Sex-Determining Mechanisms in Land Plants - Barley World

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Sex-Determining Mechanisms in Land Plants - Barley World

S62The Plant CellFigure 1. The Life Cycle of the Liverwort Marchantia polymorpha.Haploid gametophytes develop gametangia in antheridiophores and archegoniophores that produce the sperm and egg, respectively. Uponfertilization, which is facilitated by raindrops, the diploid sporophyte remains attached to the archegoniophore and produces yellow sporangia, inwhichhaploid spores are formed after meiosis. The spores are liberated and germinate to form a new gametophyte thallus. The sex of the thallus depends onwhich sex chromosome it inherits. Photographs courtesy of Katsuyuki Yamato, Kyoto University.(2000) set out to identify these factors by constructing separatemale and female P1-derived artificial chromosome (PAC)libraries and identifying clones specific to either the male or thefemale genome. Their screen resulted in 70 male-specific PACclones that hybridized only the Y chromosome by fluorescence insitu hybridization. No female-specific clones were found, indicatingthat the X chromosome does not harbor long stretchesof unique sequences, as does the Y chromosome. To date, twomale-specific PAC clones with insert sizes totaling 126 kb havebeen sequenced (Okada et al., 2001; Ishizaki et al., 2002). Thisand other analyses have revealed that approximately one-fourthto one-third of the 10-Mb Y chromosome of Marchantia consistsof an estimated 600 to 15,000 copies of an element of variablelength (0.7 to 5.2 kb) that contains other smaller repetitiveelements (Okada et al., 2001; Ishizaki et al., 2002). Of the sixputative protein-encoding genes found embedded within therepeats, all are present in multiple copies on the Y chromosomebased on DNA gel blot hybridization. Two of these genes, namedORF162 (Okada et al., 2001) and M2D3.5 (Ishizaki et al., 2002),are unique to the Y chromosome; the remaining four genes arepresent in low copy number on the X chromosome or theautosomes. ORF162 encodes a putative protein with a RINGfinger domain; M2D3.5 is a member of the same gene family.ORF162 transcripts are detectable only in the male sexualorgans, indicating that the gene family represented by ORF162and M2D3.5 may be important in the development of theantheridiophore. Of the four genes also present on the Xchromosome or the autosomes, only one (M2D3.4) is restrictedin its expression to the male gametophyte, indicating that theM2D3.4 X or automosmal homolog might be a pseudogene.M2D3.4 encodes a putative protein similar to a Lilium longiflorumgene that is expressed exclusively in the male gametic cells. Theremaining three genes are not sex specific in their expression.The functions of the Y chromosome–encoded genes are as yetunknown.Although only a small portion of the Marchantia Y chromosomehas been sequenced, it is sufficient to make meaningfulcomparisons with the euchromatic male-specific region (MSY)of the human Y chromosome, the sequence of which waspublished recently (Skaletsky et al., 2003; see also Hawley,2003). The sequence of the human MSY region and limitedcomparative sequencing of the MSY regions in great apes(Rozen et al., 2003) have added new insights to our understandingof how the mammalian testis gene families on the Ychromosome have been maintained over the course of evolution.As will be shown, the similarities between the human andliverwort Y chromosomes are striking and may reflect a commonmechanism underlying the evolution of the Y chromosome inthese two disparate organisms. The MSY region of the human Ychromosome is made up of three classes of sequences: Xtransposed, X degenerate, and ampliconic, the latter representing30% of the MSY euchromatin. Because the Marchantia Xchromosome has not been sequenced, it is only possible tomake comparisons between the human ampliconic sequences,which are Y specific, and the Marchantia Y chromosomesequences. Like the Marchantia Y chromosome sequences,the ampliconic regions of the MSY consist of highly repetitivesequences unique to the Y chromosome, although the sizes,sequences, and stoichiometries of the repetitive elements veryconsiderably between the two species. Protein-encoding genes

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