Appendage patterning in the South American bird spider Acanthoscurria geniculata (Araneae: Mygalomorphae)

Abstract

Pattern formation by the genes dachshund (dac), Distal-less (Dll), extradenticle (exd) and homothorax (hth) in spider appendages has been studied previously only in members of the higher spiders (Araneomorphae). In order to study the diversity and conservation of pattern formation in spiders as a whole, we studied homologs of these genes in embryos of the bird spider Acanthoscurria geniculata, which belongs to the Mygalomorphae, a more primitive spider group. We show that the patterns of dac and Dll are largely conserved in all spiders studied so far. We find a duplication of hth and exd genes as previously identified in the higher spider Cupiennius salei. These data suggest that pattern formation shows little diversity in all spiders, including the duplication of hth and exd that likely occurred before the split of Mygalomorphae and Araneomorphae. We also find that the legs and pedipalps bear endites of which only the pedipalpal endite expresses Dll and is retained in the adult. Similarly, the limb buds of the posterior spinnerets express Dll and become segmented appendages in the adult, whereas the anterior spinnerets lack Dll expression and are absent in postembryonic stages. In both cases, the expression of Dll or the lack of it indicates structures which will be retained as adult traits or rudimentary structures that degenerate, respectively. The presence of embryonic rudiments of leg endites in Acanthoscurria and the leg-like pattern formation in the posterior spinnerets are interpreted as primitive traits that have been lost in the Araneomorphae.

A conserved genetic mechanism specifies deutocerebral appendage identity in insects and arachnids

“…Based upon the data presented here, taken together with reported expression patterns in the millipede Glomeris marginata and multiple spider species [32,33], we infer that broad expression of hth and Dll in both proximal and distal territories of deutocerebral appendages was present in the arthropod Figure 3. hth expression patterns in embryonic appendages of (a -c) the harvestman, P. opilio; (d -f ) the scorpion, C. sculpturatus; and (g-i) the horseshoe crab, L. polyphemus.…”

“…In late states of chelicerate development, the degree of overlap between hth and Dll expression domains is unique to each appendage type [31,32]. As in D. melanogaster, this overlap is nearly complete in the deutocerebral appendages, but not in the walking legs, where hth is absent from the two distal-most podomeres [31][32][33] …”

Chelicerata

“…In other arthropod subphyla, genes including Dll , homothorax ( hth ), extradenticle ( exd ), and dachshund ( dac ) are required for appendage development, and it has been shown that these genes are also necessary for appendage development in chelicerates (Fig. 5.8 ;Prpic et al 2001Prpic et al , 2003Prpic and Damen 2004 ;Pechmann and Prpic 2009 ;Barnett and Thomas 2013b ;Sharma et al 2013 ).…”

“…In particular, two Entelegynae, the central American wandering spider Cupiennius salei and the common house spider Parasteatoda tepidariorum (formerly Achaearanea tepidariorum ; chelicerate, arthropod, and metazoan evolution and development (McGregor et al 2008a ;Hilbrant et al 2012 ). More recently, the Haplogynae Pholcus phalangioides has been employed as a satellite model to provide a comparative perspective in spider EvoDevo within Araneomorphae (Pechmann et al 2011 ), and there has also been one comparative gene expression study in a mygalomorph (Pechmann and Prpic 2009 ). The contribution of studies of gene expression (e.g., see Table 5.2 ) and gene function in spiders to our understanding of evolution and development is discussed in detail below.…”

Divergent role of the Hox gene Antennapedia in spiders is responsible for the convergent evolution of abdominal limb repression

“…In situ hybridization and nuclear staining with Sytox Green were performed as described (46,47) with minor modifications. Images were captured with a Zeiss Axioplan-2 microscope or with a Leica dissection microscope equipped with an Intas digital camera and UV light.…”