Scaling and shear transformations capture beak shape variation in Darwin's finches.

Evolution by natural selection has resulted in a remarkable diversity of organism morphologies that has long fascinated scientists and served to establish the first relations among species. Despite the essential role of morphology as a phenotype of species, there is not yet a formal, mathematical scheme to quantify morphological phenotype and relate it to both the genotype and the underlying developmental genetics. Herein we demonstrate that the morphological diversity in the beaks of Darwin's Finches is quantitatively accounted for by the mathematical group of affine transformations. Specifically, we show that all beak shapes of Ground Finches (genus Geospiza) are related by scaling transformations (a subgroup of the affine group), and the same relationship holds true for all the beak shapes of Tree, Cocos, and Warbler Finches (three distinct genera). This analysis shows that the beak shapes within each of these groups differ only by their scales, such as length and depth, which are genetically controlled by Bmp4 and Calmodulin. By measuring Bmp4 expression in the beak primordia of the species in the genus Geospiza, we provide a quantitative map between beak morphology and the expression levels of Bmp4. The complete morphological variation within the beaks of Darwin's finches can be explained by extending the scaling transformations to the entire affine group, by including shear transformations. Altogether our results suggest that the mathematical theory of groups can help decode morphological variation, and points to a potentially hierarchical structure of morphological diversity and the underlying developmental processes.

The Drosophila proboscis is specified by two Hox genes, proboscipedia and Sex combs reduced, via repression of leg and antennal appendage genes.

The proboscis is one of the most highly modified appendages in Drosophila melanogaster. However, the phenotypes of proboscipedia (pb) mutants, which transform the proboscis into leg or antenna, indicate a basic homology among these limbs. Recent genetic studies have revealed a developmental system for patterning appendages and identified several genes required for limb development. Among these are: extradenticle (exd), homothorax (hth), dachshund (dac), Distal-less (Dll) and spalt (sal). These limb genes have not been well studied in wild-type mouthparts and their role if any in this appendage is not well understood. Here we demonstrate that the homeotic gene products Proboscipedia (Pb) and Sex combs reduced (Scr) regulate the limb genes in the labial disc to give rise to a unique type of appendage, the proboscis. Pb inhibits exd, dac and sal expression and downregulates DLL: This observation explains the ability of Pb to inhibit the effects of ectopically expressed trunk Hox genes in the proboscis, to suppress leg identity in the trunk and to transform antenna to maxillary palp. Scr suppresses sal expression and also downregulates Dll in the labial discs; discs mutant for both pb and Scr give rise to complete antennae, further demonstrating appendage homology. In the labial disc, Pb positively regulates transcription of Scr, whereas in the embryo, Scr positively regulates pb. Additionally, our results suggests a revised fate map of the labial disc. We conclude that the proboscis constitutes a genetically distinct type of appendage whose morphogenesis does not require several important components of leg and/or antennal patterning systems, but retains distal segmental homology with these appendages.

Homologs of Drosophila appendage genes in the patterning of arthropod limbs.

Studies of the genes involved in patterning the appendages of Drosophila melanogaster have revealed a system of signaling and transcriptional regulation that is responsible for specifying the proximo-distal limb axis. Here we report the expression patterns of presumptive homologs of the Drosophila genes extradenticle, dachshund, nubbin, ventral veins lacking (a.k.a. Cf1-a), and Dll in the limbs of the woodlouse Porcellio scaber and the spider Steatoda triangulosa. Although the expression domains of the appendage genes roughly correspond to those of Drosophila, their relative positions and segmental affiliation are distinct. In addition, the expression patterns of the appendage genes allows a resolution of the segmental composition of different appendages within crustacean and spider embryos. We conclude that certain limb types, e.g., mouthparts, appear to be derived from a leg-like ground-plan via the elimination/fusion of the intermediate and distal podomeres. Moreover, we observe just such a modification during the transformation of the anterior legs into mouthparts in P. scaber. Although our data do not unequivocally resolve the question of homology of the arthropod leg segments, they do provide evidence for a single conserved proximo-distal patterning system in the development of noninsect arthropod limbs.

Crustacean (malacostracan) Hox genes and the evolution of the arthropod trunk.

Representatives of the Insecta and the Malacostraca (higher crustaceans) have highly derived body plans subdivided into several tagma, groups of segments united by a common function and/or morphology. The tagmatization of segments in the trunk, the part of the body between head and telson, in both lineages is thought to have evolved independently from ancestors with a distinct head but a homonomous, undifferentiated trunk. In the branchiopod crustacean, Artemia franciscana, the trunk Hox genes are expressed in broad overlapping domains suggesting a conserved ancestral state (Averof, M. and Akam, M. (1995) Nature 376, 420-423). In comparison, in insects, the Antennapedia-class genes of the homeotic clusters are more regionally deployed into distinct domains where they serve to control the morphology of the different trunk segments. Thus an originally Artemia-like pattern of homeotic gene expression has apparently been modified in the insect lineage associated with and perhaps facilitating the observed pattern of tagmatization. Since insects are the only arthropods with a derived trunk tagmosis tested to date, we examined the expression patterns of the Hox genes Antp, Ubx and abd-A in the malacostracan crustacean Porcellio scaber (Oniscidae, Isopoda). We found that, unlike the pattern seen in Artemia, these genes are expressed in well-defined discrete domains coinciding with tagmatic boundaries which are distinct from those of the insects. Our observations suggest that, during the independent tagmatization in insects and malacostracan crustaceans, the homologous 'trunk' genes evolved to perform different developmental functions. We also propose that, in each lineage, the changes in Hox gene expression pattern may have been important in trunk tagmatization.

Embryonic expression patterns of the Hox genes of the crayfish Procambarus clarkii (Crustacea, Decapoda).

Higher crustaceans (class Malacostraca) represent the most species-rich and morphologically diverse group of non-insect arthropods. The superorders Eucarida and Peracarida, two large groups that separated over 350 million years ago, encompass most malacostracan diversity. Recently, the Hox genes of the peracarid woodlouse Porcellio scaber(Isopoda) were shown to be expressed in domains that coincide with morphological boundaries of body tagmata, which differ from those in insects (Abzhanov and Kaufman 1999a,b). Moreover, observed changes in Hox expression domains during ontogeny correlate with morphological remodeling, such as a transformation of the first thoracic leg into mouthpart maxillipeds, which occurs in the trunk of the embryo. Decapods have a different modification of the malacostracan bodyplan, with up to three pairs of maxillipeds and extensive fusion and cephalization of the thorax. Here we describe expression patterns of the trunk Hox genes Scr, Antp, Ubx, abd-A and cad in the eucarid crayfish Procambarus clarkii (Decapoda). We find that the crayfish expression patterns, for the most part, resemble those of the woodlouse Porcellio scaber(Isopoda), but are more modulated and complex. Nevertheless, as in Porcellio the boundaries of the Hox expression domains do correlate with morphological features and their modulations to transformations in the embryo. Thus we propose that the trunk Hox genes were likely important in the evolution of and currently play an essential role in the development of the complex decapod bodyplan.

Homeotic genes and the arthropod head: expression patterns of the labial, proboscipedia, and Deformed genes in crustaceans and insects.

cDNA fragments of the homologues of the Drosophila head homeotic genes labial (lab), proboscipedia (pb), and Deformed (Dfd) have been isolated from the crustacean Porcellio scaber. Because the accumulation domains of the head homeotic complex (Hox) genes had not been previously reported for crustaceans, we studied the expression patterns of these genes in P. scaber embryos by using in situ hybridization. The P. scaber lab homologue is expressed in the developing second antennal segment and its appendages. This expression domain in crustaceans and in the homologous intercalary segment of insects suggests that the lab gene specified this metamere in the last common ancestor of these two groups. The expression domain of the P. scaber pb gene is in the posterior part of the second antennal segment. This domain, in contrast to that in insects, is colinear with the domains of other head genes in P. scaber, and it differs from the insect pb gene expression domain in the posterior mouthparts, suggesting that the insect and crustacean patterns evolved independently from a broader ancestral domain similar to that found in modern chelicerates. P. scaber Dfd is expressed in the mandibular segment and paragnaths (a pair of ventral mouthpart structures associated with the stomodeum) and differs from insects, where expression is in the mandibular and maxillary segments. Thus, like pb, Dfd shows a divergent Hox gene deployment. We conclude that homologous structures of the mandibulate head display striking differences in their underlying developmental programs related to Hox gene expression.

Novel regulation of the homeotic gene Scr associated with a crustacean leg-to-maxilliped appendage transformation.

Homeotic genes are known to be involved in patterning morphological structures along the antero-posterior axis of insects and vertebrates. Because of their important roles in development, changes in the function and expression patterns of homeotic genes may have played a major role in the evolution of different body plans. For example, it has been proposed that during the evolution of several crustacean lineages, changes in the expression patterns of the homeotic genes Ultrabithorax and abdominal-A have played a role in transformation of the anterior thoracic appendages into mouthparts termed maxillipeds. This homeotic-like transformation is recapitulated at the late stages of the direct embryonic development of the crustacean Porcellio scaber (Oniscidea, Isopoda). Interestingly, this morphological change is associated with apparent novelties both in the transcriptional and post-transcriptional regulation of the Porcellio scaber ortholog of the Drosophila homeotic gene, Sex combs reduced (Scr). Specifically, we find that Scr mRNA is present in the second maxillary segment and the first pair of thoracic legs (T1) in early embryos, whereas protein accumulates only in the second maxillae. In later stages, however, high levels of SCR appear in the T1 legs, which correlates temporally with the transformation of these appendages into maxillipeds. Our observations provide further insight into the process of the homeotic leg-to-maxilliped transformation in the evolution of crustaceans and suggest a novel regulatory mechanism for this process in this group of arthropods.

Chelicerate Hox genes and the homology of arthropod segments.

Genes of the homeotic complex (HOM-C) in insects and vertebrates are required for the specification of segments along the antero-posterior axis. Multiple paralogues of the Hox genes in the horseshoe crab Limulus poliphemus have been used as evidence for HOM-C duplications in the Chelicerata. We addressed this possibility through a limited PCR survey to sample the homeoboxes of two spider species, Steatoda triangulosa and Achaearanea tepidariorum. The survey did not provide evidence for multiple Hox clusters although we have found apparent duplicate copies of proboscipedia (pb) and Deformed (Dfd). In addition, we have cloned larger cDNA fragments of pb, zerknullt (zen/Hox3) and Dfd. These fragments allowed the determination of mRNA distribution by in situ hybridization. Our results are similar to the previously published expression patterns of Hox genes from another spider and an oribatid mite. Previous studies compared spider/mite Hox gene expression patterns with those of insects and argued for a pattern of segmental homology based on the assumption that the co-linear anterior boundaries of the Hox domains can be used as markers. To test this assumption we performed a comparative analysis of the expression patterns for UBX/ABD-A in chelicerates, myriapods, crustaceans, and insects. We conclude that the anterior boundary can be and is changed considerably during arthropod evolution and, therefore, Hox expression patterns should not be used as the sole criterion for identifying homology in different classes of arthropods.

Understanding the genetic basis of morphological evolution: the role of homeotic genes in the diversification of the arthropod bauplan.

Due to the segmental organization of their body plans, arthropods can be considered the paradigmatic modular organisms. In the past two decades, genetic studies of the homeotic (Hox) genes in Drosophila melanogaster have provided initial insight into the molecular mechanisms that govern the establishment of segmental identity. In this review, we will address the question of the possible role of four Hox genes: labial (lab), proboscipedia (pb), Deformed (Dfd), and Sex combs reduced (Scr) in the morphological evolution of arthropods, particularly with respect to the evolution of the head and head structures in insects. Overall, these preliminary studies illustrate the role that some of the Hox genes expressed in the insect head have played in the morphological evolution of hexapods and likely arthropods in general.