The relatively low evolution rate of the chloroplast DNA has made it an ideal tool to study evolutionary processes in plants above the species levels. However, recent studies have demonstrated that intraspecific variation in the chloroplast DNA is also common. This variation has provided useful insights into population level evolutionary processes. The polymerase chain reaction and sequencing of a noncoding chloroplast region used to classify onion lines for cytoplasmic type facilitated the identification of one sterile and two normal plastome variants in onion (Allium cepa L.). Sequence comparison revealed that differences between plastome variants included the presence of single-nucleotide polymorphisms associated with cytoplasmic type and variable numbers of tandem repeats, possibly resulting from slipped-strand mispairing. Our observations support the use of chloroplast-specific markers to assist in the selection of specific cytoplasmic types, suggest the potential to facilitate genotype determination, and demonstrate the presence of additional variation within cytoplasm type which gives insight into plastome evolution and may facilitate more accurate genotyping and selection.
Josefina Alcalá, Leonard M. Pike and James J. Giovannoni
J. Alcala, J.F. Lopes, J.J. Giovannoni and L.M. Pike
Identification and production of onion male-sterile and maintainer lines by conventional breeding takes between 4 to 8 years, due in part to the biennial nature of onion. In addition, male sterile plants and maintainer genotypes occur at a very low frequency in onion populations (Pike, 1986). A significantly shorter and more efficient alternative involves the screening of breeding lines for cytoplasmic male sterility using PCR-based technology. Thirty short-day onion breeding lines from the Texas A&M onion program were screened for type of cytoplasm (normal or sterile). Specific amplification of a fragment of chloroplast genome was achieved using the polymerase chain reaction according to Havey (1991). Forty-eight individual onion plants were screened per line. Out of thirty lines evaluated, 13 showed 100% sterile cytoplasm, 6 showed 100% normal cytoplasm, and 11 showed both types of cytoplasm. Lines showing normal cytoplasm or both cytoplasmic types were kept and reanalyzed. Only plants presenting normal cytoplasm were grown to maturity to help in the identification of maintainer lines as part of the Texas A&M onion breeding program.
J. Alcalá-Sáinz, K.S. Yoo, L.M. Pike and R.W. Jones Jr.
Fifteen shortday onion cultivars grown at two production locations (GB and ST) in the Lower Rio Grande Valley, Texas were evaluated for pungency levels using gas chromatography (GC) and pyruvic acid tests.
Significant differences (P=0.05) were observed between cultivars in the pyruvic acid and GC tests within each location. Pyruvic acid content ranged from 3.0 to 5.1 μmol·g-1 fresh wt. The amount of total sulfur volatiles measured by the GC method ranged from 28 × 103 to 58 × 103EU. The correlation coefficients between GC and pyruvic acid were 0.10*** and 0.18*** at the GB and ST location, respectively.
When the two locations were combined, no significant differences (P=0.05) were observed between cultivars or locations using the GC test. However, the pyruvic acid test showed significant differences between locations. This result indicated that each cultivar had a different response in pungency as influenced by production location or environment.