When crossing polyploid Rhododendrons we have noticed some offspring have a 2C value that is less than the C value of the seed parent + C Value of pollen parent. This can be true even when the higher 2C value is the seed parent.
To date we have no evidence to suggest Rhododendron reproduce asexually. Flow cytometry on seed indicates sexual reproduction.
Examples of lower than expected 2C values are:
tetraploid (seed parent) X diploid (pollen parent) yields in some cases diploid offspring.
2x X 4x yielding some 2x
4x X 6x yielding some 4x
4x X 8x yielding some 4x
4x X 10x yielding some 4x
6x X 6x yielding some 4x
2x X 3x yields some 2x
3x X 2x yields some 2x
Is this smaller than normal 2C value for the offspring normally related to meiosis of one of the parent prior to fertilization, abnormal development of the embryo, mitosis of the seedling after germination, or something else?
Apomixis might be one of the explanation (see details in the enclosed file).
@ I agree with the above answers. Parthenogenesis, chromosome elimination and unequal segregation of chromosomes during meiosis are likely to be the reasons.
Note: In general the offspring having these lower than expect 2C values show characteristics of both parents. In fact in one instance the offspring look much more like the pollen parent than the seed parent. Moreover in those cases where we have looked at flow cytometry for seed of these same parents, the flow cytometry for the peaks of the embryo and endosperm indicate sexual reproduction.
If apomixis is excluded, another cause to consider is related to the origin of polyploidy in the parents (auto- or allopolyploidy). During the prophase of the first meiotic division of an autopolyploid parent, some bivalents will form, but multivalents containing three or more chromosomes connected by synaptonemal complexes and univalents with a single chromosome, will also form . The aberrant distribution of chromosomal material at the end of the first meiotic division generates many aneuploid diads and gametes will not all get the same numbers of chromosomes. This imbalance will cause partial sterility upon self-fertilization and lower than expect 2C values after hybridization.
Dr. Circo De Pace, Dr. M.C Sidhu, Dr . Babar Hussain
In our work crossing triploid deciduous azaleas X tetraploid deciduous azaleas we clearly have found that the triploid provides a distributed contribution varying from 1X to 2x and some 3x. Most of the offspring of these 3x X 4x crosses have 2C values reflecting aneuploids between triploid and tetraploid. Our results very much reflect the results that Darlington got when working with triploids in the late 1920's early 1930's.
However in the lepidote species rhododendrons some of the offspring of 4x X 6x, 4x X 8x, 4x X 10x seem to be close to 2C value of the 4x yet the seedlings show hybrid characteristics of the 2 parents.
My question is,do you think that it is more likely the 6x, 8x, and 10x are only contributing 2x gamete to the zygote or that the resulting embryo is using (conserving) only 2x worth of gamete being contributed by the pollen parent?
Dr. Circo De Pace, Dr. M.C Sidhu, Dr . Babar Hussain
Note: In our work crossing tetraploid deciduous azaleas species X autooctaploid (chemically induced) deciduous azaleas, the resulting offspring are hexaploid as expected.
We hope to do crosses using these hexaploid offspring starting in spring 2015.
Note: In one instance an open pollinated triploid deciduous azalea produced 10 offspring each reflecting a flow cytometry 2C value in the typical triploid range for deciduous azalea very close to the 2C value for the seed parent. The 2C value for this triploid and its offspring were very close to the 2C values we get when we close diploid deciduous azaleas by tetraploid deciduous azaleas.There was no indication based on their flow cytometry scores that any of the offspring of this open pollinated triploid deciduous azalea were aneuploids.
Dr . Babar Hussain
In crosses involving
3x X 4x
we have seen instances where 5x offspring are produced.
However aneuploids between triploid and tetraploid are much more likely.
The percentage of pentaploids produced seems to differ depending on tetraploid parent even when the triploid seed parent is the same, However conditions such as temperature may be what is causing the differences in the percentage of pentaploids produced since we are doing these crosses outside.
Dr . Babar Hussain
In fact when doing
3x X 4x
we have seen 2C values reflecting
close to 3x
above 3x but below 3.5x
close to 3.5x
above 3.5x but below 4x
close to 4x
above 4x but below 5x (This is rare but does happen)
close to 5x
Further explanation to what you observe might come from more basic knowledge on the geographical origin of the polyploids that you are crossing, the mechanism of polyploidization of the parents, and cytogenetical observation on the chromosome pairing cofiguration at meiosis of the the parents and their hybrids.
The multitude of the Rhododendron polyploid species are not distributed at random but they fit precise geographical and altitudinal distibutions. Diploids might have had recurrent polyploidization events whenever a new ecological opportunity was offered during colonization of new habitats. Each time that migration occurred from a diploid species the genotypes that occupied the new ecological niche were different from those of the previous dispersal event although they all have risen the chromosome number to the same polyploidy level. This cause genetic differentiation among the ecotypes with the same ploidy level. When such homoploid ecotypes with different genetic make-up are crossed, abnormal mitosis in the developing embryos might occur, leading to elimination of selected chromosomes (aneuploidy) or elimination of the whole chromosome set of one of the parents. This has also been observed in crosses between 4x X 2x or 6x X 2x Poaceae species having a wide taxonomical distance. When the diploid Hordeum bulbosum or the diploid maize pollinate the tetraploid or the hexaploid wheat, diploid and triploid hybrid embryos, respectively, are formed carrying only the haploid set of wheat chromosomes. Inspection of meiosis of the ‘pollen mother cells’ of the hybrid plant will greatly help in interpreting the chromosome number observed in the hybrid root tips (in case of the wheat example, only monovalents and rare rod bivalents are observed in the resulting diploid or triploid hybrid). Chromosome painting techniques (i.e. fluorescent in situ hybridization or FISH) might help to deduce the degree of chromosome differentiation between the parents that might be causing the abnormal chromosome number observed in the hybrid progeny.
Knowing the mechanism that leads to polyploidization is also important. Allopolyploids can be distinguished because they are self fertile, only bivalents are usually formed at meiosis, and disomic inheritance is observed for biallelic genetic markers; on the other hand, autopolyploids forms multivalents which lead to reduced fertility and display polysomic inheritance. If the parents have the same ploidy level but one is allopolyploid and the other is autopoliploid, and cross fertilization occur, the progeny will show less chromosomes than that expected from the sum of the parental haploid chromosome number. Therefore, adding simple observations on taxonomy, geographc origin, pollen fertility, and chromosome pairing in parents will help in explaining the 2C values observed in the hybrid embryos.
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