Dendritic routing defects in prospero mutant embryos. (A) Anterior (red) and posterior (blue) dorsal branches stay clear of each other in wild-type embryos. (B,C) In prospero¹⁷ mutant embryos, dorsal branches may cross each other or veer sharply toward the other dorsal branch. (D) Outgrowth (red arrow) and routing (yellow arrow) defects of dorsal cluster axons in prospero mutant embryos.

Dynamic lateral branching revealed by time lapse analysis. Dendrite development beginning at 15 hr AEL was recorded at room temperature by confocal microscopy. Pictures were taken at 10-min intervals. (Arrows) One of many areas in which dendritic retraction is observed; (arrowheads) one of many areas in which dendritic extension occurs. Bar, 10 μm.

The constitutively active form of Dcdc42 promotes primary dendrite outgrowth and inhibits lateral branching. (A) Wild-type 16–18 hr AEL embryo. The neurons were labeled by Tau–GFP, which binds to microtubules. Tau–GFP expression was driven by Gal4 60. (B) 16–18 hr AEL embryo with the constitutively active form of Dcdc42 expressed in PNS neurons driven by Gal4 60. Dorsal branches fail to fully extend and the number of lateral branches is reduced. (C) One dorsal primary dendrite extends from the neuron (arrow) in wild-type 14–15 hr AEL embryo. (D) Multiple dorsal primary dendrites (arrow) extend in14–15 hr AEL embryo expressing the constitutively active form of Dcdc42.

kakapo (kak³⁰) and shrinking violet (shrv¹³) mutant embryos display reduced numbers of lateral branches in the dorsal cluster of the PNS. Wild-type embryos (A) have more anterior lateral branches than kak³⁰ mutant embryos (B). (C) The numbers of anterior lateral branches for wild-type (10.2 ± 0.2) and kak³⁰ (4.4 ± 0.2) embryos (mean±s.d. for 100 segments). (D) Some lateral branches in enabled (ena⁴⁶) embryos fail to grow toward segment boundaries, but turn dorsally instead. shrv¹³ mutant embryos exhibit fairly normal lateral branch number at stage 16 (E) but do not maintain them, resulting in fewer lateral branches at stage 17 (E). (E,F) are representative images of dorsal clusters at different developmental stages.

Dorsal md neuron dendrites develop late in embryogenesis. (A) Pictures were taken from homozygous Gal4 109(2) 80-UAS–GFP embryos at different stages. (a) Dendrite budding begins at 12–13 hr AEL; (b) dorsal dendrites extend significantly by 14–15 hr AEL; (c) dorsal dendrite extension is almost complete at 16–17 hr AEL; (d) lateral dendrites continue extending and retracting at 18–19 hr AEL; (e) branching pattern is fixed by 20–21 hr AEL; (f) lateral branches cover the hemisegment before hatching (22–23 hr AEL). (B) The time line of dorsal cluster dendrite development. The tail of each arrow indicates the approximate time point at which a specific developmental process begins. Impermeable cuticle forms before dorsal branches fully extend. Bar, 10 μm.

Drosophila PNS neurons in wild-type embryos labeled by GFP. (A) A lateral view of PNS neurons in a stage 17 wild-type embryo. In all figures, anterior is toward the left and dorsal is up. (AMX) Antennomaxillary complex; (D) dorsal cluster; (L) lateral cluster; (V) ventral cluster. (B) Schematic representation of all the dorsal cluster neurons bracketed in A. There are six md neurons (ovals), four es neurons (circles), one bd neuron (diamond), and one tracheal innervating neuron (square). Gal4 109(2) 80 drives UAS–GFP expression (green) in all but the four es neurons (black) in the dorsal cluster. (C) Schematic representation of the general dendritic pattern in the dorsal cluster. The lines represent: segment boundaries (broken black line), dorsal midline (broken brown line), dorsal dendrite branches (red), and lateral dendrite branches (blue).

Mutations affecting dendritic branching lead to changes in density of dendrites. shrub (shrb⁴) mutant embryos (B) show decreased dorsal and lateral dendritic elaboration compared with wild-type (WT) embryos (A). tumbleweed (tum¹⁴) mutant embryos exhibit increased dendritic branching resulting in increased dendritic coverage per segment (C), as well as axonal outgrowth defects (D). Time-lapse analysis of dendrite development beginning at 15 hr AEL in a tum¹⁴ embryo reveals that dendrites undergo premature and excessive branching (blue bracket) prior to complete outgrowth of the dorsal primary dendrites (E). Pictures were taken at 10-min intervals and chronological sequence in E is from left to right and from top to bottom.

Mutations causing excessive dorsal branch outgrowth. (A) A dorsal view of a late first instar larva shows dendrites from dorsal clusters of the two hemisegments meet at the dorsal midline (red line), but do not mix extensively. (B) A dorsal view of a 20–21 hr AEL wild-type embryo shows the large area near the dorsal midline not covered by dendrites (red bracket). (C) A dorsal view of a 20–21 hr AEL sequoia (seq²²) embryo reveals abnormal dendritic outgrowth (green bracket). (D) Normal muscle differentiation in seq²² embryos illustrated by anti-MEF-2 antibody staining (Red). The white arrow indicates cardioblasts of the dorsal vessel that underlie the dorsal midline. (E) Normal epidermal cell layer organization in seq²² embryos stained with anti-Numb antibody (green). The white arrow points to the dorsal midline. (F) seq²² embryos exhibit axonal outgrowth defects. The green arrow indicates the breakpoint of the axon. Dendrites in fmi⁷² mutant embryos exhibit aberrant dendritic outgrowth phenotype (yellow bracket) in G or misrouting phenotype (yellow arrow in H).