Academic journal article Genetics

The Evolution of the SEPALLATA Subfamily of MADS-Box Genes: A Preangiosperm Origin with Multiple Duplications throughout Angiosperm History

Academic journal article Genetics

The Evolution of the SEPALLATA Subfamily of MADS-Box Genes: A Preangiosperm Origin with Multiple Duplications throughout Angiosperm History

Article excerpt

ABSTRACT

Members of the SEPALLATA (SEP) MADS-box subfamily are required for specifying the "floral state" by contributing to floral organ and meristem identity. SEP genes have not been detected in gymnosperms and seem to have originated since the lineage leading to extant angiosperms diverged from extant gymnosperms. Therefore, both functional and evolutionary studies suggest that SEP genes may have been critical for the origin of the flower. To gain insights into the evolution of SEP genes, we isolated nine genes from plants that occupy phylogenetically important positions. Phylogenetic analyses of SEP sequences show that several gene duplications occurred during the evolution of this subfamily, providing potential opportunities for functional divergence. The first duplication occurred prior to the origin of the extant angiosperms, resulting in the AGL2/3/4 and AGL9 clades. Subsequent duplications occurred within these clades in the eudicots and monocots. The timing of the first SEP duplication approximately coincides with duplications in the DEFICIENS/GLOBOSA and AGAMOUS MADS-box subfamilies, which may have resulted from either a proposed genome-wide duplication in the ancestor of extant angiosperms or multiple independent duplication events. Regardless of the mechanism of gene duplication, these pairs of duplicate transcription factors provided new possibilities of genetic interactions that may have been important in the origin of the flower.

MOLECULAR genetic studies over the past 2 decades have identified a large number of regulatory genes that control early floral development (MA 1994; WEIGEL and MEYEROWITZ 1994; ZHAO et al 2001; JACK 2004). In particular, genetic studies in Arabidopsis thaliana and Antirrhinum majus have led to the proposal of the genetic "ABC model" for specifying floral organ identity (CoEN and MEYEROWITZ 1991). Most of the genes required for the ABC functions are MADS-box genes that encode putative transcription factors (THEISSEN et al. 1996; BECKER and THEISSEN 2003). In Arabidopsis, the A function requires the APETALA/ (API) gene, B function needs both the APETALA3 (AP3) and the PISTILLATA (PI) genes, and the AGAMOUS (AG) gene is necessary for C function. In Antirrhinum, the functional homologs of AP3, PI, and AG are DEF, GLO, and PLE, respectively (CoEN and MEYEROWITZ 1991; MA 1994; MA and DEPAMPHILIS 2000). These ABC MADS-box genes are expressed in the floral meristem before or at the time of organ primordia initiation in regions corresponding to the genetically defined functional domains, indicating that the expression patterns of these genes are good predictors of functional domains (MA and DEPAMPHILIS 2000). Phylogenetic analyses indicate that the ABC genes and their close homologs form several well-defined subfamilies, whose evolutionary histories have been, or are currently being, studied phylogenetically (RRAMER et al. 1998, 2004; BECKER and THEISSEN 2003; LITT and IRISH 2003; KIM et al 2004; SOLTIS et al. 2005).

Although ABC genes are critical for floral organ identity, they are not sufficient to convert leaves into floral organs, indicating that other flower-specific genes are needed. More recent genetic and molecular studies in Arabidopsis indicate that four additional MADS-box genes, SEPALLATA1/2/3/4 (SEPl/2/3/4; formerly AGL2/ 4/9/3) (MA et al. 1991; HUANG et al. 1995; MANDEL and YANOFSKY 1998), are required for the specification of the identity of all four whorls of floral organs and for floral meristem determinacy (HoNMA and GOTO 2000; PELAZ et al 2000, 2001a,b; DITTA et al 2004). The Arabidopsis SEPl, -2, and -4 genes are expressed throughout the floral meristem at stage 2, slightly earlier than SEP3, which is expressed in a region corresponding to the inner three whorls just before the initiation of floral organ primordia (FLANAGAN and MA 1994; SAVIDGE et al. 1995; MANDEL and YANOFSKY 1998; DITTA et al. 2004). Subsequently, SEPl and SEP2 expression persist in all floral organ primordia and SEP3 is expressed in the inner three whorls, whereas SEP4 expression becomes more highly expressed in the central dome than in the sepals. …

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