Figure 10

Hypothetical evolution of the mandible patterning function of cnc in embryos in the stem lineage of the mandibulate arthropods. (A) The post-antennal limbs of stem-lineage mandibulates are serially homologous biramous limbs with multiple endites, represented here as a single lobe for clarity, on the medial part of the protopodite. (B) Hypothetical expression of Dfd in a lobopod, the ancestor to all arthropods (and closely related taxa such as the Onychophora) based on expression of Dfd in an onychophoran[68]. Here, Dfd is expressed proximally in monopodial limbs with the anterior limit at the segment homologous to the first leg segment (L1) of chelicerates and onychophorans. (C) Hypothetical expression of Dfd in a hypothetical non-mandibulate ancestor to Mandibulata. (D) Expression of cnc and Dfd in a hypothetical ancestor to the mandibulate arthropods (Pancrustacea and Myriapoda). The mandibular segment identity is specified by cnc. We hypothesize that the mandible patterning function of cnc evolved in the stem lineage of the mandibulate arthropods. The mandibular and maxillary segments of mandibulates are homologous to the first and second leg segments of chelicerates and onychophorans. (E) Expression of Dfd in chelicerates based upon Dfd expression in spiders. There are two homologs of Dfd in spiders, both of which are expressed in the L1 to L4 segments. (F) Expression of Dfd in an onychophoran. (G) The mandibular gnathal edge, consisting of an incisor and molar, most likely evolved from the proximal endite (star) on the primitive biramous limb present in species such as Martinssonia. The other more distal endites were lost at some point. Labels are: antenna (An), first leg (L1), jaws (J), labrum (La), mandible (Mn), maxilla (Mx), maxilla-like mandible precursor (Mn*), primary antenna (Pa), second antenna (An2), second leg (L2), slime papilla (Sp).