Read this to get an idea. Basically, a plant forms a seed but the seed contains one or more embryos that are formed from maternal tissue. In other words, the embryo was not fertilized by pollen. An apomictic embryo will produce a plant identical to the maternal parent.
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I’ve got quite a few hc seedlings…nothing great to report back yet. The one tree had fruit this year but most of them rotted. The ones i did (the unbad parts) had that hc taste to them. I’ll try again next year. Very aggressive tree…thing is huge after 4 years and all the fruit were very high in the canopy…so maybe winter damage or spring cold damage to lower buds…have no idea… Also budded some other seedlings (last summer) onto an older apple…maybe next year i’ll get a sample.
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Please keep us up on your progress. Honeycrisp is a nice apple.
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I wonder if you could please explain (as plainly as possible) a possible cause of this study… The full article is in Russian and behind a paywall so I just copied the abstract here:
Variation in three generations of apomictic fig seedlings.
Author(s) : Arendt, N. K.; Kazas, A. N.
Journal article : Tr. Nikitsk. botan. sad 1980 Vol.80 pp.7-29 ref.15
Abstract : Application of foreign pollen or physiologically active substances to the flowers of Ficus carica resulted in the apomictic development of embryos. Viable seeds were obtained and three generations of apomictic seedlings were raised which differed in morphological and biological characters. Useful forms were selected in the first and second generations.
I read that the formation of haploid cells can result in crossing over during meiosis, which results in some variation.
https://www2.palomar.edu/users/warmstrong/pljun99b.htm
I wonder if you could also answer whether an apomictic seed that arose from a haploid cell that doubled to become diploid would then be homozygous? Or if there are any other possible ways for a heterozygous parent to produce homozygous seeds?
Start with one important concept. Apomictic seedlings are not necessarily doubled haploids, but doubled haploids represent one particular type of apomixis. Doubled haploids are an important tool in plant breeding for example in developing pure breeding lines in maize. Doubled haploids do not occur in most plant species so specific methods have been developed to induce them.
I will describe an apomictic seedling in terms of citrus which is the variant I am most familiar with. Not all citrus species produce apomictic seed. For an apomictic seed to form, the flower generally has to be pollinated. Inside the forming seed, the sequence of events is that one or more clusters of cells develop with simple cell replication, in other words, all chromosomes are duplicated. The group of cells that will form an embryo then diverges going through three separate divisions to produce 7 haploid cells and 1 diploid cell. One of the haploid cells is “imprinted” to become the future embryo. the diploid cell is imprinted to become the endosperm. The remaining haploid cells perform very specific functions associated with pollen merging with the proto-embyro and the proto-endosperm. All eight cell nuclei are required for a fully functional embryo to develop and for the endosperm to form but only one of the nuclei is fully reproductive. When the flower is pollinated, each pollen grain contains 2 nuclei. As the nuclei move down the pollen tube toward the proto-embryo, the 2 pollen nuclei split with one merging with the proto-embryo and the other with the proto-endosperm. At this point, the endosperm develops as triploid tissue storing nutrients to feed the developing seedling. I am unsure exactly what happens to the fertilized embryo at this stage, but it should stay relatively quiescent until germination is triggered.
So what goes wrong with an apomictic seedling? One mechanism is that 2 of the “helper” nuclei merge producing a diploid cell that under some conditions can germinate. If this is correct, then chromosome pairing would cause some chromosomes to be lost while others become homozygous. This could easily explain the divergence in ficus seedlings as per your article. I can’t prove this is the correct mechanism so it is entirely speculation on my part. A second mechanism that some plants exhibit is to produce a haploid embryo (with triploid endosperm!), presumably from failure of the pollen grain merging with the proto-embryo. This haploid embryo would then undergo a duplication to become diploid followed by normal cell division. The other mechanism and the one that commonly occurs in citrus is that a diploid cell initiates meiosis (to split into haploid nuclei) but the cell then fails to complete separation. The chromosomes then re-pair and the cell behaves as though it were a fertilized embryo. This commonly occurs in nucellar tissue i.e. in the tissue surrounding the normal embryo. As a result, a single citrus seed may have 1 fertilized normal embryo and half a dozen nucellar embryos. It is common for the nucellar embryos to be more vigorous than the fertilized embryo which conveys a survival advantage. Nucellar seedlings are effectively clones of the maternal parent but with the important difference that plant viruses can often be eliminated. An infected parent can easily produce a virus free clonal seedling.
There are other mechanisms described in the wiki article with documentation describing how they work.
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Thank you very much Darrel. I would like to sort out some of the various fig fruit genes and being able to make homozygous plants would really help I think, but probably not enough to go to great lengths… and persistence is seed lethal for figs so any homozygous plants may all require pollination and I thank that also means I would only get to see half of the genes fully expressed if that were the case.
I might try using mulberry and osage orange pollen again to see if any variations could be useful. Only 25% of sexually produced seedlings (with a persistent pollen parent) will be persistent females so the odds are much better also.
Thanks again.
Honeycrisp continues to gain popularity as a breeding apple more all the time.