Barley doubled haploids
A haploid plant is derived from male or female gametic cells and/or
tissues.
In barley, haploids can be derived from pollen via anther or microspore
culture or from eggs via ovary culture or an interspecific pollination
procedure known as the Hordeum bulbosum technique.
This procedure is based on selective chromosome elimination. Hordeum
bulbosum is a perennial wild barley found throughout the Mediterranean
region.
When diploid H. bulbosum (2n = 2x = 14) is used as a pollen parent
in crosses with diploid H. vulgare (2n = 2x = 14), the H. bulbosum
genome is eliminated from the developing embryo after fertilization.
This results in a haploid H. vulgare embryo. This embryo is then
transferred to a nutrient medium for haploid plant regeneration.
The haploid plants are sterile, so colchicine is used to double
the chromosome complement back to the diploid level, producing an
array of doubled haploid (DH) genotypes.
If DH lines are produced from the gametes of a heterozygous H. vulgare
plant (such as an F1), these DH lines represent a random sample of
the genetic variation present in the gamete donor and are, for all
practical purposes, equivalent to the progeny that would be obtained
after an infinite number of generations of self-pollination.
These DH lines, being completely homozygous, represent an "immortal" reference
population that can be repeatedly genotyped and phenotyped.
As a barley plant produces more pollen than eggs, androgenetic haploid
systems (i.e. pollen or microspore culture) are potentially more
efficient than gynogenetic systems (i.e. ovule culture or the H.
bulbosum technique).
However, H. vulgare genotypes vary considerably in their response
to androgenetic systems, and the extended tissue culture phases required
for current androgenetic haploid production protocols can lead to
gamete selection or somaclonal variation.
Thus, the H. bulbosum technique remains a dependable system for
producing barley mapping populations, and it is used by a number
of applied breeding programs.
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