Use of genetic resources

The need to avoid complete genetic uniformity over a long period is obvious from the examples of agricultural pathogens I've already mentioned - corn leaf blight, wheat rust, potato blight. Are there more immediate concerns about conservation of genetic resources? Yes. Consider how important use of genetic resources has been in increasing agricultural production.

• Yields of many crops increased dramatically in the half century from 1930 to 1980.
  • Rice, barley, soybeans, wheat, cotton, and sugarcane - doubled

  • Tomato - tripled

  • Corn, sorghum, potato - quadrupled

• Plant breeders use of genetic diversity accounted for at least one-half of that doubling [3]

• Even maintaining productivity requires constant input of new genetic material to overcome crop losses due to pests that become pesticide resistant.

• Consider the humble tomato
  • Resistance to at least 32 major tomato diseases have been discovered in wild relatives of the cultivated tomato.

  • Genes responsible for promoting resistance to 16 of these have been bred into commercial cultivars, allowing tomato production in areas where they could not otherwise have grown.

  • Insect resistance, tolerance to temperature extremes, salinity tolerance, drought tolerance, and tolerance of waterlogging are among the traits expressed in wild relatives that may be useful in breeding commercial tomatoes.

  • The potential economic value is great.
    • High fruit solids are important for processing-type tomatoes because less water is processed and more paste is manufactured from each ton of tomatoes.

    • Each 0.1% increase could be worth as much as $10,000,000 per year to the California tomato processing industry.

    • Breeding lines between cultivated tomato and Solanum chmielewskii have the potential to increase fruit solids from 6.2% to 8.6%, an increase of 2.4% which would be worth nearly $250,000,000 per year.

• Overall crop breeding programs add at least $1 billion per year to the value of U.S. agricultural production.

• But the most spectacular example has to be associated with the ``Green Revolution.''<sup>10</sup>
  • A cultivar of wheat, ``Norin 10,'' from Japan was of shorter stature than typical varieties of wheat being grown because of two genes, Rht1 and Rht2, that caused dwarfing. These genes were derived from a Japanese landrace ``Shiro Daruma.''

  • Norman Borlaug speculated that by breeding these genes into Mexican wheat lodging would be reduced and the plants would respond to fertilizer application.

  • As it turns out, these genes not only reduce lodging through reduced height, they have direct effects on yield as a result of more efficient nutrient uptake and enhanced tillering.

The potential of seed banks to contribute to increased agricultural production is probably much greater than what has already been realized [4]. The old paradigm for use of gene banks was simple, look for the phenotype:

1. Screen entries from a gene bank for a desired characteristic.

2. Cross with an elite cultivar to introduce the genes.

While simple, it works well only for simply inherited traits, i.e., those associated with only one or two genes. There are, of course, many traits that are agronomically important that are also influenced by more than one or two genes. Because of the severe bottlenecks associated with production of today's elite cultivars, it is unlikely that modern cultivars retain all of the genes that could contribute to high yield. The new paradigm is to look for the genes:

1. Use molecular markers to map loci that contribute to agronomically important traits, like yield.

2. Use the markers that have been identified to enhance the introgression of these multi-locus traits into an elite background.

Using this approach in cultivated tomato, genes from Solanum hirsutum were introgressed and resulted in lines that yield 48% more, have 22% greater soluble solids content, and 33% ``better'' color (even though S. hirsutum is green). The normal yearly improvement from traditional breeding techniques is less than 1%.