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Citrus Breeding
Kevin M. Crosby
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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
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Breeding Goals
• Rootstock- tree size, stress tolerance
• Scion- fruit color, size, shape, flavor, yield
• Disease resistance- CTV, Phytophthora, CVC, Alternaria, Scab, Greening, etc.
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Rootstock
• Dwarfing- Poncirus, some mandarins
• Compatability- citrus better than Poncirus
• Seedling vigor and scion yield
• Fruit quality- size, shape, flavor, juice
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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
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Carrizo Citrange
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Swingle Citrumelo
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Rootstock Fruit
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Scion
• Vigor, yield potential, cold tolerance
• Fruit type- mandarin grapefruit, orange
• Fruit quality- flavor, size, seediness, appearance, shelf-life
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Scion Hybrids
• ‘Orlando,’ ‘Minneola’ tangelos- ‘Duncan’ grapefruit x ‘Dancy’ tangerine
• ‘Page’ mandarin- ‘Minneola’ x ‘Clementine’
• ‘Oro Blanco’ grapefruit triploid- tetraploid pummelo x grapefruit
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Disease Resistance
• Viruses- CTV, Psorosis, Exocortis
• Bacteria- Citrus Variegated Chlorosis, Greening, Canker
• Fungi- Alternaria, Scab, Melanose, Phytophthora
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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
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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
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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
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Seed Structure
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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
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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
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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
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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
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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
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Biotechnology
• Protoplast fusion of different genotypes
• Gene mapping with molecular markers- gene cloning.
• Genetic transformation with novel genes to modify DNA- Agrobacterium, biolistics
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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
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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
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Map-based Cloning
GENE
RFLP RAPD AFLPSCAR
FINGERBLIGHT
RESISTANCE
1 CM .4 .6
BAC120 kb
RFLP but no RAPD
CGTTGA- part of FR gene
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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
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Texas Priorities
• Salt and drought tolerance
• CTV and Phytophthora resistance
• Cold and heat tolerance
• Fresh market fruit- size, sugars, low acid
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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
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Past Achievements in California
• Hybrid mandarins- ‘Kinnow,’ ‘Pixie’
• Triploid seedless grapefruit- ‘Oroblanco’
• Hybrid red pummelo- ‘Chandler’
• Rootstocks- citranges, citrumelos
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Past Achievements in Florida
• Tangelos- ‘Orlando,’ ‘Minneola,’ ‘Page,’
• Grapefruit- ‘Marsh seedless,’ ‘Duncan’
• Tangors- ‘Murcott,’ ‘Temple,’ ‘Fallglo,’ ‘Ambersweet’
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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
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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
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Future Goals
• CTV resistance gene in susceptible scions and rootstocks
• Phytopthora resistant, salt tolerant, high yielding rootstocks
• Fruit- sweeter, seedless, longer shelf life
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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