We maintain an onion-breeding program and are committed to the genetic improvement of onion using classical and molecular approaches. In an average year, we plant over 700 four-meter observation plots to produce and evaluate bulbs for inbred development, genetic studies, and seed increases. We are grateful to Gary and Corey Kincaid, Palmyra WI, for providing field space every year. For seed production, bulbs harvested the previous year are selected based on desired attributes or defined genetic relationships and planted at Arlington, WI, to produce seed. We generally plant at least 30 cages (larger seed increases or testcrosses) and 200 breeding plots (small self or mass pollinations). Our crosses and seed increases are done using flies. In 1999, we released three male-sterile and maintainer pairs (B1717, B1828, and B2354) for use in hybrid production. In 2007 we released the first male-sterile and maintainer inbred of red onion, a male-sterile and maintainer pair selected from the very early Japanese population ‘Sapporo-Ki’, and a synthetic population with the highest reported level of gynogenic haploid production. The vast majority of hybrid-onion seed is produced using a source of cytoplasmic male-sterility (CMS) that traces back to a single plant identified in 1925 in Davis, CA. This is an undesirable state of cytoplasmic uniformity. We identified molecular markers distinguishing onion cytoplasms and assessed cytoplasmic diversity among the world collection of onion germplasm. In 1999, we recently released a unique source of CMS, conditioned by the cytoplasm of Allium galanthum backcrossed to the bulb onion. The benefit of this new alloplasmic source of CMS is that no anthers are produced, making rouging in the field easier, as well as there are no known nuclear male-fertility restorers.
Even though the onion is the world’s second most economically important vegetable crop, little is known about its progenitor, relationships to wild relatives, and genetic diversity. We used molecular markers in the organellar and nuclear genomes to identify Allium vavilovii as the wild species most closely related to the bulb onion. We are presently cooperating with researchers in Turkmenistan to expand the world’s collection of this important wild species.
With support of major grants from the USDA, we developed the most detailed genetic maps of onion, comprised of morphological, RFLP, SSR, and SNP markers and developed the first set of onion ESTs. Our goal is develop genomic resources to answer basic questions in onion genetics and breeding.
Genetics of Production, Flavor, and Health-Enhancing Attributes of Onion
Replicated plantings of onion families were planted and bulbs produced over environments. We analyzed onion flavor (pungency), storage ability, solids (soluble and total), and in vitro antiplatelet activity. This latter attribute measures the ability of onion to inhibit the aggregation of platelets in blood, a major cause of heart attacks and strokes. We identified one major chromosome region explaining a significant proportion of the phenotypic variation for flavor, solids, and antiplatelet activity. Our long-term goal is to develop unique inbreds and hybrids for production of onion bulbs with acceptable flavor and production characteristics combined with defined heath-enhancing attributes.
Molecular-Facilitated Selection of Maintainer Lines in Onion
Using classical crosses, it takes at least four to eight years to determine the cytoplasm and nuclear genotype at the Ms locus of onion. We have developed a PCR marker distinguishing male-fertile and male-sterile cytoplasms of onion, reducing from years to a few hours the time required to establish cytoplasms. We identified molecular markers tightly linked to the nuclear Ms locus. We identified single nucleotide polymorphisms (SNPs) in the linked regions to Ms and developed PCR-based markers for easy detection.
The mitochondria of cucumber and melon are unique in that they possess the largest known genomes and show paternal transmission. We characterized the molecular basis of this huge genome expansion and revealed that small (about 100 bp), highly repetitive, dispersed sequences contributed significantly to the expansion of the cucumber mitochondrial genome. In a collaborative project, we used FISH analyses to reveal great structural diversity in the chloroplast DNA and the publication from this study was awarded “Best Paper in The Plant Cell” for the year 2001. We also study the genetic bases of unique mitochondrial mutants of cucumber conditioning strong mosaic (MSC) phenotypes, and a nuclear locus (Psm) that controls sorting of paternally transmitted mitochondrial DNAs.
Mapping Virus Resistances in Cucumber
Genes conditioning resistance to potyviruses are clustered in cucumber and were mapped using recombinant inbred lines developed from TMG1 x Straight 8. An AFLP which cosegregated with virus resistances was converted to a PCR-based marker.
Late-Blight Resistance in Mexican Diploid Solanum species
Late blight resistance and susceptibility were identified in Mexican diploid 1EBN species. Solanum pinnatisectum was crossed with the susceptible S. cardiophyllum and a BC family generated. RFLPs were identified on all chromosome arms and mapped late-blight resistance to chromosome 7 in a region not previously associated with late-blight resistance loci. We also showed that unilateral incompatibilities among Mexican 2x(1EBN) species is inherited independently of the S locus in Solanum.