What exactly are haploids, dihaploids, and tetrahaploids? Do they occur in roses?
These are great questions Mike.
In general the US scientists and some European call a haploid one that has half of the chromosomes of the parent plant and is typically formed by a sex cell giving rise to a whole organism without fertilization. THis can be an unfertilized egg, synergid (same genetic constitution as the egg) or pollen that in tissue culture can regenerate a new plant. THere are some more out of the ordinary ways this happens.
In Europe there is a strong contingent of scientists that like to use prefixes to quantify the ploidy of the haploid. So a dihaploid as a group of French researchers report generating in rose are diploid plants derrived from tetraploid roses. They also use the term haploid to mean one set of chromosomes if a prefix is not given. There is some debate as to what use of terminology is best. One can say a rose is a haploid out of some rose and then describe its ploidy (monoploid, diploid, triploid, etc.).
There is also some disagreements/confusion with other genetic terminology like n, 2n, 3n,
The more recent scientists tend to use 2n to describe the sporophyte of the plant (what we see) and n for the gametophyte of sex cell. x then is used to describe how many sets of the basic chromosome number is there. So, for a tetraploid rose like Rosa arkansana 2n=4x=28 for 28 total chromosomes. A typical sex cell is n=2x=14. Then there comes the terminology of 2n pollen. This is pollen, or a sex cell, that has the same chromosome number as the 2n or sporophyte that produced it. This mixes the terminology telling us that we are talking about the gametophyte because of saying pollen and then the chromosome number is the same as the sphorophyte that generated it. Saying n interchangably with x is something that some choose to do.
Some roses are more prone to generating haploids via unfertilized eggs than others and some crosses are more prone to generate them than others too. The pollen helps to start embryo development, but doesn’t participate in fertilization of the egg or if so, the chromosomes are kicked out. IN potato there is evidence that there is some somatic recombination between chromosomes at times and then the pollen donor’s chromosomes can be kicked out as they have found relatively few DNA segments of the pollen donor in the haploid. Sometimes the pollen nucleus and egg nucleus do not fuse and then lead to cells that have one or the other. As the embryo grows one type of nucleus will overtake the growing point. This way we can get a haploid with the DNA of the father.
My work in potatos and generating haploids of modern tetraploid potatoes was a lof of fun. If we keep our eyes open in rose we can find them. Sometimes they are just weaker than the siblings in the population and we overlook them I suspect.
Sincerely,
David
David, would you expect that a very small embryo among many other large embryos from the same hip/cross would have a greater liklihood of being a haploid?
Hi Don,
Yes, I think it would be more likely. When a synergid forms into an embryo and there are twins that share the same testa, the haploid one is typically much smaller than the normal, fertilized one. Because I germinate my seeds in baggies and transplant them I think it’s been easier as I pull apart the remaining seed coat on some to find these small embryos. I pot them up very carefully. Most are too weak and are lost, but some have made it.
David
These are very exciting informations. I am trying to get offspring of a cultivar of extremely low fertility (no hips, so I use it as pollen parent). I get hardly any seed, and all my efforts of several years led to only two seedlings.
In 2008 I found out my hip parent can set seed apomictically (demasculated flowers, no pollination led in some cases to hip formation and seed)
The seeds of my crossings were no twins. Both seedlings don’t look like the pollen parent, but also differ from their mother plant. So, even not getting proper offspring of the pollen donor, there is the possibility, that some genes of the pollen donor is in them.
Ullus
David,
Do you know of any reliable lab protocols for rose karyotyping?
Tony