Biology subjects

Bruce, H. S.

Publications and source records attributed to Bruce, H. S..

4 recordsLinked to original sources

bioRxiv2025

The molecular basis of arthropod appendage diversity

Arthropods have an incredible diversity of limbs that are modified for walking, chewing, cleaning, mating, grasping, sensing, and more. Understanding the relationships and evolutionary histories of different limbs is a central task, but their sheer diversity makes this a daunting if not impossible task using morphology alone. Here, the in situ expression patterns and CRISPR-Cas9 phenotypes for the five best-studied leg-patterning genes - Distal-less, Sp6-9, dachshund, extradenticle, and homothorax - are described for all limbs of the crustacean Parhyale. Crustaceans are well-suited for this task because their limbs are more diverse than those of other arthropods, and each individual possesses a wide range of limb types that are relevant to many other arthropods, living and extinct. These results will a) provide a template for understanding the genetic basis of limb construction in arthropods more generally based on the strong phenotypes that can be obtained with CRISPR-Cas9, and b) contribute to our understanding of the evolution and affinities of highly modified legs like mouthparts and genitalia using molecular methods to complement previous morphological and embryological approaches.

developmental biology↗

bioRxiv2021

The Daphnia carapace and the origin of novel structures

Understanding how novel structures arise is a central question in evolution. Novel structures are often defined as structures that are not derived from (homologous to) any structure in the ancestor1. The carapace of the water flea Daphnia magna is a bivalved "cape" of exoskeleton that has been proposed to be one of many novel arthropod structures that arose through repeated co-option of genes that also pattern insect wings2-4. To determine whether the Daphnia carapace is a novel structure, we compare the expression of pannier, araucan, and vestigial between Daphnia, Parhyale, and Tribolium. Our results suggest that the Daphnia carapace did not arise by co-option, but instead derives from an exite (lateral lobe) that emerges from an ancestral proximal leg segment that was incorporated into the Daphnia body wall. The Daphnia carapace therefore appears to be homologous to the Parhyale tergal plate and the insect wing5. Remarkably, the vestigial-positive region that gives rise to the Daphnia carapace appears to be present in Parhyale6 and Tribolium as a small, inconspicuous protrusion. Similarly, the vestigial-positive regions that form thoracic tergal plates in Parhyale appear to be present in Daphnia, even though Daphnia does not form thoracic tergal plates. Thus, rather than a novel structure resulting from gene co-option, the Daphnia carapace appears to have arisen from a shared, ancestral tissue (morphogenetic field) that persists in a cryptic state in other arthropod lineages. Cryptic persistence of unrecognized serial homologs may thus be a general solution for the origin of novel structures.

developmental biology↗

bioRxiv2021

How to align arthropod legs

Arthropods (insects, "crustaceans", myriapods, and chelicerates) display a fascinating diversity of ectodermal structures like plates, horns, helmets, knobs, carapaces, mimicry outgrowths, and wings. The origins and relationships of these structures has tantalized researchers for over a century: did these structures arise de novo in each lineage, or do they emerge from shared, ancestral primordia present in some or even all arthropods? One way to begin answering this is to assess the position and context of these structures on arthropod bodies: do these structures emerge from proximal embryonic leg segments that were converted into the body wall, or do they represent dorsal, non-leg-derived structures? Here, the expression of pannier, araucan, and drumstick - genes previously shown to distinguish proximal leg segments in crustaceans and insects - are examined in a chelicerate representative, the tarantula Acanthoscurria. This gene expression comparison, together with over a century of gene functional, morphological, embryological, and paleontological data, suggests that all arthropod leg segments correspond to each other in a one-to-one fashion, but that in many arthropod lineages, the base of the ancestral leg in the embryo flattens and expands to form the body wall of the adult. This would mean that many arthropod outgrowths which appear to stand on the lateral body wall, such as wings, tergal plates, and gin traps, are derived from the ancestral/embryonic leg base, and likely arose from shared, ancestral primordia present on this leg base in all arthropods. The analysis detailed here means that a simple three- or four-gene in situ expression experiment with pannier, araucan, and drumstick can elucidate the homologies of any arthropod ectodermal structure of interest, including beetle horns, treehopper helmets, and more.

developmental biology↗

bioRxiv2018

Insect wings and body wall evolved from ancient leg segments

The origin of insect wings has long been debated. Central to this debate is whether wings evolved from an epipod (outgrowth, e.g., a gill) on ancestral crustacean leg segments, or represent a novel outgrowth from the dorsal body wall that co-opted some of the genes used to pattern the epipods. To determine whether wings can be traced to ancestral, pre-insect structures, or arose by co-option, comparisons are necessary between insects and arthropods more representative of the ancestral state, where the hypothesized proximal leg region is not fused to the body wall. To do so, we examined the function of five leg patterning genes in the crustacean Parhyale hawaiensis and compared this to previous functional data from insects. By comparing gene knockout phenotypes of leg patterning genes in a crustacean with those of insects, we show that two ancestral crustacean leg segments were incorporated into the insect body, moving the legs epipod dorsally, up onto the back to form insect wings. Thus, our data shows that much of the body wall of insects, including the entire wing, is derived from these two ancestral proximal leg segments. This model explains all observations in favor of either the body wall origin or proximal leg origin of insect wings. Thus, our results show that insect wings are not novel structures, but instead evolved from existing, ancestral structures.\n\nOne Sentence SummaryCRISPR-Cas9 knockout of leg gap genes in a crustacean reveals that insect wings are not novel structures, they evolved from crustacean leg segments

developmental biology↗