Cooption of an appendage-patterning gene cassette in the head segmentation of arachnids

Abstract

The jointed appendages of arthropods have facilitated the spectacular diversity and success of this phylum. Key to the regulation of appendage outgrowth is the Krüppel-like factor (KLF)/specificity protein (Sp) family of zinc finger transcription factors. In the fruit fly, , the homolog is activated by () and establishes ventral appendage (leg) fate. Subsequently, maintains expression of the axial patterning gene (), which promotes limb outgrowth. Intriguingly, in spiders, has been reported to have a derived role as a segmentation gap gene, but the evolutionary origin and regulation of this function are not understood because functional investigations of the appendage-patterning regulatory network are restricted to insects. We tested the evolutionary conservation of the ancestral appendage-patterning network of arthropods with a functional approach in the spider. RNAi-mediated knockdown of the spider ortholog resulted in diminution or loss of expression and truncation of appendages, as well as loss of the two body segments specified by the early function. In reciprocal experiments, is shown not to be required for expression. Knockdown of (Wnt-1 coreceptor) disrupted segmentation and appendage development but did not affect the early expression domain. Ectopic appendages generated in the spider "abdomen" by knockdown of the Hox gene () expressed comparably to wild-type walking legs. Our results support () the evolutionary conservation of an appendage-patterning regulatory network that includes canonical Wnt signaling, , and and () the cooption of the regulatory cassette in arachnid head segmentation.

The evolution of selector gene function: Expression dynamics and regulatory interactions of tiptop/teashirt across Arthropoda

“…A colony of the stone centipede Lithobius atkinsoni was maintained at 26°C. Husbandry and embryo collection followed our previously published protocols (Setton et al, ; Setton & Sharma, ). Tissues were fixed in a 150 μl bubble of 4% PFA in 1× PBS, supplemented with heptane for 1 hr.…”

“…In situ hybridization for L. atkinsoni followed modified protocols (Setton & Sharma ; Setton et al, ). In addition to performing in situs on whole embryos, the germbands of some living embryos were dissected from the yolk in PBST at a range of limb bud bearing stages (3 to 8, sensu Kadner & Stollewerk, ) prior to dehydration in methanol.…”

Phylogenomic resolution of sea spider diversification through integration of multiple data classes

“…The mechanistic basis of supernumerary segment addition is currently an enigma, due to limited developmental genetic resources for sea spiders [39,40]. Future work should prioritize the gap segmentation genes of sea spiders, especially those functionally linked to gap segmentation phenotypes in chelicerate model species [41][42][43]. rRNA in such studies [12,44].…”

Evolution of limb development in cephalopod mollusks

“…Mouse gene expression: Meis1 (Gonzalez-Lazaro et al, 2014), Pbx1 (Capellini et al, 2006), Sp8 (Kawakami et al, 2004), Dlx5 (Vieux-Rochas et al, 2013), Dach1 (Salsi et al, 2008), Wnt1 , Wnt5a , Wnt7a , Sfrp2 (Witte et al, 2009), Shh (Riddle et al, 1993), Bmp7 (Choi et al, 2012) and En1 (Loomis et al, 1998). Arthropod expression based on: Chelicerates, Htx , Exd , Dll , Dac (Prpic et al, 2003), Sp8/9 (Konigsmann et al, 2017), Wg (Damen, 2002), En (Damen, 2002) and Sfrp1 /2/5 (Hogvall et al, 2018); Myriapods, Htx , Exd , Dll , Dac (Prpic and Tautz, 2003), Sp8 / 9 (Setton and Sharma, 2018), Wg , Dpp (Prpic, 2004), Wnt5 , Wnt7 , Hh , En (Janssen et al, 2004), Sfrp1/2/5 (Hogvall et al, 2018); Hexapods, flour beetle, Htx , Exd (Prpic et al, 2003), Sp8 / 9 (Schaeper et al, 2010), Dll (Beermann et al, 2001), Dac (Prpic et al, 2001), Wg , Wnt5 (Bolognesi et al, 2008), Dpp (Sanchez-Salazar et al, 1996), En (Brown et al, 1994); Hexapods, cricket, Htx , Exd , Dll , Dac , Wg , Hh , Dpp , En (Donoughe and Extavour, 2016).…”