Characterization of an unusual cytoplasmic chimera detected in bolting garlic clones

Abstract

Production of a visible flower stalk, or bolting, has been used as a major trait to categorize garlic (Allium sativum L.) clones. Analysis of mitochondrial genome variation with polymerase chain reaction (PCR) revealed differences between bolting and nonbolting clones of garlic. Screening 333 garlic accessions from diverse geographic origins revealed a 1403-bp mitochondrial DNA marker associated with bolting that the authors call "Bolt Marker" (BltM). Bolt Marker did not amplify in any of the 131 nonbolting clones, whereas amplification of this marker was observed in 127 of 130 (97.7%) garlic clones that bolted completely in Wisconsin. Seventy-two garlic clones bolted incompletely (clones in which some but not all of the plants bolted), and this marker was not amplified in 69 (95.8%) of these clones. Because of the significant association of BltM with bolting, this PCR-based marker can be used to discriminate complete-bolting garlic clones reliably from nonbolting and incomplete-bolting ones. Sequence characterization of this marker revealed that BltM is a chimera involving both mitochondrial and chloroplast DNA. The DNA sequences including and flanking both the 5' and 3' ends of this marker are consistent with an approximate to 4.8-kbp chloroplast DNA fragment having been inserted into the mitochondrial genome downstream from the mitochondrial cox3 gene. Sequence alignment of the chloroplast genes in this chimeric region with the homologous sequences in GenBank indicate the presence of deletions, insertions, and single nucleotide polymorphisms in the coding sequences, resulting in putative, incomplete open reading frames or frame shift mutations. Hence, the authors speculate that this insertion may have occurred long ago in the evolution of garlic.