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First published online November 17, 2003
doi: 10.1242/10.1242/dev.00864

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Initial state of the Drosophila eye before dorsoventral specification is equivalent to ventral

¹ Department of Molecular and Cellular Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
² Program in Developmental Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
³ Department of Ophthalmology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA

View larger version (66K): [in a new window]   Fig. 2. Pnr is not essential for DV patterning of eye during embryogeneis. Ush was misexpressed in eye by ey-GAL4; UAS-Ush (ey>Ush) and cultures were shifted to 29°C during different stages of development. (A) Three different restrictive temperature conditions used. (B,D-F) Eye discs were stained for Wg or L (blue), Elav (red) and Ey (green). (B-D) Cultures maintained at 29°C throughout development served as controls and resulted in elimination of entire eye field (B) in eye disc and (C) in adult eye. (D) Some weaker phenotypes of very small eye marked by Elav-positive cells were also observed. (E) Maintenance at 18°C during embryonic development and then shift to 29°C for subsequent development also resulted in complete elimination of eye field. (F,G) When cultures were shifted to 29°C during embryonic development to block pnr activity and later allowed to develop at 18°C, they did not show any eye suppression phenotype (F) in the eye disc and (G) in adult eye.

View larger version (56K): [in a new window]   Fig. 1. Expression of L in the larval eye disc is initiated earlier than pnr and Iro-C genes. All eye discs in this and subsequent figures are oriented anterior towards the right and dorsal towards the top. Eye disc of first- (A,A', arrows), early second- (B,B', arrow), and late second- (C,C') instar larvae stained for pnr (green), L (red) and Elav (blue). Expression of pnr (green, arrow) begins in late first- to early second-instar disc (B,B') in a small group of cells at the dorsal margin. Expression of L (red) and mirr (green) in first- (D,D', arrowhead), early second- (E,E') and mid-second- (F,F') instar eye disc, respectively. mirr is expressed in a broader domain in the dorsal eye as compared with the pnr. Individual channels of the images are also shown ('). Magnifications of the images are same.

View larger version (63K): [in a new window]   Fig. 3. Loss of function of L suppresses ventral eye development. (A) L mutant disc showing loss of ventral eye pattern. Loss-of-function clones of L were marked by absence of GFP (green) in the eye disc (B) and by absence of white gene expression in adult eye (C). Eye discs were stained for 22C10 or Elav. Clone in the dorsal eye shows no effect on eye fate in disc (B,B', arrow). Ventral clone caused suppression of eye fate as seen by absence of 22C10 in disc (B,B', arrowhead) and in adult (C, arrow). (D,D') Ventral loss-of-function clone of L also showed ectopic induction of Ey where eye fate is blocked (arrowhead). (E,E') Early loss-of-function clone of L showed complete elimination of eye fate as evident by absence of Elav (red). Note that eye field (EYE) is highly reduced, whereas antennal (AN) development was not affected.

View larger version (57K): [in a new window]   Fig. 4. Loss of Ser gene function can abolish the eye fate. (A) Graphical presentation of eye phenotypes generated by targeted misexpression of ey-GAL4; UAS-SerDN (ey>SerDN) along the time course of temperature shifts for the samples collected at every 12 hours of interval until the late third instar. Effect of dominant-negative Ser (SerDN) was scored for its effect on eye fate in the discs and phenotypes observed were classified into three main categories: complete loss of eye fate (blue); loss of ventral eye pattern (purple) and generation of two antennal fields (yellow). For each time window, at least 20 discs were scored. (B,D,E,G) Eye discs were stained with Elav (red) and with Ey (green) and Wg (blue). Misexpression of ey>SerDN for 12-72 hours resulted in complete elimination of eye disc (B), adult eye (C) and eye disc with a few photoreceptors (D) (shown in blue in A). For 96-108 hours, ey>SerDN resulted in preferential loss of ventral eye in disc (E) and adult (F) (shown in purple in A). It has been suggested that loss of Ser using the same ey>SerDN caused the homeotic transformation of the antenna to the eye fate (Kumar and Moses, 2001). (G) ey>SerDN primarily showed suppression of the eye field and also occasionally (3/35) results in the generation of two antenna fields (shown in yellow in A). This may be due to `splitting' of the antenna field, as evident from the mirror image duplication of Wg expression in the ventral sector of antenna disc (AN).

View larger version (99K): [in a new window]   Fig. 5. Loss-of-function of pnr and Iro-C changes dorsal eye sensitivity to ventral. (A,B) Overexpression of L by ey>L causes selective ventral eye suppression in disc (A, arrow) and adult (B, arrow). Loss-of-function clones of pnr were generated in eye using EGUF approach, which resulted in dorsal eye enlargement (C) in disc and (D) in adult. In the ventralized disc with EGUF pnr clones in dorsal when L (ey>L) was overexpressed resulted in small eye because of suppression of eye fate on both dorsal and ventral margin of (E) disc (arrows) and in (F) adult eye (arrowhead). Misexpression of SerDN (ey> SerDN) in the ventralized disc with EGUF pnr clones completely abolished the entire eye fate (G) in disc and in (H) adult eye. Loss-of-function clones of Iro-C show dorsal eye enlargement in (I) disc. (J) EGUF Iro-C clones in eye disc also result in dorsal eye enlargements. (K,L) In the ventralized eye disc mutant for Iro-C, overexpression of ey>L (K) or ey>SerDN (L) results in suppression of eye on both DV margins and complete removal of the eye fate, respectively.

View larger version (29K): [in a new window]   Fig. 6. Larval eye primordia arise from an initial state comprising a group of cells that require L/Ser function for growth and maintenance. Removal of L/Ser function in these initial cells can completely eliminate eye. During late first instar of development, the pnr+ cells emerge and initiate the expression of downstream Iro-C genes, which results in DV specification of eye. Establishment of DV lineage in eye restricts the L/Ser requirement to the ventral cells only. pnr+ and Iro-C+ cells become independent of L/Ser requirement. Therefore, the initial state prior to DV specification is probably equivalent to ventral eye in nature.

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