Mushroom Body Evolution - Figure 2

Click on inline image to retrieve larger version (approx. 152k)

Figure 2: The evolution of the mushroom bodies. A strict parsimony tree has been derived from an analysis of 100 neural characters and 26 taxa (outgroups omitted; see, Strausfeld, 1998). The tree widely separates chilopods from diplopods (often combined in other trees as a group called the Myriapoda). Diplopods emerge as sister to the Onychophorans. Pycnogonids (L. hilgendorfii) are unambiguously placed into the clade Cheliceriformes. Platyhelminthes are here shown basal to coelomates but are know known from molecular studies to be grouped with annelids and gastropods into the Lophotrochozoa. All arthropods, along with nematodes, belong to the Ecdysozoa (Rosa et al., 1999). Nonmalocostracan crustaceans (branchiopods) are more closely related to archaeognathan insects than to any arthropod group, thus establishing insects and crustaceans as sister groups (Strausfeld, 1998). Heavy lines connect taxa possessing mushroom bodies. Crustacea, along with the basal archaeognathan insects, and the Collembola stand apart (see text). However, it is likely that the hemiellipsoid bodies of crustaceans are equivalent to the mushroom bodies of insects although the two may have evolved independently.

Characters mapped are: Globuli cells (1); discrete lobes comprising parallel fibers (2); chemosensory afferents ending in glomeruli (3); lateralization of lobes (4); metameric repetition of glomeruli (5); postoral appendage (the antenna; secondarily pre-oral in crustaceans) (6); postoral appendage chemosensory (7); segmental glomeruli retained in first postoral ganglion (8); glomeruli lost in all ganglia (9); glomeruli retained in first abdominal/last thoracic neuromere (10); glomeruli retained in first two postoral ganglia (11); glomeruli retained in second postoral neuromere (12); loss of characters 1-5 (13); preoral appendage (antennule) (14); wedge-shaped glomeruli (15); homoplastic origin (re-expression?) of globuli cells (16); dense, layered, non-retinotopic neuropil in eye stalk (hemiellipsoid body) (17); second glomerular neuropil (termed accessory lobe) (18); hemiellipsoid body in midbrain (19); homoplastic re-expression of globuli cells (20); parallel fibers comprising bilateral lobed neuropils (21); antennae acquire olfactory receptors (22); glomerular neuropil supplied by antenna (23); calyces (24); absence of features 22-24 (25).

Character assemblages 1 with 2, and 20 with 21, accord with the Flögel-Kenyon criteria for mushroom bodies. Characters 8-11 show a general trend in the reduction of segmental glomeruli (but these basal characters are retained in Limulus and pycnogonids). Mushroom bodies, sensu Flögel-Kenyon, are shared by the annelid-onychophoran-diplopod-cheliceriform-chilopod (AODCC) assemblage but are absent in crustaceans and archaeognathan insects. Their reappearance in thysanuran insects suggests homoplasy and convergent evolution with the AODCC assemblage. The character assemblage 22, 23, 24 is unique to neopteran insects. The character assemblage 14-18 is unique to malacostracan crustaceans. Boxed character (event) 13 presumes character loss. Circled characters 16, 20 indicate two possible homoplastic origins of globuli cells.

All contents are copyright © 2000 Evolbrain or their original publication (as noted).
Images or text may be used with proper acknowledgement.

AAxxxxx