Subdivision boundaries are placed differently in polytene chromosome maps of D. melanogaster.
Saura, Anja O. Department of Biosciences, Division of Genetics, P.O. Box 56 (Viikinkaari 5), FIN-00014 University of Helsinki, Finland. E-mail: anja.saura@helsink
The Bridges' drawn maps of the polytene salivary gland chromosomes of D melanogaster (C.B. Bridges, 1935, 1938; P.N. Bridges, 1941a,b, 1942; Bridges and Bridges, 1939; reprinted in Lindsley and Zimm, 1992, and in Sorsa, 1988) have served as a universal reference for cytogenetic analysis since they were published. Likewise, the photographic maps of Lefevre (1976) have become a self-evident tool for identification of chromosome arms and regions together with the Bridges' drawings . As Ashburner (1989) notes, "The most useful procedure is to get a rough idea where a breakpoint or in situ site is from Lefevre's photomaps and then analyze the region carefully with the appropriate revised map." Yet "The revised maps are neither easy to use nor, in some regions, as accurate as one would wish."
One difficulty in using the revised maps is that several division and subdivision boundaries have been shifted in the process of revision. Boundaries were moved primarily to make use of heavier, darker bands making some regions easier to find and, secondarily, to even up the lengths of divisions and subdivisions as much as possible (Bridges, 1942). The shifts were marked by dashed lines below the map drawings on the maps of the X, 3L and 2L chromosomes. Six or seven boundary shifts were also made in 2R (Bridges and Bridges, 1939) and one or two in 3R (Bridges, 1941a), but these slight changes are not marked on the revised maps of 2R and 3R.
In addition to the above standard maps, there are polytene chromosome maps made later both with light microscopic (LM) and electron microscopic (EM) methods. A prerequisite for the use of the other maps is that they agree, i.e., that the about 600 subdivision boundaries of the maps are placed exactly the same way as in the Bridges´ revised maps. They are not. This was observed when the electron micrograph maps based on thin sections of salivary gland chromosomes, published in 1979-1996, were made (references in Saura, 1986; Sorsa, 1988; Ashburner, 1989; Zhimulev, 1996; Saura et al., 1997).
I have checked the boundary locations in 1) the electron micrograph map from whole mounted salivary gland (SG) chromosomes (Ananiev and Barsky, 1985); 2) the photo-map of the polytene fat body (FB) chromosomes (Richards, 1980); 3) the LM and EM division maps of the salivary gland chromosomes (Sorsa, 1988); 4) the light microscopic photomap (in Heino et al., 1994).
Results and Discussion
Table 1 lists the subdivision boundaries of the above maps which differ from the revised maps of the Bridges’. The table also suggests how the boundaries should be corrected. Some additional notes about shifts, discrepancies and the correspondence of subdivisions (after the changes) are given in the following.
The X chromosome. Four subdivision boundaries, 7C/D, 13F/14A, 14A/B, 16A/B, were shifted slightly to the right on the revised map (Bridges, 1938), and subdivisions 20E and 20F (borders 20D/E and 20E/F) were added to the proximal end of the chromosome. The changed borders are, in general, easy to locate. According to Richards (1980), the border 13F/14A in the FB map appears more like the original SG map than the revised version in band intensities. In the SG chromosome maps the
borders in divisions 17 and 18 need to be attended to. The region 17A in the map of Ananiev and Barsky (1985) corresponds to 17A and B of Bridges’ map, their 17B equals Bridges’ 17C, their 17C equals to Bridges’ 17D, and their 17D+E equals Bridges’ 17E. The boundaries 18D/E, 18E/F, 18F/19A in the LM map (in Sorsa, 1988) should be also changed; these corrections have been made in the LM map of Heino et al. (1994).
2L chromosome. The following 12 subdivision boundaries were dashed (and shifted?) in the 1942 map (Bridges, 1942): 27F/28A, 28B/C, 29B/C, 33B/C, 34B/C, 34D/E, 36D/E, 37E/F, 38A/B, 38E/F, 38F/39A, and 39B/C. According to Wright et al. (1976) and Ashburner (1978) P.N. Bridges (1942) indicated some boundaries as being altered, when, in fact, they were not (39B/C) and, furthermore, altered some boundaries without so indicating (38C/D, 39D/E). The boundary line 39F/40A should also be dashed (Saura 1986). The list of differently placed boundaries of Saura (1986) included more borders of 2L than is presented here (Table 1). The now omitted boundaries may, after all, be correctly placed. Taken together, 2L is the hardest of the major chromosome arms to analyse.
2R chromosome. Contrary to what Lefevre (1976) states, 6-7 subdivision boundaries were changed in the revised map (Bridges and Bridges, 1939), but these changes are not indicated by dashed boundary lines. According to Saura (1986), the shifts may include at least some of the following: 42A/B, 43C/D, 43D/E, 44D/E, 47E/F, 50C/D, or perhaps 41C/D and 44C/D. The boundaries 43C/D and 43D/E, which are incorrectly placed in the division maps (Sorsa 1988) have been corrected in Heino et al. (1994).
3L chromosome. Bridges (1941a) made 18 changes in the revised map: 63B/C, 66A/B, 67A/B, 67C/D, 70D/E, 72B/C, 73A/B, 73B/C, 73E/F, 74C/D, 74E/F, 75A/B, 75C/D, 75D/E, 77B/C, 77E/F, 78F/79A, 79A/B, and added subdivisions 80D, 80E, and 80F. The boundaries included in Table 1 should be moved only slightly to the right or left. According to Richards (1980), the assignment of the 70C and 70D boundaries is made difficult by the deeply staining bands in 70C5-10.
3R chromosome. One or two subdivision changes were made in the revised map of Bridges (1941b) even though Lefevre (1976) stated that no boundaries were changed. The changes are not, however, marked in the map. The shifted boundaries might be (or not) 84E/F, 88A/B or perhaps 100D/E and 100E/F. The boundary 88C/D is marked too far to the right in the map of Ananiev and Barsky (1985) but too far to the left in the map of Richards (1980). In addition, other changes should be done in the maps (Table 1). The changes alter the subdivision names. For example 98C in the map of Ananiev and Barsky (1985) equals 98C+D on Bridges’ map, their 98D = 98E and their 98E+F = 98F. The border 98F/99A is marked too far to the right in two maps (Table 1).
Chromosome 4. Subdivision boundaries of 4R are relatively easy to identify (Saura et al., 2002). In the map of Ananiev and Barsky (1985) there are three boundary differences.
Table 1 includes a total of 62 boundary lines of the three chromosome maps, which are probably marked differently from the Bridges’ revised maps. The boundaries 64B/C, 65F/66A, 88C/D and 98F/99A were marked incorrectly in a similar fashion in two maps, other borders only in one map.
The revised map of 2R contains the least amount of discrepancies and is thus the easiest chromosome arm to analyse. Among the different maps, the map of Ananiev and Barsky (1985) had most discrepancies, while the LM map of Heino et al. (1994; not included in Table 1) is (almost) correct. Finally it should be noted that the boundary sites which should be corrected in our 1979-1996 electron micrograph maps (references in Saura et al., 1997) are not included here.
Acknowledgments: This work has been supported by grants from the Jenny and Antti Wihuri Foundation and the University of Helsinki Research Funds.
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