Homology: a synthetic concept of evolutionary robustness of patterns

Homologues and homology and their related terms in phylogenetic systematics

In the field of phylogenetic systematics, the terms homology and homologue and their relationship to cladistic terms such as character, character state, synapomorphy and symplesiomorphy, as well as their relationships to each other, have been and are still discussed frequently. A recent re-emergence of concepts of homology/homologue free of any reference to explanatory hypotheses prompted us to explore these concepts and their relationships to each other as well as to the concept of morpheme, as introduced recently. All concepts are examined with regard to their ontological status and their bearing in the epistemological process in morphology and phylogenetic systematics. To us, morphemes, homologues and in partem character states refer to things (concrete objects in the ontological sense). However, although morphemes are exclusively descriptive, the latter two represent objects of causal explanations. Homologue always refers to the things themselves, yet a character state also can be a property or the absence of a thing. In this context, a character as a transformation series of character states does not represent a thing but a natural kind. Character states of one character are connected by homology relationships, i.e. common descent. Synapomorphy and symplesiomorphy represent different states of a single transformation series. A nonexplanatory, purely descriptive, concept of homologues is contradictory to its original as well as the post-Darwinian, evolutionary, concept which refers to causal relationships between parts of organisms and their correspondences in the archetype or ancestor, respectively. Character states, homologues and synapomorphies/symplesiomorphies can only be approximated in the form of hypotheses. We argue that the high value of the concept of homology and its related concepts for evolutionary biology should be maintained by acknowledging their explanatory nature and that dilution with nonexplanatory (even idealistic) definitions should be avoided.

New insights into the evolution of portunoid swimming crabs (Portunoidea, Heterotremata, Brachyura) and the brachyuran axial skeleton

Portunoidea (Heterotremata) is a morphologically disparate taxon of true crabs (Brachyura) best-known for many of its representatives being considered "swimming crabs". The term "swimming crab", however, sometimes refers to a distinct taxon (traditionally to Portunidae within Portunoidea), and sometimes to a certain morphotype in which the 5th pereiopod (P5) has a specific shape that facilitates swimming. We use the term "P5-swimming crab" or "P5-swimmer" herein, not only to restrict it to the morphotype, but also to distinguish the swimming in question from other kinds of swimming in Brachyura. The evolution of P5-swimming crabs has not yet been satisfactorily investigated. In particular, it is not known whether the morphotype evolved several times independently in different lineages of Portunoidea or whether it evolved only once and was lost in several lineages. Ours is the first approach combining molecular with morphological data to result in a new phylogenetic positioning of some members of Portunoidea. For the first time, data from the axial skeleton and extrinsic musculature are used. Morphological examinations reveal that the axial skeleton and extrinsic musculature in P5-swimming crabs are more diverse than previously thought, with the exception of the P5 anterior coxa muscle, which originates at the median plate in all P5-swimmers. Ancestral state reconstructions based on parsimony reveal that the stem species of Portunoidea already showed the morphotype of a P5-swimming crab, but with a long merus which probably resulted in less effective P5-swimming than in extant P5-swimming crab species with a short merus. Several other extant taxa represent a reversal of the P5-swimmer morphotype to varying degrees, with some extant species showing a complete reversal of unambiguous P5-swimming crab character states-one example being the well-known common shore crab Carcinus maenas. The absence of a connection between interosternite 7/8 and the sella turcica (the secondary loss of the "brachyuran sella turcica") in the stem species of Heterotremata, resulting in a junction plate which forms a cavity that offers room and attachment sites for the P5 extrinsic musculature is uncovered as preadaptation to the P5-swimmer morphotype in Heterotremata. This preadaptation is missing in Podotremata and Thoracotremata, the other two traditional main taxa of Brachyura.

Evolutionary morphology of coxal musculature in Pseudoscorpiones (Arachnida)

Pseudoscorpions are an ancient and globally distributed lineage of arachnids with more than 4000 species. Despite being present in virtually all terrestrial habitats, their morphology and anatomy has rarely been studied to date, which hampers homology statements both within and between other arachnid orders. All pseudoscorpions share a morphological peculiarity, the fixation of the coxae of all the walking legs. The same morphological condition is seen in certain other arachnid taxa, such as Solifugae or Scorpiones - potential sistergroups of Pseudoscorpiones. To investigate the musculature apparatus of this unusual feature, we reconstructed the musculature in the coxae of walking legs in three species of pseudoscorpions that represent the three major clades within this order. Using micro-computed tomography (μCT), we show that pseudoscorpions have the highest number of coxal muscles amongst the arachnid orders (12 vs. fewer than 10 in others), and that the muscular composition of the first two legs differs from that in the hind legs, correlating with the difference in function, i.e. pulling in the front legs and pushing in the hind legs. Pseudoscorpions are also unique amongst the arachnids in lacking endoskeletal structures (coxal apodeme or costa coxalis) inside the coxae. We observed that within pseudoscorpions, there is a trend towards a reduction of the number of coxal muscles, with the most basal-branching taxon having the highest number and more derived taxa exhibiting lower counts. We hypothesize the muscular ground pattern for Pseudoscorpiones and discuss the evolution of this system by comparing it to the (scanty) data on other arachnids available in the literature.

Evolutionary and functional substitution of extrinsic musculature in Solifugae (Arachnida)

The locomotory system of Solifugae is distinct from that of other Arachnida in several ways. Only three pairs of legs are involved in locomotion, while the first pair function as sensory appendages. Morphologically, the proximal region of the locomotory system in Solifugae is characterized by fused coxae. Within the prosoma of Solifugae, an endosternite is missing: in other Arachnida, this endosternite serves as the proximal attachment site for a portion of the extrinsic musculature. How then do these skeletal modifications influence the muscular anatomy in the proximal region of the locomotory system? To answer this question, we studied the skeletomuscular anatomy of Galeodes granti at the interface between the prosoma and legs, reinvestigating the complex muscular anatomy of this body region for the first time in over 80 years and-for the first time-using detailed micro-computed tomography scans to analyze the skeletomuscular morphology. Specimens of three further species were checked for comparison. The analysis revealed differences in the number and composition of coxa-trochanter muscles in each of the four pairs of legs. These are compared in the light of serial homology. The comparison between the proximal locomotory system of Solifugae and that of other Arachnida unveils a series of analogies. Primarily, the coxa-trochanter joint is the most proximal joint to move the leg relative to the prosoma. Therefore, we argue that from a morpho-functional point of view, the coxa-trochanter muscles in Solifugae should be considered secondary extrinsic musculature. Thus, the legs gain a stable, articulated joint in the most proximal region of the leg to the prosoma, which might be advantageous for agile locomotion.

Complex Homology and the Evolution of Nervous Systems

We examine the complex evolution of animal nervous systems and discuss the ramifications of this complexity for inferring the nature of early animals. Although reconstructing the origins of nervous systems remains a central challenge in biology, and the phenotypic complexity of early animals remains controversial, a compelling picture is emerging. We now know that the nervous system and other key animal innovations contain a large degree of homoplasy, at least on the molecular level. Conflicting hypotheses about early nervous system evolution are due primarily to differences in the interpretation of this homoplasy. We highlight the need for explicit discussion of assumptions and discuss the limitations of current approaches for inferring ancient phenotypic states.

Evo-devo in the era of gene regulatory networks

Advanced genomics tools enable powerful new strategies for understanding complex biological processes, including development. By extension, we should be able to use these methods in a comparative fashion to capture evolutionary mechanisms. This requires a capacity to go deep and broad, to analyze developmental gene regulatory networks in many organisms, especially nontraditional models. As we usher in a new era of next-generation GRN (gene regulatory network) analysis, it is important to ask how to evaluate the evolution of network interactions. Particularly problematic, as always, is defining "independence": Are two character traits found together because they are functionally linked or because of historical accident? The same basic question applies to understanding developmental GRN evolution. However, the essential difference here is that a GRN defines a causal chain of events. An understanding of causal relations--how Genes A and B work in concert to drive expression of Genes C and D to create a new Territory E--gives hope for establishing "trait independence" in a way that purely correlative arguments--the association of the expression of Gene D in Territory E--never could. Insight into causality provides the key to interpretation, as seen in this simplified scenario. Real-world networks bring new degrees of complexity, but the elucidation of causal relations remains the same. Has the day arrived when a single laboratory has the wherewithal to conduct multiorganism gene network projects in parallel? No. However, we argue that day is closer than one might suppose. We describe how a speedboat GRN project in one's favorite nonmodel organism(s) might look and provide a framework for comparative network analysis.

The mouthparts of Scutigerella immaculata: Correspondences and variation among serially homologous head appendages

Descriptions of the skeletomuscular system of the symphylan head are characterized by a high number of incongruencies among authors. The increased accuracy and transparency of 3D-reconstruction methods allows for an allocation of some of the incongruencies to (i) misinterpretations of literature descriptions, or (ii) different views of what constitutes a separate muscle. A structural comparison allows to homologize muscles among the serially arranged mouthparts of Symphyla.

The formation of the theory of homology in biological sciences

Homology is among the most important comparative concepts in biology. Today, the evolutionary reinterpretation of homology is usually conceived of as the most important event in the development of the concept. This paradigmatic turning point, however important for the historical explanation of life, is not of crucial importance for the development of the concept of homology itself. In the broadest sense, homology can be understood as sameness in reference to the universal guarantor so that in this sense the different concepts of homology show a certain kind of "metahomology". This holds in the old morphological conception, as well as in the evolutionary usage of homology. Depending on what is (or was) taken as a guarantor, different types of homology may be distinguished (as idealistic, historical, developmental etc.). This study represents a historical overview of the development of the homology concept followed by some clues on how to navigate the pluralistic terminology of modern approaches to homology.