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In the Fern Ceratopteris Richardii Copyright 0 I996 by the Genetics Society of Arnerica Genetic Interactions Among Sex-Determining Genes in the Fern Ceratopteris richardii James R. Eberle' and Jo Ann Banks Department of Botany and Plant Pathology, Lilly Hall of Life Sciences, Purdue University, West Lafayette, Indiana 47906 Manuscript received September 26, 1995 Accepted for publication December 4, 1995 ABSTRACT Haploid gametophytes of the fern Ceratopteris are either male or hermaphroditic. The determinant of sex type is the pheromone antheridiogen, which is secreted by the hermaphrodite and directsmale development of young, sexually undetermined gametophytes. Three phenotypic classes of mutations that affect sex-determinationwere previously isolated and include the hermaphroditic (her),the transformer (tra) and feminization (fem) mutations. In the present study, linkageanalysis and tests of epistasis among the different mutants have been performed to assess the possible interactions among these genes. The results indicate that sex determination in Ceratopteris involves at leastseven interacting genesin addition to antheridiogen, the primary sex-determining signal. Two models describing how antheridiogen may influence the activity states of these genes and the sex of the gametophyte are discussed. APLOID gametophytes ofmany homosporous male thallus divide and form a hermaphroditic thallus. H ferns, including Ceratopteris richardii, develop as The capacity of the male to switch its sex in response either males or hermaphrodites. The sexual fate of the to changes in the primary sex signal indicates that the developing gametophyte is determined by pheromone male program of sex expression, unlike the hermaphro- antheridiogen (DOPP 1950; NAF 1979; NAF et al. 1975), ditic program of sex expression, is reversible during which is referred to as &,E in Ceratopteris. In the ab- development and that &;E is required for bothinitiating sence of &,E, individual Ceratopteris spores develop as and maintaining the male program of sex expression. hermaphroditic gametophytes that produce egg-bear- Although the structure of &:E is unknown, the &:E re- ing archegonia, sperm-bearing antheridia and a well- sponse can be blocked by inhibitors of gibberellin bio- organized multicellular meristem (i.e., aremeristic). synthesis as well as abscisic acid (ABA), indicating that The hermaphrodite produces and secretes ACE into its &:E either is or is similar to a gibberellin (HICKOK1983; surroundings when it is no longer competent to re- WARNEand HICKOK1989). spond to it. When grown in the presence of &;E, spores Genetic and molecular studies of sex determination develop as male gametophytes with numerous an- and differentiation in animals, particularly Drosophila theridia but no archegonia or organized multicellular melanogaster and Caenorhabditis elegans, have shown that meristem (i.e.,are amerisitic).Because hermaphrodites the control of sex type in these organisms involves a form a meristem and males do not, the two sexes can number of genes that interactwithin a well-defined reg- be easily distinguished by their size and shape as well ulatory heirarchy, which has at its apex the primary sex- as type of sex organ produced. determining signal (reviewed in BAKER1989; HODGKIN Physiological studies of the I\CE response in Ceratop- 1990; CLINE 1993).Although the primary sex-determin- teris (BANKSet al. 1993) have shown that the sex of the ing signal in Ceratopteris (a pheromone) is clearly dif- gametophyte is initally determined early in develop ferent from that in Drosophila and Caenorhabditis (the ment (stage 2) as the spore wall cracks and the cell X:A ratio),Ceratopteris has manyuseful features in inside begins to divide. If &:E is absent during this stage common with Drosophila and Caenorhabditisthat of development, the gametophyte will develop as a her- make it well suited for the study of sex determination maphrodite even if exposed to exogenous ACE at later in plants. First, the determinant of sex type is known. stages of development. If ACE is present duringthis and Second, individuals are sexually dimorphic, a property subsequent stages of development,the gametophyte of Drosophila and Caenorhabditis that has expedited will develop as a male. However, if &E is withdrawn the study of the differences between two individuals of from a male gametophyte, undifferentiated cells of the different sexes. Third, Ceratopteris is amenable to the genetic analysis of mutations that affect a variety of de- Corresponding author: Jo Ann Banks, Department of Botany and velopmental processes (reviewedin CHASEN1992; PlantPathology, Lilly Hall, Purdue University, West Lafayette, IN 47906. E-mail: [email protected] HICKOKet al. 1995) including sex determination (BANKS ' Present addre.w Department of Biology, Saint Francis College, 2701 1994; EBERLEet al. 1995). A principal advantage of Spring St., Fort Wayne, IN 46808. Ceratopteris as a geneticsystem is that mutationsaffect- Genetics 142 97.7-985 (March, 1996) 974 J. R. Eberle and J. A. Banks ing haploid gametophyte development,either domi- ture water was added to the males. The water and decrease nant orrecessive, are apparentimmediately after muta- in temperature cause the sperm to be released from the an- theridia within minutes. Drops of water containing sperm genesis of the single-celled spore and growth of the were then added toeach individual female or hermaphrodite. gametophyte under selective conditions. While the hap A cross was considered successful only if the sperm swarm loid nature of the gametophtye facilitates the selection to the archegonia. To obtain sperm from a population of of mutations, it is also a limitation of the system because hermaphrodites, room temperature water was added to 3-4 traditional complementationexperiments cannot be week-old cultures previously maintained at 29". Water con- taining the spermreleased fromhermaphrodites was removed performed in a haploid organism. We have used a ge- within minutesbefore the sperm could swarmto the her- netic approach similar to that used in Drosophila and maphroditesfrom which they were being removed then Caenorhabditis to understand how the pheromone &:E added to the appropriate female donor. Because less sperm governs the sexual phenotype of the Ceratopteris ga- is obtained when using a hermaphrodite than a male gameto- metophyte (BANKS1994). This analysis has led to the phyte, there is a greater chance that the hermaphrodite sen- ing as a female donor could be self-fertilized. To insure that identification of three phenotypic classesof sex-de- sperm was present from a population of hermaphrodites, the termining mutants that exhibit insensitivity to &E as water containing the sperm was visually inspected for sperm well as an altered sexual phenotype. These include the and was also added to fern1 females since prnl females cannot hermaphroditic (her) mutants thatdevelop as meristic her- self-fertilize. maphrodites, the transfmmerl (tral) mutant that devel- Antheridiogen, which is not available in purified form, was obtained as a crude aqueous filtrate of medium that had sup- ops as an amerisitic male, and the feminization1 (feml) ported thegrowth of gametophytes fromwild-type Hnn spores mutant that develops as a meristic female. (40 mg/liter medium) for 14 days. Each batch of medium The mutant phenotype of each sex-determination was assayed for &:F. activity according to BANKSet al. (1993) gene, in conjunctionwith information from physiologi- and only batches of &:E that induced >YO% Hnn wild-type cal and developmental studies of sex determination in males grown in isolation were used. Because the number of wild-type hermaphrodites that form in a population of ga- this species, have been used to construct a model to metophytes grown in the presence off&, is negligable, the describe how genetic and epigenetic factors might in- calculated expected gene frequencies of wild-type males teract to form a regulatory hierarchy controlling the grown in the presence of &:E was assumed to be 100%. sex of the gametophyte (BANKS1994). This model was Batches of medium lacking &:k, were also tested to insure that based on the assumption that TRAl and EM1 act in a the medium was not contaminated with &:E by sowing wild- type spores in this medium in isolation. Only medium that positive manner to direct differentiation of female and resulted in 100% hermaphroditic development was used. male traits, respectively, and is therefore referred to as The presence and abundance of antheridia, particularly in the positive model. This model makes specific, testable the intersexualgametophytes, was assessed by staining ga- predictions about the epistatic interactions among the metophytes with Hoechst's dye and then viewing under UV sex-determining genes. The goals of the present study fluorescence on a Zeiss Axiophot microscope according to BANKSet al. (1993). were to test the predictions of the positive model and to determine the number of gene loci represented by RESULTS these mutations. While the results of this study support many predications of the positive model, they do not Linkage analysis among the her and fml mu- support themall. An alternative model thatis consistent tants: The her mutants were originally selected as her- with the genetic evidence is described. maphrodites that grow in the presence of &:E, a condi- tion that normally promotes male development (BANKS 1994). Most her mutations are completely insensitive to MATERIALSAND METHODS &:E; i.e., when a her hermaphrodite is self-fertilized
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