Plant Breeding

Huseyin Tombuloglu, Phd.GBE 304 Spring 2015

Basic Terms:• Outbreeding: Sexual reproduction between

individuals (cross-pollination). (It involves two individual plants)

• Inbreeding: Sexual reproduction within an individual (self-pollination). (It involves one individual plant)

Outbreeder or Inbreeder??

Often one can tell just by looking at a flower whether it cross-pollinates or self-pollinates

OUTBREEDER INBREEDERTraitIncompatibilityFlower #Flower sizeFlower colorNectoriesScentNector guidesAnther positionPollen #Style positionStigma

self-incompatible self-compatiblemany flowers few flowerslarge flowers small flowersbright colors mono-colorednectaries present nectaries absentscented flowers unscented flowersnectar guides present nectar guides absentanthers far from stigma anthers close to stigmamany pollen grains fewer pollen grainsstyle exserted from flower style included in flowerstigmatic area well-defined stigmatic area poorly-defined

Outbreeding (Cross-pollination)

•Advantages:

•Disadvantages:

Increases genetic variability

Strong evolutionary potential Adaptation to changing conditions

Successful long-term

Can destroy well-adapted genotypes

Relies on effective cross-pollination, seed dispersal and establishment

Inbreeding (Self-pollination)

• Advantages:

• Disadvantages:

Preserves well-adapted genotypes

Insures seed set in the absence of pollinators

Single colonizing individual possible

Decreases (or maintains) genetic variability

Evolutionary dead-end

Cannot adapt to changing environmental conditions

Successful short-term

What is hybridisation?the process of combining different varieties of organisms to create a hybrid

What is hybrid?

In biology a hybrid is mix of two animals or plants of different breeds, varieties, species or genera

Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids.

Hybrids between different species within the same genus (such as between lions and tigers) are sometimes known as interspecific hybrids or crosses.

interspecific hybrids and intraspecific or F1 hybrids

Hercules, a "liger", a lion/tiger hybrid.

A "Zonkey", a zebra/donkey hybrid

A "Jaglion", a Jaguar/Lion hybrid.

A mule, a Domestic Canary/Goldfinch hybrid.

The offspring of an interspecific cross are very often sterile

They cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed.

HYBRID PLANTS

An example of an interspecific cross is the New Rice for Africa (NE-RIC-A).

In West Africa two rice species co-exist: African rice (Oryza glaberrima) and Asian rice (Oryza sativa).

Many of the Asian rice varieties grown in Africa are highly susceptible to insect pests, diseases, weeds and toxic soils.

NERICA was developed by crossing Asian and African rice varieties to find stable and fertile breeds that would combine the yields of Asia’s plants and the toughness of Africa’s.

NERICA

Maize F1 hybrids

F1 hybrids tend to be bigger, develop faster, and more fertile than either parent.F1 hybrids are selected for high yields and for improved grain qualityA particular F1 hybrid line can be selected for specific pest and disease resistances or for tolerance to stress (such as drought)

Why?  Hypotheses:

masking of undesirable recessive alleles> they make some combinations of alleles .

ADVANTAGESF1 hybrids are uniform in appearance and behaviour:

–enables farmer to treat and harvest crop at the same time uniformity (all the plants in the population will grow and develop at the same rate)

–has marketing advantages when sold to buyers with strict quality standards

F1 hybrids have hybrid vigour or heterozygote advantage (makes crops more competitive with weeds)

Hybrids are sometimes stronger than either parent variety, a phenomenon known as hybrid vigor (orheterosis).

An economically important example is hybrid maize.

An additional advantage of F1 hybrids which is very important to farmers is their uniformity:. all the plants in the population will grow and develop at the same rateThis agronomic characteristic makes the crops easier and cheaper to manage.

The seeds of F1 hybrids are not sterile. However, subsequent generation will not be uniform, and due to genetic segregation of alleles during sexual reproduction, not all the individuals derived will contain all the desired agronomic characteristics found in the hybrid. This is why farmers are advised to either buy or develop new hybrid seeds each time they plant.

Maize Reproduction

Kernels in a cob have the same mother but can have different fathers A Maize cross protected by a bag

What is a maize f1 hybrid?

How do you make an F1 hybrid? 

The first stage of hybrid seed development is the generation of the parent inbred lines. Selected plants are carefully pollinated by hand by taking pollen from the tassel (the male flower, drawn in pink in the illustration) and placing it on top of the ears (female flowers, yellow in the illustration).

The resulting seeds are harvested and planted, and the process (called selfing or inbreeding) is repeated for several generations (up to 7 or 8) to achieve genetic uniformity in the lines. This means that all the plants that are grown from the seeds from one plant look and respond the same way to the environment and to pests and diseases. Due to inbreeding depression – the reduced fitness of the plants due to low levels of genetic diversity- inbred lines also tend to be smaller and with lower fertility levels.

Inbred lines are screened for desired agronomic characteristics to match the requirements of the sites for which F1 hybrid seeds are being developed: suitability to environmental conditions such as soil type, climate and resistance to prevalent pests and diseases.

The best lines are taken to the second stage of F1 hybrid seed development, called test-crossing.

Two different inbred lines are manually crossed to each other by taking the pollen of a plant from one inbred line and placing it on the ears of a plant of a different inbred line.

The resulting seeds are collected, and the resulting F1 hybrid plants are grown the following year for assessment. A large number of crosses are made with the aim of selecting the cross that result in F1 hybrid seeds that produce plants combining all the agronomic requirements needed, and higher yields than both parent. As with the previous step, most crosses are discarded because they do not represent an improvement on existing seed varieties.

The last stage of F1 hybrid seed development is large-scale seed production for commercial sale.

Two selected inbred lines are planted in the same fields in alternated rows: for example one row of the line selected as the male parent, and four rows of the inbred line selected as a female parent. On the female seed rows, the tassels (male flowers) need to be removed to prevent contamination from the pollen. Tassels can be removed by a detasseling machine, but subsequent detasseling by hand is also required as not all tassels develop at the same time, so some may be missed by the machine and could compromise the quality of the seed. Pollen from the male plants is allowed to fertilise the female rows by the wind. The seeds from the female parents are harvested and packaged for distribution and sale. This process needs to be repeated each year.

Genetic uniformity of F1 plants can be a problem if the conditions are bad (such as extreme weather conditions, new pest or disease).

(genus Brassica)

AllopolAllopoliiploidploid

AmphidiploidAmphidiploid; doubled diploid; doubled diploid

amphidiploid Triticale

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