebrates are included, what specific problems are in view? Marine invertebrates were not just relevant via generality to questions of adaptation, biogeography, classification, and speciation but gave special insight to questions about evolvability, novelty, and variation.
They display an amazing diversity of larval forms and developmental variation in the context of biphasic life cycles across twenty-three phyla Young ; Table 6; cf. Brusca and Brusca These larval forms are not only distinct from one another, but also extremely divergent morphologically when compared with their adult forms Figure 2. The advantages that accrue from a recognition of the morphological modes of relation between ontogeny and phylogeny [i. First is the fact that the material on which the factors of evolution work is not restricted to a single type in each species, but consists of all the structures presented at all the stages of the life-history of each species.
In other words, an organism shows variation in its own life-history. Second, the effects of heterochrony produce important results even without the introduction of numerous evolutionary novelties in the form of new structures. And when new structures and functions are added to the effects of heterochrony, especially those described under the name of paedomorphosis, such evolutionary changes may be very important and far-reaching de Beer , The larval and juvenile stages are sometimes more drastically affected by evolutionary innovations than the adults.
The pelagic larvae of marine invertebrates, for instance, are exposed to high selection pressures. However, most of the larvae feed during dispersal and this opens a new niche for the species.
Larvae are just another example of adaptation; explanations of adaptation in terms of natural selection subsume the problem of explaining the origin of novelty, which implies that no developmental information is necessary Mayr ; cf. Love Unfortunately, most marine invertebrate biologists interested in importance of development for evolution and active in the middle part of the 20th century e.
These researchers are thematically contiguous with contemporary Evo-devo research cf. Love and Raff ; Raff and Love , but were absent from a Modern Synthesis that was desensitized to the significance of particular problems like evolvability or novelty because of a bias in its preferred model organisms. The practical exclusion of marine invertebrates and their ontogeny had an impact on the theoretical formulation and structure of the Modern Synthesis.
It resulted in an overall lack of generality but, more importantly, a lack of overall completeness with respect to specific evolutionary problems. Thus, the evidence we have seen for a practical exclusion thesis in terms of the absence of standard model organisms from comparative embryology marine invertebrates led to a theoretical exclusion of problems that were central in comparative embryology and elsewhere evolvability, novelty, and variation. This does not imply that the arrow did not sometimes run in the other direction. It also does not diminish the ways in which concepts from comparative or experimental embryology were reinterpreted so that their significance for evolution was dramatically minimized Davis et al.
But the epistemic value of explanatory completeness provides a bridge of implication for seeing the links between practical and theoretical exclusions of ontogeny relevant to the Modern Synthesis. Model Organisms in Evo-devo and Evolutionary Theory The introduction of epistemic values that are still in operation today brings the issues of exclusion from the Modern Synthesis into contemporary research.
A key consequence of this analysis is that theoretical generality cannot be the only concern when selecting model organisms. We must also ask about completeness at different levels. It is not just a question of whether the model organism is representative theoretical generality but what biological processes it is supposed to represent explanatory completeness.
When anomalies are identified between model systems and their targets, such as null mutations in orthologues yielding different phenotypes for mice and humans Liao and Zhang , a necessary question to ask is whether the anomaly occurs with respect to the biological processes of interest. The importance of completeness holds for model systems chosen in Evo-devo research, as well as for the study of development and disease Bier and McGinnis For instance, the practical preference for model species with short generation times can encourage a conceptual separation between embryonic development and life history.
Marine invertebrate larvae that spend weeks actively foraging in a pelagic environment prior to metamorphosis fall outside the scope of investigation unless we consider the entire trajectory as ontogeny. This implies that the standard criteria for choosing developmental model organisms e. Altering these criteria would be difficult since conforming to them is less expensive Burian . Financial constraints may generate intellectual barriers to a robust investigation of particular biological questions.
The lesson of completeness also has a particular application in the case of excluded problems that were salient to comparative embryologists. This involves two distinct components. First, can investigations of particular novelties be generalized to other research on different novelties? Do explanatory principles invoked to explain the origin of segmentation also apply to the origin of avian flight?
Second, can investigations of experimental organisms be generalized to the long extinct taxa relevant to the innovation or novelty under scrutiny? Is the developmental patterning of the branchial arches in zebrafish a relevant model for comprehending jaw origins Metscher and Ahlberg ? The latter issue often comes into view because it is part of a commitment to generality for model systems used in developmental research. But the former question reveals that generalization from the present to the past may be less difficult than generalizing from one case to another due to phylogenetic position differences relevant to the character of interest.
As a result, there may need to be a sheer proliferation of models. Choosing a model organism not only requires paying attention to completeness at the level of a problem agenda the origin of novelty , but also at the level of more restricted questions within its compass the origin of auricularia larvae versus the origin of avian flight. If we compare this perspective with recent discussions of model organism choice in Evo- devo, we find both congruence and incongruence.
Milinkovitch and Tzika argue for a consensual list of model organisms based on the characters being investigated, as well as other criteria like phylogenetic position and ease of maintenance. This approach is sensitive to the value of theoretical generality, especially the need to expand the number of model organisms. It is also congruent with the problem of novelty requiring attention to the characters being studied. But the proposal is less sensitive to explanatory completeness.
Particular model organisms may be more or less ideal for specific biological problems so that no single consensus list is achievable. Collins et al. Their concern of representation includes more than theoretical generality because they link model organism choice with specific problems such as evolvability, novelty, and variation. But of the five new Evo-devo models they explore, only one is a marine invertebrate Nematostella.
This fits with the emphasis on particular characters in the study of novelties but misses that these require special emphasis on phylogenetic position. Jenner and Wills explicitly recognize the significance of phylogenetic position for investigating novelties and additionally argue that model organisms should be chosen to illuminate themes in Evo-devo. The missing element in these arguments is what we observed historically in the practical exclusion thesis for marine invertebrate larvae—some grades of organisms illuminate problems more effectively than others.
Working with marine invertebrates in scientific practice makes problems like evolvability and novelty salient, encouraging research that addresses these types of questions e. Going only from problems to model organism choice privileges theory over practice. Given that the suitability of a model for addressing a problem cannot be guaranteed in advance and that model organisms can shift the interest of researchers onto other problem agendas Burian  , there is good reason to learn from the history of model organism practice in evolutionary research rather than merely repeat it.
This means coming to terms with the financial, intellectual, pedagogical, and technological costs associated with studying marine invertebrates. Adopting them as models demands increased expense, familiarization with relatively alien morphology, curricular incorporation for the next generation of researchers, and potentially the sacrifice of experimental tools or more time devoted to accommodating them to new applications. These calls flow in part from the perspective of a theoretical exclusion thesis, supplemented by new discoveries over the past sixty years.
Numerous studies explore the bearing of development on evolution in groups such as cephalochordates Holland , cephalopods Lee et al. New marine invertebrate models are coming online regularly Boell and Bucher But the presence of research on marine invertebrate ontogeny in Evo-devo is not enough to encourage a synthesis of this information into an overarching evolutionary framework.
Many of the papers focused on ecological genetics, invasive species, molecular evolution, phylogeography, speciation, and systematics 20 of 30 papers. The questions asked by researchers show a similar slant away from those most salient to biologists focused on marine invertebrate larvae: evolvability, novelty, and variation. This is why it is critical to keep the epistemic values of generality and completeness constantly in view because together they underwrite the concerns expressed with respect to conceptual content and experimental practice.
Only when we explicitly interrogate evolutionary theory with the questions that arise from these values will we move toward an extended synthesis that is theoretically general and explanatorily complete. Conclusions Was development left out of the Modern Synthesis?
Some of the empirical content of ontogeny from experimental embryology and particular interpretations of heterochrony were incorporated, although in such way that led to a minimization of the significance of development for evolutionary theorizing Horder ; Davis et al. A, anterior; P, posterior; R, right; L, left. Online version in colour. Although the molecular mechanisms underlying cortical inheritance are still not completely understood, the cortical unit appears to play an important role in it.
The transversal microtubule ribbon is located on the left side of the basal body, and the post-ciliary microtubule ribbon points in a posterior direction [ 7 ]. During cell division, the basal body is duplicated with strict polarity. Next, cytoskeletal appendages form at the peribasal site, confined within the cortical unit [ 7 ]. Thus, in ciliates, properties of the cortical unit itself are sufficient for self-assembly into high-order subcellular structures, such as cytoskeletal organelles and networks [ 7 ].
However, nature is even more complex and interesting than one might think. Even in the mirror-image doublets, the mirror-image enantiomorphic form of the cortical unit has never been observed [ 10 ]. For example, the position of the ciliary rootlet is not the mirror image in the doublet cell [ 10 ]. Therefore, in addition to the self-assembly of cortical units, there must be local cues to induce this planar rotation of the cortex. In addition, it was shown that when regions of the cell are placed in abnormal positions relative to one another, the cell intercalates these regions to restore their normal orientations in the membrane by the shortest permissive route [ 19 — 22 ].
These observations led to the proposal that the reversed anteroposterior axis of the oral apparatus in the mirror part of the doublets may be owing to the abnormal juxtapositioning of right and left marginal cortical units [ 10 ]. Regardless of the details, cortical inheritance suggests that the LR asymmetric morphology of a cell is dictated by molecular chirality.
That is, these observations demonstrate that the chirality of subcellular structures can direct the chirality at the whole-cell level. Recent studies revealed that cell chirality is not exclusively found only in protozoans, but also exists in metazoans. The Drosophila embryonic hindgut is invaginated from an epithelial monolayer and first forms as a bilaterally symmetric structure. Because the hindgut looping is the first visible sign of LR asymmetry in Drosophila , the directional rotation of the hindgut appears to break the LR symmetry.
Taniguchi et al. These cell surfaces have more leftward-tilted cell boundaries than rightward-tilted ones. The cell chirality is evident not only in the overall shape, but also in organelle and protein distributions inside the cells.
The neuroendocrine system of invertebrates: a developmental and evolutionary perspective
The centrosomes of hindgut epithelial cells tend to be located in the right-posterior region of the cell, and a cell adhesion molecule Drosophila E-cadherin DE-cadherin is more abundant along the rightward-tilted cell boundaries than along the leftward-tilted ones at the apical cell surface [ 25 ]. The involvement of the cell chirality in promoting the LR asymmetric rotation of the hindgut was supported by an in silico simulation, which showed that the introduction and subsequent dissolution of cell chirality in a model epithelial cell tube is sufficient to recapitulate the directional rotation of the model hindgut [ 25 ].
Cell chirality and LR asymmetric morphogenesis in Drosophila. Before the onset of the rotation, hindgut epithelial cells show chirality with more frequent leftward-tilted cell boundaries than rightward-tilted ones. The chirality disappears when the rotation is completed. Distribution of D E-cadherin green also shows chirality.
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Epithelial cells in the A8a segment of male genitalia show chirality just before and during the LR asymmetric rotation. These cells have more frequent rightward-tilted cell boundaries and a higher expression of Myosin II along the rightward-tilted cell boundaries. Schema is adopted from Sato K et al. The adult gut develops from larval primordia called the imaginal ring, consisting of H1 and H2 segments. The cell chirality determinant Myo31DF is required only in the H1 segment during larval stages.
Cell chirality is observed in the H2 segment only after the H1 segment is eliminated. The handedness determined by Myo31DF in the H1 segment might be conveyed to the H2 segment through atypical cadherins, Dachsous and Fat. The Myo31DF gene was identified in a Drosophila screen for gene mutations affecting the LR asymmetry of the embryonic gut [ 27 ]. The inversion phenotypes in both hindgut rotation and cell chirality were rescued by over-expressing wild-type Myo31DF in the hindgut epithelial cells [ 25 , 28 ].
Cell chirality is an intrinsic property of individual cells, and Myo31DF switches the direction of cell chirality. Left: wild-type embryos show rightward looping of the hindgut and dextral cell chirality. Middle: in Myo31DF mutant embryos, both the hindgut looping and cell chirality are inverted. Right: when cells expressing wild-type Myo31DF are randomly introduced into the Myo31DF hindgut, only the cells expressing wild-type Myo31DF show the normal dextral chirality, indicating that cell chirality is formed intrinsically in each cell.
Myo31DF is a member of the unconventional myosin I class; these molecules consist of an N-terminal head domain containing an ATP-binding motif, a neck domain containing two calmodulin-binding IQ motifs, and a short C-terminal tail domain [ 27 , 29 , 30 ]. Myosin 1d Myo1d is a rat orthologue of MyoID.
Recently, analyses of a Myo1d knockout rat revealed that Myo1d is required for the formation of planar cell polarity in multiciliated epithelial cells, but not for LR asymmetric organ development [ 32 ]. Thus, the roles of MyoID family proteins in LR asymmetric organ development are not evolutionarily conserved in mammals, although their biochemical functions in cell chirality may be widely maintained. In addition to LR inversion in the embryonic gut, Myo31DF mutants exhibit inversion in the looping of the adult gut and testes, and in the rotation of the male genitalia [ 27 , 29 ].
Sato et al. The chirality of the A8a cells is reversed in the Myo31DF mutant [ 23 ]. A computer model demonstrated that the biased cell boundary rearrangement, attributed to the biased expression of Myosin II, is important for the directional rotation of the male genitalia [ 23 ]. As Drosophila undergoes metamorphosis, the adult gut is developed from larval primordia called the imaginal ring. The imaginal ring consists of two segments H1 and H2. Epithelial cells in the H2 segment proliferate during the pupal stages and form the adult gut with dextral looping, whereas the H1 segment is eliminated during the pupal stages [ 24 ].
Myo31DF activity is required only in H1 during the late larval stages [ 24 ]. In the Myo31DF mutant tissues in which LR asymmetry is the mirror image of wild-type, the cell chirality is also switched from dextral to sinistral default. Evidence suggests several possible mechanisms for these events. In the epithelium of the Drosophila embryonic hindgut, Myo31DF is required for the chiral distribution of DE-cadherin [ 22 ].
Alternatively, Myo31DF may switch the chirality of the structure or function of actin cytoskeleton, given that disrupting the actin cytoskeleton abolishes cell chirality, and that Myo31DF is required for the chiral distribution of Myosin II in Drosophila [ 22 , 34 ]. Cell chirality—associated phenomena are observed in the blastomeres of various invertebrate species [ 35 ].
A spiral cleavage that is conserved in many members of the lophotrochozoan taxa, referred to as Spiralia, often involves chiral blastomeres, especially in the early cleavage stages. In some cases, the chirality of the blastomeres determines the handedness of the embryo. Snails, which belong to the Mollusca phylum of the lophotrochozoa, undergo spiral cleavage [ 36 — 39 ].
A formin activity plays a critical role in creating the blastomere chirality in a snail [ 41 ], which is reminiscent of a formin-dependent chirality formation seen in mammalian cells, as discussed below [ 42 ]. The handedness of the spiral cleavage can be reversed by surgical manipulation at the eight-blastomere stage; these embryos exhibit a mirror-image handedness of their entire body [ 43 ].
Therefore, the positioning of blastomeres at the eight-cell stage or earlier determines the handedness of the snail body. Cell chirality in snails and C. Bottom: Pulmonata is a snail species with counter clockwise-coiling shells and internal organs that mirror those of Lymnaea. In Pulmonata , blastomere spindles slant anticlockwise as viewed from the animal pole, resulting in a counter clockwise blastomere rearrangement at the eight-cell stage.
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In both cases, blastomere chirality determines the shell coiling direction and LR asymmetry of the body. Schema is adopted from Shibazaki et al. Bottom: at the six-cell stage, changing the LR-asymmetric arrangement of blastomeres to their mirror-image positions results in situs inversus. Thus, in both snails and C.
In Pulmonata, mutations affecting the handedness of the shell coiling and internal organs have been found in natural populations [ 44 , 45 ]. In mutants with LR inversion of the shell-coil direction, the early blastomere cleavage pattern is first symmetrical and then becomes a mirror image of the stereotypical cleavage pattern. In Pulmonata evolution, species occasionally emerged with anticlockwise-coiling shells and internal organs that were mirror images of those in the dextrally coiling snails [ 39 ].
These studies showed that the handedness of the spiral cleavage is correlated with the direction of shell coiling and of the internal organs [ 39 ]. Interestingly, the first cleavage in Xenopus is accompanied by a slight anticlockwise torsion of the two blastomeres [ 46 ]. A chemical treatment can dramatically increase this cortical anticlockwise torsion, and pharmacological analyses suggested that the torsion requires F-actin [ 46 ].
Thus, the cortex of an egg undergoing radial cleavage has intrinsic chirality, supporting the idea that cell chirality is a common property in metazoans. Caenorhabditis elegans C. As in snails, the first sign of LR asymmetry in C. At the eight-cell stage, the embryo midline tilts rightward from the anterior—posterior axis; this positioning is induced by LR-asymmetric blastomere protrusion and migration [ 48 ].
These events involve differentially regulated cortical contractility in the sister blastomeres that are bilateral counterparts [ 48 ]. Thus, as with snails, the relative LR-asymmetric blastomere positioning is completely responsible for the subsequent LR-asymmetric body development in C. That is, intercellular interactions responsible for the subsequent LR-asymmetric development depend on the LR-asymmetric blastomere configuration in the early cleavage stages.
In summary, blastomere chirality is a common mechanism driving LR asymmetric development in various invertebrates. Although the molecular mechanisms underlying blastomere chirality formation are not well understood at present, it may have common features with other cases of cell chirality formation, such as the involvement of formin and actin, as discussed below.
Cell chirality was recently observed in various vertebrate cultured cells. For example, murine myoblast C2C12 cells, human umbilical vein endothelial cells hUVECs and vascular mesenchymal cells VMCs show a chirally polarized cell shape when plated on a micropattern [ 49 , 50 ]. Whether the handedness is dextral or sinistral depends on the cell line [ 49 ]. Chirality in the nuclear shape and the involvement of E-cadherin in transmitting a chiral bias to neighbouring cells were shown using Madin—Darby canine kidney epithelial cells [ 51 , 52 ].
Cell chirality is also observed in the dynamics of cultured cells. Human promyelocytic leukaemia HL60 cells, which are neutrophil-like cells, show a leftward-biased migration in the absence of spatial cues [ 53 ]. Genetic and pharmacological analyses revealed that microtubules are involved in this process [ 53 ]. Fibroblasts from human foreskin seeded on a micropattern and cultured zebrafish melanophores show chiral swirling [ 42 , 54 ]. In these processes, the actin cytoskeleton is important, but microtubules are not [ 42 , 54 ].
Tee et al. This process was found to require the radial growth of the radial fibres, which depends on actin's polymerization by formin [ 42 ]. Chiral shape and swirling of cultured mammalian cells. Left: cultured murine myoblasts top and vascular mesenchymal cells bottom demonstrate intrinsic chirality when plated on a substrate with a ring or stripe micropattern. Schemas are adopted from Wan et al. Right: fibroblasts from human foreskin seeded on a micropattern show anticlockwise chiral swirling. Radial actin fibres initially situated in a radial pattern eventually start to tilt rightward top.
The clockwise rotation of actin filaments in radial fibres generated by formin may cause the rightward tilting bottom. Schemas are adopted from Tee et al.
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Directional LR asymmetry of the body structure is broadly observed in ecdysozoans, lophotrochozoans and deuterostomes. In addition, cell chirality is observed in these three groups of animals. Thus, it is possible that the mechanisms by which chiral morphology develops, including cell chirality, can be traced back to the ancestral bilateralia. In the cases of cell chirality observed so far, the actin cytoskeleton appears to play a profound role.
Matrotrophy and placentation in invertebrates: a new paradigm
In particular, formin, which drives the unidirectional rotation of F-actin, is indispensable for the formation of cell chirality in snail, frog and mammalian cells [ 41 , 42 ]. Thus, chirality in the structure or function of actin cytoskeleton may be an important determinant of cell chirality.
During animal development, most cells differentiate and exhibit functions at specific parts of the embryo, which are determined by positional information based on the dorsoventral and anteroposterior axes. Given that some of these cells have intrinsic cell chirality and are positioned in a specific part of the embryo, these cells can define the LR polarity, leading to LR asymmetric development, as found in Drosophila.
This scenario is supported by the absence of any observed LR-asymmetric gene expression in Drosophila. Therefore, cell chirality may serve as a mechanism for inducing organ-intrinsic LR asymmetry in the absence of an established LR axis [ 23 — 25 ]. Left: in vertebrates, LR morphogenesis occurs according to an established body LR axis.
In addition to directly informing the decisions of population managers, their results—which include the demonstration of gonochory, and identification of the spawning season—will be extremely useful in designing studies to study other parameters e. Photograph by Nancy Sheridan.
Volume , Issue 3 Volume , Issue 4 Cave habitats are unusual among terrestrial habitats because most of their trophic resources are ultimately derived by transport from surface communities. The amount and distribution of these resources in caves may have strong effects on the biology of cave-dwelling species. In this issue, Manenti et al. They also quantified a variety of cave physical parameters and the abundance of a prey species, the fly Limonia nubeculosa. Across all caves surveyed, spiders were most likely to be found in caves with the highest abundance of prey species.
Their results suggest that the distribution of trophic resources plays a large role in controlling the distribution of this predator; further, they suggest that the abundance of the easily surveyed spider may be a good indicator of the quantity of trophic resources available in a cave. Photograph by Enrico Lunghi. Female spiders deposit eggs within egg sacs made primarily of a type of silk produced in tubuliform glands. Silk produced in these glands is used only for egg sac production. In spiders that reproduce repeatedly, the silk-synthetic activity of the secretory cells of the tubuliform gland may be linked to the stage of the reproductive cycle.
In this issue, Herrera et al. They observed striking and reproducible changes in the structure of the gland over the reproductive cycle. These granules presumably consist of silk proteins, as they appear to be exocytosed in the gravid spider. The striking cyclical patterns in cell activity and form Herrera et al. Transmission electron micrograph by Merri L. These data are particularly useful given the recent release of the whole genome of the ctenophore Mnemiopsis leidyi , along with phylogenomic analyses suggesting that ctenophores are the earliest living animals, a finding important for our understanding of the evolution of mesoderm and nervous systems.
In addition to providing a timely review of the role of cilia in the life of ctenophores, Tamm also provided our cover illustration, a cut paper collage showing a lobate ctenophore like Mnemiopsis leidyi dining on copepods printed and distributed by Local Colors Gallery, Woods Hole, Massachusetts at www. Invertebrate biologists have long been making observations of the fine structure of invertebrate sperm, with the aims of using these data to evaluate phylogenetic hypotheses and to understand how the functional requirements of sperm transfer and fertilization have shaped the evolution of sperm form.
These cells have several unusual features, including a highly coiled nucleus. This is clearly seen in the cover illustration, a transmission electron micrograph of numerous sperm packed together in the spermatheca of a female of S. In addition, mature sperm in the male contain numerous membrane-bounded inclusions; these are absent from sperm stored in the female spermatheca, suggesting a function during transfer or storage. Additional comparative studies are required to fully understand the phylogenetic significance of these observations, and functional studies are needed to understand how sperm form is related to sperm function in this species.
Micrograph by Kevin Eckelbarger and Alan Hodgson. Volume , Issue 3 Volume , Issue 4 The evolution of segmentally arranged structures has long been a focus of intense research, especially in chordates, arthropods, and annelids. In this issue, Oliveira et al. While the dorsal integument of nearly all peripatids is composed of a conserved number of 12 annulations, or plicae, per segment, members of one species — Plicatoperipatus jamaicensis — have 24 annulations per segment.
Oliveira et al. However, 12 additional rows of dermal papillae which they term pseudoplicae; orange are inserted between the plicae later in development. The pseudoplicae differ in position and structure from the plicae, as evidenced, for example, by the distribution of hyaline organs purple. While in other peripatids these structures occur in the furrows between plicae, they lie on the pseudoplicae in P.
The functional significance if any of these novel structures is unknown. Comparative studies of the development of peripatids may be useful for understanding the molecular basis of this evolutionary novelty. Scanning electron micrograph by Ivo de Sena Oliveira. One common method for testing hypotheses of structural homology is to compare the developmental processes that generate those structures: homologous structures are generally expected to be formed by similar morphogenetic processes.
In this issue, Tilic et al. Tilic et al. They found numerous differences in the formation of the chaetae of lumbrinerids compared to those of capitellids and spionids, and conclude that the hooded hooks of these two sets of taxa did indeed evolve independently. Their conclusion is consistent with inferences based on phylogenetic analyses. Such analyses will eventually help us to understand how annelid chaetae, which are critically important in the systematics of the group, have evolved.
Light micrograph by Ekin Tilic. For 11 years, they monitored populations of this species on Cousine Island, the only Seychelles granitic island that is free of potentially predatory invasive mammals. Millipedes were most abundant and active during high rainfall months, leading the authors to suggest that any population manipulations e. In addition, they suggest that control efforts for the invasive African big-headed ant, which involve the use of a pesticide that may affect millipedes, should be confined to low rainfall months, when millipede activity is at its lowest.
Image by James Lawrence. Most living cephalopods can alter the color of their skin on fine spatial scales and rapid time scales, producing stunning visual displays that are important in prey capture, predator avoidance, mating, and communication. These color changes are driven in large part by the expansion or retraction of many thousands of pigmented chromatophore organs. In cephalopod chromatophore organs, pigment cell expansion is controlled by the contraction of radially arranged muscle cells.