Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 372

The first step in using doubled haploids in breeding is identifying the source of haploids.

 Natural SourcesHaploids originate in nature through the phenomenon of parthenogenesis (gamete formation without fertilization). The haploids may be maternal or paternal in origin. It is estimated that haploids occur in corn at the rate of 1 in 1000 diploids, 99% of which arise from parthenogenesis of maternal origin. Spontaneous doubling occurs in corn at the rate of 10% of haploids developed. The key is distinguishing between haploid and diploid plants. A marker system for this purpose was first developed by S.S. Chase based on seedling color, purple plants being encoded by the dominant gene (P) while normal green plants are recessive (p). A cross of F1(pp) × PP would yield 999Pp (purple diploids) and 1 pp (green haploid). Another marker used is the purple aleurone color.To use this marker system, the breeder should cross a heterozygous female to a male with marker genes. The seed from those with dominant endosperm marker of the male is saved and planted, discarding seedlings with the dominant male marker. Next, cytological evaluation of plants with the recessive female marker (by root tip squash) is conducted. The haploid plants are retained and grown in the greenhouse or field, and self‐pollinated to produce diploids.

 Artificial sourcesHaploid production through interspecific and intergeneric crosses is in use, one of the most well‐known being the barley system (previously discussed). After doubling the chromosome, the diploid plants are grown to maturity. Seeds are harvested for planting plant rows. Because diploids produced by this method are normally completely homozygous, there is no need for growing segregating generations as obtains in conventional programs.