XXII CNIE - Accademia nazionale italiana di Entomologia

are closer to the Entomostraca than any other crustacean group. In short, molecular
studies support the view that insects and entomostracans, not malacostracans, share a
common ancestor. In contrast, neural phylogenies do not show this. Instead, analyses
indicate that either the malcostracan or entomostracan affinities of the hypothetical
ancestor of the insects is unresolved or, as shown more recently with added data, that
insects are closer to the Malacostraca than to the Entomostraca (Strausfeld, 2005).
Such major discrepancy between two proven approaches, in this case molecular
phylogenetics and neural phylogenetics, often typifies scientific endeavor and challenges
us to search for alternative explanations of the data. Might molecular phylogenetics fail
to resolve certain affinities, such as those of remipedes and malacostracans, but not
others? If, however, molecular phylogenetics is unassailable then another explanation is
needed for why insect brains seem to be more similar to those of malacostracans than of
entomostracans. One possible explanation for this is that there has been evolutionary
convergence of brain structures in insects and malacostracans or a secondary loss of
structures in the entomostracans, such that all of those living today have drastically
reduced brains. One way of examining the plausibility of either of these alternatives is
by comparing the occurrence of specific brain areas across representative species and
then considering whether similarities can be best explained on the basis of common
ancestry or on the basis of homoplasy, the evolutionary convergence of similar
characters.
The most cited example of homoplasy in neurobiology is that of the olfactory system
(Strausfeld and Hildebrand, 1999). The olfactory bulbs of vertebrates (chordates) have
subunits, glomeruli, the neurons of which are almost identical in organization and
connectivity to those of glomeruli in the olfactory lobes of insects (ecdysozoans).
Glomerular olfactory lobes are also found in the brains of polychaete worms
(lophotrochozoans), suggesting that these distinctive centers have evolved independently
at least three times. Yet, recent studies contradict this: specific genes that control
developmental processes in one phylum may have homologues in another that control
comparable processes. In the fruit fly Drosophila, a gene called ‘empty spiracles’ is
required for the proper development of the deutocerebrum, that part of its brain
containing the antennal lobes. Later during development the same gene is required for
controlling the correct number of relay neurons that connect glomeruli to other parts of
the brain and then later again the gene is required for setting up specific connections
onto the dendrites of some of these neurons. There is a homologous gene in the mouse
called Emx1/2 that is required for the development of olfactory circuits in the mouse
olfactory bulb (Lichtneckert et al., 2008). This is most unlikely to be a coincidence.
Homologous genes involved in such similar developmental pathways suggests that a
common ancestor of flies and mice, something pretty simple that crept around on the sea
bed about 600 million years ago, possessed the antecedent of this genetic program.
With that example cautioning against assumptions of homoplasy, I will now review three
structures shared by the brains of insects and malacostracan crustaceans, none of which
are present in the brains of entomostracan crustaceans. These are: olfactory lobes that are
subdivided into discrete subunits; optic lobes that comprise four retinotopic neuropils,
three of which are connected by chiasmata; and thirdly, a central neuropil called the
central complex that in basal insects and in malacostracan crustaceans shares an almost
identical arrangement of layers and local chiasmata. There are also other specific
attributes that typify crustacean and insect brains, distinguishing them from myriapods
and chelicerates. However, it is these three centers that offer the greatest challenge to
received opinion about the origin of the insects.
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