A layman's look at the basics behind variety development
Mitch McGrath is research geneticist with the USDA-ARS Sugarbeet & Bean Research Unit, Michigan State University, East Lansing. This article is based on one appearing in the 2009 Research Trial Results booklet published by Michigan Sugarbeet REACH (Research & Education Advisory Council).
This primer is intended as an introduction to sugarbeets from a breeding standpoint. Topics include the history of the crop, the genetics of breeding and agronomic characters, and the potential impact of new technologies.
Breeding new varieties is a timeconsuming effort — often taking in excess of 10 years before germplasm with new characters is available to growers in the form of new varieties.
History of the Crop
Sugarbeet, one of the newer crops of significant economic importance, is a product of the Industrial Revolution in Europe. During the latter half of the 18th century, sucrose was discovered in the roots of red and white beets used for animal fodder. Subsequently, beets with higher sugar levels were selectively bred, measures for the cultivation of beets for sugar were described, an extraction process was developed, and the first European sugarbeet factories were constructed.
Sugarbeet is classified as Beta vulgaris, which includes fodder beet, red beet, Swiss chard and a variety of wild
forms found along European and Mediterranean coastlines. They are known as subspecies maritima types.
Most Beta vulgaris types are diploid with 18 chromosomes in each cell (though some sugarbeet varieties are triploids). There are few or no barriers to cross fertilization among these types. The maritima types have contributed some of the most valuable disease resistances, including resistance to Cercospora leafspot and the rhizomania virus — and have the potential to contribute a great deal more.
The majority of sugarbeet varieties grown today trace back to the early selections performed during the late 18th and early 19th centuries. They probably originated from fodder beets grown in Poland from a type known as White Silesian. At that time, fodder and red beets were grown for animal feed and human consumption, a practice dating back to the Middle Ages.
Leafy beets (similar in use to today’s Swiss chard types) were grown in the gardens of Babylon and ancient Egypt, and were the ancestors of all cultivated beets. During their early history, hybridization with wild beets undoubtedly occurred naturally. New types were probably selected from the progeny of those inadvertent outcrosses. But while a great deal of genetic variation exists within Beta vulgaris, the germplasm base of sugarbeet is relatively narrow.
Sugarbeet is biennial. Vegetative growth during the first year is geared toward bulking storage reserves (mainly sucrose in the roots) for the following year’s reproductive growth. Sugarbeets behave as a perennial if flowering is not induced. Induction of flowering occurs after a period of cool temperatures and long nights — a process known as vernalization. Vernalization, can which occur at any time during the plant’s growth, can be problematic for growers who plant their crop too early, leading to plants “bolting” in the field, accompanied in turn by a loss of sucrose yield.
In practice, beets harvested from selection plots are placed in a 4°C (40°F) cooler for 12 to 16 weeks toeffect vernalization. Flowering commences within five weeks after removing the plants from vernalization.
In most commercial U.S. seed production — which takes place almost exclusively in the coastal valleys of western Oregon — seeds are field planted in late summer, and plants vernalize in the field during winter with little risk of freezing (although it can happen). Flowering, seedset and seed harvest are complete by August of the next year in the field. In the greenhouse, it is possible to obtain seed for testing the year following field selection of mother roots.
Beets, which are wind pollinated, have perfect flowers. A complex selfincompatibility system serves to limit pollen germination and growth when it lands on its own flower; but there are a number of exceptions that allow for self-fertilization. These exceptions (e.g., pseudo-self-fertility, genetic selffertility) are often used for breeding purposes. In all cases, commercial seed is obtained in isolation plots separated by at least one mile from one another to prevent excessive pollen contamination from other varieties.
Beet seed is unusual from a botanical standpoint. The seed that is planted is actually the entire flower, which develops into a woody fruit. During seed processing, that fruit is polished, graded and, in today’s world, primed and coated. Priming occurs when seeds are imbibed and then dried before the radicle emerges. Within the fruit or seedball, one (monogerm) to five (multigerm) seeds will arise from the fusion of separate flowers borne in the leaf axils.
All commercial beet seed used in developed countries is monogerm. Monogermity is a single-gene character expressed by the seed parent. Multigerms are used as pollinators for hybrids due to their generally better vigor and ease of mass selection (for disease resistance in particular).
The monogerm character is one of the few recessive genes common in breeding programs. Two other recessive genes are required that result in CMS (cytoplasmic male sterility) in a sterile cytoplasm.
Incorporating these three genes is one of the bottlenecks in developing better seed parents for hybrid varieties. Prior to the development of CMS and maintainer lines 50 years ago, commercial varieties were openpollinated and multigerm.
Hybrids are made using a system of cytoplasmic male sterility. In this system, normal pollen development is disrupted by an unknown mechanism associated with a defect in the mitochondria (the energy-producing machinery of the cell). Mitochondria are inherited maternally; and in this case, only the seed parent will contain a sterile cytoplasm.
For CMS to be expressed, two genes present in the cell’s nucleus must be recessive. If either of those genes is dominant or the cytoplasm is normal, the plant will be pollen-fertile.
Generally, male-sterile CMS lines are maintained by crossing with a similar genotype with a normal cytoplasm. These are known as maintainer or Otype lines. For each CMS, there needs to be a corresponding O-type line.
In hybrid beet seed production, monogerm CMS pollen-sterile seed parents are interplanted with fertile monogerm or multigerm pollen donors. Seed is harvested exclusively from the CMS line. Because the monogerm trait is expressed by the seed parent, all hybrid seed will be monogerm.
Seed parents must possess at least four characteristics to be useful: monogerm, CMS, lacking two nuclear restorer genes, and have an O-type maintainer line.
A great deal of effort and expense is expended in identifying suitable seed parent lines. Coupled with the requirements for high sucrose and high tonnage yields, perhaps the most difficult phase of sugarbeet breeding is producing good seed parent lines. If disease resistance needs to be homozygous (i.e., two copies of the gene) in the hybrid, such traits also need to be incorporated in the seed parent.
Sugarbeet breeding for agronomic characters has relied mainly on mass selection. This strategy works well for
traits that are easily scored and relatively insensitive to environmental fluctuations.
A variation on this theme — recurrent selection — has been practiced to some extent. With this method, selections
are made and crossed with a common parent. The progeny are evaluated, and the best-performing families or lines are identified. Those seed parents whose progeny showed high performance are then intercrossed and advanced to another round of selection. Frequently, progeny testing occurs with a promising pollinator crossed with a series of CMS tester lines.
Performance is measured in various ways. Agronomic characters such as sucrose percent and yield are measured at the end of the growing season. Disease nurseries are employed to evaluate performance under disease pressure. Visual evaluations for the number of crowns and sprangled roots, relative vigor, color, shape and root smoothness are sometimes performed.
In general, the breeding data typically collected in many breeding programs have bee insufficient for examining the genetics of agronomic traits. Unfortunately, the available information is often dated or inadequate. So a re-examination of these questions with the more-precise methodologies available today should be a high priority. Here’s a brief summary of the number of genes controlling a trait, as well as their proposed mode of gene action:
• Sucrose — Percent sucrose in beets ranges from 4-6% in some wild species and up to 20% or more in elite sugarbeet germplasm. Vegetable beets (red beet and Swiss chard) are generally intermediate in sucrose concentration, commonly between 6-10%.
Increasing the percent sucrose in sugarbeet from “intermediate” to “high” levels probably occurred within the first 50 years of sugarbeet breeding. The inheritance of sucrose concentration is highly heritable and amenable to mass selection. Among crosses of sugarbeets and other types, it was inferred that three or four genes control sucrose concentration.
• Yield — Yield, expressed either as weight of the beet or per-unit area, is an unpredictable trait. Both high and low yielders can be retrieved from the progeny of either low-yielding or high-yielding beets, indicating nonadditive gene action. In practice, highyielding hybrids must be determined via trial and error by crossing seed parents with many prospective pollen parents to determining a parent’s combining ability. That is a laborious and expensive task.
• Disease Resistance — Disease resistance in beets is generally dominant in its expression, due to the nature of the breeding system and the reliance on mass selection as a breeding tool. Many resistances are controlled by dominant genes at more than one locus. For example, tolerance to the most prevalent type of Cercospora leafspot is controlled by at least five independent genes. Similarly, tolerance to Rhizoctonia root rot is controlled by two or more genes.
From these numbers it is clear that breeding for Rhizoctonia tolerance should be easier than breeding for Cercospora — and in practice, this does appear to be the case.
For other major diseases, however, the pattern of inheritance is not as clear. Exceptions include that of: (1) rhizomania resistance, where a single dominant gene is being widely used (e.g., the “Holly” gene, also known as Rz1), and (2) a single gene for resistance to the beet cyst nematode.
With the exception of Rhizoctonia tolerance, each of these resistances’ origin can be traced back to wild beets or other species. Resistance to other diseases (sometimes near-immunity) has been identified among the wild species. It is not clear whether these resistances are the same or different from the ones currently in use.
A great deal of effort is expended by public breeding programs such as USDA-ARS to identify and incorporate new sources of disease resistance into commercial-ready sugarbeet parents through germplasm enhancement. Enhanced germplasm is released to commercial seed companies for reselection and hybrid development.