Developing New Varieties
Larry Darrah
Research Geneticist and Adjunct ProfessorUSDA-ARS Plant Genetics Research Unit
and Department of Agronomy, UMC
Self- vs. cross-pollinated crops
• Self-pollinated: Uses pollen and egg from the same plant to produce seed. Examples—Wheat and soybeans.
• Cross-pollinated: Uses pollen from one plant to fertilize an egg from another plant. Examples—Corn and squash.
Self- vs. cross-pollinated crops
• Soybean—Selfs easily and crosses with great difficulty; 3-4 seeds per pollination, if you are good! Only about 10% of the crosses set any seed.
• Wheat—Selfs easily and crosses with difficulty; 10-15 seeds per pollination. 95% of the crosses will set seed.
• Corn—crosses and selfs easily; 300-400 kernels per pollination. Anyone can pollinate corn!
• Determines what type of product is available to the farmer (hybrid vs. variety).
Corn anatomy
Tassel - ♂♂• Sheds pollen at maturity
Ear - ♀♀• Each silk is attached to
one ovule.
• Pollen tube grows down silk and fertilizes ovule.
Shoot bagging
Cutting back the ear
Tassel bagging
Pollination
Terminology
• Inbred: A plant that is produced through self-pollination over many generations.
• Hybrid: A plant that is produced by cross-pollinating two inbreds.
Does plant breeding work?
You betcha it does!
Lets look at corn yields over time.
Corn yields 1870-2000(10 tons/ha = 159 bu/a)
1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Year
0
1
2
3
4
5
6
7
8
9
10
Ave
rage g
rain
yie
ld (t/ha)
Varieties; b = -0.00 t/ha/year
How’ed they do that?
Population improvement/basis of gain from selection
Stalk lodging damage
Rind penetrometer use
Rind penetrometer in action
Divergent selection results
Response to selectionin MoSCSSS
0 1 2 3 4 5 6 7 8 9 10 11
Rind penetrometer resistance bins (each 0.25 kg)
0
100
200
300
400
500
Fre
quen
cy
2530 data points for Cycle 0Mean = 4.37 for B73 x Mo17Mean = 4.40 for Cycle 0
2699 data points for B73 x Mo17
B73 x Mo17
Cycle 0
0 1 2 3 4 5 6 7 8 9 10 11
Rind penetrometer resistance bins (each 0.25 kg)
0
100
200
300
400
500
Fre
quen
cy
2446 data points for high2508 data points for low
High sel.
Low sel.
Hig
h m
ea
n =
6.5
4
Lo
w m
ea
n =
3.0
9
Cycle 0 and B73 x Mo17 Cycle 6 low and high
Rind penetrometer resistance
0 1 2 3 4 5 6 7 8 9 10 11 12
Cycle of selection
1
2
3
4
5
6
7
8
9
10
Rin
d pe
netr
omet
er r
esis
tanc
e (k
g pl
ant-1
)
High = 3.60 + 0.17X + 0.20X2
Observed high
Observed low
Low = 3.60 - 0.24X + 0.01X2
Avg = 3.60 - 0.04X + 0.01X2
R 2 = 0.88
R2 = 0.88
Recurrent selection
• Generate families.
• Test family performance.
• Recombine selected families to complete a cycle of selection.
• Extract inbred lines by selfing in selected families and testcrossing at S2
or S3.
Family structure
• Selfed plants (S1 or S2).
• Crossed plants (Half-sib families with various testers.
• Selfed and crossed plants (S1 or S2 testcrosses to various testers.
• Reciprocally crossed plants (two half-sib testcrossing schemes.
Family evaluation (Yield)
• Test 100-300 families.• 2-row plots spaced 30” apart and about 7” between
plants for a total of 60 plants; 26,000-30,000 plants/a.• 4-9 replications at 2-6 locations in one season (3
reps. at 3 locns., 2 reps. at 4 locns., or even 1 rep. at 6 locations might be used.
• Locations are representative of the region of adaptation.
• Combine harvest to obtain grain weight, test weight, and moisture.
Recombination of selected families
• Bulked pollen, 1 male for 2 females.
• Diallel: 1 x 2, 1 x 3, . . . , 9 x 10; in paired rows or by chain crossing where most rows are used as both a male and female where the species allows (as in corn).
Partial Diallel
6 7 8 9 10
1 1 x 6 1 x 7 1 x 8 1 x 9 1 x 10
2 2 x 6 2 x 7 2 x 8 2 x 9 2 x 10
3 3 x 6 3 x 7 3 x 8 3 x 9 3 x 10
4 4 x 6 4 x 7 4 x 8 4 x 9 4 x 10
5 5 x 6 5 x 7 5 x 8 5 x 9 5 x 10
♂♂♀♀
Germplasm sources for population improvement
• Existing varieties (landraces and improved populations).
• Crosses within heterotic groups of existing elite lines for corn.
• Synthetic populations (contain varieties, lines, other synthetics, etc.). For example, germplasm with resistance to the European corn borer.
Look at many—save a few!
Wheat variety ‘Ernie’
• Came from pedigree selection in a cross of ‘Pike’ and Exp. Mo9965.
• Pike is an old variety with a very mixed pedigree.
Triticale: A “new” crop
• Triticale is a cross of wheat (female) and rye (male).
• Confers traits of high yield and baking quality.• Confers traits of tolerance to acid soils and
salinity, drought tolerance, winter hardiness, rust and mildew resistance, and higher lysine.
• Grown on 7.5 million acres (acid and marginal soils) in the world—primarily Australia, Brazil, France, Germany Poland, and South Africa.
New birdsfoot trefoil with rhizomes
• U.S. trefoil has persistence problems because of root and crown rot.
• U.S. varieties lack rhizomes.• Paul Beuselinck, USDA-ARS, Columbia,
collected a rhizomatous birdsfoot trefoil in Morocco and has bred it into U.S. germplasm resulting in release of ARS-2620.
• Grazing studies show increased persistence in pastures.
No rhizomes Rhizomes
Pedigree selection in self-pollinated crops
Variety A x Variety B
F2
F1
F3F3F3 F3 F3
F4
F5
F6
Can split apart further at F3 or subsequently
Where do most new varieties come from?
• Selfing out of existing varieties and testing.• Crosses among existing lines and varieties
followed by selfing and testing.
Yes, this approach seems to be self-limiting (funnel), but it has worked well thus far in many crops. See the following data from the Kenya Maize Breeding Project:
Genetic variance estimated from ear-to-row selection in Kitale Composite A (E7) from 1965 to 1974
• Cycle 0 355• Cycle 1 287• Cycle 2 242• Cycle 3 470• Cycle 4 316• Cycle 5 283• Cycle 6 586• Cycle 7 213• Cycle 8 1148• Cycle 9 263