Pre-cheliceral region patterning in a spider provides new insights into the development and evolution of arthropod neurosecretory centres

Comparing head development among arthropods has helped identify ancestral aspects of brain patterning and structure in animals more generally. Most understanding of arthropod head patterning has been learned from insects and the myriapod Strigamia maritima. Chelicerates represent an outgroup to mandibulate arthropods and can provide a valuable perspective to arthropod evolution and development. We assayed the expression of key markers of head patterning and neurosecretory centres from mandibulates in the pre-cheliceral region of embryos of the spider Parasteatoda tepidariorum. We found that, like mandibulates, this spider likely has a pars intercerebralis, marked by six3.2 and visual system homeobox/chx. We also found some evidence for another neurosecretory centre, the pars lateralis, marked by six3.2 and fasciclin 2. Furthermore, we identified anterior-medial cells in the spider pre-cheliceral region that express six3.2, foxQ2 and collier1, suggesting they may be pioneer neurons. However, these spider cells do not appear to be equivalent to the central pioneer neuronal cells identified in S. maritima because they lack expression of other key markers. Taken together, our study of spider pre-cheliceral region patterning adds a new chelicerate perspective to understanding the development and evolution of the arthropod head.