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Doubled Haploids

Tuesday, January 4, 2011
filed under: Research and Development

Thirty years ago, the average U.S. corn yield was around 95 bu/ac. Now it’s approaching 155-160 bushels. Put another way, the nation’s corn yields have trended upward by an average of two bushels per year across the past three decades.

While there are a variety of factors behind this impressive track record — Bt corn, transgenics and improved management practices among them — “I think the bigger piece is the breeding,” suggests USDA-ARS sunflower research geneticist Brent Hulke. And prominent within the corn breeding realm has been the employment of doubled-haploid technology.

What’s a doubled haploid, and how has it helped breeders of corn and a variety of other crops?

Traditionally, a hybrid’s parental lines (inbreds) have been developed through a multi-year process of self-pollination, backcrossing to male sterility, and evaluation. It’s a process that can involve seven, eight or even more generations. Plus, even once a very good inbred has been developed, its genetic purity may not be absolute.

Doubled-haploid breeding is a much faster way of developing an elite inbred line. And, it results in 100% genetic purity — meaning the inbred’s genetic traits are “set in stone.”

In doubled-haploid breeding, the parent lines are pollinated with a haploid inducer — which results in a plant that possesses just one chromosome, not the normal two, from each pair of parents. The haploids are then subjected to a special treatment that causes the single chromosome to double — hence producing a duplicate that is genetically identical to the original.

So the haploid — which in the case of corn had 10 single chromosomes — now has 10 double chromosomes. The plant that is grown from this seed is referred to as a doubled-haploid inbred. The inbred can be used as a hybrid parent year after year — all the while remaining genetically pure.

The process is similar with sunflower, with the difference being that sunflower has 17 pairs of chromosomes, not 10.

The bottom line is that (1) desired inbreds can be developed much faster than through conventional methods, and (2) these inbreds possess 100% genetic purity.

Doubled-haploid breeding is a new idea in the sunflower sector and is not yet being used in breeding, as it is in corn. USDA-ARS geneticists Hulke, Lili Qi and C.C. Jan are trying to perfect the art to get this useful tool into sunflower breeders’ hands. “If we can get doubled-haploid technology together with an effective marker system that’s correlated with very specific traits (think ‘SNP’), we can be the ‘next corn,’ ” suggests Hulke. “We have so much in the way of genetic resources; and we have effective people in both industry and the public sector. If all this comes together, we could be making improvements much quicker than we are right now.”

Doubled haploids are, at their essence, a time-reducing strategy, the ARS geneticist points out. Marker-assisted selection is all about efficiency of selection — allowing one to do more within a given time frame. One is the “speed” component; the other is the “quality” component. “And they are perfectly complementing technologies.”

The hybrid seed industry has contributed to a doubled-haploid fund within the National Sunflower Association. A postdoctoral scientist, Shuelin Fu, is now on staff under the guidance of the ARS unit’s geneticists, working specifically on identifying the most efficient method of producing doubled-haploid sunflower. — Don Lilleboe
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