Unequal Crossing-Over Associated with Asymmetrical Synapsis

Proc. NatL Acad. Sci. USA Vol. 80, pp. 5017-5021, August 1983 Genetics

Unequal crossing-over associated with asymmetrical synapsis between nomadic elements in the Drosophila melanogaster genome (gene duplication/transposable elements/chromosome pairing) MICHAEL L. GOLDBERG*, JENQ-YUNN SHEENt, WALTER J. GEHRINGt, AND M. M. GREEN§$ *Section of Genetics and Development, Cornell University, Ithaca, New York 14853; tThe Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138; tBiozentrum der Universitaet Basel, CH-4056 Basel, Switzerland; and §Department of Genetics, University of California, Davis, California 95616 Contributed by M. M. Green, May 2, 1983 ABSTRACT The molecular structure of reciprocal duplica- a deficiency or a duplication for all or part of the white locus tions and deficiencies produced by unequal crossing-over at the (5-10). However, in the absence of independent genetic or cy- white (w) locus of Drosophila melanogaster females heterozygous tological evidence for duplicated sequence homologies of the for the alleles wa and wtA has been examined. A transposable, co- white locus that would result in such unequal exchanges, this pia-like element is found at the rearrangement breakpoints. Fur- phenomenon and its implications for the mechanism of chro- ther characterization indicates that asymmetrical pairing between mosome pairing have remained obscure. two copies of this element, which are at least 60 kilobases apart The Drosophila genome is now known to contain many re- in the parental chromosomes, followed by a crossover within the iterated transposable sequences, which may appear at different paired elements, is responsible for the duplication and deficien- chromosomal locations in different strains (see ref. 13 for re- cies observed. The frequency of these events is high compared with normal homologous exchange, implying that synaptic pairing view). We reasoned that such nomadic elements might provide during meiosis must be sufficiently flexible as to allow efficient regions of homology near the white locus that would allow the recognition of sequences located in nonidentical positions on ho- occurrence of these novel asymmetrical recombinational events mologous chromosomes. These results suggest a possible mecha- (14). Such a model would explain the requirement for particular nism for the generation of tandem duplications in eukaryotic or- heterozygous configurations of white alleles, as chromosomes ganisms. carrying different white mutations also might exhibit different distributions of such transposable elements. When homologous Genetic exchange in eukaryotic organisms almost invariably oc- nomadic elements are located at distinct sites within the same curs in association with the synaptic pairing of homologous region of the two parental chromosomes, unequal crossing-over chromosomes. Although cytological observations of chromo- could ensue. The model is further supported by recent findings some behavior during the pachytene stage of meiosis show that that many mutations at the white locus are associated with the homologous chromosomes are intimately paired throughout their insertion of transposable elements (15-17). length, the genetic and molecular properties of this process are In this report, we examine at the molecular level the struc- not understood. In particular, in the face of this seeming ri- ture of the reciprocal duplication and deficiency products of gidity of pairing, many examples of unequal crossing-over are unequal exchange in females heterozygous for the alleles white- known (1-12). The unexpected frequency and variety of these apricot and white-apricot 4 (Wa/Wa4). Our results are com- events have further raised the possibility that unequal ex- pletely in accordance with the hypothesis that nomadic ele- changes are controlled by novel, illegitimate mechanisms of re- ments have supplied the DNA sequence homology required for combination (12). unequal recombination. These results will be further discussed The first suggestion of flexibility in meiotic pairing is found in terms of possible mechanisms for the synaptic pairing of ho- in the observations of Sturtevant (1) on Drosophila homozy- mologous chromosomes during meiosis. gous for Bar (B), a genetic locus correlated with a tandem duplication of part of the X chromosome. Asymmetrical synapsis MATERIALS AND METHODS between these duplicated sequences may occasionally yield nonparental chromosomes, with either one or three copies of Strains and Materials. For an explanation of genetic sym- this region. The occurrence of unequal exchange at several other bols used see Lindsley and Grell (18). loci, including Beadex (Bx) (2), Star (S) (3), and bobbed (bb) (4) W-55) a deletion of the white locus abbreviated in the text also has been well documented. Each case is similarly associ- as wrl, was obtained as a white-eyed spl son of a y2 SU(Wa) Wa ated with a tandem duplication of a portion of the Drosophila spl/a4; Cy/+;Ubx/+ female. Genetic experiments estab- genome. These events suggest that infrequent mispairing of lished this w deletion to be associated with a small, cytologically chromosomes may lead to unequal exchange given sequence invisible loss of a part of the white locus (5). w-82W1G') spl (w-r2) homology at the novel site of association. was independently obtained as an exceptional male offspring of In the vicinity of the white (w) locus of Drosophila melano- a female of the same genotype some 25 years later. The second gaster, unequal crossing-over occurs regularly in females het- and third chromosome balancers Cy and Ubx, respectively, were erozygous for certain combinations of w alleles. These events introduced to increase the frequency of crossing-over on the X are characterized by the genetic exchange of markers flanking chromosome (19). the white locus, and yield reciprocal products, containing either Dp (w+R), abbreviated w+R, was obtained originally as a zeste- eyed female in a cross of y2 wa4 spl/y w(+);Cy/+;Ubx/+ fe- The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertise- Abbreviation: kb, kilobase(s). ment" in accordance with 18 U.S.C. §1734 solely to indicate this fact. I To whom reprint requests should be addressed.

5017 Downloaded by guest on October 2, 2021 5018 Genetics: Goldberg et aL Proc. NatL Acad. Sci. USA 80 (1983) males to Dp(l)z*males (10). [w()is a wild-type recombinant de- of event. (ii) Independently derived deficiencies or duplica- rived from 0a/twch females, the wch allele mapping centromere tions, such as w-r and war, should have the same structure, proximal of wa. Therefore, the sequences proximal to the white reflecting the similarity in their origins. (iii) A transposable ele- locus on the w +)-containing chromosome must have originated ment should be found within the white locus region on the wa4- from the wa-conitaining chromosome (10, 20).] Sons of the zeste- containing chromosome, and a homologous element should be eyed females wiere Dp(l)z and y w+R spl When the z mutant present at a different location on the a- or wH+)-containing was crossed on to the w+R-containing chromosomes, males ex- chromosomes (Fig. 1). Genetic experiments indicating that w+R pressed the zeste eye color, suggesting that W+R is a small tan- is duplicated only for the centromere proximal part of the white dem duplicatiorn of the proximal region of the white locus re- locus (10) suggest that the putative nomadic sequence in w or ciprocal to the deletion associated with wrl. w+ should reside proximal to the locus. These elements should The bacteria1 strain K802 was used for the propagation of exhibit the same proximal-to-distal orientation, as recombina- bacteriophage clones (21). The bacteriophage vector A1059 was tion between two elements inverted with respect to each other used in the consstruction of recombinant libraries from the w-rl would result in the reciprocal inversion of the intervening DNA and we" strainss (22). Other recombinant clones were isolated rather than in the formation of deficiencies and duplications. from a library cconstructed from sheared embryonic Canton S (iv) One copy of the same nomadic element should occur in the (wild-type) DN,A by using the bacteriophage A vector Charon deficiency and in the duplication, again with the same proxi- 4 supplied by J Lauer and T. Maniatis (23). mal-to-distal polarity, but flanked by different sequences from Methods. Drrosophila genomic DNA was purified from em- the white locus region (see Fig. 1). bryos of the strrains Oregon R (wild type), Wa, Wa4, w , and We had previously cloned DNA sequences from the region w+R by establis]hed procedures (24). Drosophila genomic DNA of the D. melanogaster X chromosome containing the white lo- was purified fr(am adults of strains w-r2 and w(+) by the tech- cus (15), providing the initial material necessary for a molecular nique of Bingham et al. (25). Genomic DNA was partially di- examination of the products of unequal exchange. To position gested with restriction endonuclease Sau3A and inserted into the white locus relative to the cloned sequences, we had at that BamHI-cut A1C)59 vector DNA as described (15). time localized the distal breakpoint of the w`rl deletion. Frag- Techniques ffor the selection of recombinant phage and for ment A, purified from a bacteriophage clone containing white the analysis of nitrocellulose transfers of restriction endonu- locus DNA sequences in the Canton S (wild type) genome (Fig. clease digests c)f bacteriophage or genomic DNAs by hybrid- 2a), was on this.basis chosen as a probe to select clones spanning ization with 32p'-labeled probes also have been detailed (15). the deletion breakpoint from a recombinant library constructed with w-ur genomic DNA. Three overlapping clones containing RESULTS novel sequences not found adjacent to fragment A in the Can- ton S genome were purified, and their restriction maps are pre- Our model for tthe structural basis of asymmetrical pairing and sented in Fig. 2. DNA sequences repeated several times in the unequal crossirig-over is schematically presented in Fig. 1. Drosophila genome are adjacent to fragment A in these clones, Several specific predictions of the hypothesis are easily testable as shown by the strong hybridization of labeled Drosophila to- by using the esstablished techniques of molecular biology. (i) tal genomic DNA to the restriction endonuclease-generated Sequences that are deleted in w-rl should exactly correspond fragments indicated in Fig. 2b (data not shown). with the sequeinces duplicated in W+R; that is, the deficiency The model predicts that fragment B, which lies adjacent to and duplication should be the reciprocal products of one type fragment A in Canton S DNA but which is deleted from the w-rl genome (15), will be present in two copies in the tw+R-co- Wa4 taining chromosome. The more distal of the copies should be X Affiffiffiffiffi~ffi-f-B C D Z adjacent to fragment A, while the proximal copy should border the same repetitive sequence found in the clones spanning the w r' breakpoint (see Fig. 1). Fig. 2 demonstrates that both pre- wa or w( + A B D + dicted configurations of clones selected with fragment B are ------obtained. In particular, clones containing what we presume to copia I be the more proximal copy of fragment B (Fig. 2c, map ii) pos- ri COP8 Isess sequences that. hybridize with the repetitive DNA found deficiency X A D + at the breakpoint of wrl. Moreover, the polarity of this element relative to fragment B is exactly that required by the model. If the repetitive sequence associated with these rearrange- w +R ments, which we term BEL (for the Babylonian king Belshaz- duplication + A B ------C C D Z zar), is transposable, it should be possible to find strains in which fragments A and B are not separated by the insertion of the FIG. 1. Schennatic representation ofa model for the molecular events element. As has been shown, BEL is not inserted between frag- underlying uneqi ual crossing-over in the vicinity of the white locus (w). ments A and B in Canton S or W+R DNAs. It is similarly pos- Left to right in tlhis figure corresponds to a direction from the X-chro- sible to verify that fragment D, a nonrepetitive sequence flank- mosome telomeree to the centromere (distal to proximal); distances are ing BEL in warl is directly juxtaposed in the Canton S genome not drawn to scale. A, B, C, and D indicate DNA sequences around the to DNA sequences adjacent to BEL W.".in Fragment D, de- white locus; X, Z and + indicate mutant and wild-type alleles of ge- ' * frl w netic markers fla the white locus.----,Extent of either by mking sequences anved from thel cloneA-rs3, was labeled neck-translation deleted or duplie ated as-a result of recombination between two copies and used to select clones from a Canton S recombinant library. of a Drosophila transposable element (EM). The deficiency and dupli- Fragment C of the selected phage (Fig. 2d) is indeed homol- cation used in thisstudywere actually generated infemales of-different ogous to DNA flanking BEL in the w+Rcontaining genome, as genotypes: in ad(dition to different distributions of markers flanking shown in Fig. 2c. the white locus, t,he parental w. and w .containing chromosomes are Verification that the clones we have obtained are repre- distinguished by the insertion of a copia element close to the-junctions _ of regions A and B in w0 (ref. 12 and this article). Replacement of the sentative ofo sequences in the u r and w genomes is pre- wa-containing chiromosome with the w(+)-containing chromosome ap- sented in Fig. 3. Restriction fragments homologous with frag- pears not to infltuence the occurrence of unequal exchange (10). ments B and C are absent from the w-rl genome, whereas two Downloaded by guest on October 2, 2021 Genetics: Goldberg et al. Proc. Natl. Acad. Sci. USA 80 (1983) 5019

Skb a I I

e cc _- 34 M434 cc ao F4 0 -4 -ia8 0 'Ua xx x u 0 (0)CD xx 0 I ul 11 l I I I 11 I 11II I I~~~l I ~~~~~~~~~~~~~~PIlA

copia

34" 4 c c " 0 b 0 00 0 "4 ~ !2: E oooa E xx x w CD cocain n x us x x CD a

(A) BEL D

I A-rl j X-r12

0- F L4 o 0 w

c 0 0 0 0 C U= 0 0'0 xx w (a i III-II I I _o.9 I II 11 i (A) (B) I1+Rl J X+R2

E a a a 0 o E a;_; owu x ul x x ID xx ul 00 iI I I I I II BEL (B) (C) E A+R3

X Z 0 0

c A

d 0 a I 8 'U x us°WO0 x a us l I iI b _ I

I .)Cs1

FIG. 2. Molecular characterization of DNA sequences at deficiency (b) and duplication (c) rearrangement breakpoints. Restriction fragments in this figure are lettered to correspond with the chromosomal regions in Fig. 1. All maps in this figure are drawn to the same scale. (a) Wild type. Restriction map of the Canton S white locus region, adapted from ref. 12. For reference, the position of the copia insertion in Wa is indicated. Frag- ments A and B () were purified from restriction endonuclease digests ofrecombinant phage as described (12). (b) Deficiency. Composite restriction map ofrecombinant wrl clones homologous to fragment A (the extent of homology is indicated by the shaded bar marked "A"). The BEL transposable element (I) is defined by those restriction fragments that hybridize strongly to labeled Canton S whole-genome DNA (data not shown). (c) Duplication. Restriction maps of two regions from the w#R genome homologous to fragment B. The area of homology to fragment B appears to be extended because an EcoRI site defining the fragment is absent from the wR genome. The same polymorphism occurs in certain wild-type strains of D. melanogaster such as Oregon R (unpublished results). E, Extent in ii of homology with clone A-r13. (d) A region of the Canton S wild-type genome homologous to fragment D isolated from w-r1 recombinants. This region does not overlap clones we have purified containing about 50 kilobases (kb) ofDNA contiguous to the white locus in the proximal direction (unpublished results), indicating that the BEL elements under study must lie a minimum of 60 kb apart.

different w+R restriction fragments hybridize to each of the same Wurl DNAs appear to be identical in length. Consistent pat- probes. The lengths of the restriction fragments homologous to terns are obtained by examination of the other hybridizations fragments A or D in the deficiency DNA correspond with pre- shown in Fig. 3; additional experiments with other restriction diction. For example, hybridization with fragment A illumi- endonuclease digests of genomic DNAs give similar results (data nates bands of the same size in the w04 and w-rl genomes, not shown). The lengths of restriction fragments within the seg- whereas fragment D-homologous sequences in the w0, w , and ments we have cloned (Fig. 2) are the same as those of the ge- Downloaded by guest on October 2, 2021 5020 Genetics: Goldberg et aL Proc. Natl. Acad. Sci. USA 80 (1983)

O f1 W r2 4 OR CS wo OR wfri-r w W+R w *} wa A -- 15. aim ..w.

-...... ; FyIn I* e of... $Is .4 ... B .,. FIG. 4. BEL sequences intheDrosoph- ila genome. Fragment F was prepared from ...1. . i. . - . A-r13 DNA (Fig. 2), labeled by nick-trans- .C lation, and hybridized to nitrocellulose 40 transfers of Xho I-digested whole genomic C~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DNAsfom strains Oregon Rwildtype (OR), Canton S wild type (CS), and white-apricot -4 (Wa). Arrows indicate the length of Xho I fragments internal to the BEL element 40. Is' (Fig. 2).

erties of the well-characterized copia-like dispersed repetitive elements in Drosophila (13).

v~~~~%OA DISCUSSION The data presented here firmly support the hypothesis that ur- equal crossing-over at the white locus is mediated by DNA se- D quence homology provided by nomadic elements in the Dro- sophila genome. We believe that the occurrence of such events is not restricted to the vicinity of the white locus and would be observed at other locations in the genome if phenotypic dis- tinctions between the parental chromosomes and the resultant deficiencies and duplications were available. Moreover, the BEL family is unlikely to be the only group of repetitive elements that may be involved in asymmetrical pairing and recombina- FIG. 3. Hybridization toEcoRI-digested genomic DNAs. Whole ge- tion. Several other heterozygous combinations of white alleles nome DNA from strains Oregon Rwild type (OR), W-rl, -r2 Wa4)W also regularly exhibit this phenomenon (6-9), and it may be in- WG, EcoRI, frac- and w were digested with restriction endonuclease ferred from recent molecular data that other sequences such as tionated on 0.5% agarose gels, and transferred to nitrocellulose filters are associated with these events (15, 16). Our character- as described. Identical filters were individually hybridized with the 32p copia labeled probes detailed in Fig. 2. (A) Fragment A. (B) Fragment B. (C) ization of BEL, though incomplete, suggests that its organi- Fragment C. (D) Fragment D. Lengths of the genomic fragments are zation closely resembles the structural paradigm of copia-like shown in kb. In A and B, fragments of novel size in the wa genome are elements in Drosophila; no unique features are evident. More- revealed, corresponding to EcoRI sites internal to the element copia, over, unequal crossing-over between yeast Ty transposable ele- which were not diagrammed in Fig. 2 for the sake of clarity. ments has recently been observed (26), further suggesting that asymmetrical exchange potentially can be mediated by many nomic fragments revealed in these blotting experiments. More- different repetitive sequence families. over, the patterns exhibited by w-rl and w-r2 DNAs in these BEL does not appear to be a member of any copia-like family blotting experiments appear equivalent, indicating that un- characterized to date. The BEL element does not hybridize to equal recombination through BEL elements may regularly oc- cloned Drosophila segments containing the copia, 297, 412, cur. NEB, or FB-NOF elements (refs. 13 and 15; unpublished re- As established by whole-genome Southern analysis (Fig. 4) sults), nor does the restriction map of BEL appear to corre- and in situ hybridization to polytene chromosomes (results not spond to those of the elements mdgl, mdg3, gypsy, or roo (27- shown), sequences homologous to BEL are present at :25 lo- 29; W. Bender, personal communication). In terms of the re- cations dispersed over the Drosophila genome. The chromo- striction map and length of the repetitive element at wa4, our somal locations of BEL vary in different Drosophila strains. data are slightly at variance with a previous study that associ- Nonetheless, at the majority of these sites, the sequence or- ated the w04 mutation with the insertion of novel sequences ,ganization of the element must be conserved to at least some (16). The discrepancies are likely to be insignificant, repre- degree. This point is substantiated by the strong hybridization senting small differences in the analysis of the region by the signals evinced by genomic restriction fragments apparently in- direct cloning used in these experiments relative to the ge- ternal to the element when probed with BEL-specific se- nomic blotting techniques previously used. quences (Fig. 4). The two ends of the BEL elements are them- The frequency of unequal exchange in Wa/lu) heterozygous selves homologous, as a restriction fragment containing the right females is approximately 0.02% (refs. 5 and 10; unpublished end of BEL from clone A+R3 (fragment E in Fig. 2c) hybrid- results). Given that the homology between two BEL elements izes to radioactive A-rll DNA, which only contains the left end can be at most 7.2 kb of DNA, this figure seems remarkably of BEL (data not shown). On the basis of these criteria, it would high. Although the frequency of recombination may be influ- on the av- appear that the BEL family of elements exhibits several prop- enced by several factors, it has been estimated that, Downloaded by guest on October 2, 2021 Genetics: Goldberg et al. Proc. Natl. Acad. Sci. USA 80 (1983) 5021 erage, a DNA sequence about 3.7-3.8 kb long in homologous between elements separated by a pairing site. chromosomal locations would undergo symmetrical exchange at Asymmetrical exchange between transposable elements would a rate of 0.01% (30). Perhaps a more accurate comparison can provide an efficient mechanism for the generation of tandem be made to the frequency of intracistronic recombination within duplications required as an initial step in the evolution of clus- the white locus itself, as the rate of recombination is not uni- tered multigene families (34). Further, it is not unlikely that the form over the length of the X chromosome (19). Transposable appearance of several spontaneously arising tandem duplica- elements associated with the wa and wOP (white-spotted-i) mu- tions associated with mutant phenotypes in Drosophila, such as tations are inserted at locations approximately 5 kb apart (16), Bar (35), Hairy wing (36), Confluens (37), and Beadex-recessive while these alleles are genetically separated by 0.014 centi- (38) was similarly associated with recombination between no- morgans (31). The determination of the genetic distance sep- madic sequences. arating wa and wSP" was performed in females heterozygous for inverted second and third chromosome balancers sim- We thank Dr. Tom Maniatis of Harvard University for extending the multiply hospitality of his laboratory to M. L.G. and J.-Y.S., Dr. Renato Paro for ilar to those present in the wa/wa4 females used in our exper- helpful discussions, and Drs. Ross MacIntyre and David Goldberg for iments. Thus, this comparison of the frequency of symmetrical critical evaluation of the manuscript. This work was supported by Pro- and asymmetrical recombination in the vicinity of the white gram Project GrantS P01 GM29301 to T. Maniatis and M. L. G., by U. S. locus is at least partially standardized for the interchromosomal Department of Agriculture Hatch Project Grant 186416 to M.L.G., and effects of autosomal balancer chromosomes, known to increase by National Institutes of Health Grant GM2221 to M. M.G. the rate of recombination between X chromosomes (19). These comparisons, though inexact, nonetheless indicate that 1. Sturtevant, A. H. (1924) Genetics 10, 117-147. 2. Green, M. M. (1953) Z. Ind. Abt. Vererbungslehre 85, 435-449. the rate of asymmetrical crossing-over between two BEL ele- 3. Lewis, E. B. (1945) Genetics 30, 137-166. ments is similar to that of normal homologous exchange be- 4. Schalet, A. (1969) Genetics 63, 133-153. tween any chromosomal segments of the same length. The same 5. Green, M. M. (1959) Z. Vererbungslehre 90, 375-384. relationship holds for unequal recombination observed be- 6. Green, M. M. (1959) Genetics 44, 1243-1256. tween yeast Ty elements (26). It is possible that these DNA se- 7. Judd, B. H. (1959) Genetics 44, 34-42. quences are in some manner prone to recombination so that 8. Green, M. M. (1961) Genetics 46, 1555-1560. 9. Judd, B. H. (1961) Proc. Nati Acad. Sci. USA 47, 545-550. infrequent asymmetrical recognition and pairing of these re- 10. Green, M. M. (1963) Z. Vererbungslehre 94, 200-214. gions would result in unusually high rates of exchange. If, as we 11. Finnerty, V. (1968) Dissertation (Univ. of Connecticut, Storrs). believe, this is not the case, it must be concluded that the pair- 12. Gelbart, W. M. & Chovnick, A. (1979) Genetics 92, 849-859. ing of homologous chromosomes must be sufficiently flexible 13. Spradling, A. C. & Rubin, G. M. (1981) Annu. Rev. Genet. 15, 219- so that sequences in different chromosomal locations can re- 264. combine as efficiently as when they occupy homologous posi- 14. Gehring, W. J. & Paro, R. (1980) Cell 19, 897-904. 15. Goldberg, M. L., Paro, R. & Gehring, W. J. (1982) EMBO J. 1, tions. This presumed flexibility of pairing must extend over a 93-98. minimum of 60 kb separating the BEL elements studied here 16. Zachar, Z. & Bingham, P. M. (1982) Cell 30, 529-541. (Fig. 2). 17. Levis, R., Collins, M. & Rubin, G. (1982) Cell 30, 551-565. The ability of reiterated sequences in nonidentical chro- 18. Lindsley, D. L. & Grell, G. H. (1968) Genetic Variations of Dro- mosomal regions to recombine must nonetheless be to some sophila melanogaster (Carnegie Inst. of Washington, Washing- extent constrained. The genome of D. melanogaster contains ton, DC). 19. Schultz, J. & Redfield, H. (1951) Cold Spring Harbor Symp. Quant. -80 different families of repetitive elements; on the average, Biol 16, 175-197. each is present about five times on each of the five major eu- 20. Bingham, P. M. & Judd, B. H. (1981) Cell 25, 705-711. chromatic chromosomal arms (32). Were each element able to 21. Blattner, F. R., Williams, B. G., Blechl, A. E., Thompson, K. D., synapse with the other members of the same family on the same Faber, H. E., Furlong, L. A., Grunwald, D. J., Kiefer, D. O., chromosomal arm at the frequencies observed here, it could be Moore, D. D., Schumm, J. W., Sheldon, E. L. & Smithies, 0. estimated that =50% of all gametes would contain resultant du- (1977) Science 196, 161-169. 22. Karn, J., Brenner, S., Barnett, L. & Cesareni, G. (1980) Proc. Nati plications or deficiencies, many of which are likely to be del- Acad. Sci. USA 77, 5172-5176. eterious. The occurrence of unequal crossing-over in the ge- 23. Maniatis, T., Hardison, R. C., Lacy, E., Lauer, J., O'Connell, C., nome is unquestionably rarer. For example, based upon the Quon, D., Sim, G. K. & Efstratiadis, A. (1978) Cell 15, 687-701. frequency of spontaneous appearance of tandem duplications 24. Wensink, P. C., Finnegan, D. J., Donelson, J. E. & Hogness, D. in the vicinity of the rosy (ry) locus of Drosophila melanogaster, S. (1974) Cell 3, 315-325. it was estimated that the genomic frequency of maternal un- 25. Bingham, P. M., Levis, R. & Rubin, G. M. (1981) Cell 25, 693- 704. equal exchange between nonsister homologues is about 0.1% 26. Roeder, G. S. (1983) Mol Gen. Genet. 190, 117-121. (12). The tandem duplications found near ry were considerably 27. Ilyin, Y. V., Chmeliauskaite, V. G., Anaiev, E. V., Lyubomir- larger than w+R. Thus, it would appear that the efficiency of skaya, N. V., Kulguskin, V. V., Bayev, A. A., Jr., & Georgiev, G. asymmetrical pairing is sensitive to increasing distances be- P. (1980) Nucleic Acids Res. 8, 5333-5346. tween homologous sequences. These considerations further 28. Bayev, A. A., Jr., Krayev, A. S., Lyubomirskaya, N. V., Ilyin, Y. suggest that the genetic burden of unequal exchange operates V., Skryabin, K. G. & Georgiev, G. P. (1980) Nucleic Acids Res. as a selective 8, 3263-3273. pressure, simultaneously limiting the number of 29. Meyerowitz, M. & Hogness, D. S. (1982) Cell 28, 165-176. elements of a reiterated family and distributing these elements 30. Lefevre, G., Jr. (1971) Genetics 67, 497-513. as far apart as possible in the genome, to minimize synapsis. An 31. LeFever, H. M. (1973) Drosoph. Inf. Serv. 50, 109-110. additional constraint on the occurrence of unequal crossovers 32. Young, M. W. (1979) Proc. Natl Acad. Sci. USA 76, 6274-6278. is suggested by the recent work of Hawley (33), which pos- 33. Hawley, R. S. (1980) Genetics 94, 625-646. tulates the existence of selected sites on the X chromosome 34. Long, E. 0. & Dawid, I. B. (1980) Annu. Rev. Biochem. 49, 727- for the initiation of A 764. necessary pairing. testable implication of 35. Tice, S. C. (1914) Biol BulL 26, 221-230. this hypothesis is that unequal crossing-over between nomadic 36. Demerec, M. & Hoover, M. E. (1939) Genetics 24, 291-299. sequences should occur relatively freely within chromosomal 37. Schultz, J. (1941) Drosoph. Inf. Serv. 14, 54-55. regions defined by two pairing sites but should occur only rarely 38. Green, M. M. (1952) Proc. NatL Acad. Sci. USA 38, 949-953. Downloaded by guest on October 2, 2021