Chapter 5 Genetic Analysis of Apomixis - cimmyt

F.... S....lity to ""'Iis: Maleu. oat! Geoetk ",....., 181Arabidopsis, relatively little is known about thegenetic control of megasporogenesis. Recently,the isolation of 270 Arabidopsis mutants withdefective spore development (megasporogenesis-defective,msd) was reported (Schneitzet al. 1997). Mutants of the msd class do notproduce a megagametophyte, however,sporophytic ovule development proceedsnormally. The developmental defects duringmegasporogenesis have not been characterizedin detail. All of these mutants also affectmicrosporogenesis and, therefore, are maleand female sterile. They may affect meiosis perse rather than female specific processes.Usually, only a single archesporiaI cell, andconsequently a single megasporocyte,differentiates in an ovule. However, theoccurrence of multiple megaspore mother cellsin some species (Eames 1961; Walters 1985;Sumner and van Caseele 1998) and of twomegasporocytes in abou t 5% of the wild typeArabidopsis ovules suggest that severalnucellar cells have the potential to enter themeiotic pathway. Once a cell is committed, itappears to inhibit neighboring cells fromdoing the same (Grossniklaus and Schneitz1998). This view is supported by a recentlyidentified mutant in maize. Plantshomozygous for multiple archegonial cellsl(mac1) contain between three and 21megasporocytes in a Single ovule (Sheridanet al. 1996). Thus, mad is only likely to beinvolved in megaspore mot~~r celldetermination. The phenotype shows certainsimilarities to apospory, in which multipleaposporic initials form around the sexualmegaspore mother cell. However, unlike inapomicts where microsporogenesis is usuallyunaffected, mad mutants also show abnormalmale sporogenesis (Sheridan et al. 1999).The genetic regulation of meiosis has beenextenSively studied in maize and the yeastSaccharomyces cerevisiae (Golubovskaya 1979;Golubovskaya et al. 1992; Mitchell 1994;Roeder 1995). In yeast, a large body ofknowledge on the molecular mechanismscontrolling meiosis has been amassed. Manyyeast mutants that regulate the entry intomeiosis and differentiate between meiotic andmitotic division have been isolated. Thesemutants share some characteristics withapospory or diplospory of the Antennaria type(Koltunow 1993), and their plant homologscould be instrumental in the engineering ofapomixis.Many genes that play roles in yeast meiosishave been characterized, generally byidentifying mutants with specific meioticdefects and studying the level of transcriptsof the corresponding genes during meiosis(Mitchell and Bowdish 1992; Mitchell 1994).These meiotic genes act at different stages ofmeiosis. Genes acting early in the pathway andregulating the decision between mitotic andmeiotic divisions are of particular interest.Among the products of early meiotic genes,the meiotic activator IMEl is a master controlgene required for the expression of the genesacting in the early phase of meiosis (Kassir etal. 1988; Smith and Mitchell 1989; Mitchell etal. 1990; Kawaguchi et al. 1992). To befunctional, IMEl has to becomephosphorylated by RIMll (Bowdish et al.1994). Upon phosphorylation, the earlymeiotic genes are activated and a starveddiploid cell undergoes meiosis to produce fourhaploid spores. In the fission yeastSchizosaccharomyces pombe, the Mei3 gene isinduced by nutrient deprivation. Mei3 inhibitsthe protein kinase Pat1, that then triggers theentry into meiosis (reviewed by Yamamoto1996). The Patl kinase, in tum, represses theMei2 protein, which is an essential positivefactor for entry into meiosis. Thus, regulatorynetworks involving phosphorylation and dephosphorylationevents responding toenvironmental signals playa crucial role.in thecommitment to the meiotic pathway.