Figure 4.

Two models for the origin of the lobula (LO) and internal chiasma. (A) (1) Hypothetical ancestral form. Medulla (ME) neuron antecedents to Mi and Tm neurons (red) in the ancestral ME/LO receive input in distal strata and provide input to undifferentiated ancestral T4/T5 LOP cells (blue). These cell pairs constitute two combined classes that are not initially differentiated, but which later duplicate. Gray arrows in each neuropil indicate direction of accretion of new columns to the neuropil, from first to last, corresponding to the posterior-anterior axis of the visual field; (2) T4/T5 cells duplicate, and their dendritic zone (pink) segregates into two. Then ingrowing Tm terminals separate the two layers between what will become the M10 and Lo1 strata, one for each differentiated T4 and T5 cell type, respectively. The original ME and the newly developed LO strata are thus arranged in tandem; (3) The newly formed LO separates from the ME, segregating T5 from T4 cell populations, and generating what will become the internal chiasma; (4) The LO rotates 90° in a counter-clockwise direction (gray dashed arrow in A3), the Tm axons (red) generating a chiasma between the ME and LO. Now the Tm cell axons must have changed the direction of their entry to the LO, to enable the LO to become rotated to its current position, parallel to the LOP. This causes the chiasma to form between ME and LO. (B) Alternative “VPN-duplication” model. Model A offers an explanation for the close similarity between ME stratum M10 (with T4 dendrites) and LO stratum Lo1 (with T5 dendrites), but not for the chiasma of Tm cell axons between them. (1) Ancestral form as in A1, but with additional projections from ancestral columnar VPNs in the proximal ME to the central brain. The numbers on the VPN CB represent the order of cell generation from a proposed proliferation center (1 = early, 4 = late). The direction of cell displacement is shown by a white arrow; (2) Some classes of VPNs may have duplicated in an anti-parallel direction (white arrows), originating from the proliferation center. It is proposed that the formation of a new neuropil, the LO (dashed black box), was induced by newly generated VPN populations. The polarity of the LO could then be defined by the order of VPN generation, as in the ME (Figure 1); (3) LO is now innervated by Tm cells from ME, which were generated by the duplication of Mi cells, as well as by T5 cells from LOP. Insofar as the anterior-posterior axis is inverted in LO from ME, the axons of Tm cells have to cross to innervate LO, causing the inner chiasma to form.