Figure 5. Prdm16 and Mecom define secondary fast motor neurons.

(A) Schematic of a transverse cross section of the zebrafish tail showing (left) the four dorsal-ventral muscle quadrants (black solid line with labels I, II, III, and IV) as defined by primary MN innervation pattern and the different muscle functional fiber types (solid light gray as slow, dashed dark gray outline as fast) and (right) the nerves that innervate them (slow muscle by septal nerves [light gray] and fast muscle by medial nerves [dark gray]). Primary MNs (p-type, light blue), responsible for escape behaviors, project along the medial nerve to innervate one of four fast muscle quadrants. m-type secondary MNs (dark blue) innervate fast muscle via the medial nerves. ms-type secondary MNs (purple) innervate both fast and slow muscle via the medial and septal nerves. s-type secondary MNs (light gray) innervate slow muscle via the septal nerves. Each individual secondary MN can innervate from one to four quadrants (I–IV) of muscle.

(B) Transverse view of the zebrafish tail showing muscle innervation in Tg(prdm16::GFP);Tg(olig2:dsRed2) larvae at 5 dpf. Scale bar, 10 μm.

(C) Average GFP fluorescence from Tg(prdm16::GFP) within each muscle quadrant normalized to the fourth quadrant within each fish at 5 dpf. Gray lines indicate individual fish. Black points indicates average with SEM.

(D) Pie chart showing subtype proportions labeled in pan-MN sparse label experiment.³¹ Inner subtype distribution numbers detailed in Table S1.

(E) Pie chart showing subtype proportions labeled in Tg(prdm16:GAL4).

(F) Example images of all sparsely labeled subtypes in (E). Number in parentheses indicates number of times subtype was labeled.