Novel Function of Distal-less as a Gap Gene during Spider Segmentation

Abstract

Despite many aspects of the regulation of segmentation being conserved among arthropods, the evolution of novel gene functions has played an important role in the evolution of developmental regulation and the emergence of new segmental structures. Moreover the study of such novel gene functions can be informative with respect to the patterns and direction of evolutionary changes in developmental programs. The homeobox gene Distal-less (Dll) is known for its conserved function in appendage development in metazoans. In arthropods, Dll is required for the specification of distal appendage structures. Here we describe a novel and unexpected role of Dll in the spider Achaearanea tepidariorum. We detect At-Dll transcripts not only in the appendages, but unexpectedly also in an anterior domain during early development, prior to the specification of the limb primordia. A similar early Dll domain is present in the distantly related spider Pholcus phalangioides. In A. tepidariorum this early At-Dll expression is required for head segmentation. RNA interference results in spiders that lack either the first or the first and the second walking leg segments. The early At-Dll expression is also required for the activation of the segment polarity genes engrailed and hedgehog in this region. Our work identifies the Distal-less gene as a novel factor in anterior spider segmentation with a gap gene-like function. This novel role of Dll is interesting because Dll expression is reduced in this region in crustaceans and the homologous insect segment, the mandible segment, does not express Dll and does not require this gene for patterning. We therefore discuss the possible implications of our results for understanding the evolution and diversification of the mandible segment.

The Wnt and Delta-Notch signalling pathways interact to direct pair-rule gene expression via caudal during segment addition in the spider Parasteatoda tepidariorum

“…This result is consistent with previous analysis of these and other pair-rule orthologues in the Central American wandering spider Cupiennius salei (Damen et al, 2000(Damen et al, , 2005. Our data provide further evidence for differences in the regulation of prosomal and opisthosomal segments in spiders, whereby gap and pair-rule gene orthologues respectively direct the formation of segments in these tagmata (Damen et al, 2000(Damen et al, , 2005Pechmann et al, 2009Pechmann et al, , 2011Schwager et al, 2009). Interestingly, this also indicates that the roles of eve and run-1 in spiders is restricted to formation of more posterior segments than, for example, in the insects D. melanogaster and Tribolium, and the myriapods Strigamia maritima and Glomeris marginata, in which eve is expressed in a segmental pattern in four segments more anterior to O1/T2 (Frasch et al, 1987;Brown et al, 1997;Janssen et al, 2011;Brena and Akam, 2013).…”

“…Firstly, there is evidence that hunchback (hb) and Distal-less (Dll) perform gap gene-like functions during formation of the prosomal segments of spiders Pechmann et al, 2011). Secondly, the orthologues of Drosophila pair-rule genes are also expressed in the SAZ and segments of short-germ arthropod embryos, which is consistent with roles in segmentation in these animals, although it is likely that these genes were expressed in single rather than double segmental periodicity ancestrally in arthropods (Frasch et al, 1987;Sommer and Tautz, 1993;Patel et al, 1994;Damen et al, 2000Damen et al, , 2005Davis et al, 2001;Dearden et al, 2002;Chipman et al, 2004b;Schoppmeier and Damen, 2005b;Choe et al, 2006;Damen, 2007;Mito et al, 2007;Chipman and Akam, 2008;Janssen et al, 2011;Sarrazin et al, 2012;Brena and Akam, 2013;Green and Akam, 2013).…”

“…The formation of prosomal segments requires travelling waves of hedgehog and orthodenticle expression and more posteriorly hb and Dll (Pechmann et al, , 2011Schwager et al, 2009;Kanayama et al, 2011); whereas the Wnt8 (Pt-Wnt8) and Delta-Notch signalling pathways are required for formation of the SAZ and development of opisthosomal segments (Oda et al, 2007;McGregor et al, 2008b). It was shown previously that knockdown of Pt-Wnt8 or Pt-Delta (Pt-Dl) using parental RNAi results in strongly reduced expression of Ptcad expression and gives rise to truncated embryos with malformed or even an absence of posterior segments (Oda et al, 2007;McGregor et al, 2008b).…”

Evolutionary crossroads in developmental biology: the spiderParasteatoda tepidariorum

“…Two different approaches have been developed to knock down gene expression using eRNAi in Parasteatoda. Pechmann and colleagues (Pechmann et al, 2011) have established the injection of doublestranded RNA into the perivitelin fluid that surrounds the embryo (typically at stage 4). This technique results in global knockdown of gene expression in subsequent stages and can therefore be used in conjunction with pRNAi to distinguish between the early and late functions of a gene of interest (Pechmann et al, 2011).…”

“…Pechmann and colleagues (Pechmann et al, 2011) have established the injection of doublestranded RNA into the perivitelin fluid that surrounds the embryo (typically at stage 4). This technique results in global knockdown of gene expression in subsequent stages and can therefore be used in conjunction with pRNAi to distinguish between the early and late functions of a gene of interest (Pechmann et al, 2011). This global eRNAi approach is complemented by the application of eRNAi to subsets of cells, which was facilitated by the development of single-cell microinjection of early cleavage stage Parasteatoda embryos (Kanayama et al, 2011;Kanayama et al, 2010) (Fig.…”