Molecular and cytogenetic characterization of aposporous-apomixis in Pennisetum squamulatum
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Apomixis is an asexual reproduction system through which plants propagate themselves through seeds without fertilization. Progenies are genetically identical to their maternal parent, morphologically uniform, and stable between generations. Employment of apomixis could revolutionize plant breeding programs and hybrid production. Unfortunately, apomixis has not been reported in any of our major crops. The transfer of apomixis from wild relatives into crops was hindered by linkage drag of undesired traits for the last 30 years. Molecular studies indicated that aposporous embryo sac development, one type of apomixis, was associated with a complex chromosome unit, which was defined as an Apospory-Specific Genomic Region (ASGR) in Pennisetum squamulatum, a relative of pearl millet (P. glaucum). Serious suppression of recombination was observed in the ASGR. Molecular and cytogenetic studies were carried out to investigate the extent of recombination suppression over the genome of P. squamulatum, to identify the ASGR-carrier chromosome, and to follow the transfer of this chromosome through multiple backcross (BC) generations. A linkage map was established for the polyploid genome of P. squamulatum with amplified fragment length polymorphism (AFLP) markers. The linkage map comprised 182 single-dose markers, 44 linkage groups, and covered 2600 cM. The estimated genome size of P. squamulatum was 4632 cM with an expected genome coverage of 73.7%. Four AFLP markers associated with the trait for aposporous embryo sac development were clustered between close flanking markers on a single linkage group. The total map distribution of AFLP markers indicated that recombination suppression was limited to the ASGR. One chromosome from P. squamulatum that carried the ASGR was identified with fluourescence in situ hybridization (FISH) using apomixis- linked markers. The ASGR was located on the distal end of a metacentric chromosome. There was no other chromosome in P. squamulatum that showed hybridization signal indicating the hemizygosity of the ASGR in P. squamulatum. Using genomic in situ hybridization (GISH), three chromosomes that hybridized with P. squamulatum genomic DNA were detected in a BC3 generation with a recurrent background of tetraploid pearl millet. The P. squamulatum chromosomes in BC5, BC6, and BC7 ranged from one to three. The expression of aposporous embryo sac development was associated with the ASGR-carrier chromosome and no change in reproductive phenotype was detected with the presence of other P. squamulatum chromosomes. The P. squamulatum chromosomes in backcrosses behaved as univalents and no associations between P. squamulatum chromosomes themselves or with P. glaucum chromosomes were observed.