Fate of Epipods/Gills epipods/gills
Fate of Epipods/Gills epipods/gills wings (loss) book gills book lungs, tracheae and spinnerets spinnerets tubular tracheae book lung
The insect wing and spider spinnerets may have evolved from a dorsal branch of an ancestral multibranched limb. (a) The proposed structural homology between the insect wing and (b) an ancestral dorsal leg branch (red). (c) In a crayfish, the Pdm protein is expressed throughout a dorsal branch of the third thoracic limb and in rings in the primary branch (walking leg). (d) The last common ancestors of all arthropods were aquatic creatures with branched appendages. The ventral branches (blue) of these appendages were used mostly for locomotion (e.g. legs), while the dorsal branches, called epipods (red), were used mostly for respiration and osmoregulation (gills). Endopods/legs are preserved in most arthropods. Epipods/gills are preserved in aquatic arthropods but modified or lost in terrestrial groups. In terrestrial arachnids (spiders and scorpions), a series of related primordia arise in posterior segments of the body. In spiders, the first primordium fails to develop further, the second gives rise to book lungs, the third gives rise to book lungs or to the lateral tubes of the tubular tracheae (depending on the group of spiders), and the more posterior ones give rise to the spinnerets. For simplicity, some appendages or appendage parts are not shown (for example antennae, exopods). Source: Parts b-d from Damen WG, Saridaki T, et al. Curr Biol 2002; 12: 1711-1716. Copyright (2002), reprinted with permission from Elsevier.
Chelicerates (Horseshoe Crabs)
book gills Chelicerates (Spiders)
book gills Chelicerates (Spiders)
In the brine shrimp Artemia, pdm and a homolog of the insect wing dorsal compartment selector gene, apterous, are expressed in a dorsal lobe of the thoracic limbs. The parallels between the expression domains of pdm and apterous in crustacean epipodites and in insect wings suggest that these structures share a common history. This dorsal limb branch is used for respiration and osmoregulation in some extant crustaceans, consistent with the evolutionary origin of insect wings from a gill-like structure with respiratory function.
Comparisons with chelicerates suggest that dorsal limb epipodites have been repeatedly modified to form structures with highly divergent morphologies and functions. Horseshoe crabs, chelicerates that have retained an aquatic lifestyle, bear gill-like appendages on their opisthosoma segments. These book gills show uniform expression of pdm. Similarly, the tubular tracheae and book lungs (internal respiratory structures) of spiders express pdm as well as apterous. Thus, the respiratory role of the ancestral arthropod gill has been retained in some chelicerate as well as crustacean lineages. Spider spinnerets develop from the serially homologous opisthosomal primordia that give rise to these respiratory structures and they also show expression of pdm and apterous genes. These observations are consistent with the idea that spider spinnerets are derived from an ancestral respiratory structure, which was modified when early arachnids adopted a terrestrial lifestyle.
Examination of diverse arthropod species reveals that overlapping expression domains of pdm and apterous appear to be characteristic of structures that are derived from a specific epipodite or gill (Fig. 6.2d). Developmental genetic similarities between these disparate appendages suggest that regulatory evolution might explain the morphological differences between these structures. Given the similarities in the expression domains of the developmental regulatory genes apterous and pdm, downstream changes in target gene regulation may have contributed to the divergent morphologies of chelicerate respiratory structures, crustacean limb branches, spider spinnerets, and insect wings.
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