Post by Deleted on Aug 1, 2019 15:12:08 GMT -5
Here is some information that might be useful for citrus hybridization:
As many of you know, many types of common citrus cultivars are considered to be polyembryonic (that produce seeds which are genetic clones of the parent). This can cause problems for breeding because most of the seeds in a polyembryonic cultivar will just be clones of the parent, rather than inheriting any traits from the parent the tree was pollinated with. However, not absolutely all of the seeds in a polyembryonic citrus variety will be clones, a few will be zygotic (the result of sexual recombination).
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data in this table comes from Frost and Soost (1968)
H.B. Frost, R.K. Soost, (1968) Seed reproduction: development of gametes and embryos.
In: W. Reuther, L.D. Bachelor, H.J. Webber, The Citrus Industry, Volume II. Division of Agricultural Sciences. University of California Berkeley, pp 292-334
Clementine and Temple Orange are two varieties that have frequently been used in breeding as the female parent because they produce all zygotic seed.
_________________________________________
According to one study, out of 67 Yuzu seedlings examined, 19.4 percent were zygotic.
(Isozyme analysis of citrus rootstock populations to identify zygotic seedlings, S. Fatta Del Bosco, G. Matranga, G. Geraci, Advances in Horticultural Science, Vol. 8, No. 2 (1994), pages 71-74 )
Calamondin was 7 percent zygotic.
Sour orange 26.7 percent.
_________________________________________
Crossing a tetraploid citrus with a regular diploid citrus will result in a triploid citrus. Triploids are generally sterile and seedless. This is a strategy that has been used in many cases to breed seedless citrus varieties (although there are other strategies).
It turns out that most regular citrus varieties—the ones that are polyembryonic producing clonally from seed—will convert to tetraploids at least once in a while. That is, if you grow 300 seeds, at least one of them will likely turn out to be tetraploid, even though it is otherwise a clone of its diploid parent. Tetraploids are often slightly bigger and slightly deeper in color than their corresponding diploid parent.
Some citrus varieties are more likely to convert to tetraploids than others. For example, out of 78 seeds grown from a Duncan grapefruit, 5 turned out to be tetraploids.
'Mapo' tangelo (7 out of 73) and 'Tardivo di Ciaculli' mandarin (2 out of 38) also had high rates of tetraploid seed. For Minneola Tangelo, 3 tetraploids were observed out of 166 seeds.
Something else notable, it appeared that plants producing fruit in colder conditions in marginal climatic areas had an increased likelihood of producing tetraploid seed.
Troyer citrange had very high rates of polyploidy, as high as 10-20 percent of the seedlings. (Carrizo was almost as high too)
_______________________________________
P. Ollitroult found half diploid and half triploid among the seeds of 'Oroblanco'.
Citrus genetics, breeding and biotechnology, Iqrar A. Khan, p205
Only diploids were found in the progeny of clementine fertilized by Oroblanco pollen, the same source says.
Triploid pollen can induce the formation of haploid embryo from the female fruit parent; the single chromosome set would always come from the female parent in such a case.
"This study reports haploid plantlet regeneration through gynogenesis in Citrus clementina Hort. ex Tan., cv. Nules, induced by in vitro pollination with pollen grains of Oroblanco, a triploid cultivar of grapefruit. It indicates that parthenogenesis induced in vitro by triploid pollen can be an alternative method to obtain haploids in monoembryonic cultivars of Citrus."
( Gynogenetic haploids of Citrus after in vitro pollination with triploid pollen grains, M.A. Germanà, B. Chiancone. Plant Cell, Tissue and Organ Culture, July 2001, Volume 66, Issue 1, pp 59–66 )
_________________________________________
One study looked at the seeds of triploid citrus (this particular cultivar having resulted from a tetraploid cross between Hamlin orange x rough lemon, which then was crossed with diploid tangerine) which was open-pollinated, and out of 25 well-developed seeds, 13 were triploid, 10 were diploid, 1 was tetraploid, and 1 was aneuploid. What was particularly interesting was that all of the triploids appeared to be nucellar.
( Ploidy variation and genetic composition of openpollinated triploid citrus progenies, Shi-Ping Zhu, Jian-Kun Song, Botanical Studies (2009) 50: 319-324 )
The findings suggest very low likelihood of triploid meiosis being able to produce a diploid gamete, and so presumably the single tetraploid seed must have resulted from an unreduced triploid (3n) gamete being sexually combined with a normal haploid (1n) gamete. The zygotic seeds of a triploid variety would be diploid because two normal haploid (1n) gametes combine together as normal.
_________________________________________
"Isozyme analysis was the basis for determining the frequency of occurrence and the characteristics of zygotic plants in Swingle citrumelo seedling populations from various sources of open-pollinated seeds, in a commercial nursery of Swingle citrumelo before and after roguing, and in commercial orchards and rootstock trials where this rootstock was used. Most zygotic seedlings identified by isozyme analysis could be distinguished by careful examination of morphological characteristics. Frequencies of zygotic seedlings varied among seedling populations, but were in the range (≈5% to 10%) found in previous studies. Roguing based primarily on size and growth habit of seedlings was effective in removing some, but not all, zygotic seedlings.
… prior studies showed that the frequency of zygotic plants in Swingle seedling populations maybe as high as 18% (Hutchison, 1974; Moore and Castle 1988, Xiang and Roose, 1988 )."
Isozymic Identification of Zygotic Seedlings in Swingle Citrumelo Citrus paradisi × Poncirus trifoliata Nursery and Field Populations, Catalina M. Anderson, William S. Castle, University of Florida, J. AMER. SOC. HORT. SCI. 116(2):322-326, 1991
journal.ashspublications.org/content/116/2/322.full.pdf
"The percentage of zygotic seedlings was <10 in C-32 citrange, Gomeri and Indio rough lemon, 10–30 in C.P.B. 4475 citrumelo, Cuban shaddock, Volkamer lemon and Yuma Ponderosa lemon, and >30 in Sacaton and Terra Bella citrumelos, Taiwanica sour orange and Yuma citrange. Small seed had lower germination and seedlings were smaller than those derived from normal seed. In all rootstocks except Yuma citrange, the frequency of zygotics in seedlings from small seed was not significantly different from that in populations derived from normal seed. Zygotic seedlings were generally shorter than nucellar seedlings. The distribution of height of zygotics considerably overlapped that of nucellars in most rootstocks, so that roguing by height alone was relatively ineffective. For the polyembryonic accessions studied, zygotic seedlings are as likely to occur in seeds producing 2 seedlings as in those producing 1 seedling."
Frequency and characteristics of nucellar and zygotic seedlings in 12 citrus rootstocks, C.Xiang, M.L.Roose, Scientia Horticulturae, Volume 37, Issues 1–2, November 1988, pages 47-59
As many of you know, many types of common citrus cultivars are considered to be polyembryonic (that produce seeds which are genetic clones of the parent). This can cause problems for breeding because most of the seeds in a polyembryonic cultivar will just be clones of the parent, rather than inheriting any traits from the parent the tree was pollinated with. However, not absolutely all of the seeds in a polyembryonic citrus variety will be clones, a few will be zygotic (the result of sexual recombination).
seed parent | Seedlings/seed | % nucellar |
Lemon: Eureka, Lisbon, etc | 1.05-1.06 | 32-33 |
Rough Lemon | 1.24-1.96 | 54-98 |
Mexican Lime | 1.29 | 78 |
Mandarin: Dancy, Kara | 1.37-1.71 | 100 |
Mandarin: Satsuma | 1.44 | 90 |
Mandarin: Kishu | 1.00 | 0 |
Mandarin: King, Ponkan | 1.01-1.42 | 21-98 |
Grapefruit: Marsh | 1.08 | 96 |
pummelo: 11 cultivars | 1.00 | 0 |
Sweet orange: 4 cultivars | 1.09-2.00 | 39-97 |
Sour orange | 1.21 | 85 |
Tangelo: Orlando, Minneola | 1.31-1.49 | 83-97 |
Trifoliate orange | 1.03-1.26 | 13-73 |
data in this table comes from Frost and Soost (1968)
H.B. Frost, R.K. Soost, (1968) Seed reproduction: development of gametes and embryos.
In: W. Reuther, L.D. Bachelor, H.J. Webber, The Citrus Industry, Volume II. Division of Agricultural Sciences. University of California Berkeley, pp 292-334
Clementine and Temple Orange are two varieties that have frequently been used in breeding as the female parent because they produce all zygotic seed.
_________________________________________
According to one study, out of 67 Yuzu seedlings examined, 19.4 percent were zygotic.
(Isozyme analysis of citrus rootstock populations to identify zygotic seedlings, S. Fatta Del Bosco, G. Matranga, G. Geraci, Advances in Horticultural Science, Vol. 8, No. 2 (1994), pages 71-74 )
Calamondin was 7 percent zygotic.
Sour orange 26.7 percent.
_________________________________________
Crossing a tetraploid citrus with a regular diploid citrus will result in a triploid citrus. Triploids are generally sterile and seedless. This is a strategy that has been used in many cases to breed seedless citrus varieties (although there are other strategies).
It turns out that most regular citrus varieties—the ones that are polyembryonic producing clonally from seed—will convert to tetraploids at least once in a while. That is, if you grow 300 seeds, at least one of them will likely turn out to be tetraploid, even though it is otherwise a clone of its diploid parent. Tetraploids are often slightly bigger and slightly deeper in color than their corresponding diploid parent.
Some citrus varieties are more likely to convert to tetraploids than others. For example, out of 78 seeds grown from a Duncan grapefruit, 5 turned out to be tetraploids.
'Mapo' tangelo (7 out of 73) and 'Tardivo di Ciaculli' mandarin (2 out of 38) also had high rates of tetraploid seed. For Minneola Tangelo, 3 tetraploids were observed out of 166 seeds.
Something else notable, it appeared that plants producing fruit in colder conditions in marginal climatic areas had an increased likelihood of producing tetraploid seed.
Troyer citrange had very high rates of polyploidy, as high as 10-20 percent of the seedlings. (Carrizo was almost as high too)
_______________________________________
P. Ollitroult found half diploid and half triploid among the seeds of 'Oroblanco'.
Citrus genetics, breeding and biotechnology, Iqrar A. Khan, p205
Only diploids were found in the progeny of clementine fertilized by Oroblanco pollen, the same source says.
Triploid pollen can induce the formation of haploid embryo from the female fruit parent; the single chromosome set would always come from the female parent in such a case.
"This study reports haploid plantlet regeneration through gynogenesis in Citrus clementina Hort. ex Tan., cv. Nules, induced by in vitro pollination with pollen grains of Oroblanco, a triploid cultivar of grapefruit. It indicates that parthenogenesis induced in vitro by triploid pollen can be an alternative method to obtain haploids in monoembryonic cultivars of Citrus."
( Gynogenetic haploids of Citrus after in vitro pollination with triploid pollen grains, M.A. Germanà, B. Chiancone. Plant Cell, Tissue and Organ Culture, July 2001, Volume 66, Issue 1, pp 59–66 )
_________________________________________
One study looked at the seeds of triploid citrus (this particular cultivar having resulted from a tetraploid cross between Hamlin orange x rough lemon, which then was crossed with diploid tangerine) which was open-pollinated, and out of 25 well-developed seeds, 13 were triploid, 10 were diploid, 1 was tetraploid, and 1 was aneuploid. What was particularly interesting was that all of the triploids appeared to be nucellar.
( Ploidy variation and genetic composition of openpollinated triploid citrus progenies, Shi-Ping Zhu, Jian-Kun Song, Botanical Studies (2009) 50: 319-324 )
The findings suggest very low likelihood of triploid meiosis being able to produce a diploid gamete, and so presumably the single tetraploid seed must have resulted from an unreduced triploid (3n) gamete being sexually combined with a normal haploid (1n) gamete. The zygotic seeds of a triploid variety would be diploid because two normal haploid (1n) gametes combine together as normal.
_________________________________________
"Isozyme analysis was the basis for determining the frequency of occurrence and the characteristics of zygotic plants in Swingle citrumelo seedling populations from various sources of open-pollinated seeds, in a commercial nursery of Swingle citrumelo before and after roguing, and in commercial orchards and rootstock trials where this rootstock was used. Most zygotic seedlings identified by isozyme analysis could be distinguished by careful examination of morphological characteristics. Frequencies of zygotic seedlings varied among seedling populations, but were in the range (≈5% to 10%) found in previous studies. Roguing based primarily on size and growth habit of seedlings was effective in removing some, but not all, zygotic seedlings.
… prior studies showed that the frequency of zygotic plants in Swingle seedling populations maybe as high as 18% (Hutchison, 1974; Moore and Castle 1988, Xiang and Roose, 1988 )."
Isozymic Identification of Zygotic Seedlings in Swingle Citrumelo Citrus paradisi × Poncirus trifoliata Nursery and Field Populations, Catalina M. Anderson, William S. Castle, University of Florida, J. AMER. SOC. HORT. SCI. 116(2):322-326, 1991
journal.ashspublications.org/content/116/2/322.full.pdf
"The percentage of zygotic seedlings was <10 in C-32 citrange, Gomeri and Indio rough lemon, 10–30 in C.P.B. 4475 citrumelo, Cuban shaddock, Volkamer lemon and Yuma Ponderosa lemon, and >30 in Sacaton and Terra Bella citrumelos, Taiwanica sour orange and Yuma citrange. Small seed had lower germination and seedlings were smaller than those derived from normal seed. In all rootstocks except Yuma citrange, the frequency of zygotics in seedlings from small seed was not significantly different from that in populations derived from normal seed. Zygotic seedlings were generally shorter than nucellar seedlings. The distribution of height of zygotics considerably overlapped that of nucellars in most rootstocks, so that roguing by height alone was relatively ineffective. For the polyembryonic accessions studied, zygotic seedlings are as likely to occur in seeds producing 2 seedlings as in those producing 1 seedling."
Frequency and characteristics of nucellar and zygotic seedlings in 12 citrus rootstocks, C.Xiang, M.L.Roose, Scientia Horticulturae, Volume 37, Issues 1–2, November 1988, pages 47-59