Developmental choices in mating-type interconversion in fission yeast

[]~EVIEWS 21 Stem, C. (1954) Am. Sci. 42, 213-247 22 Simpson, P. (1990) Development 109, 509-519 23 Campos-Ortega, J.A. and Jan, Y.N. (1991) Annu. Rev. Neurosci. 14, 399-420 24 Heitzler, P. and Simpson, P. (1991) Cell 64, 1083-1092 25 de Celis, J.F., Mari-Beffa, M. and Garcia-Bellido, A. (1991) Proc. Natl Acad. Sci. USA 88, 632-636 26 de Cells, J.F., Mari-Beffa, M. and Garcia-Bellido, A. (1991) Wilhelm Roux's Arch. Dev. Biol. 200, 64-76 27 Moscoso del Prado, J. and Garcia-Bellido, A. (1984) Wilhelm Roux's Arch. Dev. Biol. 193, 246-251 28 Huang, F., Dambly-Chaudiere, C. and Ghysen, A. (1991) Development 111, 1087-1095 29 Dambly-Chaudiere, C., Ghysen, A., Jan, L.Y. and Jan, Y.N. (1988) Wilhelm Roux's Arch. Dev. Biol. 197,

419-423 30 Garrell, J. and Modolell, J. (1990) Cell 61, 39-48 31 Ellis, H.M., Spann, D.R. and Posakony, J.W. (1990) Cell

61, 27-38 32 Garcia-Alonso, L. and Garcia-Bellido, A. (1988) Wilhelm Roux's Arch. Dev. Biol. 197, 328-338 33 Jim~nez, F. and Campos-Ortega, J.A. (1987) J. Neurogenet. 4, 179-200 34 Martin-Bermudo, D., Martinez, C., Rodriguez, A. and Jim~nez, F. (1991) Development 113, 445-454 35 Jim~nez, F. and Campos-Ortega, J.A. (1990) Neuron 5,

81-89

The molecular mechanisms of the developmental choices in cell differentiation are best understood in two unicellular yeasts. Baker's or brewer's yeast, Saccharomyces cerevisiae, grows by budding, while the rod-shaped yeast Schizosaccharomyces p o m b e divides by fission (Schizo for fission). The name p o m b e is a Swahili word for brewing and the yeast is found in cultures used for alcoholic fermentation in East Africa. Investigations of both yeasts have led the way toward understanding the regulation of cell type, as both undergo an interesting cell-autonomous process of mating-type interconversion. Since these yeasts are only distantly related, it is useful to study and compare molecular details of cell type determination in both organisms. A more comprehensive comparison of the two systems can be found in recent reviews 1-3. This review is restricted to recent studies of how the execution of developmental choices at the single-cell level generates a highly regulated pattern of matingtype changes in cell pedigrees.

Mternative alleles of the mating-type locus S. p o m b e is primarily a haploid organism and produces two sexual cell types, plus (P) and minus (M). These cell types are indistinguishable in cultures growing in rich medium. Under poor growth conditions, especially deprivation of a nitrogen source, cells of opposite mating type recognize each other and fuse to form a diploid zygote cell. The zygote cell stops growing and undergoes meiosis and sporulation to produce an ascus containing four haploid meiotic spores. Two of the spores are of the P type and the other two are of the M type. P and M cell types are conferred by the alternative alleles of the mating-type locus, mat1 (Ref. 4).

36 Romani, S., Campuzano, S. and Modolell, J. (1987) EMBO

J. 6, 2085-2092 37 Cabrera, C.V., Martinez-Arias, A. and Bate, M. (1987) Cell

50, 425~i33 38 Cabrera, C.V. (1990) Development 110, 733-742 39 Skeath, J.B. and Carroll, S.B. Development (in press) 40 Dambly-Chaudiere, C. and Ghysen, A. (1987) Genes Dev.

1, 297-306 41 Brand, M. and Campos-Ortega, J.A. (1989) Wilhelm Roux's Arch. Dev. Biol. 197, 457-470 42 Tortes, M. and S~nchez, L. (1989) F~BOJ. 8, 3079-3086 43 Erickson, J.W. and Cline, T.W. (1991) Science 251,

1071-1074 44 Parkhurst, S.M., Bopp, D. and Ish-Horowicz, D. (1990) Cell 63, 1179-1191 45 Mlodzik, M., Baker, N.E. and Rubin, G.M. (1990) Genes Dev. 4, 1848-1861 46 Boulianne, G.L. et al. (1991) EMBOJ. 10, 2975-2983 47 Blochlinger, K., Bodmer, R., Jan, L.Y. and Jan, Y.N. (1990) Genes Dev. 4, 1322-1331 48 Lo, L. et al. (1991) Genes Dev. 5, 1524-1537 49 Campuzano, S. et al. (1985) Cell 40, 327-338 S. CAMPUZANOAND J. MODOLELLARE IN THE CENTRODE BIOLOG£AMOLECULAI~CSIC AND UNIVERSIDADAUIONO~L4 M.4DR~ 28049 MADRX~SPAIN.

Developmental choices in mating-type interc0nversi0n in fission yeast AMARX.S. KI.AR Fission yeast cellsfollow a specific pattern of mating (cell) type switching in single cell pedigrees. Asymmetric cell divisions producing sisters of different developmental fates result from inheritance of specific paretffal DNA strands accordiag to the classical model of semiconservative replication and segregatiom

Instability at the matl locus Naturally occurring strains are called homothallic because a single cell can produce a colony containing a mixture of both cell types, as a result of interconversion between m a t l - P and m a t l - M alleles. An exciting series of genetic experiments, conducted primarily in the laboratories of Leupold, Egel and Gutz, led to the proposal of a transposition model for switching 5,6. It was proposed that switching occurs by transposition of a copy of a P or M allele from the nearby m a t 2 / m a t 3 locus into the mat1 locus. Molecular cloning and sequence determination established and extended this model 7q° (Fig. 1). The m a t 2 locus contains an unexpressed P allele and is located 15 kb distal to the mat1 locus. The mat3 locus, containing an unexpressed M allele, is 15 kb distal to

T1GJUNE1992 VOL 8 NO. 6

[]~EVIEWS mat2. Switching involves a unidirectional gene conversion/recombination event in which the expressed allele at mati is replaced by a copy derived from the mat2 or mat3 donor locus (Fig. 1). As a result, only the mat1 locus switches, while the donor loci remain unchanged. Each alternative mat1 locus actually consists of two genes that are divergently transcribed from the promoters) located in the middle of the cassette.

orM SAS1,2 mat 2-P H2 H1 .j) H2 H1 -~15kb ~ .....~. mat 1-P

mat 3-M

H2 H1

Initiation of switching by a double-strand cut in mat l /qG Inl Southern blot analysis of DNA isolated Switching occurs by the transposition and substitution of a replica of 1.1 kb from homothallic yeast showed that there is a site-specific double-strand break (DSB) at of mat2 or mat3 allele-specific genetic information into mat1. H1 (59 bp) matl. The break occurs at the junction of the and H2 (135 bp) are homology boxes presumably required to pair cassettes allele-specific and H1 regions of both matl-P during recombination. SAS1 and SAS2(switch-activating sequences; open boxes) are cis-acting sequences essential for generating the double-strand and matl-M alleles. The same sites in the break (DSB) at matl. The vertical wavy arrow indicates the site of the I)SB. donor loci are not cleaved, howeverT,~, u. In Horizontal arrows denote the direction of transposition. Figure is not drawn exponentially growing cultures, nearly a quar- to s~ale. ter of the cells possess the DSB and this level remains constant throughout the cell cycle 7. There are three classes of switching-defective efficiently, as 72-94% of the four-cell pedigrees con(swi-) mutations that define the trans-acting genes retained one zygote. This is termed 'Miyata's rule' or quired for recombination. Mutations in one class, repthe 'one-in-four switching rule'. Apparently, two conresented by swil, swi3 and swi7, reduce the number secutive developmentally asymmetric cell divisions are of cells carrying the DSB and such mutants are conrequired to generate the observed programme of sidered defective in the initiation step of recombiswitching. The first division of an unswitchablc cell nation. In the second class of mutants, represented by (Pu, Fig. 2) produces one unswitchable (Pu) daughter, swi2, swi5 and swi6, switching is reduced but the while the other is switchable (Ps) in 80-90% of cell number of cells with the DSB is not affected. This class divisions. The Ps (switchable) daughter in turn proof genes is thought to be required for the utilization of duces one daughter that is switched to the M type. the DSB for recombination. The third class of muThe question arises whether the sister of the tations, in the swi4, swi8, swi9, swilO and rad22 genes, recently switched cell is switchable or unswitchable. affects proper resolution of the gene conversion event, The procedure used by Miyata and Miyata ~7 could not as these mutations generate rearrangements at matl determine this, because that cell is lost to zygote for(Refs 8, 12-14). Several of these mutations cause cells mation when it mates to the switched sister. Studies to become sensitive to ultraviolet radiation and 7-rays, with diploid cells, containing one wild-type marl indicating that the genes are required for general locus whose switching is investigated, and on tile recombination and repair as well as mating-type other homologue the nonswitchable mutant matl switching 13. allele, extended the pattern of switching past the fourIn addition to the trans-acting functions, at least cell stage. In these studies, the ability of individual two cis-acting sites (SAS1 and SAS2), which span diploid cells to undergo meiosis and sporulation (in150 bp and are located distal to matl, just outside the stead of the mating test used by Miyata and Miyata ~7) H1 region (Fig. 1), are required for generation of the DSBS,tS,16. The absence of these sequences at mat2 and mat3 prevents cleavage and switching at these loci.

\

Programmed patterns of switching in cell pedigrees At present, the most interesting aspect of general importance in the mating-type interconversion system is the precise pattern of switching within a cell lineage. Miyata and Miyata 17 found that switching in a cell lineage occurs in a highly regular fashion. On solid nitrogen-deficient medium where growth is limited and mating is induced, they followed growth and matings between progeny of individual cells under the microscope. Among four granddaughters of a given cell only two mated with each other to produce one zygote, and two zygotes were never found. In other words, only one in four granddaughters (of a Pu cell, Fig. 2) must have switched. The switching occurred

Mu

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TIG JUNE 1992 VOL. 8 NO. 6

20~

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[~EVIEWS was determined by growing them on solid sporulation medium and microscopically observing ascus formation in cell pedigrees. Diploid cells homozygous for m a t l - P or m a t l - M fail to sporulate because sporulation requires heterozygosity at mat1. But once a switch has occurred in the switchable allele, generating the m a t l - P / m a t l - M heterozygous constitution, then that cell forms an ascus. By this procedure, the switching potential of the sister of the recently switched cell can be assayed in cell pedigrees. It was found that the sister of the recently switched cell is switchable (Ps, Fig. 2) in 80-90% of cell divisions18,19. This is called the 'recurrent switching rule'. As switchable (Ps) cells switch to the M type in 80-90% of cell divisions, the choice of donor must be nonrandom. This is called the 'directionality rule'. In summary, a Pu cell produces one daughter like itself (Pu), while the other (Ps) is advanced in its developmental programme as it has acquired the potential to switch (Fig. 2). The Ps cell itself produces one daughter like itself and one daughter that is switched to M. Both these patterns are analogous to the mammalian stem cell lineage, where one daughter is like the parent cell, while the other daughter is developmentally advanced. M cells also switch following the same three rules.

The strand-segregation model and its tests The stem cell pattern of mating-type switching prompts the question of why two daughters of a cell almost always acquire/execute different developmental fates. A simple explanation is that any one of the 11 known functions required for m a t l interconversion, or other as yet unidentified functions, may be asymmetrically distributed, expressed, stabilized or modified between daughters. What, in that case, would regulate their differential activity? Perhaps another regulator? Such an answer only sets the question one step back. Recent studies, however, have identified a primary basis for developmental asymmetry by arguing that switching potential is segregated with a specific parental DNA chain at matI. The first evidence suggesting that switching potential in cell pedigrees segregates in cis with the mat1 locus came from a study of diploids. Egel 2° found that competence for switching is independently determined for mat1 loci in the two homologues, indicating a chromosomal basis for the segregation of switching competence. The mat1 locus was proposed to be imprinted, thereby making it switchable in a particular cell. To explain the rules of one-in-four and recurrent switching, consider a specific strand-segregation model 19,21 (Fig. 3A). The basic premise of the model is that sister cells with different development potential have inherited specific chains of parental DNA at mat1. To differentiate between the two strands, it was hypothesized that heritable site- and strand-specific imprinting at m a t l DNA occurs in each cell. The term imprinting indicates an epigenetically inherited nonmutational alteration of a chromosome that predetermines its behaviour later in deyelopment. The imprinting event is predicted to be a precursory state of the DNA that allows cleavage of the specific chromatid inheriting the imprinted DNA chain, perhaps during

replication. Further, replication of the previously cleaved chromosome will generate one specific chromatid with the switched allele, while the sister cell inheriting the previously imprinted strand will inherit the cleaved DNA, thereby allowing efficient recurrent switching of that daughter. Several features of the model have been tested by constructing and analysing a strain containing a tandem inverted duplication of the mat1 cassette (Fig. 3B). Two molecular predictions are suggested by the model. First, such a strain should have twice as many chromosomes cleaved as in wild-type cells. Second, in a given chromosome one or the other mat1 allele should be cleaved but never both simultaneously, as imprinting occurs on only one strand at each cassette. The results obtained with cultures containing the duplication verified both molecular predictions 21. Furthermore, and even more importantly, the model predicts that, instead of the usual switching of one in four granddaughters of a wild-type cell (Fig. 3A), two (cousins and not sisters) in four granddaughters of a duplication-containing cell should switch (Fig. 3B). In other words, both daughters of a cell should become switchable. Second, the inverted cassette itself should switch by the usual one-in-four rule. Results satisfying these predictions were obtained in singlecell pedigree assays 19. These results provide direct evidence that inheritance of specific parental DNA chains dictates the observed lineage of switching in cell pedigrees. Specific details of the model remain to be worked out. The imprinting event itself has not been demonstrated. However, if such an event did not exist it is hard to imagine why only one of the two specific chromatids in wild-type strains, and both chromatids (in different cassettes) in strains containing the inverted duplication, should be cleaved in the same nucleus. One unusual requirement of the model is that when the chromosome with the DSB is replicated, one chromatid is healed by switching, while the other one still has the break. How can the cells replicate through the break? Genetic evidence suggests that a new cut is made in cells undergoing recurrent switching. The imprinted strand is first sealed, then replicated, and at or soon after the time of replication the resulting chromatid is cleaved again, alleviating the replication difficulty 15. One result that is apparently inconsistent with the model should be noted. The model (Fig. 3A) predicts that 50% of the cells of a wild-type culture should have the break, at least under growth conditions where cells are known to switch efficiently. Under such conditions in nitrogen-deficient medium, the generation time is as long as 6-8 hours. However, only 25% of cells growing in rich medium, with a generation time of about 2 hours, were found to have the cut TM. The very different growth conditions used to test switching and to determine the number of cells with the DSB are presumed to lead to this apparent contradiction of the prediction 19. Remarkably, in strains deleted for both m a t 2 and mat3 the break is produced and repaired even without switching and without any deleterious effect on the cell 1°. Furthermore, the cut ends are probably protected in vivo and

TIGJUNE1992 VOL. 8 NO. 6

EWEWS [ A) Strand Segregation Model Pu

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The strand-segregation model and its predictions. (A) Imprinting (*) of, say, the 'Watson' (W) strand at mat1 occurs in the 'parent' Pu cell. During the replication of Pu, the chromatid inheriting the old imprinted W strand is cleaved to generate a switchable Ps locus. At or soon after the replication of Ps, one cleaved Ps chromatid (containing the old imprinted strand) is produced while the other chromatid is repaired as it receives the switched Mu allele. Thus, in the left-hand part of the family tree, one granddaughter is switched. The other chromatid inheriting the old 'Crick' (C) strand and the newly symhesized, unimprinted W strand is uncleavable, as it is not imprinted, thereby generating the Pu 'daughter'. The Pu cell will be imprinted by the time it is to be replicated and will produce Ps and Pu 'granddaughters', like its Pu parent. The Mu cell will similarly start the switching programme to produce P cells. (B) Segregation of DNA strands of the chromosome containing an inverted mat1 duplication. The predictions of this model are discussed in the text. Wide arrowheads in (A) and (B) indicate the orientation of mat1 in the chromosome; small arrows indicate segregation and inheritance of particular chains of DNA. The cassettes are placed about 4.0 kb apart. Adapted from RefS 19, 21.

may be held together, since they are resistant to exonuclease digestion in vitro (A. Klar, unpublished).

Directionality of switching Switching-competent cells switch to the opposite allele in 80-900/0 of cell divisions. This implies a nonrandom choice of donor: the donor locus containing genetic information different from that existing at m a t l is usually selected. In wild-type strains, the mat2 donor always contains the P allele, while mat3 contains the M allele. Another interesting and unusual feature of the m a t region is that no meiotic recombinfition events have been found in the 15 kb m a t 2 mat3 interval, at a resolution of 0.001 centimorgans (cM) 6. This region thus acts as a 'cold spot' of recombination. Recombination is also rare in the m a t l - m a t 2 interval 22. Furthermore, no essential gene is located in this interval, since the deletion of this region is not lethalS, 10. An attempt to explain directionality and the cold spot of recombination led us to propose that the donor loci are made accessible in vivo by a higher order chromatin structure that promotes the intrachromosomal folding of donors onto mat1 (Ref. 22; Fig. 4).

This suggested intrachromosomal folding may preclude interaction between homologues in the m a t 2 mat3 interval, thereby causing the meiotic cold spot of recombination. Three recent genetic observations favour this model. (1) The donor loci in diploid cells are inefficient in donating information to the m a t l locus on the homologous chromosome, arguing against a diffusible intermediate of switching 22. (2) An 18 kb deletion of mat2, mat3 and the intervening sequences paradoxically results in a nearly 30-fold increase in recombination in the flanking regions 10.22. (3) A swi6 mutation does not affect the level of the DSB, yet it reduces switching efficiency, presumably because the break is used inefficiently for switching 12. An interesting finding is that inhibition of recombination in the mat region is lifted in swi6- strains (Ref. 22; A. Lorentz, L. Helm and H. Schmidt, pets. commun.). Mutations in another gene, rikl, have pleiotropic effects, one of which is to allow recombination in the cold spot 23. Furthermore, mutations in the clrI (cryptic loci regulator) locus, which is implicated in silencing of mat2 and mat3 cassettes, also allow recombination in this interval 24. Collectively, these observations

-ri6 june 1992 vet. 8 No. 6

[k~EVIEWS mat2-P

M cell

4

4

x

matl-M mat3-M

1I mat2-P

P cell

matl-P X 4 mat3oM

FIGFI A looping model for directionality. In P cells, maU-M is made more accessible by specific chromatin organization of the mating-type region. Likewise, a different chromatin structure causes increased accessibility, for example by looping, of mat2-P in M cells. 'X' denotes the preferred interaction, indicated by the increased thickness of the interacting lines that represent the chromosome.

suggest that in M cells mat2-P is more accessible than mat3-M (Fig. 4). The converse should be true in P cells. Perhaps the chromatin structure of the donor loci is altered between P and M cells, thereby promoting preferential selection of the appropriate donor

lOCUS22,24.

Comparisonwith mating-type switching in $. cerevisiae Budding yeast also undergoes a highly regulated pattern of mating-type switching. The alternative alleles of the mating-type locus, MATa and MATs, are interchanged using distantly located (over 120 kb away) HMLa and HMRa loci as donors (recently reviewed in Ref. 2). As in S. pombe, a DSB at MATinitiates the recombination event 25. The pattern of switching in S. cerevisiae is highly regular and is significantly different from that of S. pombe. In a pair of sister cells, only the larger, older ('mother') cell is switchable and the smaller, younger ('daughter') cell never switches. Moreover, the mother cell produces daughters that are both switched 26,27. The mother-daughter difference in the ability to switch is ascribed to the differential expression of the HO gene 2s, which encodes the site-specific endonuclease that cleaves MAT 29. Two models have been developed to explain the restriction of HO expression to mother cells. One model invokes the differential action of the positive regulatory SWI5 trans-acting function, which is required for HO gene expression30. The other model is based on the difference ha the cell size or the G1 phase of the cell cycle of motl~er and daughter cells31. Unequiw~cal evidence supporting either of these models has not been obtained thus far (see, for example, Ref. 32).

The directionality of switching is also an interesting issue in S. cerevisiae. It is k n o w n that MAT~z cells preferentially choose the HMRa donor, while MATs cells choose HMLa (Ref. 33). Consequently, cells switch mostly to the opposite cell type. Furthermore, a given cell type chooses a donor locus on the basis of the position of the locus rather than its genetic content. For example, HMLa HMRo~ strains switch MATa to M A T s and MATs to MATs efficiently, but only inefficiently switch to the opposite allele. Our recent experiments suggest a similar p h e n o m e n o n in S. p o m b e (G. Thon and A. Klar, unpublished). H o w both systems choose the particular donor locus remains an interesting but enigmatic issue. Our proposal is that cell type dictates the chromatin arrangement of donor loci, regulating their accessibility z224. Alternatively, specific sequences around donor loci may promote specific interaction with the active locus during recombination. In summary, the primary event making sisters different in S. p o m b e is dictated by the inheritance of a parental DNA chain at m a t l that makes the site cleavable in vivo. In contrast, in S. cerevisiae the control lies with the expression of the endonuclease-encoding HO gene, which is expressed only in mother cells. Why the HO gene is not expressed in daughter cells is not known. Once the break is made, on the basis of their cell type, cells preferentially select a particular donor to effect a switch usually to the opposite mating type. Understanding the molecular details of developmental choice in these highly evolved and distantly related systems should provide us with a framework for understanding cellular differentiation in more complex multicellular biological systems.

Acknowledgements I am indebted to present and past colleagues for valuable discussions and experimental contributions. I also thank H. Schmidt for communicating results before publication, R. Frederickson for the artwork, A. Arthur for editorial suggestions and P. Hall for manuscript preparation. My apologies to colleagues for the necessarily limited number of citations. Research in my laboratory was sponsored by the National Cancer Institute, DHHS under contract No. N01-CO-74101 with ABL. The contents of this publication do not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government.

References 1 Egel, R. (1989) in Molecular Biology of Fission Yeast (Nasim, A., Johnson, B. and Young, P., eds), pp. 31-73, Academic Press 2 Klar, AJ.S. (1989) in Mobile DNA (Berg, D.E. and Howe, M.M., eds), pp. 671-691, American Society for Microbiology 3 Gutz, H. and Schmidt, H. (1990) Semin. Dev. Biol. 1, 169-176 4 Leupold, U. (1958) ColdSpring HarborSymp. Quant. Biol. 23, 161-170 5 Egel, R. and Gutz, H. (1981) Curr. Genet. 3, 5-12 6 Egel, R. (1984) Curr. Genet. 8, 199-203 7 Beach, D.H. (1983) Nature 305, 682-688 8 Beach, D.H. and Klar, AJ.S. (1984) EMBOJ. 3, 603-610

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it

[]~EVIEWS 9 Kelly, M. et al. (1988) FJ4BOJ. 7, 1537-1547 10 Klar, A.J.S. and Miglio, L.M. (1986) Cell46, 725-731 11 Nielsen, O. and Egel, R. (1989) F~IBOJ. 8, 269-276 12 Egel, R., Beach, D.H. and Klar, A.J.S. (1984) Proc. Natl Acad. Sci. USA 81, 3481-3485 13 Schmidt, H., Kapitza, P. and Gutz, H. (1987) Curr. Genet.

11, 3O3-308 14 Schmidt, H., Kapitza-Fecke, P., Stephen, E.R. and Gutz, H. (1989) Curr. Genet. 16, 89-94 15 Klar, A.J.S., Bonaduce, M.J. and Cafferkey, R. (1991) Genetics 127, 489-496 16 Arcangioli, B. and Klar, A.J.S. (1991) EMBOJ. 10,

3025-3033 17 Miyata, H. and Miyata, M. (1981)J. Gen. Appl. Microbiol.

27, 365-371 18 Egel, R. and Eie, B. (1987) Curr. Genet. 12, 429-433 19 Klar, A.J.S. (1990) F~IBOJ. 9, 1407-1415 20 Egel, R. (1984) Curr. Genet. 8, 205-210 21 Klar, A.J.S. (1987) Nature 326, 466--470 22 Klar, A.J.S. and Bonaduce, M.J. (1991) Genetics 129,

1033-1042

23 Egel, R., Wilier, M. and Nielsen, O. (1989) Cztrr. Gepwt.

15, 407~i10 24 Thon, G. and Klar, A.J.S. Genetics (in press) 25 Strathern, J.N. et al. (1982) Cell 31, 183-192 26 Hicks, J.B. and Herskowitz, I. (1976) Genetics 83, 245-258 27 Strathern, J.N. and Herskowitz, I. (1979) Cell 17, 371-381 28 Nasmyth, K.A. (1983) Nature 302, 670476 29 Kostriken, R. et al. (1982) Cell 35, 167-174 30 Nasmyth, K.A., Seddon, A. and Ammerer, G. (1987) Cell 49, 549-559 31 Sternberg, P.W., Stern, M.J., Clar, I. anti Herskowitz, I. (1987) Cell 48, 567-577 32 Moll, T. et al. (1991) Cell 66, 743-758 33 Klar, A.J.S., Hicks, J.B. and Strathern, J.N. (1982) Cell 28, 551-561 A.J.S. KLARtS lX THENCI-FREOERICKCANCERRESEARCHamo DEVELOPMENTCENTER~A B I ~astc RESEARCH PROGRAM, P O B o x 1~ BLDa 539, FREDEmCK, M D 21 702-1201, USA.

Models of plant-path0gen P l a n t defense against pathogens often involves a single-gene resistance factor. Pathogens can escape this resistance if they carry a matching single-gene virulence factor. This gene-for-gene interaction between a host and a pathogen can occur at more than 20 separate loci, leading to genetic battles between host and pathogen populations and to extensive genetic polymorphisms for resistance and viruler ~e 1. The extensive polymorphisms are, at first glance, rather puzzling. Why should a plant population maintain variability for disease resistance? The resistance is presumably advantageous and should therefore spread to fixation. Likewise, why should pathogens be polymorphic for the ability to overcome host defenses? An avirulent pathogen cannot attack a host, cannot reproduce, and does not contribute genes to future generations. To make matters more complex - and more interesting - preliminary data suggest that the frequency of each resistance gene varies widely over space: it may be absent in one place, fixed in another, and polymorphic in a third 2,3. The pathogens vary in a similar way. These observations, and some related theories, lead to an intriguing conjecture: disease polymorphisms are the result of continual cycles of coevolution woven through time and space 3-5. Against this idea of shifting polymorphisms, nearly all authors suggest that the distribution of disease in wild populations is a fragile equilibrium between hosts and pathogens - a delicate balance of nature. According to this view the frequencies of polymorphic genes are held nearly constant in most situations. Epidemics and fluctuating gene frequencies result from environmental disturbances caused, more often than not, by human infraction 6. This review summarizes the complex polymorphisms in gene-for-gene systems and the theories that attempt to explain these polymorphisms.

c0ev01uti0n S.A. FRANK Plant populations are often genetically polymorphic for resistance to pathogens. The effectiveness of this resistance is limited because the pathogens are, in turn, polymorphic for virulence genes that can evade plant resistance. Theoretical models and intriguing preliminary data suggest that these plant-pathogen polymorphisms are maintained by continual cycles of coevolution within populations, combined with occasional immigration of new virulence and resistance genes from distant populations.

Observations Plant defense against pathogens includes specific major-gene resistance, which is effective against certain genetic races of pathogens, and general polygenic resistance, which is effective against a broader range of pathogens 1,7. This review focuses on the major-gene factors. In this section I condense the vast literature on plant-pathogen genetics to a few general observations that any theory of disease polymorphism must explain. Gene-for-gene systems

During the 1940s and 1950s, H.H. Flor studied the inheritance of specific resistance and virulence factors in flax and its fungal pathogen flax rust s. The interaction between host and pathogen genotypes turned out to have simple properties that Flor referred to as a 'gene-for-gene' system. In an idealized gene-for-gene system, each pair of resistance and susceptibility alleles in the host has a matching pair of virulence and avirulence alleles in the pathogen. Tables 1 and 2

T1GJUNE 1992 VOL. 8 NO. 6 ©1992 Elsevier Science Publishers Lid (11K)