MARTIN A. YEBOAH

Introduction Molecular markers

Yam Stats. Linkage mapping

Germplasm F1 mapping population

Botany Future trends

Biological constraints Conclusion

Polyploidy and mapping

Breeding history

Yam improvement in IITA

Breeding scheme

Rapid propagation

The world population has passed 6 billion and continues to grow. Hunger , poverty and malnutrition are major challenges to mankind.

Half the world — nearly three billion people — live on less than two dollars a day.

Technology advances in agriculture and food need to continue to meet these challenges.

―If we fail to keep agriculture moving in

the less developed nations, poverty

will continue to grow, and the social

upheaval that will ensue will become

a global nightmare.”

Norman Borlaug, 1970 Nobel Peace

Price Laureate.

According to UNICEF, 26,500-30,000 children die each day due to poverty. (source www.globalissues.org).

Iron and vitamin A deficiencies, and infectious diseases continue to devastate people of the developing world.

Non-communicable diseases attributable to obesity are increasingly common in developed and developing countries.

Yam diets providing micronutrients and health-promoting phytochemicals could alleviate both under-nutrition and obesity.

Diversification into yam production can contribute to poverty alleviation through several ways.

1. Household food security at the domestic and community level can be achieved through increased yam production, improved handling after harvest, processing, and marketing.

2. Yam can be consumed in the household or sold to generate income to purchase household goods and pay for education of the youth.

3. Yam can be processed using appropriate technologies. The processed products can be consumed within the household or sold as part of value-added income generation.

Table 1: Production (in ‘000 tons) and Area (in’ 000 ha) of yam by

Continents

Continent Production(in’000

tons)

Area (in’000 ha) % of world

production

Africa 44514.5 4598.6 94.9

Asia 234.3 15.4 0.5

Europe2.7 0.16 0.005

Caribbean 486.8 77.8 1.03

Oceania 343.7 52.9 0.73

South America599.9 62 1.27

World total 46920.7 4781.1 100

Source: FAO,2007

Continent Countries

Africa Benin, Burkina Faso, Burundi, Cameroon, Central African

Rep., Chad, Comoros, DR. Congo, Ethiopia, Garbon,

Ghana, Guinea, Kenya, Liberia, Mali, Nigeria, Sudan,Togo

Asia Japan, Philippines

Europe Portugal

Caribbean Cuba, Barbados, Dominica Republic, Guadeloupe, Haiti

Oceania Papua New Guinea, Solomon Island, Tonga, Vanuatu

South

America

Brazil, Colombia, Guyana, Venezuela

Yam belongs to the genus Dioscorea of family

Dioscoreaceae

The genus contains some 600 species with more

than 10 species cultivated for food and pharma-

ceutical use (Ake Assi ,1998).

Important staples in many areas:◦ West Africa, southeast Asia, Pacific and Caribbean

Islands

Yams have been cultivated for over 5000 years in

tropical Africa.

#

##

#

#

#

##

##

##

#

##

##

#

23

4

56

7

1011

12

13

14

1

8

915

16 17

18

Niger

BornoYobe

TarabaOyo

Bauchi

kebbi

Kogi

Kaduna

Kwara

Benue

Edo

ZamfaraKano

Adamawa

Sokoto

JigawaKatsina

Delta

Plateau

Nasarawa

OndoOgun

Gombe

Cross River

Rivers

Osun

FCT

Imo

Ekiti

Enugu

Ebonyi

Lagos

Lake Chad

Akwam Ibom

400 0 400 800 Kilometers

N

EW

S

ForestDerived savannaGuinea savanna

#

Major Yam Producing Areas in Nigeria

Six important staples include

1. White yam ( D. rotundata)

2. Water yam (D. alata)

3. Yellow yam (D. cayenensis)

4. Trifoliate yam (D. dumetorum)

5. Aerial yam (D. bulbifera)

6. Chinese yam (D. esculenta)

IITA –the largest world collection 8 spp >3000 accessions (391 core collections).

CTCRI in Tryvandrum india

VASI in Hanoi ,Vietnam

PhilRootCrops in Babay ,Philippines.

VARTC in Santo, Vanuatu

INRA and CIRAD in Guadeloupe, West Indies

China and Japan

Dioscorea spp. (true yam)

Most popular cultivated spp.

D. rotundata - West Africa

D. alata - Asia

Wild/semi-domesticated spp.

D. abyssinica, D. praehensilis etc

Vegetatively propagated

Deiocious

Allo - , auto-polyploid or Diploid?

Long life cycle

Dioecy and polyploidy

Poor to non-flowering

Vegetative propagation

Juvenile phase

Yam mosaic disease

Anthracnose disease

Terauchi et al.,1992, proposed that D. rotundata was domesticated from a wild species that shared the same chloroplast genotype, and that D. cayenensis is a hybrid origin and should be considered as a variety of D. rotundata.

However, Mignouna et al., 2005a, classified guinea yam into seven morphotypes and therefore separated D.cayenensis and D. alata into two separate groups.

Nutrient D. alata(s=16)

D. esculenta(S=99)

D. rotundata(S=3)

Moisture % 77.3 74.2 65.3

Protein % 2.06 2.04 1.52

Starch % 16.7 19.3 30.2

Sugars % 1.03 0.55 0.32

Fat % 0.08 0.06 0.09

Ca (mg/100g) 8.2 7.5 4.6

P (mg/100g) 38 39 28

Fe (mg/100g) 0.60 0.75 0.60

Zn (mg/100g) 0.39 0.46 0.30

Cu(mg/100g) 0.15 0.17 0.12

Vitamin A (mg/100g)

0.018 0.017 0.8

Bradbury and Halloway,1988

Country Varieties

(n)

Dry

matter

Minerals Starch Sugars Amylose Protein

Papua

New

Guinea

43 23.5 5.1 67.5 3.3 17.5 12.0

CV% 16.4 14.7 7.8 49.1 11.4 32

Vanuatu 48 23.4 3.3 73.1 1.85 17.2 11.9

CV% 17.15 15.2 9.1 91.3 11.6 17.8

Fiji 19 25.2 4.25 68.5 2.46 18.6 8.03

CV% 18.2 18.8 6.1 26.4 5.7 21.7

Source: SPNY,2003

species A diploid

(fertile)

X

autotetraploid(fertile)

spontaneousgenome duplication

Causes of genome duplication:a) meiotic non-reduction of gametes (both in egg and sperm)

b) genome duplication w/o cytokinesis (after fertilization)

Autopolyploidy arises from genome duplication

aborted gamete production

Duplicated genomes are fertile !!Botanical term: Allopolyploids

spontaneousgenome duplication

(fertile)

successful cell division

Hybrid AABB

“allopolyploid”

Hybrid AB

during meiosis

Hybrid AB

body cells

species A

species BX

II. Allopolyploidy arises from hybridization plus genome duplication

III. Homologous pairing is predominant in allopolplyoids

homologous pairing homeologous pairing

genomes maintained separately

selfing generations

VI. Diploid vs. Allopolyploid hybridization

1. Because allopolyploids involves a merger of two fully

differentiated genome, pairing behavior during meiosis

is expected to resemble a diploid and disomic

segregation occurs.

2. In autopolyploid, during meiosis pairing can occur either

between randomly chosen pairs of homologous

chromosome call bivalent or between more than two

homologous pair of chromosomes (multivalent) and

polysomic inheritance occurs.

Chromosome pairing in tetraploids can occur that only homologue pair or such that any two homeologue may pair.

This two type of pairing may affect the segregation pattern e.g. diploid or tetraploid genetics.

AFLP markers segregated like a diploid in cross pollinated population, suggesting D. rotundata is an allotetraploid 2n=4x=40,

(Mignouna and Asiedu,1999)

a) Disomic inheritance: Allotetraploid

Strictly bivalent pairing

If AAaa is selfed, there are 2 possibilities

1. Homologues are homozygous:

e.g. AA and aa; implies all gametes are Aa;

progeny are all AAaa.

2.Homologues are heterozygous: Aa,Aa

gametes are in ratio of 1AA:2Aa:1aa

Progeny are 15A-:1aaaa (1AAAA:4AAAa:6AAaa:4Aaaa:1aaaa)

3. AAaa test cross

1. Homologues are homozygous: AA ,aa all gametes are Aa with all progenies being Aa.

2. If homologues are heterozygous Aa, Aa then gametes are = 1AA:2Aa:1aa

All progenies are 3A-:1aaaa

B) Tetrasomic inheritance: polysomic polyploidy (autotetraploids)

1. Any chromosome can pair with up to 3 homologues therefore we can have higher order pairings e.g. quadrivalent.

AAaa selfed: produces 1AA:4Aa:1aa gametes

Progeny ratio of 35A-:1aaaa

(1AAAA:8AAAa:18AAaa:8Aaaa:1aaaa)

However AAaa testcross (x aaaa) gives progeny 5A-:1aaaa.

With tetraploids five different genotypes and multiple alleles are possible:

1. AAAA:quadriplex 2.AAAa: triplex

3. AAaa: duplex 4.Aaaa: simplex

5.aaaa nulliplex

Complex segregation e.g.

1.Selfing a duplex AAaa gives :

1/36 AAAA: 2/9 AAAa:1/2AAaa: 2/9 Aaaa: 1/36 aaaa.

2.While selfing a diploid Aa gives: 1/4 AA: 1/2Aa:1/4 aa.

The situation becomes more complex at higher ploidy level.

It may not always be possible to distinguish each of the heterozygous genotypes or distinguish them from the homozygous dominant depending on the type of marker used.

With a dominant marker, the genotype AAAA, AAAa, AAaa, Aaaa, can not be distinguished from one another.

Therefore selfing a duplex AAaa will give a segregation ratio of 35/36 [A] and 1/36 [a]

With co-dominant markers genotype AAAA and aaaa can be distinguished from heterozygous AAAa, AAaa, Aaaa genotypes.

Also the intensity of the electrophoretic band may discriminate among the three heterozygotes forms (Dubreuil et al.,1999.)

The segregation of a duplex will be informative , neglecting the homozygous genotypes.

Therefore the segregation ratio of

2/9 AAAa:1/2AAaa: 2/9 Aaaa

is observed being close to that of a diploid 1/4AA:1/2Aa:1/4aa

According to Wu et al., 1992, analysis of the segregation should be based on the presence or absence of a fragment in the progeny.

A fragment represented by a single dose in a parent is equivalent to an allele in the heterozygous simplex state (Mmmm) M for presence and m for absence.

Half of the gamete will contain the allele and half will not.

A cross between a simplex plant and a nulliplex plant (no fragment) will give a ratio of 1:1 segregation regardless of the ploidy level.

Double dose restrictive fragment (DDRF) genotypes (MMmm) can also be considered in the same way to yield 1/6MMmm:2/3Mmmm:1/10 mmmm

However, triple dose fragment (MMMm) will not be informative because no segregation will result if it is crossed to a plant with absent or no fragments.

Dosage Diploid Tetraploid Hexapod Octaploid

1 1/2 1/2 1/2 1/2

2 1 5/6 4/5 11/14

3 1 19/20 13/14

4 1 1 39/70

Source: Ripol et al., 1999

1. Improvement in agronomic traits e.g. vegetative organs.

2. Increase in the differences between extreme genotypes at each locus leading to greater genetic variance.

3. Increase in genetic variability due to presence of more than two alleles at one locus with interactions between more than two alleles.

4. Greater homeostasis in varying and variable environment due to buffering capacity.

1. Several International Research have contributed to breeding.

2. Most researched species include D. alata D. cayenensis and D. rotundata

3. Environment for research includes Nigeria, India , Guadeloupe and Vanuatu

4. Other cultivated spp. are D.bulbifera,D. esculenta, D.nummularia,D.opposita, D. pentaphylla, D. transversa and D. trifida.

Significant breeding effort for D.trifida made by INRA in 1960 in Guadeloupe

Selections obtained in 1971 for yield of 30t/ha unstaked

IITA yam breeding and selection since 1970 focusing on D. rotundata.

Principal objectives :

1. High stable yield of marketable tubers

2. Suitability to cropping systems

3. Good quality e.g. DM, texture ,taste etc.

4. Resistant to biotic stresses in the field.

5. Good postharvest storage.

The long term objective are:

to release genotypes adapted to non-stake conditions and to partial or complete mechanical harvesting. Tubers with shallow settings, oval or round , tough skinned, several tubers /plant are preferred

The objectives of INRA, CIRAD and CTCRI for D. alata are:

1. Major diseases e.g. Anthracnose cause by C.gloeosporioides.

2. Physico-chemical characteristics of D. alata

Goal: Develop and disseminate improve technologies to increase the productivity of yam based system in partnership with NARES through:

1. strategies for integrated control of pests and diseases in the field, during storage and soil management.

2. reduced labor input in yam base system

3. manipulation of tuber dormancy to increase efficiency in propagation and flexibility in crop cycle

3. Expand utilization opportunities through processing into value added product.

4. Improving market channels to improve productivity

Specific objectives:

1. High stable yield of marketable tubers

2. Host plant resistance for nematodes, viruses, and fungi e.g. anthracnose

3. High tuber quality and characteristics preferred by consumers.

4. Suitability to the cropping system and tolerance to abiotic stress i) nutrient responsiveness and ii) tolerance to terminal drought etc.

Problem of sexual hybridization

1. Sparse flowering

2. Poor synchronization of male and female phase

3. Poor pollination mechanism

Achievement on sexual hybridization

1. Many parental genotypes that combine good agronomic trait with reliable flowering identified

2. Techniques to manipulate the flowering period to enhance synchronization and extended pollination established.

3. Anthesis period of pollination viability and stigma receptivity have been determined for the relevant species.

4. Pollen storage over two years has been demonstrated

1. Rapid propagation of introduced genotypes as parents in selection cycle.

2. Rapid propagation of improved hybrids for advanced clonal evaluation or for distribution.

3. Best ways are the use of the mini-sett technique, rooted stem cuttings and in vitro growth of nodal segments.

Determine Objectives.

Identify Source of Genetic Variation/ Genetic Recombination.

Selection of Superior Progenies/ Generation Advance.

Testing of Experimental Varieties/ Release

1. Conventional plant breeding

2. Biotechnology (molecular markers , wide crosses, double haploids) to overcome species barrier/improve breeding efficiency.

3. Interdisciplinary collaboration

Characterization and germplasm evaluation

1. field performance

2. tuber quality

3. morphology

4. ploidy status

Selection of parents for hybridization through biparental crosses.

Open pollination among selected clones planted in isolation.

Seedling evaluation in nurseries

Clonal trial for selection of superior genotypes

1. Unreplicated observational trial

2. Preliminary yield trial

3. Advance yield trial etc.

Evaluations of cooking quality ,processing etc.

Multiplication of propagules

Regional collaborative trial with partners

Yam improvement scheme

HYBRIDIZATION BLOCKS

CLONAL EVALUATION

PRILIMINARY YIELD TRIAL

ADVANCED YIELD TRIAL

MULTIPLICATION, VIRUS ELIMINATION, DISTRIBUTION

V

CLONAL COLLECTION

Evaluation and selection

CLONAL COLLECTION

Evaluation and selection

CLONAL COLLECTION

SEEDLING NURSERY

HYBRIDIZATION BLOCKS

Evaluation and selection

CLONAL COLLECTION

CLONAL EVALUATION

SEEDLING NURSERY

HYBRIDIZATION BLOCKS

Evaluation and selection

CLONAL COLLECTION

PRILIMINARY YIELD TRIAL

CLONAL EVALUATION

SEEDLING NURSERY

HYBRIDIZATION BLOCKS

Evaluation and selection

CLONAL COLLECTION

Send to

NARS

ADVANCED YIELD TRIAL

PRILIMINARY YIELD TRIAL

CLONAL EVALUATION

SEEDLING NURSERY

HYBRIDIZATION BLOCKS

Evaluation and selection

CLONAL COLLECTION

Send from

NARS

Evaluation and selection

Evaluation and selection

Evaluation and selection

Evaluation and selection

REGIONAL COLLABORATIVE TRIAL WITH NARS

MULTIPLICATION, VIRUS ELIMINATION, DISTRIBUTION

V

ADVANCED YIELD TRIAL

PRILIMINARY YIELD TRIAL

CLONAL EVALUATION

SEEDLING NURSERY

HYBRIDIZATION BLOCKS

Evaluation and selection

CLONAL COLLECTION

Year 1 evaluate resistance to diseases

and pests

Year 2-3 evaluate resistance to diseases

and pests

Year 4 evaluate resistance to diseases

and pests ; tuber conformation and yield

Year 5-6 evaluate resistance to

diseases and pests ;tuber

conformation , yield and quality

Evaluate resistance to diseases and

pests; tuber conformation, yield

and quality

Isozymes:

1. Low cost , allows screening of large number of accessions

2. Low polymorphism

DNA markers (RFLP, AFLP,SSR and RAPD)

1. More accurate

2. Expensive

3. Labor -intensive

Molecular markers: characterization and early screening.

Tissue culture: haploidization and mapping population development.

Genome studies: ploidy , QTL mapping

Plant genetic transformation: gene transfer

1. Two heterozygous parents (P1, P2) are mated to produce a full sib F1 family which is subsequently replicated through cloning (tissue culture)

2. QTL mapping is conducted using phenotypic measurements on the F1 clones.

3. Suitable for species like yam where full sib crosses is difficult , vegetative propagation is easy and hybrids are heterotic .

4. Mainly use dominant markers for pseudo test cross analysis.

1. With dominant markers the design can be reduced to the paternal and maternal backcross mating types hence the name pseudo test cross (PTC).

2. The PTC mating has Aa and aa genotypic classes which can be discerned with dominant markers.

3. Expedient for spp. not widely studied as a genetic models or poor pedigree records.

4. Failed PCR not disquishable from null allele.

Could also apply to co-dominant markers for

Intercross, maternal and paternal informative mating types.

Easy exchange or sharing of germplasm with other countries and institutions.

Marker assisted selection (MAS) should be given priority for resistance breeding for both biotic and abiotic stresses.

Varieties suitable for low inputs eg fertilizer, pesticide, weedicide etc. should be bred for the resource poor farmers.

Interspecific hybridization of wild spp. and cultivated spp for disease resistance breeding .

Application of haploids in breeding should be investigated to speed up breeding process .

Varieties with improved shelf life, rich in nutritive values and suitable for processing should be developed e.g.pro-vitamin A (β-carotene) Fe, Ca and Zn (nutrient fortification) .

Embryo rescue to unlock genetic potential in wild yam via wide crosses

Varieties with increased opportunities for market for the fresh and value added products e.g. High quality flour, starch, storage , taste , flavor , anthocyanin, starch for tablets, baby food etc.

Acceptable varieties as dietary source of pro-vitamin A, Fe, Zn to address nutrition and health issues.

Need for the improvement of starch and carbohydrate quality of yam, since high glycemic index starches (high amylopectin with low amylose content) are related with conditions such as type 2 diabetes and insulin resistance.

Modification of starch in yam to increase amylose and amylopectin ratio would improve the glycemic index (effect on blood sugar level) to improve the nutritional quality and subsquently have effect on health.

Food and ManufacturerGI serve (g)

carb/serve

(g) GL

Yam, peeled, boiled 35 150 36 13

Yam 54 150 36 19

Yam, steamed 51 150 36 18

Yam (Dioscorea spp.), boiled 74 150 38 28

Yam (Dioscorea spp.), boiled, consumed with

4.24 g salt 74 150 38 28

Coco yam (Xanthosoma spp.), peeled, cubed,

boiled 30 min 61 150 46 28

Lucea Yam (Dioscorea rotundata), peeled,

cubed, boiled 30 74 150 27 20

Lucea Yam (Dioscorea rotundata), peeled,

roasted on preheated charcoal77 150 38 29

Source http://www.glycemicindex.com/

Table 6: The Glycemic index of Yam

From a technical point of view, it may be concluded that the key step for enhancing the standard of yam breeding is to meet its objectives is to build a bridge between conventional breeding and molecular techniques .

Where molecular markers linked to target genes can be identified accurately so that breeders can make selection based on the genotype of each plant by molecular markers.