Terri A Williams

Trinity College · Biology

Simple Summary The engrailed (en) and invected (inv) paralogs play a fundamental role in arthropod segmentation. Previous research suggests that knockdown of either en or inv in sequentially segmenting insects leads to an unexpected and variable loss of segments but does not mimic the segment polarity defects seen in Drosophila en mutants; the cons...

Background: Segmentation in arthropods typically occurs by sequential addition of segments from a posterior growth zone. However, the amount of tissue required for growth and the cell behaviors producing posterior elongation are sparsely documented. Results: Using precisely staged larvae of the crustacean, Thamnocephalus platyurus, we systematic...

We describe the dynamic process of abdominal segment generation in the milkweed bug Oncopeltus fasciatus We present detailed morphological measurements of the growing germband throughout segmentation. Our data are complemented by cell division profiles and expression patterns of key genes, including invected and even-skipped as markers for differen...

Virtually all arthropods all arthropods add their body segments sequentially, one by one in an anterior to posterior progression. That process requires not only segment specification but typically growth and elongation. Here we review the functions of some of the key genes that regulate segmentation: Wnt, caudal, Notch pathway, and pair-rule genes,...

Wnt genes are a family of conserved glycoprotein ligands that play a role in a wide variety of cell and developmental processes, from cell proliferation to axis elongation. There are 13 Wnt subfamilies found among metazoans. Eleven of these appear conserved in arthropods with a pattern of loss during evolution of as many as six subfamilies among he...

Segmented animals are found in major clades as phylogenetically distant as vertebrates and arthropods. Typically, segments form sequentially in what has been thought to be a regular process, relying on a segmentation clock to pattern budding segments and posterior mitosis to generate axial elongation. Here we show that segmentation in Tribolium has...

The structural diversity of crustacean limbs is enormous, and their evolution is hotly debated. Attempts have been made to understand the developmental patterning mechanisms that generate distinct types of adult limbs by analyzing genes known to regulate limb development in the arthropod model system, Drosophila. This has led to the discovery of de...

The field of Evolutionary Developmental biology arose with the promise of new approaches to answering longstanding questions of comparative biology. Here we review the fruits of that promise some decades later. We chose three areas of arthropod Evo Devo-evolution of body plans, segment number, and appendage morphology-to provide an overview for the...

Repeated body segments are a key feature of arthropods. The formation of body segments occurs via distinct developmental pathways within different arthropod clades. Although some species form their segments simultaneously without any accompanying measurable growth, most arthropods add segments sequentially from the posterior of the growing embryo o...

The branchiopod crustacean Triops longicaudatus has paddlelike thoracic appendages with few joints and multiple marginal lobes. Here, we explore the degree to which the Triops limb is patterned by the same network of genes known to pattern the uniramous, multi-jointed insect appendage. Insect leg patterning proceeds through a process of subdividing...

Western blot of Triops and Drosophila extract probed with DAC α-serum and HTH α-serum. a DAC polyclonal antibody detected three prominent bands (two smaller than 37 kDa and one ~60 kDa) in Triops larval extract (stage 1–4) whereas two distinct bands (~32 and 42 kDa) were detected in Drosophila embryo extract (0–16 h). Drosophila Dachshund protein h...

Distal-less (Dll) plays a well-known role in patterning the distal limb in arthropods. However, in some taxa, its expression even during early limb development is not always limited to the distal limb. Here, I trace the expression of Distal-less in a crustacean (Thamnocephalus platyurus) from the early limb bud to later stages of limb development,...

Crustacean limbs exhibit highly diverse morphologies. One major route of diversification is in the number and position of branches arising from the proximal part of the limb. Here I describe development of larvae of the branchiopod crustacean, Thamnocephalus platyurus and describe in detail the development of the thoracic limbs. The thoracic limbs...

Setae are a prominent feature of arthropod limbs. In taxa where the limbs develop during the larval phase, developing setae are an integral part of the developing limb bud and their differentiation cannot easily be separated from the early patterning and formation of the overall limb. Here I describe the morphogenesis and adult setae in a branchiop...

Arthropod bodies are formed by a series of appendage-bearing segments, and appendages have diversified both along the body axis within species and between species. Understanding the developmental basis of this variation is essential for addressing questions about the evolutionary diversification of limbs. We examined the development of serially hom...

The developing leg of Drosophila is initially patterned by subdivision of the leg into proximal and distal domains by the activity of the homeodomain proteins Extradenticle (Exd) and Distal-less (Dll). These early domains of gene expression are postulated to reflect a scenario of limb evolution in which an undifferentiated appendage outgrowth was s...

Segmentation is one of the most salient characteristics of arthropods, and differentiation of segments along the body axis is the basis of arthropod diversification. This article evaluates whether the evolution of segmentation involves the differentiation of already independent units, i.e., do segments evolve as modules? Because arthropod segmental...

Segmentation is one of the most salient characteristics of arthropods, and differentiation of segments along the body axis is the basis of arthropod diversification. This article evaluates whether the evolution of segmentation involves the differentiation of already independent units, i.e., do segments evolve as modules? Because arthropod segmental...

Arthropods exhibit highly diverse limb morphologies ranging from unbranched walking legs to multibranched swimming paddles. Understanding morphogenesis in structurally diverse limbs can be useful for ascertaining homologies between limbs. Structurally similar limbs have been produced by different evolutionary modifications of morphogenesis in certa...

In Drosophila, Distalless (Dll) is critical in establishing the proximal/distal axis of the leg. Lack of proper Dll expression causes distal limb structures to be truncated or lost. Dll expression was examined through the course of development in the limbs of two crustaceans, Triops and Nebalia. Because the limbs of these two species are branched,...

Recent advances in developmental genetics of Drosophila have uncovered some of the key molecules involved in the positioning and outgrowth of the leg primordia. Although expression patterns of these molecules have been analyzed in several arthropod species, broad comparisons of mechanisms of limb development among arthropods remain somewhat specula...

Developmental genetics has revealed several molecular players critical for patterning the adult leg in Drosophila. Comparisons throughout insects and crustaceans reveal that some of the molecular mechanisms that initiate limb patterning are conserved: limbs seem to always consist of anterior/posterior compartments, and their initial proximal/distal...

We have studied the process of post-embryonic segmentation in the anostracan crustacean Artemia franciscana using a specific antibody raised against the engrailed protein of this organism. Three cephalic segments are specified during embryonic development, before larval hatching, whilst trunk (thoracic) segmentation begins after the first stage fre...

The expression patterns of homeotic genes in a crustacean — the brine shrimp Artemia franciscana — provide a new perspective on the evolution of arthropod body plans.

Newly hatched Artemia larvae use one pair of limbs to locomote. During development they gradually add additional limbs along the elongating trunk. As larvae grow, body length increases from about 0.4 mm to 4 mm, mean swimming speed increases from 1.8 mm s(-1) to 9.9 mm s(-1), and frequency of antennal beat decreases from 9.5 to 6.7 Hz. As new limbs...

A physical model of the swimming appendage (second antenna) of a larval Artemia was oscillated and translated through a tank of glycerine to determine how such a shape may be used to generate thrust at the intermediate Reynolds numbers at which it operates. Force on the model was measured by strain gauges and used to calculate coefficients of drag...

SYNOPSIS. The crustacean nauplius larva is a development stage characterized by the presence of three pairs of head appendages. All crustaceans pass through the naupliar stage whether embryonically or as freeliving larvae. The nauplius is thought to be the phylotypic stage and represent a fundamental developmental constraint in crustaceans. However...