Genetic basis and improvement of reproductive traits

Aynalem Haile and Mourad Rekik (ICARDA)

EIAR-DBARC-ICARDA-ILRI (LIVES)-FAO Training on Reproduction in Sheep and Goat, Debre Berhan, Ethiopia,

13-15 October 2014

Breeding and Reproduction---Objectives

• Improved lamb production• More lambs per lambing• More frequent lambing• Increased percent of total sheep nos.• Reducing death losses

Reproduction: is a complex composite trait influenced by many components including puberty,

ovulation, estrus, fertilization, embryo implantation, pregnancy, parturition, lactation, and

mothering ability.

The production systems

• Crop-livestock systems where genetic interventions can make a difference

• Pastoral & semi-pastoral systems in which adaptation is critical

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• Crop-livestock systems– Medium to high potential areas– Individual/family enterprises– Limited land– Medium to high productivity breeds

• Pastoral & semi-pastoral systems– Large herds/flocks– Dictates of climate– Mobility– Indigenous breeds– Strong community values

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Scenarios • Fluctuation and poor

quality of the feeding resources

• Insufficient health care

• Poor housing conditions

• Fragile economic asset

Reproduction is adversely affected

Sheep and goat breeds of arid and semi arid zones are

often late-maturing animals, have a delayed puberty, shorter

production life-time later than in more favourable natural

conditions.

Productive outputs are limited, within other causes, by

long anoestrus periods and low fertility and prolificacy.

Furthermore, in utero undernutrition, a very common event

when pregnant dams are inadequately fed under arid and

semi arid conditions, contributes to a reduced reproductive

fitness of the progeny

Consequences for reproduction

Large genetic differencesBetween species

• Goats are more strict seasonal breeders than sheep

• Goats are in average more prolific than sheep

Within breed variability

Between breeds

• Late maturing vs. early maturing breeds

• Existence of natural prolific strains

Characteristics of the reproductive traits to be improved

• Economically important (fertility, litter size vs. return to oestrus)

• Expression at the individual level (litter size vs. Prolificacy)

• Easiness of measure (litter size vs. ovulation rate)

• Cost of measure• Existence of variability !!!!!!

Factors affecting reproduction in the ewe

• Heredity• Age• Photoperiod (seasonal)• Temperature and humidity• Nutrition and Exercise• Parturition and lactation• Disease and parasites• Fertility of & assoc. with the ram

Factors affecting the reproduction in the Ram

• Breeding soundness exam• Palpation of the testicles, epididymis, and

penis and visual appraisal of feet, legs, eyes and jaws.

• Semen evaluation• Disease prevention• Heat stress

Desirable traits for accelerated lambing

• Ewes can breed year round• Ewes that can mate while lactacting• Ewes that have a good lambing rate (ie

twinning)• Sires that produce a desirable market lamb

and have the libido and fertility for conception year round

Genetic effects• Although component traits of reproduction are

under the influence of many genes, a limited number of major genes associated with separate components of reproduction have been reported in sheep

• Expressions of the genetic effects on reproduction are affected by numerous environmental factors such as season, climatic conditions, management, health, nutrition, ram to ewe breeding ratio, age of ewe, and ram libido and fertility. Because genetic and environmental factors interact, genetic improvement of reproduction is very complicated.

• Selection for a single component of reproduction such as ovulation rate, litter size at birth or number of lambs weaned has commonly been practiced. However, selection for a single component of a composite trait does not always result in an overall improvement of a complex trait such as reproduction

• The relevance of the different reproductive traits is not the same and also differs among species.

• In meat sheep production, litter size and days to lambing are two of the most important traits

Sex ExpressionMainly in females

• Age at puberty• Age at first lambing• Fertility• Litter size at lambing• Litter size at weaning• Lambing interval• Productive lifetime

Little attention in males

• Scrotal diameter???• Libido and sexual

aggressiveness???

Heredity basis of reproductive traits

• For most breeds, reproductive traits are quantitative traits: progress is obtained by transmission of the additive effects of genes

• In some breeds or strains, litter size is influenced by major genes: alleles polymorphism in some known genes

(≈ Mendelian trait)

Quantitative trait

Composition of the phenotype variance

δ2P = δ2

G δ2E δ2

GE

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2 categories of genetic effects

• Due to the effect of random halving of the genome, we have 2

fundamentally different categories of genetic effects:

• Effects that come into play by mating an individual to a random

sample of the population and are effective as the average of the

offspring (= additive effects).

• Effects determined by specific combination of gametes in a particular

individual, but not the offspring (= dominance and epistatic effects).

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Splitting the genetic effect

g = ga + do + ep

• ga = "additive" gene effect (relevant for breeding value) – ga is the part of g, which comes into effect when the individual

is mated to a representative sample of the population in the average of its offspring.

• do = "dominance effect"– do is the part of g, not explained by ga and due to interactions

of alleles of the same locus within a particular individual.

• ep = "epistatic effect"– ep is the part of g, not explained by ga or do and due to

interactions of alleles of different loci within a particular individual.

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Concept of heritability h2

Heritability is the proportion of variation in a phenotype (trait, performance) that is thought to be caused by genetic variation among individuals. The remaining variation is usually attributed to environmental factors. A measure of the degree to which the variance in the distribution of a phenotype is due to genetic causes. - h2 < 10%: low heritability, low genetic progress by direct selection- 10% < h2 < 30% : moderate heritability, slow genetic progress by direct selection- h2 > 30% : high heritability, significant progress by direct selection

Estimates of heritability for basic and composite traits (Rosati, 2002)

Conception Rate 0,06

Number of lambs born 0,10

Number of lambs born alive 0,05

Number of lambs alive at weaning 0,01

Litter mean weight per lamb born (kg) 0,13

Litter mean weignt per lamb weaned (kg) 0,15

Number of lambs born per ewe exposed 0,09

Number of lambs weaned per ewe exposed 0,07

Total litter weight at birth (kg) 0,4

Total litter weight at weaning (kg) 0,17

Total litter weight born per ewe exposed (kg) 0,13

Total litter weight weaned per ewe exposed (kg)

0,11

Lamb survival at weaning (%) 0,12

Major genes affecting litter size

1. The BMPR 1B (Bone Morphogenetic Protein Receptor type 1B) gene has been mapped to sheep chromosome 6 (“hyperprolific phenotype” of the Booroola sheep, Assaf??)

2. The BMP15 (Bone Morphogenetic Protein 15) gene, has been mapped to sheep chromosome X (Rasa Aragonesa, Lacaune, Galway)

3. The sheep GDF9 (Growth Differentiation Factor 9) gene maps to chromosome

The challenge• Regarding genetic improvement, available work has mainly

focused on phenotypic selection, rather than using information on specific genetic factors (genotypic information) affecting these traits.

• Nevertheless, reproductive traits are characterized by low heritabilities and a complex genetic basis and are thus difficult to improve using traditional selection methods.

• Moreover, these traits are recordable only in one sex and late in the animal’s life.

• These limitations have led to a growing interest in the identification and characterization of specific genes and genomic regions implicated in the variability and regulation of reproductive processes.

Breeding structures

Modern Breeding Structures

Breeders

Commercial flocks, community or base

Breeders

Base flocks

Multipliers

Gene flow (males)

Mueller, 200824

No breeding structures

– Occurs in low input systems – Difficult for the breeder to detect the best animals with

high precision – Difficult to organize a stratified mating in his flock– Less chance for gene inflow – Mating is at random within the flock– Genetic progress is slow, if any– Difficult to follow

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Nucleus Breeding Structure

Base

Nucleus

Females Males

“Open” to gene flow in any direction

Mueller, 2008

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Centralized and dispersed nucleus

centralnucleus

Participating flocks Participating flocks

dispersednucleus

Mueller, 2008

Mueller, 2008

Mueller, 2008

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The higher the dissemination, the higher should be the BV and its

accuracy

Records

Records on relatives

30%

60%

90%

Genomic

100%Visual

Random

0%

Performance test

Progeny test

BLUP analyses

BLP analyses

Sophistication of selection system

Sele

ction

acc

urac

y

Thank you