North American Fruit Explorers mailing list at ibiblio

["Hal Love" <lovehd@comcast.net>]

Jerry,

 

The pairing of chromosomes is usually a fertility problem in the hybrids, and not an issue in trying to produce the hybrids. The difficulty in crossing may be related to pollen compatibility or pollen tubes not growing properly, or embryos aborting. The sterility often seen in hybrids that contain the same number of chromosomes is in part due to a relatively high similarity, but with some rearrangements of the DNA within the chromosomes, such as translocations and inversions. Also, some chromosomes may not pair up at all, and get randomly distributed. This results in improper pairing during meiosis, where one chromosome may pair up with two chromosomes or pair up in an uneven manner. The result is gametes that have gained or lost whole chromosomes or contain abnormal chromosomes. Occasionally, just by chance, the chromosomes may not pair up completely, and they get distributed randomly, with a few gametes containing the correct number of unique chromosomes (with 90 chromosomes, this is not likely to happen very often). When cross-pollinated with good fertile pollen, the occasional good seed is produced. Pollen production is often abnormal due to very few good pollen grains, so the hybrids are frequently pollen sterile. Continued back-crossing and selection may eventually restore full fertility. Keep plugging away at it, and you may very well succeed (but it may take a long time!)

 

Hal

 

 

 

 

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A tool often used in hybridizing as Lon wrote is doubling the chromosome numbers thus creating tetraploids, or 2 sets of each chromosome.  But if you cross this to a diploid the result is a triploid (three chromosomes of each) which resulting plant or animal is sterile. The interesting question is, why are the 90 chromosome American persimmon and the 90 chromosome kaki persimmons nearly impossible to hybridize? Both are hexiploid, or three sets of each chromosome. There should be lots of chromosomes to match up easily. But they don't, at least not easily. The Russian hybrid of kaki x virginiana, Rosseyanka, does produce a few seeds and some of them produce seedlings. Whether they are apomictic or a recombination with pollen is still not known for sure, but think they are out crosses. None of the about 10 growing here have born fruit yet, but the leaf differences would indicate outcrosses. 

For what it's worth!

Jerry

PS. Japanese research scientists have found that some kaki are 120 chromosome. They are breeding these for larger fruiting kaki. As 120 C. would have larger pollen grains they are sifting pollen through stretched silk cloth. The smaller grains (likely 90 C.) fall through and the larger grains remaining in the cloth are used for breeding.