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Citrus Breeding

Citrus Breeding. Kevin M. Crosby. History. Early agriculturalists selected natural hybrids and mutants for seed propagation 1800’s- grafting and cuttings became popular to propagate best varieties 1900’s- artificial cross-pollination practiced. Seedless fruit of the Tahiti lime .

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Citrus Breeding

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  1. Citrus Breeding Kevin M. Crosby

  2. History • Early agriculturalists selected natural hybrids and mutants for seed propagation • 1800’s- grafting and cuttings became popular to propagate best varieties • 1900’s- artificial cross-pollination practiced

  3. Seedless fruit of the Tahiti lime More than 70 percent of all citrus fruits grown in the US are varieties developed by the ARS citrus breeding program.

  4. Breeding Goals • Rootstock- tree size, stress tolerance • Scion- fruit color, size, shape, flavor, yield • Disease resistance- CTV, Phytophthora, CVC, Alternaria, Scab, Greening, etc.

  5. Rootstock • Dwarfing- Poncirus, some mandarins • Compatability- citrus better than Poncirus • Seedling vigor and scion yield • Fruit quality- size, shape, flavor, juice

  6. Rootstock Hybrids • ‘Carrizo,’ ‘Troyer’ Citrange- navel orange x Poncirus, very popular in FL, CA • ‘Swingle’ Citrumelo- grapefruit x Poncirus, very popular in Florida, salt intolerant • ‘Sunki’ x ‘Swingle’ tf- semi-dwarfing, salt-tolerant, possible replacement for SO in TX

  7. Carrizo Citrange

  8. Swingle Citrumelo

  9. Rootstock Fruit

  10. Scion • Vigor, yield potential, cold tolerance • Fruit type- mandarin grapefruit, orange • Fruit quality- flavor, size, seediness, appearance, shelf-life

  11. Scion Hybrids • ‘Orlando,’ ‘Minneola’ tangelos- ‘Duncan’ grapefruit x ‘Dancy’ tangerine • ‘Page’ mandarin- ‘Minneola’ x ‘Clementine’ • ‘Oro Blanco’ grapefruit triploid- tetraploid pummelo x grapefruit

  12. Interspesific hybridisation

  13. Disease Resistance • Viruses- CTV, Psorosis, Exocortis • Bacteria- Citrus Variegated Chlorosis, Greening, Canker • Fungi- Alternaria, Scab, Melanose, Phytophthora

  14. Breeding Techniques • Cross-pollination- combine genes from different parents in hybrid progeny • Self-pollination- fix genes of interest in one line to stabilize phenotype (inbreeding) • Mutation- natural or induced genotypic modification

  15. Cross Pollination • Combine desirable traits from different genotypes/species and exploit heterosis • Swingle- one of earliest to make extensive crosses for rootstock improvement • Many hybrid rootstocks between Poncirus and Citrus- citrange, citrumelo, citrandarin

  16. Self-pollination • Natural mechanism for species to maintain genetic uniformity- pummelo, mandarins • Serious inbreeding depression in citrus overcome by apomixis- nucellar embryony • Important for gene inheritance and function studies

  17. Seed Structure

  18. Mutation • Natural mutations- ‘sports’ of buds or limbs: ‘Ruby Red,’ most orange varieties • Gamma rays- chromosome breaks cause genotypic changes: ‘Star Ruby,’ ‘Rio Red’ • Chemical and t-DNA- interrupt single genes

  19. Population Development • Pedigree- all progeny from specific cross carefully evaluated, limited genetic base • Recurrent selection- diverse populations improved by selection and intercrossed • Mass selection- large population evaluated for a few outstanding individuals

  20. Pedigree Method • Most citrus varieties developed by this method- few crosses • Relatively few parents (monoembryonic) as females; various males • Each progeny evaluated from each family

  21. Recurrent Selection • Each population developed for important traits- good genetic diversity • Crosses between individuals from improved populations evaluated for superior traits • Most productive over long period

  22. Mass Selection • Characterized or heterogeneous populations from relatively few crosses screened • Focus mainly on quantitative traits or genes with incomplete penetrance • Labor intensive but rapid improvement

  23. Biotechnology • Protoplast fusion of different genotypes • Gene mapping with molecular markers- gene cloning. • Genetic transformation with novel genes to modify DNA- Agrobacterium, biolistics

  24. Protoplast Fusion • Isolate cell protoplasts from callus or leaf tissue and fuse in vitro to form hybrids • Mostly polyploid plants regenerated from tissue culture- genetic hybrids • Overcome barriers to sexual reproduction

  25. Gene Cloning • Mapping genes in DNA with molecular markers- RAPD, RFLP, AFLP, etc. • Chromosome walking- locate DNA markers adjacent to gene of interest, clone gene inside bacterial plasmid • cDNA cloning- isolate genes from mRNA

  26. Map-based Cloning CGTTGA- part of FR gene 120 kb BAC RFLP but no RAPD RAPD RFLP AFLP SCAR .4 .6 1 CM FINGERBLIGHT RESISTANCE GENE

  27. Genetic Transformation • Insertion of cloned gene sequence into DNA (genome) of desirable plant • Modify single trait while maintaining good attributes of parent- SO with Ctv gene • Insertion point in genome not targeted

  28. Texas Priorities • Salt and drought tolerance • CTV and Phytophthora resistance • Cold and heat tolerance • Fresh market fruit- size, sugars, low acid

  29. Past Achievements in Texas • ‘Ruby Red Grapefruit’- bud sport of Thompson in LRGV, changed market • ‘Star Ruby’- irradiated seedling of ‘Hudson,’ darkest red grapefruit • ‘Rio Red’- irradiated budwood of ‘Ruby Red,’ most popular red grapefruit today

  30. Past Achievements in California • Hybrid mandarins- ‘Kinnow,’ ‘Pixie’ • Triploid seedless grapefruit- ‘Oroblanco’ • Hybrid red pummelo- ‘Chandler’ • Rootstocks- citranges, citrumelos

  31. Past Achievements in Florida • Tangelos- ‘Orlando,’ ‘Minneola,’ ‘Page,’ • Grapefruit- ‘Marsh seedless,’ ‘Duncan’ • Tangors- ‘Murcott,’ ‘Temple,’ ‘Fallglo,’ ‘Ambersweet’

  32. Ambersweet, a cold-hardy orange variety Shasta Gold, a large, seedless tangerine released 2002

  33. Current Variety Development • New triploid, seedless mandarins- CA, FL • New salt tolerant, dwarf rootstocks- CA,TX • New low acid grapefruits- CA • New fusion product rootstocks- FL

  34. Current Molecular Research • CTV resistance gene cloning- CA,TX,FL • Low acid gene mapping- CA • Fruit development gene mapping- CA,FL • Chromosome Isolation and Fusion-TX

  35. Future Goals • CTV resistance gene in susceptible scions and rootstocks • Phytopthora resistant, salt tolerant, high yielding rootstocks • Fruit- sweeter, seedless, longer shelf life

  36. Breeding Strategy for Texas • Increase effort in transgenics development • Increase emphasis on fruit quality and earliness for fresh market expansion • Increase research into genetic cold tolerance

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