Arabidopsis female meiotic mutant

The life cycle of plants alternates between a haploid gametophyte and a diploid sporophyte. In higher plants the gametophyte is reduced to a small number of cells surrounded by sporophytic tissue. The female gametophyte is contained within the ovule and consists of an embryo sac which harbors the egg cell and other associated haploid cells required for fertilization and development of the embryo sac and endosperm (Maheshwari, 1950; Willemse and van Went, 1984; Reiser and Fisher, 1993). The molecular mechanisms underlying female gametophyte development have begun to be elucidated with the identification of genes that act in the gametophyte and are necessary for its development (Drews et al., 1998; Grossniklaus and Schneitz, 1998). Mutations in genes that are required in the haploid phase of female gametophyte development would be expected to cause semisterility and show non-Mendelian segregation arising from reduced transmission through the affected sex. Female gametophytic mutants affecting development of the embryo sac have been identified in both Arabidopsis and maize (Singleton and Manglesdorf, 1940; Nelson and Clary, 1952; Redei, 1965; Kermicle, 1971; Castle et al., 1993; Kieber et al., 1993; Niyogi et al., 1993; Springer et al., 1995; Feldman et al., 1997; Moore et al., 1997; Christensen et al., 1998). Another class of gametophytic mutants that affect embryo and endosperm development have also been identified (Ohad et al., 1996; Chaudhury et al., 1997; Grossniklaus et al, 1998). The recent cloning of gametophytic genes affecting female gametophyte development (Springer et al, 1995), as well as genes affecting embryo and endosperm development (Grossniklaus et al., 1998; Luo et al., 1998; Ohad et al., 1999) has provided valuable insights into the molecular events underlying this fascinating stage of the plant life cycle. Less is known about genes that are involved in the control and elaboration of early steps in the female reproductive pathway, during megasporogenesis. The first identifiable step in the reproductive pathway at the anatomical level is the enlargement of a single cell in the subepidermal layer at the tip of the ovule primordium to form an archesporial cell. In Arabidopsis the archesporial cell directly forms the megaspore mother cell 197 Development 127, 197-207 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV0267