21 Sharma P.P.

Schwager E.E.

Extavour C.G.

Giribet G. Evolution of the chelicera: a dachshund domain is retained in the deutocerebral appendage of Opiliones (Arthropoda, Chelicerata).

22 Prpic N.M.

Tautz D. The expression of the proximodistal axis patterning genes Distal-less and dachshund in the appendages of Glomeris marginata (Myriapoda: Diplopoda) suggests a special role of these genes in patterning the head appendages.

23 Mittmann B.

Scholtz G. Distal-less expression in embryos of Limulus polyphemus (Chelicerata, Xiphosura) and Lepisma saccharina (Insecta, Zygentoma) suggests a role in the development of mechanoreceptors, chemoreceptors, and the CNS.

24 Briggs D.E.G.

Siveter D.J.

Siveter D.J.

Sutton M.D.

Garwood R.J.

Legg D.A. Silurian horseshoe crab illuminates the evolution of arthropod limbs.

Figure 4 Dorsal Outgrowth and Neurogenesis in Phalangium opilio Show full caption Expression of Po-Dll in intermittent developmental stages shows coalescence of ocularial domains (black arrowheads). Transient appearance of ventrolateral domains (white arrowheads) develops after stage 10 (A), being first observed in stage 14 embryos (B) and waning by stage 18 (C). Abbreviations are as in Figure 1 . In (A) and (B), arrows indicate the labrum. Scale bars represent 200 μm. See also Figure S1

We reasoned that the presence of multiple eye types in Tetrophthalmi (median and lateral) may have resulted in transient retention of this morphology during embryogenesis of modern species. We therefore investigated the developmental genetics of eye-associated outgrowths (ocularium and ozophores) in a member of Phalangida, Phalangium opilio. The appendage-patterning gene Distal-less (Dll) is commonly co-opted to pattern nonappendage outgrowths. In the model harvestman Phalangium opilio, recent work has demonstrated that Po-Dll is expressed in domains that give rise to the ocularium. Po-Dll knockdown results in the loss of the ocularium []. We reinvestigated embryonic Po-Dll expression during several consecutive stages of development. In addition to the ocularial domains ( Figures 4 A–4C, black arrowheads) , we observed multiple transient, paired expression domains of Po-Dll in the lateral fields ventral to the semilunar grooves ( Figures 4 A–4C, white arrowheads). These domains peak in expression strength in stage 14 embryos and wane thereafter. In all Opiliones, these tissues ultimately form the secretory ozopore (the opening of the repugnatorial glands), which is situated at the tip of an outgrowth, the ozophore, in Cyphophthalmi ( Figure 1 C). In P. opilio, an outgrowth does not form in this region in wild-type embryos, and Po-Dll knockdown demonstrates no associated loss-of-function phenotype. These data suggest that the domains in the lateral eye fields represent vestiges of ozophores. Similar transient Dll domains are reported to signify vestigial outgrowths in other arthropods. For example, they are observed in the intercalary segment of millipedes, which putatively bore true appendages in the ancestor of mandibulate arthropods [], and in the vestigial exopods of extant horseshoe crabs [].