Substoichiometric shifting in the fertility reversion of cytoplasmic male sterile pearl millet.

Cytoplasmic male sterility (CMS) represents an important agricultural trait in pearl millet [Pennisetum glaucum (L.) R. Br.] with a value to the seed industry in facilitating economical hybrid seed production. Among the CMS systems available in millet, the A1 source is the most commonly used for hybrid production, but it can undergo low frequency reversion to fertility. Plant mitochondrial genomes are highly recombinogenic, becoming unstable and prone to ectopic recombination under conditions of tissue culture, somatic hybridization, or interspecific crossing. Similarly, CMS systems prone to spontaneous fertility reversion experience sporadic mitochondrial genome instability. We compared mitochondrial genome configurations between the male-sterile A1 line and fertile revertants of pearl millet to develop a model for millet mitochondrial genome reorganization upon reversion. Relative copy number of a subgenomic molecule containing the CoxI-1-2 junction region, a component of the recombination process for reversion, is amplified tenfold following reversion, relative to the CMS A1 line. We propose that increased copy number of this molecule in a small number of cells or at low frequency triggers a recombination cascade, likely during reproductive development. The proposed recombination process initiates with ectopic recombination through a 7-bp repeat to produce a novel CoxI-3-2 junction molecule and an unstable recombination intermediate. Subsequent intra-molecular recombination stabilizes the intermediate to form a new copy of CoxI accompanied by a deletion. This study furthers the argument that substoichiometric shifting within the plant mitochondrial genome plays an important role in the evolution of the mitochondrial genome and plant reproductive dynamics.

Production and evaluation of a synthetic hexaploid in blueberry.

One synthetic hexaploid clone (Hex-DT) was produced by placing 3-node stem cuttings of triploid blueberry clone FL 81-19 [V. corymbosum (4x) x V. elliottii (2x)] on blueberry micropropagation medium supplemented with 0.6% agar, 24.6 µM 2ip, and 0.02% colchicine. The most effective chromosome-doubling was achieved with colchicine over 6 days, for 6 h per day. Pollen stainability and pollen germination in the hexaploid Hex-DT were 42.2% and 13.0%, respectively, versus 87.9% and 50.9% in Hex-F1, a hexaploid F1 hybrid between V. ashei (6x) x FL 81-19 (3x). The number of seedlings produced per flower pollinated with V. ashei pollen in Hex-F1 was similar to the number obtained from V. ashei x V. ashei crosses, and at least twice the number produced by Hex-DT pollinated with V. ashei pollen. Observed mean numbers of univalents, bivalents, trivalents, quadrivalents, and hexavalents per PMC at metaphase I in Hex-DT were 1.29, 20.30, 2.31, 2.63, and 2.11, respectively. No univalents were observed in Hex-F 1, but the mean frequencies of bivalents, trivalents, quadrivalents, and hexavalents were 29.14, 0.87, 1.91, and 0.58, respectively. Irregularities such as chromosome lagging and unequal disjunction were observed at both anaphase I and II in Hex-DT. Anaphases I and II in Hex-F1 were normal with few irregularities. The high fertility observed in Hex-F1 suggests a high level of homology among the three species contributing to its makeup.

Production and viability of unreduced gametes in triploid interspecific blueberry hybrids.

Three triploid (2n=3x=36) blueberry hybrids were obtained by hand-pollinating approximately 7,000 flowers of tetraploid highbush blueberry cultivars (based on Vaccinium corymbosum L.) with pollen from the diploid species V. elliottii Chapm. Meiotic analysis of these triploids revealed trivalents, bivalents and univalents in all metaphase I cells, with lagging chromosomes evident at anaphase I. Pollen of the three triploids was mostly aborted and did not stain with acetocarmine. However, the three triploids did produce from 0.9%-1.3% giant pollen grains that stained with acetocarmine and were present as monads, dyads or triads, rather than the normal tetrads. Pollination of 10,853 flowers of hexaploid V. ashei Reade cultivars with pollen from the triploids produced 266 berries, which averaged fewer than two fully-developed seeds per berry. One triploid clone showed partial female fertility when crossed to hexaploids, self-pollinated, or intercrossed with other triploids. Ploidy levels of the resulting hybrids were determined.