Genes Genet. Syst. (2005)

80

, p. 395–401

An anther- and petal-specific gene

SlMF1

is a multicopy gene with homologous sequences

on sex chromosomes

Sachihiro Matsunaga

1

*, Sabine Lebel-Hardenack

2

, Eduard Kejnovsky

3

,Boris Vyskot

3

, Sarah R. Grant

2

and Shigeyuki Kawano

4

1

Department of Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka,Suita 565-0871, Osaka, Japan

2

Department of Biology, University of North Carolina, Chapel Hill, NC 27599-3280, USA

3

Institute of Biophysics, Academy of Sciences of the Czech Republic, CZ-612 65Brno, Czech Republic

4

Department of Integrated Biosciences, Graduate School of Frontier Sciences,University of Tokyo, Kashiwanoha 5-1-5, Kashiwa,

Chiba 277-8562, Japan

(Received 25 August 2005, accepted 2 November 2005)

A male flower-specific gene

SlMF1

was isolated from male flower buds of thedioecious plant

Silene latifolia

.

SlMF1

is expressed in all the floral meristems atthe very early stage of development in both male and female flower buds. At themature stage of development in male flower buds,

SlMF1

transcripts were specif-ically accumulated in pollen mother cells, tapetal cells, and the developing tips ofpetals. Genomic Southern hybridization revealed that

SlMF1

was a multicopygene with a Y chromosome-linked homologous sequence. PCR analyses with flow-sorted chromosomes showed that

SlMF1

was localized on both autosomes and theX chromosome.

Key words:

dioecious plant, male flower, sex chromosomes

INTRODUCTION

Although sex chromosomes are found in most animals,they are limited to dioecious plants in which staminateand pistillate flowers are produced in separate individu-als.

Plant sex chromosomes have arisen independentlyin taxonomic groups from liverworts to angiosperms(Okada et al. 2001; Ishizaki et al. 2002; Charlesworth2002). Heteromorphic sex chromosomes have beenreported in several dioecious species of angiosperms(Vyskot and Hobza 2004). The Y chromosome of

Silenelatifolia

in the family

Caryophyllaceae

is the most well-studied sex chromosome in plants (Matsunaga andKawano 2001; Negrutiu et al. 2001). The Y chromosomeis largely euchromatic with the exception of centromericand subtelomeric regions (Buzek et al. 1997; Matsunagaet al. 1999a; Kazama et al. 2003). As compared with themammalian Y chromosome that emerged approximately320 million years ago, the Y chromosome in the genus

Silene

emerged quite recently, that is, during the last 10million years. Analyses of the Y chromosome deletionsinduced by

g

-rays or X-rays suggest that the Y chro-mosome has three functions: suppression of pistil deve-lopment and initiation and completion of stamendevelopment (Negrutiu et al. 2001). Recently, many Y-linked genetic markers were isolated from the malegenome of

S. latifolia

(Donnison et al. 1996; Zhang et al.1998; Nakao et al. 2001; Obara et al. 2002). In particu-lar, analyses of male-specific AFLP markers confirmedthe functions assigned to the regions of the Y chromosome(Lebel-Hardenack et al. 2002). Flow sorted X chromo-somes derived from female cultured cells are useful forrapid confirmation of the linkage of the X chromosome(Kejnovsky et al. 2001). PCR analyses with flow sortedX chromosomes revealed that a male reproductive organ-specific gene

MROS3

had a paralogue on the X chromo-some (Matsunaga et al. 1999b; Kejnovsky et al. 2001).

After floral identity genes determine the developmentalfate of each whorl, the first, second, third, and fourthwhorl develop sepals, petals, stamens, and carpels,respectively (Coen and Meyerowitz 1991). In dioeciousplants, after floral identity is determined, unisexual flow-ers develop through the suppression or promotion of cell

Edited by Minoru Murata* Corresponding author. E-mail: sachi@bio.eng.osaka-u.ac.jp

Note: Nucleotide sequence data reported are available in theDDBJ Data Library under the accession numbers AB238691(MROS4) and AB238692 (SlMF1).

396 S. MATSUNAGA et al.

division activity in each sex primordium (Matsunaga etal. 2004a). In male flowers, the fourth-whorl gynoeciumprimordium is suppressed and later becomes a rudimen-tary gynoecium in the form of a filamentous rod that lacksan ovary and pistils (Grant et al. 1994). In the thirdwhorl of female flowers, the growth of stamen primordiais arrested and the tissues degenerate before the floweropens (Uchida et al. 2003, 2005). Flower bud-specificgenes were expected to contribute to the regulation of sex-specific development of unisexual floral organs.

There-fore, several researchers attempted to characterize repro-ductive organ-specific genes (Barbacar et al. 1997; Lebel-Hardenack et al. 1997; Matsunaga et al. 1996, 1997; Scuttet al. 2002; Sugiyama et al. 2003) or orthologs of

Arabi-dopsis thaliana

genes that regulate flower development(Hardenack et al. 1994; Ageez et al. 2003; Matsunaga etal. 2003, 2004b). We have isolated a reproductive organ-specific gene

SlMF1

from male flower buds, and analyzedits expression pattern in flower buds and its sex chromo-some linkage. Interetingly,

SlMF1

was an X chromo-some-linked gene that was specifically expressed indeveloping anthers and petals.

MATERIALS AND METHODS

Plants

We used 2 inbred

S. latifolia

lines–the Japaneselaboratory strain K1 and the USA laboratory strainMR4X64 for the molecular experiments. The plantswere grown in a temperature-controlled chamber or agreen house at 22

∞

C.

Differential display and isolation of

SlMF1

full-length cDNA

Differential display was performed onmRNA isolated from young male and female flowerbuds. Total RNA and mRNA isolation were performedin accordance with the method of Moore et al. (2003).The oligonucleotides in an RNA image kit 2 (Genehunter,Nashville, TN, USA) were used for differential display.Amplified fragments were subcloned using a TOPO TACloning kit (Invitrogen). The nucleotide sequences weredetermined using an ABI 3100 genetic analyzer and aBigDye terminator cycle sequencing kit (Applied Biosys-tems).

We obtained full-length cDNAs from the screen-ing of our constructed Tripl Ex2 cDNA library (Clontech)of male flower buds (Matsunaga et al. 2003). The fulllength cDNA sequence was confirmed by RACE-PCRusing a SMART RACE cDNA amplification kit (Clontech).

Expression analyses

Northern hybridization wasperformed using an AlkPhos Direct Labelling andDetection System with CDP-Star (Amersham) with 15

m

gof total RNA from male and female flower buds andleaves.

Quantitative RT-PCR was performed in accor-dance with the method of Matsunaga et al. (2003) using aSmart Cycler (Takara) with a QuantiTect SYBR green kit

(Qiagen). The gene for the GTPase beta subunit (

SlGb

),which is expressed constitutively in all organs, was usedas an internal standard to estimate the relative expressionof mRNA. The relative expression of mRNA for a given

Fig. 1. Expression analyses of

SlMF1

. (a) Northern hybridiza-tion analysis of

SlMF1

. The upper panel shows a Northernhybridization image using an AlkPhos Detection System withCDP-Star (Amersham). The lower panel shows an electro-phoresis image of rRNA bands stained with ethidium bromide.Lanes 1, 2, 3, and 4 contain 15

m

g of total RNA from male flowerbuds, female flower buds, male leaves, and female leaves. (band c) Quantitative RT-PCR analyses of the

SlMF1

and

MROS4

transcripts. The upper and lower panels show values repre-senting the relative expression of the

SlMF1

and

MROS4

tran-scripts that were standardized based on the expression of theGTPase

b

subunit, which is expressed constitutively in allorgans. Bars 1 to 7 represent the relative expression levels inmale sepals, male petals, male stamens, female sepals, femalepetals, female styles, and female ovaries, respectively. Dataare expressed as mean

±

SEM (

n

= 3).

397A male flower-specific gene on plant sex chromosomes

tested gene was defined as its mean mRNA expressionvalue divided by that for

SlGb

, with the same cDNA usedas template. Relative expression values and correspond-ing standard deviations for the transcripts were calculatedfrom three experimental replicates. The oligonucleotideprimer sets used for quantitative RT-PCR were as follows:5'-GACATGGTGACAGCCATAGCAACA-3' and 5'-TCAC-GAGAAGCAGAGACTATCTGT-3' for

SlGb

; SlMF1-F: 5'-GTGTCACCCAATTCTACTCA-3' and SlMF1-R: 5'-ACGA-CGCCACTGGAAGATAG-3' for

SlMF1

; MROS4-F: 5'-TAGTTGTGCAAATGGCTCCC-3' and MROS4-R: 5'-TC-CGAAACACAATGGCCTTC-3' for

MROS4

. Preparationof biotin-labeled probes for

SlSF1

was performed in accor-dance with the method of Matsunaga et al. (2004a). Insitu hybridization was performed as described previously(Matsunaga et al. 2003), with an automatic ISH robotAIH-101B (Aloka) using tyramide amplification in theGenPoint system (Dako).

Genomic distribution analyses

Genomic DNA wasisolated from young leaves using an automatic DNA iso-lation system PI50 (Kurabo). Twenty five micrograms of

male and female genomic DNA were digested with

Hin-

dIII, resolved in 1% agarose gel, blotted onto a membrane,and hybridized with the full-length cDNA of

SlMF1

as aprobe. Southern hybridization was performed using anAlkPhos Direct Labelling and Detection System withCDP-Star (Amersham).

Flow-sorted chromosomes were collected as describedin Kejnovsky et al. (2001). The set of oligonucleotideprimers for the 390-bp

SlMF1

fragment consisted ofSlMF1-F and SlMF1-R.

RESULTS

SlMF1

is specifically expressed in stamens and pet-als

We performed differential display with 48 combina-tions of 7 primers using very early young male and femaleflower buds and identified 38 male flower bud-specificfragments. Northern hybridization using total RNAfrom male and female flower buds revealed that two RNAfragments strongly hybridized to the total RNA of maleflower buds. One fragment was termed

SlMF1

(a

Silenelatifolia

male flower-specific gene) and further analyzed.

Fig. 2. In situ hybridization of the

SlMF1

gene. Longitudinal sections of young male flower buds are allowed to hybridize with thebiotin-labeled probe. Hybridization signals appear brown. Sections were allowed to hybridize with the antisense probe of

SlMF1

(aand c) and the sense probe of

SlMF1

(b and d). (a and b) Male flower buds at stage 3; (b and d) male flower buds at stage 9. Po,pollen mother cell; Pe, petal; Ta, tapetum. Bars in (b) and (d) represent 50

m

m or 150

m

m, respectively.

398 S. MATSUNAGA et al.

We isolated the full-length cDNA of

SlMF1

from thecDNA library of male flower buds. The sequence of thefull-length cDNA was confirmed using the 5' RACEmethod. The length of the cDNA excluding the polyAtail was 744 bp (DDBJ accession no. AB238692), and itslongest ORF encoded a 50-amino acid protein that did notshow significant homology with other reported proteins.Northern hybridization using total RNA from male andfemale flower buds and leaves showed that the

SlMF1

transcript preferentially accumulated in male flower buds(Fig. 1a). The transcript was also detected to a smallextent in female flower buds; however, accumulation ofthe transcript in male and female leaves was less thanbelow the detection limit. Analysis of its expression infloral organs (male and female sepals and petals, malestamens, female styles and ovaries) was examined usingquantitative RT-PCR (Fig. 1b). This analysis indicatedthat the

SlMF1

transcript was preferentially expressed inmale and female petals and male stamens, with strongexpression in male stamens. In contrast, its expressionin male and female sepals and female ovaries was verylow. Quantitative RT-PCR with specific primers for amale reproductive organ-specific gene

MROS4

(DDBJaccession no. AB238691) using the same RNA samplesshowed that the transcript preferentially accumulated inmale stamens (Fig. 1c). This result is consistent withthe previously reported profile of

MROS4

expressionusing Northern hybridization (Matsunaga et al. 1996).This suggests that the results of our quantitative RT-PCRprecisely reflect the accumulation level of transcripts infloral organs.

To analyze the expression profile in greater detail, weperformed in situ hybridization using longitudinal sec-tions of young and mature male flower buds at stages 3and 9 described by Grant et al. (1994) (Fig. 2). Thebiotin-labeled probe was detected using tyramide ampli-fication and the signal appeared brown. When the sec-tion of the young male flower bud at stage 3 was allowedto hybridize with the antisense probe, significant signalscould be detected in the whole floral meristem (Fig.2a). When the young female flower bud at the samestage was used for in situ hybridization, the same signaldistribution was found in the whole floral meristem (datanot shown). When a section of the mature male flowerbud at stage 9 was allowed to hybridize with the anti-sense probe, the signals could be detected in anthers andpetals (Fig. 2c). In particular, strong signals could bedetected in pollen mother cells and tapetal cells. Whenthe mature female flower bud at the same stage was usedfor in situ hybridization, the signals were detected inpetals, but were not detected in vestigial stamens (datanot shown). In contrast, the sense probe produced nohybridization signal above background in the sections ofboth young and mature male flower buds (Fig. 2b and d).

Fig. 3. The chromosomal locations of

SlMF1

. (a) GenomicSouthern hybridization analysis using genomic DNA digestedwith

Hin

dIII. Each lane indicates DNA from male (1) and female(5) parents or male (2–4) and female (6–8) offspring of the USAlaboratory line. (b) Genomic Southern hybridization analysisusing genomic DNA digested with

Hin

dIII. Each lane indicatesDNA from male (1) and female (6) parents or male (2–5) andfemale (6–10) offspring of the Japanese laboratory line. (c) PCRanalyses on flow-sorted chromosomes. The PCR products wereamplified with the set of SlMF1-F and SlMF1-R primers for the390-bp fragments of

SlMF1

. PCR was performed using female(♀ ) and male (♂ ) genomic DNA, flow-sorted autosomes (A), Xchromosomes (X), or no chromosomes (0) as templates. M repre-sents a 100-bp ladder that was used as a DNA length marker.

399A male flower-specific gene on plant sex chromosomes

SlMF1

is a multicopy gene and is located on sexchromosomes

To confirm the genomic distribution of

SlMF1

, we performed Southern hybridization using maleand female genomic DNA from a pair of male and femaleparents and three progeny of the USA laboratory strain(Fig. 3a), and from a pair of male and female parents andfour progeny of the Japanese laboratory strain (Fig.3b). Six fragments of 20.0, 5.8, 4.9, 4.7, 3.6, and 2.9 kbhybridized strongly and three fragments of 15.0, 8.0, and7.3 kb hybridized weakly in the male genome. Thisresult shows that

SlMF1

is a multicopy gene. The 5.8-kb fragment was detected only in the male genomic DNAsof both strains. This indicates that the 5.8-kb fragmentis linked to the Y chromosome.

Regardless of whetherthe plants were male or female, the 4.9- and 4.7-kb frag-ments were detected in some plants, but were notdetected in others. This suggests that these fragmentsare autosomal or pseudoautosomal.

To determine the X chromosomal location of

SlMF1

, weperformed PCR with flow-sorted X chromosomes andautosomes using the SlMF1-F and SlMF1-R primers forthe 390-bp region of

SlMF1

cDNA. This PCR methodcan clearly confirm X chromosome linkage without theeffects of contaminated autosomes in without the effectsof contaminated autosomes in the flow-sorted X chro-mosomes as described previously (Kejnovsky et al. 2001).When PCR was performed using male and femalegenomic DNA, a single 290-bp fragment was amplified(Fig. 3). This genomic sequence is completely consistentwith the cDNA sequence, indicating that the region hasno intron. The fragments could be amplified from bothflow-sorted X chromosomes and autosomes. The ampli-fied sequences from both X chromosomes and autosomeswere completely matched. The amplified sequenceswere also consistent with those from both male andfemale genomes.

DISCUSSION

SlMF1

was expressed in all the whorls of young flowerbuds; however, its expression was limited to the petalsand anthers of mature male flower buds and the petals ofmature female flower buds. This expression profile sug-gests that

SlMF1

functions not only in the formation ofyoung flower buds but also in the development of anthersand petals during maturation of the flower buds. Thefirst morphological difference between males and femaleswas found in the size of the fourth whorl of flower budsat stage 5 (Grant et al. 1994). Before morphological sexdifferences are clearly visible, certain differences in geneexpression patterns are observed between male andfemale flower buds at stage 3. In situ hybridizationanalysis with two orthologs of floral identity genes–a PIS-TILLATA

homolog

SLM2

and an

APETALA3

homolog

SLM3–

revealed that these are expressed more closely

toward the center of the fourth male whorl compared withthe fourth female whorl at stage 3, suggesting that thesetwo genes correlate with a reduction in the size of thefourth whorl (Hardenack et al. 1994). In contrast,

SlMF1

was expressed in all the whorls of both male andfemale flower buds at stage 3. This result suggests that

SlMF1

is not directly involved in sex determination. Atstage 9 of the male flower bud, pollen mother cells andtapetal cells begin to develop in the anther and petal pri-mordia start to elongate rapidly. Two cell divisiongenes,

SlH4

and

SlCycA1

, were detected in both theanthers and petals, indicating that cells in the anthersand petals divide actively at this stage (Matsunaga et al.2004). In particular, dividing cells could be detected inthe upper region of the petal primordia. Consequently,the

SlMF1

transcript accumulated in cells in the upperregion of the petal primordia. This suggests that

SlMF1

functions in the development of anthers and petal primor-dia.

Until now, six functional Y-linked genes have beencharacterized in this species.

DD44

,

SlY1

,

SlY3

,

SlY4

,and

Slss

are housekeeping genes with X-linked paralogsthat are ubiquitously expressed in both sex organs (Mooreet al. 2003; Delichere et al. 1999; Atanassov et al. 2001;Nicolas et al. 2005; Filatov et al. 2004). Only

SlAP3Y

has no X-linked homolog; however, it has an autosomalparalog (Matsunaga et al. 2003). The expressed sequ-ence tags (ESTs)–Men-153 and Men-470 of two maleflower buds have Y-linked homologous sequences (Scuttet al. 2002). When Southern hybridization was per-formed with each EST as a probe, male-specific fragmentscorresponding to Y-linked sequences were detected.However, the full-length cDNAs of these ESTs have notbeen reported. Genomic Southern hybridization with

SlMF1

shows a pattern similar to that observed withthese ESTs. This indicated that

SlMF1

was a multicopygene with homologous sequences on autosomes and the Xand Y chromosomes. With the exception of individualpolymorphisms, the 390-bp nucleotide sequences thatwere amplified from RNA, genomic DNA, flow-sortedautosomes and X chromosomes, were completely matchedwith

SlMF1

cDNA sequences. Although we cannot com-pletely exclude the possibility that either the paralog onthe autosomes or the X chromosome may become apseudogene, it is highly possible that

SlMF1

has at leasttwo paralogs on autosomes and the X chromosome. If so,these paralogs must have duplicated recently because nodifference was observed in the 290-bp sequences betweenautosomal and X-linked

SlMF1

. Quantitative RT-PCRshowed that the

SlMF1

transcript was detected not onlyin male anthers and petals but also in female petals.This result demonstrates that

SlMF1

is not derived from

only a Y-linked gene. Moreover, we could not determinewhether or not the 5.8-kb Y-linked sequence seen inSouthern hybridization analyses is expressed.

Charac-

400 S. MATSUNAGA et al.

terization of the X- and Y-linked genomic fragments infurther studies will provide information regardingwhether the sex chromosome-linked genes arefunctional. Moreover, evolutionary analyses throughcomparison between the X- and Y-linked sequences of

SlMF1

will reveal the degree of degeneration of the Ychromosome.

This work was supported by Grants-in-Aid for ScientificResearch to S. M. (No. 16770049) from the Ministry of Educa-tion, Science, Culture, Sports, Science and Technology, Japan,and by Grant Agency of the Czech Republic (No. 204/05/2097).

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