I'd like to discuss problems with the 59E1;60D1 breakpoints currently reported for Df(2R)bwVDe2LPxKR (FBab0005117). This Df was generated by meiotic recombination within the inverted segments of In(2R)bwVDe2 and In(2R)PxK. Both have proximal breakpoints in centric heterochromatin and distal breakpoints in distal 2R. Consequently, the Df(2R)bwVDe2LPxKR chromosome has one deletion that extends from the distal In(2R)bwVDe2 breakpoint to the distal In(2R)PxK breakpoint and it has either a duplicated or deleted segment of centric heterochromatin. The 59D6-E1 breakpoint cited for In(2R)bwVDe2 (FBab0005115) appears to be accurate. The 60C7-D1 breakpoint for In(2R)PxK (FBab0010181) is, however, incorrect. It came from an oversimplified interpretation of complementation tests involving Df(2R)bwVDe2LPxKR (see below). As far as I can tell, the only direct cytological analysis of In(2R)PxK is found in Ken Kempheus' dissertation where the breakpoint was placed roughly in 60C-D. The main reason for doubting the presence of a single Df extending from 59E to 60D is the presence of at least one haplolethal gene in 60B documented first in Bruce Reed's dissertation work. The haplolethal region was narrowed by the work reported in FBrf0219066 to the five-gene region between the distal end of Df(2R)BSC136 and the proximal end of Df(2R)BSC356. It likely corresponds to the adjacent genes RpL12 and/or RpL39 as proposed in FBrf0205398. Complementation data between Df(2R)bwVDe2LPxKR and various P element insertions generated mostly by the Berkeley Drosophila Genome Project and recorded in the Df(2R)bwVDe2LPxKR entry (FBab0005117) suggest that a segment encompassing RabX1 to Phm (or roughly 59E2 to 60A16) is deleted, but genes in 60B to distal 60C are not deleted. My cytological observations of polytene squashes show that 60A15 is definitely absent and 60C7,8 is definitely present. It is difficult to score bands juxtaposed to centric heterochromatin, but I suspect 60B1,2 was absent and 60C1,2 present. Other bands in 60B were not discernible. These observations suggest Df(2R)bwVDe2LPxKR deletes roughly 59E1 to 60B1. The observations that led to the distal breakpoints of Df(2R)bwVDe2LPxKR and In(2R)PxK being placed near 60D1 are the non-complementation of Df(2R)bwVDe2LPxKR and P insertions in bs and Rpn8. It seems more likely that the Df(2R)bwVDe2LPxKR and In(2R)PxK chromosomes carry a cytologically invisible deletion in distal 60D encompassing these genes. A rearrangement involving bs would be consistent with longstanding suspicions that bs and Px are related. -- Kevin R. Cook, Ph.D. Bloomington Drosophila Stock Center Department of Biology Indiana University
