Experience-dependent developmental plasticity in the optic lobe of Drosophila melanogaster

Abstract

Early experience can affect nervous system development in both vertebrate and invertebrate animals. We have now demonstrated that visual stimulation modifies the size of the optic lobes in the laboratory fruitfly Drosophila melanogaster. Monocular deprivation (painting over one eye) decreases the aggregate volume of the lamina, medulla, and lobula plate by up to 6%. The laminae of control flies kept in complete darkness showed a more robust volume difference that could be as much as 30%. An electron microscopy study revealed that the changes in the lamina are largely attributable to an increase in the terminals of the photoreceptor cell axons. The volume of the lamina increases during the first 24 hr after emergence, and it grows more in the light than in darkness. When flies are kept in the dark for the first 12 hr of their adult life and are then brought back to light for the next 3.5 days, the lamina is almost as small as in flies raised for 4 d in constant darkness. Twelve hour dark shifts at a later time are less effective. This finding suggests a critical period for lamina development during day 1 of the adult. The lamina depends on visual stimulation to maintain its size during the first 5 d after emergence. Dark-rearing for 1 d or more at any stage during that period decreases its volume to the level of flies raised in constant darkness. A lamina that is once reduced in size seems not to recover.

Fig. 1.

A cross-section of the lamina close to its proximal margin demonstrates the basic organization of a cartridge. Six photoreceptors (R) surround two monopolar cells in the middle (L1, L2). The cartridge in turn is surrounded by epithelial glial cells (g). Magnification, 4860×. Scale bar, 1.0 μm.

Fig. 2.

Effects of monocular deprivation and rearing in darkness. A, Effects of painting one eye (monocular deprivation) on the optic lobes. Except for the lobula, the neuropils underneath the open eye had a larger volume than under the occluded eye if the flies were kept in LD conditions (striped bars;n = 130). As a control for the effects of painting, some of the monocularly painted flies were reared in DD (solid bars; n = 80). Thus, the painting itself did not reduce neuropil volume. B, Rearing in DD (n = 24) gave a more obvious volume difference than monocular deprivation in the LD cycle (n = 37).

Fig. 9.

A, Effects of a 12 hr dark shift at different ages after emergence. Dark-rearing during the first 12 hr (B) leaves the lamina as small as that found after 4 d in darkness (G) but similar dark shifts at later ages exert smaller effects (groupsC–E). Dark-rearing during the last 12 hr (F), i.e., during the subjective night of the animals, reduces size even less (n_A = 11,n_B = 10, n_C = 11,n_D = 10, n_E = 10,n_F = 10, n_G = 11). B, A reduction in volume compared with LL flies (n = 14) could represent a more natural response, because flies kept in an LD light regime (n = 26) have a lamina size that is intermediate between LL and DD flies (n = 22).

Fig. 3.

A, Electron micrographs of lamina cross-sections demonstrating the volume increase and shape modification in the light. Distally, LL flies had significantly larger cartridges than DD flies. No significant difference was found at an ∼20 μm more proximal level. Magnification, 2260×; scale bar, 1.0 μm.B, Scale drawing to illustrate the changes in the shape of the lamina resulting from the differential growth effects in light (left) and darkness (right). Note that the calculated radius (r) of the sphere is three times larger in DD than in LL flies. The calculation is based on the data in Table 1, assuming a distance of 20 μm between the distal and proximal levels.

Fig. 4.

A, Antihistamine immunoreactivity of a horizontal section of a fly’s head showing the optic neuropils. The terminals of photoreceptor axons from R7 and R8 are heavily stained, allowing the subdivision of the medulla in a distal (dM) and proximal (pM) portion. L, Lamina;Lo, lobula; Lp, lobula plate.B, Volume changes resulting from monocular deprivation are found in the lamina and in the entire medulla and its distal part in LD flies (n = 24), suggesting that the photoreceptor terminals in the distal part contribute more to the overall effect in the medulla than its proximal layers. In DD flies (n = 16), no such effects could be found.

Fig. 5.

Effects of rearing in different light regimes for wild-type CS flies and the mutants norpA^P24and hdc^jk910. A, ThenorpA^P24 mutant (n_DD = 17; n_LD = 13) lacks volume changes in its lamina. The laminae of both groups innorpA^P24 are as small as the lamina of DD flies in wild-type CS (n_DD = 22;n_LD = 26). B, In contrast, the lamina in both wild-type CS andhdc^jk910 shows a significant difference between flies reared in DD and those reared in LL (n = 12 for all groups). C, In the lobula plate, hdc^jk910 fails to show a difference between DD and LL flies. Compared with wild-type CS, the lobula plates in both groups are significantly reduced (n = 12; the same animals were analyzed as inB).

Fig. 6.

Development of the lamina and lobula plate in wild-type CS flies during the first 48 hr after emergence.A, The lamina enlarges in both groups during the first 24 hr but more so in LL flies. The first significant difference between DD and LL flies was at 12 hr after emergence (n_LL = 20_(1hr), 10_(3hr), 10_(6hr), 11_(9hr), 8_(12hr), 6_(24hr), 9_(48hr);n_DD = 20_(1hr), 10_(3hr), 9_(6hr), 8_(12hr), 10_(24hr), 11_(48hr)). B, The lobula plate develops during the first 24 hr as well, and the first significant difference between DD and LL flies appears at 9 hr after emergence (same animals as in A).

Fig. 7.

The effects of rearing in darkness at different times after emergence suggest a critical period for the development of the lamina. Dark-rearing during the first 6 hr after emergence (B) prevented the lamina from increasing to the size seen in controls (A). Any other period of dark-rearing (C–E) resulted in a significant volume difference compared with the LL control group (A) (n_A = 17, n_B = 19, n_C = 11, n_D = 15, n_E = 16, n_F = 25).

Fig. 8.

Rearing in darkness during the last 2 d (right-hand experimental group: n = 17; control:LL_{4 days}: n = 12,DD_{4 days}: n = 17) or even for only the last day (left-hand experimental group:n = 16; control: LL_{4 days} = 17; DD_{4 days} = 25) significantly reduces the volume of the lamina in 4-d-old flies. Whereas a dark shift of only 1 d leaves the lamina intermediate in volume between that in LL flies (open bars) and that in DD flies (solid bars), 2 d of darkness reduce the size to the volume found in DD controls. The data came from two independent experiments so that the absolute size of the laminae varies considerably.

Fig. 10.

Effects of extended periods of darkness in 6-d-old flies indicate that the lamina becomes less responsive compared with earlier ages (compare Figs. 7 and 9). Dark-rearing between days 2–4 (C) and 4–6 (D) reduces the lamina’s volume to that seen in DD control flies (F), whereas a dark shift during just the last day (E) no longer affects lamina volume. As before, dark-rearing for the first 2 d (B) leaves the lamina as small as in DD flies (F) (n_A = 10, n_B = 10, n_C = 8, n_D = 13, n_E = 10, n_F = 7).

Fig. 11.

Effects of rearing in different light regimes in wild-type CS (n_DD = 15,n_LL = 13) and in three learning and memory mutants [dunce¹(dnc;n_DD = 13, n_LL = 14), rutabaga¹ (rut;n_DD = 13, n_LL = 13), amnesiac (amn; n_DD = 12,n_LL = 11)]. A, In all three mutants, the lamina exhibits a significant difference between DD and LL flies, ranging from 15 to 30%. B, In the lobula plate, the three mutants exhibit significant volume differences between DD and LL flies, of from 15 to 20%. In this experiment, however, the corresponding difference in the wild-type CS control was not quite significant.