Great points Paul!! It is amazing the variability out there. It seems like ‘Tangerine Jewel’ has pollen the diameter expected for unreduced or 2n pollen. n refers to life stage and x to sets of chromosomes or ploidy. So n= the sex cell or gamete and 2n= the sporophyte or plant we see. So when we say 2n pollen we kind of mix the terms and say a gamete (it is pollen) and that is has the same chromosome count that the parent has.
When the original cells starts out (microspore mother cell for pollen or megaspore mother cell leading to the egg) it is a normal cell and has every chromosome duplicated. It is kind of like a cell with duplicated chromosomes ready to divide into two daughter cells for two identical copies (mitosis). However, for meiosis there are two rounds of division I and II as mentioned before. During I copies of like chromosomes (contains the same genes generally, although the alleles or forms of the gene can differ) find each other and sometimes exchange arms for crossing over and added variability. At the end of Meiosis I the matched chromosomes get divided up and pulled to opposite poles. With three of each chromsome (triploid), as mentioned it doesn’t happen evenly for complete sets of all the chromosomes moving together. One side may have one each of every chromosome plus an extra copy of just some.
In Meiosis II those chromosomes which are of course duplicated with the copies stuck together have the two copies split apart to the four final gamete cells. Typically then each gamete has half as many chromosomes as the parent plant. At the end of meiosis II the poles that all the chromosomes get pulled to and spindle fibers are typically at a 60 degree angle. Cytokinesis or cell division and membrane development typically happens at 90 degree angles to the spindle fibers and directional movement of the chromosomes. Each group of chromosomes are being pulled slightly different directions and we have four gametes that form (four pollen grains form typically and on the female side all but one survive to eventually make the egg and associated cells).
In meiosis II if the spindle fibers separating the two groups fo chromosomes after meiosis I ar parallel instead of at 60 degrees, then as cytokinesis occurs and we know it happens perpendicular to the direction of movement of the chromosomes, we can get just two gamete cells forming and no matter how the chromosomes divided in meiosis I we should have three complete sets in each of these two gametes which is nice. This is called 2n gamete formation due to parallel spindles. There are other mechanisms too. This helps triploids have fertile balanced gametes at times which have 3 sets of chromosomes.
4n gametes can arrise if the starting cell with its duplicated chromosomes just let them separate and everything stays in the one cell and it becomes a gamete. So the gamete then has twice as many chromosomes as the starting parent plant.
These are just ways for higher ploidy, balanced gametes that can help triploids (other ploidy levels too) to have balanced gametes. There are other ways too and of course we know triploids can have 1x or 2x pollen too.
It is really fun to think about and the genetic consequences of transferring much of the parent plants genetics directly into resulting offspring.
Hopefully I was complete enough in the description for people to follow where I was going.
David