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Dominance, mating and crossbreeding

Dominance, mating and crossbreeding. Chapter 10. Mating. Mating is pairing selected sires and dams = comes after selection Mating determines how selected alleles combine within individuals Benefit from non-additive genetic effects. Mating strategies. Mating within populations

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Dominance, mating and crossbreeding

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  1. Dominance, mating and crossbreeding Chapter 10

  2. Mating • Mating is pairing selected sires and dams • = comes after selection • Mating determines how selected alleles combine within individuals • Benefit from non-additive genetic effects

  3. Mating strategies • Mating within populations • Which individuals to mate together? • Mating between populations • = crossbreeding • Which lines to cross with each other?

  4. Mating within populations • Affects genetic make-up of next generation of selection candidates • Does not affect long-term genetic improvement

  5. Major mating strategies • Random mating • Assortative mating • Mating based on family relationships • Mate selection

  6. Random mating • = Absence of any specific mating strategy • Mates chosen at random within selected sires and dams • NOT same as random selection!

  7. Assortative mating • Mating based on phenotypes or EBVs • Two types: • Positive assortative mating • Negative assortative mating

  8. Positive assortative mating • = Mating of similar individuals • Progeny? • More extreme •  Increases genetic variance (temporarily) • Practical use limited • Also need to mate poorer animals with other poorer animals!

  9. Negative assortative mating • = Mating of dissimilar individuals • Progeny? • More intermediate •  Decreases genetic variance (temporarily) • Compensatory/corrective mating • Aims to correct faults of parents in offspring • Dairy cattle

  10. stierenkaarten

  11. Mating on family relationships • Preferential mating of relatives • = Inbreeding • Avoidance of mating of relatives

  12. x Minimum kinship mating • Avoiding mating of relatives • Calculate coefficient of kinship for all possible combinations of sires and dams • Choose matings to minimise average coefficient of kinship • Postponesoccurrence of inbreeding • Reduces long-term rate of inbreeding

  13. x Minimize kinship mating in practice • In real life, minimum kinshipmating is complicated but canbedone • Manybreedingprogrammesusesomesort of mate selectiontorestrictkinship - avoidmatingsbetween close relatives • Full sibs • Half sibs • cousins

  14. Mate selection • Combines selectionandmating in a single step • Calculateexpectedprogeny performance foreverypossiblemating of candidates • Do matingswithhighestexpectedprogeny performance • Utilisesnon-additive geneticeffects but you have toknowthem.....

  15. Summary: Mating within… • …populations means deciding which sire to mate with which dam • You can benefit from • Corrections in phenotype • non-additive genetic effects • Main use: avoid highly inbred offspring • full sibs • half sibs • cousins

  16. Mating between populations • Mating between populations: • Used to produce final production animals • Pure production strategy, no effect on breeding population

  17. Crossbreeding • Breeding system where sires and dams originate from different lines • Lines within breed • Different breeds • Many ways to combine lines • Benefit as much as possible of heterosis • Breed complementarity • Common in pigs, chicken and beef cattle

  18. A tropical breed: Nelore short hair, skin folds, long ears, no body fat reserves, long legs

  19. A temperate breed: Red Agnus Coarse hair, subcutaneous fat, short stature

  20. corssbred: productive in tropical climate, high tick resistance! X

  21. Crossbreeding in dairy cattle • Crossbred (F1)between indigenous cattle and Holsteins perform well in a number of developing countries • Should crossbreeding be implemented? • Consider population with 100.000 F1-cows

  22. Crossbreeding in dairy cattle • Large pure-bred cow population needed for the production of F1-replacements. • Bottleneck: • Reproductive rate males: no problem • Reproductive rate females: very low • 1 offspring per year • 4 offspring during life time

  23. 100,000 F1 animals (Zebu*Holstein) • Parental breeds: • Holsteins: imported (semen) • Zebu: local pure-bred cow population • Replacements for F1 population? • Every year 25% of cows (F1 and purebred) replaced • 25,000 F1-heifers needed each year. • 50,000 Zebu cows need to be inseminated with Holstein semen. • 50,000 Zebu cows need to be inseminated with Zebu semen for maintaining Zebu population

  24. Crossbreeding between breeds: concluding remarks • In developing countries: • Quick fix: improved performance, • No long term genetic improvement! • Who maintains the local breed? • Only when the local breed serves a purpose!

  25. Time for a break

  26. Crossbreeding: Atlantic Salmon 25 ♀-Gaspe 25 ♀- Mowi 25 ♀-Laks Milt 15 ♂ Gaspe (CD) Mowi (NO) Laks (UK) eggs eggs eggs eggs eggs eggs eggs eggs eggs Crosses:

  27. Cross Breeding: salmon Ranking for GF3 from smolt to 4.5 kg The results of the SP tests are used to direct the crosses in future spawning's for production.

  28. Heterosis • Crossbred offspring is better than parent average • Also called hybrid vigour (plant breeding) • Caused by dominance (and epistasis) • “Specific combining ability of lines” • Can be expressed in trait units • HF1 = µF1 – ½(µsire + µdam) • Usually expressed as percentage • HF1 = 100% x (crossbred mean – parent mean) (parent mean)

  29. Example: Heterosis in salmon • Harvest weight • Mowi-line: harvest weight = 3.8 kg • Laks-line: harvest weight = 4.0 kg • Mowi x Laks F1: harvest weight = 4.1kg What is the heterosis? • HF1= 4.1 - ½(3.8 + 4.0) = 0.2 kg • HF1 % = 0.2 kg / 3.9 kg x 100% = 5.1%

  30. Genetic basis of heterosis • Crossbreeding leads to more heterozygotes if there is a difference in allele frequency () • Line1 = MM, line 2 = LL,  = 1 • F1 = ML = maximum heterosis • 1-locus: d > 0  F1 will be better than parent average = ML > (MM + LL)/2

  31. Example: Fillet% in Salmon • Line 1, p(M) = 0.8; Line 2, p(M) = 0.3 •  = 0.8 - 0.3 = 0.5 • GMM = 60%, GML = 64%, GLL = 65% • o = (65+60)/2 = 62.5% • d = 64 - 62.5 = 1.5% • HF1 = d x 2(eq 10.10) = 1.5 x 0.52 = 0.375 fillet%

  32. Heterosis: two types • Direct or individual heterosis • Maternal heterosis

  33. Breeding line D D dam line C sire line Breeding line C (♂ & ♀!) (♂ & ♀!) Selection on growth and meat quality CxD F1-offspring Selection on reproduction Production Population Crossbreeding: two-way scheme

  34. Two-way system • More uniform F1 production population • Breed/line complementarity • Heterosis in (CxD) progeny • Note: F1 animals are not used for pure line breeding! •  All selection within pure lines D and C

  35. Two-way system • Heterosis for growth (CxD) progeny? • Direct: growth • Maternal: -- • Hgr = 4% • Gr (C) = 700 g/d; Gr (D) = 680 g/d • Gr (CxD)= 1.04 x (700+680)/2 = 718 g/d

  36. Two-way system • Heterosis for litter size in (CxD) progeny? • Direct: vitality • Maternal: --! • Hgr = 4% (vitality) • nr Pl (C) = 10; nr Pl (D) = 11 • Nr Pl (CxD)= 1.04 x (11) = 11.44 Pl

  37. Line A Line C Line D F1 sow C×D Fattening pig A × (C × D) Crossbreeding: three-way scheme Selection & Replacement Selection & Replacement Selection & Replacement Selection on reproduction Selection on growth and meat quality

  38. Three-way system: heterosis • Direct: growth in (A x (CxD)) pigs • Mean parental lines? • (750 + ½(700+680))/2 • 1.04 x 720 = 749 g/d • Heterosis CxD is not heritable!!

  39. Three-way system: heterosis • What about litter size? • Direct: 4% (vitality of piglets) • Maternal: 6 % (nr of piglets born) • Mean litter size • A: 8 • C: 10 • D: 11

  40. Three-way system: heterosis • Litter size: parent mean • CxD ! • (10+11)/2 = 10.5 • Maternal H: 0.06 x 10.5 • Direct H: 0.04 x 10.5 • Sum: 10.5 + 1.05 = 11.55 PL

  41. Three-way system • Uniform end product (A x (CxD)); breed complementarity • Heterosis in (A x (CxD)) pigs • Maternal heterosis in (CxD) sows •  better reproduction • Protection of genetic material

  42. Line A Line C Line D F1 sow C×D Info Info Fattening pig A × (C × D) What is the breeding goal? Selection & Replacement Selection & Replacement Selection & Replacement Selection on growth & litter size Selection on growth, backfat and meat%

  43. Summary: Crossbreeding… • … = production strategy • Crossbred individuals are (usually) not parents • Crossbreeding itself does not contribute to G • Aims to benefit from heterosis & line complementarity • Genetic improvement within pure lines • Should aim at crossbred performance

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