Sex determination in plants Cristina Juarez and Jo Ann Banks*

Sex determination in plants Juarez and Banks 71Although the sex of the Ceratopteris gametophyte isepigenetically determined by the presence or absenceof ACE, many of the genes that are regulated by ACEhave been identified by isolating mutations that alter thesex of the gametophyte. The epistatic interactions amongthese genes have been examined and a genetic regulatorypathway controlling sex determination in Ceratopterisdefined [1,35,36°°]. In this pathway, there are two masterregulatory genes, or sets of genes, that regulate thesexual phenotype of the gametophyte. One includesthe TRANSFORMER or TRA genes which, when active,simultaneously promote femaleness (the development ofarchegonia and meristem) and repress maleness (thedevelopment of antheridia). The other includes theFEMINIZATION1 or FEM1 gene. When active, the FEM1gene simultaneously promotes maleness and repressesfemaleness. The TRA genes are thought to repressmaleness by indirectly repressing the activity of the FEM1gene, while the FEM1 gene represses femaleness byrepressing the activity of the TRA genes. The repressionof FEM1 by TRA requires another gene called MAN1(for many antheridia); the TRA genes positively regulateMAN1, which then negatively regulates FEM1 [37]. Thesex of the gametophyte, male or female,, ultimatelydepends on which of the two master regulatory genes isexpressed first. This decision is determined by ACE, whichactivates the ACE signal transduction pathway defined bythe HERMAPHRODITIC or HER genes. When activatedby ACE, the HER genes, together with FEM1, repressTRA and the gametophyte develops as a male. WhenACE is absent, the TRA genes are not repressed, FEM1is repressed, and the gametophyte develops female traits.While the sex determining pathway is useful for understandinghow the sex determining genes interact todetermine the sex of ~he Ceratopteris gametophyte, thispathway reveals a few fundamental points about sexdetermination that might also be expected to occur or thathave been observed in other plant species. The first isthat sex determination in Ceratopteris involves both geneticfactors (defined by mutation) and pheromones/hormones,or epigenetic factors. The same is true for maize, aspreviously described. The second point is that themechanism involved in sex determination in Ceratopterisis inherently plastic in that the sex of a gametophyte canbe reversed in response to changes in the environment.A good example of another species showing labile sexexpression is Thymelaea hirsuta, where individuals maychange their sex from year to year in response tochanges in the environment [38,39°]. The final pointis that the sex of an individual involves two masterregulatory genes, each promoting one sex and repressingthe other sex. Thus, for an individual to be female, thefemale-promoting regulatory gene(s) must be expressedand the male-promoting regulatory genes(s) must berepressed; to be male, the opposite must occur. InCeratopteris and maize, the two plants where these masterregulatory genes have been identified, the evidence thusfar indicates that the simultaneous promotion of one sexand repression of the opposite is accomplished by thetwo master regulatory genes which, directly or indirectly,exclude each other's expression. Whether and how thesepredicted interactions occur can be tested once thesegenes have been cloned.ConclusionsThere has been significant progress in our understandingof sex-determining mechanisms in plants, particularlyin species that are amenable to a genetic analysis ofmutations that affect sex expression. Continued studiesof sex determination in diverse species that have beenidentified as model systems, such as S. ]atifolia, Z. maizeand C. richardii, are important for two reasons. First,the diversity of sex-determining mechanisms in plantscan only be understood by studying several speciesdisplaying different patterns of sex expression. Second,the problem of sex determination gets to the heart of thefundamental but poorly understood question of how plantcells choose between two developmental fates (male orfemale). Studying the genetic and molecular basis of sexdetermination in several species of plants should revealhow such decisions are made, and how each decision isor can be influenced by external (environmental) factorsand internal (genetic) factors.References and recommended readingPapers of particular interest, published within the annual period of review,have been highlighted as:= of special interest== of outstanding interest1. Banks J: Sex determination in the fern Ceratopteris. Trends P/his Sci 1997, 2:175-180.paper describes how the sex pheromone antheridiogen controls thesexual phenotype of gametophytes of the fern Ceratopteris. Included is asummary of the sex-determining genes and a model of how the pheromoneis thought to influence the activity of these plants in controlling sex type.2. Dellaporta SL, Calderon-Urrea A: The sex determination processin maize. Science 1994, 266:1501-1505.3. Freeman D, Doust J, EI-Keblawa A, Miglia K, McArthur E: Sexual• , sPecialization and inbreeding avoidance in the evolution ofdioecy. Bot Rev 1997, 63:65-92.This paper discusses the selective forces that drive the evolution of dioecy.While it is commonly thought that the advantage of dioecy is the avoidance ofinbreeding, the authors also examine and explain how the selection for sexualspecialization can be advantageous to some plants given their ecology.4. Grant S, Houben A, Vyskot B, Siroky J, Pan W, Macas J, SaedlerH: Genetics of sex determination in flowering plants. DevelGenet 1994, 15:214-230.5. LebeI-Hardenack S, Grant S: Genetics of sex determination in• flowering plants. Trends Plant Sci 1997, 2:130-136.This paper reviews the genetics of sex determination and in several dioeciousangiosperm species as well as the role of hormones and MADS-boxhomeotic genes in regulating sex expression.6. Irish, E: Regulation of sex determination in maize. BioEssayshis 1996, 18:363-369.paper focuses on the sex-determining genes and their interactions inmaize.7. Warmke H: Sex determination and sex balance in Melandrium.Amer J Bot 1946, 33:648-660.8. Westergaard M: The relation between chromosome constitutionand sex in the offspring of triploid Melandrium. Hereditas 1948,34:255-279.