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GENETICS, GENOMICS, AND MOLECULAR EVOLUTION

High-Resolution Physical Mapping in Pennisetum squamulatum Reveals Extensive Chromosomal Heteromorphism of the Genomic Region Associated with Apomixis¹

Department of Horticulture (Y.A., J.A.C., S.G., P.O.-A.) and Department of Crop and Soil Sciences (W.W.H.), University of Georgia, Tifton, Georgia 31793–0748; and Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843 (D.T.M., J.E.M)

Gametophytic apomixis is asexual reproduction as a consequence of parthenogenetic development of a chromosomally unreduced egg. The trait leads to the production of embryos with a maternal genotype, i.e. progeny are clones of the maternal plant. The application of the trait in agriculture could be a tremendous tool for crop improvement through conventional and nonconventional breeding methods. Unfortunately, there are no major crops that reproduce by apomixis, and interspecific hybridization with wild relatives has not yet resulted in commercially viable germplasm. Pennisetum squamulatum is an aposporous apomict from which the gene(s) for apomixis has been transferred to sexual pearl millet by backcrossing. Twelve molecular markers that are linked with apomixis coexist in a tight linkage block called the apospory-specific genomic region (ASGR), and several of these markers have been shown to be hemizygous in the polyploid genome of P. squamulatum. High resolution genetic mapping of these markers has not been possible because of low recombination in this region of the genome. We now show the physical arrangement of bacterial artificial chromosomes containing apomixis-linked molecular markers by high resolution fluorescence in situ hybridization on pachytene chromosomes. The size of the ASGR, currently defined as the entire hemizygous region that hybridizes with apomixis-linked bacterial artificial chromosomes, was estimated on pachytene and mitotic chromosomes to be approximately 50 Mbp (a quarter of the chromosome). The ASGR includes highly repetitive sequences from an Opie-2-like retrotransposon family that are particularly abundant in this region of the genome.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.033969.

^* Corresponding author; e-mail ozias{at}tifton.uga.edu; fax 229–386–3356.

Received September 28, 2003; returned for revision January 7, 2004; accepted January 7, 2004.

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