The source work (Strausfeld, Wolff and Sayre, bioRxiv 2019, published as eLife 2020) argues that lobed mushroom bodies, not lobeless hemiellipsoid bodies, represent the ground pattern of the higher olfactory center shared by crustaceans and insects, and that crustacean hemiellipsoid bodies are evolved transformations of this ground pattern. The five unresolved issues the paper itself flags are: the circuit-level organization of the highly modified anti-DC0-immunoreactive territories in true crabs (Brachyura) is unknown and its homology is actively disputed; the functional significance of the transformation from columnar mushroom body to hemiellipsoid body, and its proposed correlation with planar versus three-dimensional behavioral ecologies, lacks causal or broad comparative testing; the hypothesis that divergence of the crustacean and insect olfactory pathways at receptor, glomerular and projection-neuron levels explains divergent mushroom body evolution is untested; the conundrum that remipede brains resemble malacostracan brains while molecular phylogenies place Remipedia close to Hexapoda is unresolved and compounded by ongoing phylogenomic instability; and the functional role of the reniform body relative to the mushroom body in eumalacostracans is not established. The source raises these as separate open points and propositions under one common question: establishing the homology, evolutionary transformations, and functional significance of the pancrustacean higher olfactory center.

Background

Pancrustacea is the clade uniting crustaceans with hexapods (insects and their closest relatives), supported by all recent molecular phylogenies. In insects, the mushroom bodies are paired higher brain centers famous for their role in learning and memory: they consist of many thousands of tiny intrinsic neurons (globuli or Kenyon cells) whose dendrites form a cup- or cap-shaped calyx receiving sensory input and whose axon-like processes form columnar lobes partitioned into discrete input and output domains. In crustaceans, the corresponding paired centers in the lateral protocerebrum (the most anterior, eyestalk-associated brain region) are traditionally called hemiellipsoid bodies: dome- or cap-shaped, lobeless neuropils long treated as a crustacean-specific character distinct from insect mushroom bodies, although they likewise receive olfactory input from the deutocerebral (antennular) olfactory lobes via a characteristic tract of projection neurons, the olfactory globular tract (OGT). A key neuroanatomical marker in this debate is anti-DC0, an antibody against the catalytic subunit of Drosophila protein kinase A (the gene product of DC0), which selectively labels mushroom body columnar neuropils in insects and their putative homologues in other arthropods. A further relevant structure is the reniform body ('kidney-shaped body'), an anti-DC0-immunoreactive integrative neuropil of the eumalacostracan lateral protocerebrum, connected to the lobula of the optic lobes and to the mushroom body, which is distinct from both the mushroom body and the hemiellipsoid body and has been implicated in visual habituation. Against the received doctrine that hemiellipsoid bodies are a derived crustacean trait and mushroom bodies a derived hexapod trait, Wolff et al. (2017) demonstrated mushroom bodies meeting strict insect-style morphological criteria in stomatopod crustaceans (mantis shrimps); Sayre and Strausfeld (2019) found the same in the caridean shrimp Lebbeus groenlandicus; and Strausfeld and Sayre (2020) showed that reptantian crustaceans (lobsters, crayfish, hermit crabs) variously retain, transform, or lose the columnar lobes, with hermit-crab hemiellipsoid bodies incorporating lobe-type circuitry into the calyx. The source work synthesized this into the claim that the lobed mushroom body, not the hemiellipsoid body, provides the ground pattern for the pancrustacean higher olfactory center, with hemiellipsoid bodies arising by incorporation of columnar-lobe circuits into lobeless calycal centers, and proposed that this transformation correlates with behavioral ecology: retention of columnar lobes in mobile species negotiating complex three-dimensional habitats (e.g., reefs and escarpments) versus calycal transformation in species of predominantly planar sea-floor ecologies, with the featureless off-shore water column instead associated in the source with reduction of the center (as in Penaeus). A more cautious comparative position, reviewing the same evidence, treats the mushroom body / hemiellipsoid body correspondence as an open genealogical question rather than settled homology (Harzsch and Krieger, 2018, 2021). The problem also has a phylogenetic edge: Remipedia are eyeless cave-dwelling crustaceans whose brains (Fanenbruck et al., 2004; Stemme et al., 2012, 2016) show a malacostracan-type organization with prominent hemiellipsoid-body-like olfactory centers, while molecular phylogenies place Remipedia in Allotriocarida, the clade (with Cephalocarida and Branchiopoda) closest to Hexapoda (Oakley et al., 2013; Schwentner et al., 2017) — a conundrum for using neuroanatomy to reconstruct the ancestral pancrustacean brain.

Problem Statement

Establish the homology, circuit organization, evolutionary transformations, and functional significance of the higher olfactory centers of the pancrustacean lateral protocerebrum — the insect mushroom bodies and the crustacean hemiellipsoid bodies — by resolving, with explicit evidence, the five linked issues left open by Strausfeld, Wolff and Sayre (eLife 2020): (1) determine the exact arrangement of intrinsic and output neurons in the highly modified anti-DC0-immunoreactive territories of Brachyura (shore crabs and fiddler crabs), including which brachyuran center is a mushroom body homologue, which is the reniform body, and how they relate to the disputed higher-order center of Neohelice granulata; (2) establish the functional significance of the source's proposed evolutionary transformation from columnar mushroom body to lobeless hemiellipsoid body morphology, and test the proposed correlation between this transformation and behavioral ecology (planar versus three-dimensional environments); (3) test whether the documented differences between crustacean and insect olfactory pathways at the olfactory receptor, glomerular, and projection neuron levels may underlie the divergent evolution of mushroom body morphology; (4) resolve the conundrum that remipede brain organization is typical of Malacostraca while molecular phylogenies place Remipedia in Allotriocarida close to Hexapoda — that is, determine whether remipede neuroanatomy reflects ancestral pancrustacean organization, convergent or retained malacostracan-type characters, or homoplasy in the molecular trees; and (5) determine the functional role of the reniform body relative to the mushroom body in eumalacostracans, including whether it participates in learning and memory as well as visual habituation and whether it corresponds to any insect lateral protocerebral center such as the lateral horn. These components are thematically linked facets of the source's single question — reconstructing how the pancrustacean higher olfactory center evolved and what its transformations do. All five components are mandatory: each counts as resolved by a defensible determination supported by cited evidence, or by a reasoned argument that the available evidence is insufficient to decide it, and the task is complete only when all five have been addressed in one of those two ways.

References

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