Academic journal article Genetics

Molecular Cytogenetic Characterization of the Antirrhinum Majus Genome

Academic journal article Genetics

Molecular Cytogenetic Characterization of the Antirrhinum Majus Genome

Article excerpt

ABSTRACT

As a model system in classical plant genetics, the genus Antirrhinum has been well studied, especially in gametophytic self-incompatibility, flower development biology, and transposon-induced mutation. In contrast to the advances in genetic and molecular studies, little is known about Antirrhinum cytogenetics. In this study, we isolated two tandem repetitive sequences, CentA1 and CentA2, from the centromeric regions of Antirrhinum chromosomes. A standard karyotype has been established by anchoring these centromeric repeats on meiotic pachytene chromosome using FISH. An ideogram based on the DAPI-staining pattern of pachytene chromosomes was developed to depict the distribution of heterochromatin in the Antirrhinum majus genome. To integrate the genetic and chromosomal maps, we selected one or two molecular markers from each linkage group to screen an Antirrhinum transformation-competent artificial chromosome (TAC) library. These genetically anchored TAC clones were labeled as FISH probes to hybridize to pachytene chromosomes of A. majus. As a result, the relationship between chromosomes and the linkage groups (LGs) in Antirrhinum has been established.

THE genus Antirrhinum (2n = 16) has >20 species, most found around the Mediterranean Sea and in North America (STUBBE 1966). Among them, only Antirrhinum majus has been domesticated as an ornamental. Most species of the genus Antirrhinum are characterized by gametophytic self-incompatibility, regulated by a single multialleleic S locus (EAST 1940), which has been used as a model system for studying self-incompatibility (XUE et al. 1996; LAI et al. 2002; MA et al. 2002; ZHOU et al. 2003; QIAO et al. 2004).

Mutation instability and variegation have been well documented in A. majus, which led to the isolation of the first transposon in plants and its further application in gene tagging (reviewed by SCHWARZ-SOMMER et al. 2003a). In the first decade of the twentieth century, Baur and his colleagues isolated some unstable mutations in A. majus. During the 1950s to the 1970s, Harrison's group began to work with the unstable mutants related to two genes, NIVEA (NIV) and PALLIDA (PAL; STICKLAND and HARRISON 1974; HARRISON and CARPENTER 1979). By cloning NIV from an unstable niv allele, Hans Sommer's group was the first to isolate an autonomous transposon, Tam1, in plants (BONAS et al. 1984). Afterward, several other transposons were isolated from the NIV locus including Tam3, which was in turn used as a molecular tag to isolate PAL (MARTIN et al. 1985; SOMMER et al. 1985). The accumulated information on various transposons in Antirrhinum provided the basis for a better understanding of the function and structure of transposons in maize (SCHWARZ-SOMMER and SAEDLER 1985). Moreover, several genes, especially those related to flower development, have been cloned in A. majus using the transposon tagging system (COEN et al. 1990; SOMMER et al. 1990; BRADLEY et al. 1996).

The first classical genetic map of A. majus covered a genetic distance of ~420 cM, including 57 morphological markers scattered on eight linkage groups (STUBBE 1966). A molecular marker-based genetic linkage map has been constructed recently on the basis of an F^sub 2^ population of 92 individuals derived from an interspecific hybrid A. majus × A. molle (SCHWARZ-SOMMER et al. 2003b). These maps, together with its relatively small genome size, make positional cloning feasible in Antirrhinum (LAI et al. 2002).

In contrast to the significant advances in the genetic and molecular study of A. majus, very limited work has been done on the karyotype and cytogenetic structure of the A. majus genome. The genetic linkage groups have not been integrated into individual chromosomes. In recent years, extensive molecular and genomic resources have been established in A. majus (SCHWARZ-SOMMER et al. 2003a). Bacterial artificial chromosome (BAC) and transformation-competent artificial chromosome (TAC) libraries representing the Antirrhinum genome are available now. …

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