Hop Breeding And Selection is the process by which plant breeders develop new hop cultivars. Many forces play a role in driving the continuous development of new hop cultivars. These include the introduction of new pests and pathogens, the evolution of a pest or pathogen to overcome plant resistance, changing market conditions, entirely new market opportunities, and a desire to minimize the environmental impact of hop production practices. Constant change is normal, so hop breeders engage in an ongoing effort to alter currently available elite cultivars to satisfy the hop industry and to broaden the spectrum of hop flavors available to brewers by breeding entirely new and genetically distinct hop cultivars.
Many believe that hop cultivation began in central Europe in Bohemia (now part of the Czech Republic), Slovenia, and Bavaria, Germany, during the 8th and 9th centuries. Hop cultivation spread to other continents, such as North America and Australia, as Europeans migrated there. Most likely, early hop cultivars were not the product of deliberate breeding efforts but careful selections by hop growers of indigenous wild hops until they had found varieties that were suited for local growing conditions. In time, a select few genotypes became dominant because they possessed the characteristics most sought after by local brewers and growers. Eventually, hops became known as types named after the location where they were found or grown, such as Hallertauer, Tettnanger, or Saaz. Systematic plant breeding programs eventually developed in the late 19th century to replace the simple local selection practices of hop farmers. This was the beginning of formal hop cultivar development and the proliferation of specialized hop varieties.
The earliest known organized hop breeding effort was made in Germany in 1894 and again in 1898. The United States Department of Agriculture (USDA) attempted to establish a hop breeding program in 1904 and 1908 that showed some promise but eventually was abandoned because of federal budget cuts, the onset of World War I, and Prohibition.
Humulus genetic material that is available for hop breeders to work with has a long and global history.
Cultivated hop is dioecious, which means that male and female reproductive organs are borne on separate plants. Hops, just as humans, have one set of chromosomes that determine the sex of the individual. If a plant has an XY chromosome pair, it is male; if it has an XX chromosome pair, it is female. Only female hop plants are used in commercial cultivationbecause only they produce the cones used in brewing. Male genotypes are only used in plant breeding programs because they do not produce cones.
Effective plant breeding in a cross-pollinated species like the hop relies partly on an understanding of relatedness among individuals so that appropriate crosses can be planned to minimize inbreeding. Inbreeding, the mating of related individuals, can uncover negative traits that may result in agronomically unacceptable individuals. Because breweries with successful brands prefer replacement hop cultivars that behave functionally or organoleptically much like the ones they are currently using in their recipes, hop breeders often have to work within narrow genetic confines when developing new cultivars. The danger in this approach is that cultivars can become too closely related, which can hamper future breeding efforts and render the pool of currently grown cultivars susceptible to catastrophic events such as a devastating outbreak of disease or pests. To minimize the chances of such negative outcomes, hop breeders assess the genetic diversity among the genotypes in their breeding collection by evaluating pedigree records and variation in plant traits and using modern molecular biology tools such as genetic markers. This allows breeders to select unrelated parents for crossing in the hope that they will produce genetically diverse progeny.
Although pedigree records are useful for assessing the relatedness among hop genotypes, this information is not always reliable because the origins of many older hop cultivars are not known for certain, especially if they have unidentified indigenous wild plants in their genetic background. Also, some cultivars were developed by open pollination, a technique where the breeder does not pollinate a female flower with a known male pollen source but rather pollination occurs naturally from random males. Although this technique has been used successfully to create genetically diverse progeny, such as Galena, the anonymity of the male parent can complicate future parental selection decisions when using pedigree information.
Molecular biology has provided plant breeders with powerful tools to elucidate genetic structures within available breeding lines and cultivars. Among the tools now available are DNA sequence markers, relatedness computations between individuals, genotype fingerprinting for identification, and gene deactivation to suppress undesirable traits in individuals that might be desirable otherwise. Molecular DNA markers associated with a particular trait allow the hop breeder to select for desirable characteristics much earlier in the plant’s life cycle than traditional selection techniques. This can speed up the breeding process and limit expensive field evaluations of individuals known to carry desirable genes. Although hop breeders and researchers currently do not perform genetic manipulations, that is, the insertion of non-hop DNA into hop plants for commercial production, they do use a wide array of genetic analysis tools to make the breeding process more efficient and successful.
In recent years, a number of hop researchers have analyzed DNA extracted from numerous cultivars, breeding lines, and male genotypes in an effort to more fully understand the ancestry of the current breeding stock. This research has shown that, in general, modern hop accessions fall into one of two categories: those with a European-based ancestry and those that are hybrids between European and wild American ancestry. Within these two larger groups, a number of smaller subgroups have been identified based on traits such as sex or regional adaptation. This research has allowed hop breeders to corroborate pedigree records and, in some cases, even clarify disputed ancestry lines. These advancements have improved the planning of appropriate crosses to minimize inbreeding and to maximize potential heterosis (hybrid vigor).
Heterosis is a genetic concept where progeny from a cross outperforms both parents for a given trait. Historically this technique does not appear to have been used much in hop breeding, although it has proven useful in other crops, such as corn, and may benefit hop breeding programs in the future. Heterosis typically occurs when unrelated genotypes are mated and produce diverse progeny with unique gene combinations. The genetic assumption is that the unique alleles (a form of gene) that are brought together biochemically interact in such a way as to enhance the expression of the desired trait. Such traits may be yield, vigor, or disease resistance. Together with pedigree records and molecular marker-based genetic distance estimates, this technique is now considered to have some merit for hop breeding.
A number of hop research programs have begun creating genetic linkage maps that can be used to physically locate—or “map”—important traits on hop chromosomes. Genetic mapping is an important and powerful statistical tool that places numerous molecular markers in linkage groups. Mapping can tie each linkage group to a particular chromosome. Once the order and arrangement of the molecular markers are known, the resulting map can be used by plant breeders and geneticists to associate traits such as disease resistance, which are normally controlled by only one or a few genes, with a marker and then mapped to a physical chromosome location. This allows hop breeders to simply select for the associated marker in the seedling stage, perhaps during the winter in a greenhouse, and be reasonably certain that the selected individuals will be resistant to the respective disease or pest when grown under field conditions.
Some traits, however, are governed by the expression of many genes in combination, and several of these genes may interact in complex ways to influence the trait. Yield is the classic example of a genetically complex trait controlled by many genes. Because markers are physically mapped to a specific chromosomal region, they can be associated with gene clusters that are involved in the expression of genetically complex traits. These types of markers are termed quantitative trait loci (QTL). They allow hop breeders to select individuals in the seedling stage that contain clusters of genes important for expressing the complex trait of interest. Mapping QTLs requires considerable research effort initially to identify and place molecular markers on a linkage map. The greater the number of markers placed on the genetic map, that is, the higher the marker density, the more easily breeders can associate gene clusters with specific markers that are useful in the selection process. Thus, as molecular marker technology improves, so will the quality of the genetic maps constructed with marker technology. Ultimately, molecular markers based upon DNA sequence information will likely predominate, and this will give hop breeders new, powerful techniques for selecting desirable genotypes.
Plant transformation(or genetic modification) may also prove useful in hop breeding, should public opinion ever favor such an approach. Generating a genetically modified organism (GMO) involves inserting a piece of DNA into the host plant, usually via a ballistic device or a bacteria vector. The inserted DNA fragment may be from an entirely different organism or from the host species itself but with some modification to its function. This technique is useful in situations where a desirable individual is deficient in some critical trait such as disease resistance, but is otherwise desirable in its other traits. If a known resistance gene is available, it can be inserted into the desired individual and the transformed individual gains disease resistance to the target pathogen. Transformation can also be used to insert DNA fragments that suppress the expression of undesirable genes. However, current public sentiment would likely not support beer made with GMO hops. Therefore, it remains an open question whether this technology will find use in hop breeding.