Pioneering molecular work on chelicerate visual system development in the horseshoe crabLimulus polyphemussurprised with the possibility that this process may not depend on the deeply conserved retinal determination function ofPax6transcription factors. Genomic, transcriptomic, and developmental studies in spiders now reveal that the arthropodPax6homologseyelessandtwin of eyelessact as ancestral determinants of the ocular head segment in chelicerates, which clarifies deep gene regulatory and structural homologies and recommends more unified terminologies in the comparison of arthropod visual systems. Following this phylotypic stage, chelicerate visual (...) system development differs fundamentally from other arthropods during the compartmentalization of the ocular segment in that eye and optic neuropil primordia originate independently from each other. Comparative analyses of this phase identified further gene regulatory homologies but also major differences, most notably the possibly compensatory replacement ofPax6byPax2in lateral eye specification. Also see the video abstract here:https://youtu.be/Hdfr3z5kEXU. (shrink)

Deep‐level arthropod phylogeny has been in a state of upheaval ever since the emergence of molecular tree reconstruction approaches. While a consensus has settled in that hexapods are more closely related to crustaceans than to myriapods, the phylogenetic position of the latter has remained a matter of debate. Mitochondrial, nuclear, and genome‐scale studies have proposed rejecting the long‐standing superclade Mandibulata, which unites myriapods with insects and crustaceans, in favor of a clade that unites myriapods with chelicerates and has become known (...) as Paradoxapoda or Myriochelata. Here we discuss the progress, problems, and prospects of arriving at the final arthropod tree. (shrink)

Most chelicerates operate the world with two kinds of visual organs, the median and lateral eyes of the arthropod ground plan. In harvestmen (Opiliones), however, members of the small and withdrawn suborder Cyphophthalmi lack eyes except for two genera with lateral eyes. In the other suborders (Eupnoi, Dyspnoi, and Laniatores), lateral eyes are absent but median eyes pronounced. To resolve the phylogenetic history of these contrasting trait states and the taxonomic position of a four‐eyed harvestmen fossil, visual system development was (...) recently studied in the daddy longleg Phalangium opilio (Eupnoi). This effort uncovered not only a highly regressed and internalized pair of lateral eyes but also a similarly cryptic pair of additional median eyes. After recounting the evo‐devo discovery journey of uncompromising harvestmen taxonomists, this review explores comparative evidence that the enigmatic P. opilio relict eyes might serve the multichannel zeitgeber system of the biological clock. (shrink)

The Drosophila Bolwig organs are small photoreceptor bundles that facilitate the phototactic behavior of the larva. Comparative literature suggests that these highly reduced visual organs share evolutionary ancestry with the adult compound eye. A recent molecular genetic study produced the first detailed account of the mechanisms controlling differential opsin expression and photoreceptor subtype determination in these enigmatic eyes of the Drosophila larva. Here, the evolutionary implications are examined, taking into account the dynamic diversification of opsin genes and the spatial regulation (...) of opsin homolog expression in other insects. It is concluded that, consistent with their common evolutionary roots, the Drosophila larval and adult eyes use the same mechanisms for the regulation of opsin expression and photoreceptor cell fate specification. Strikingly, the structurally highly derived Bolwig organs retained a more ancestral state of opsin expression and regulation. Inconspicuous in size, the Drosophila larval eyes deliver useful lessons in the reconstruction of homology between neuronal cell types with gene expression data, and on the conservative nature of gene regulatory network evolution during the emergence of novel organs from ancestral templates. BioEssays 30:980–993, 2008. © 2008 Wiley Periodicals, Inc. (shrink)