I've got my hands on a number of other articles relating to this subject matter. Most of them are usually paid for when acquired. The first article I'd like to discuss is written by two Japanese and a Korean. It's really interesting stuff, but its quite technical as well. Let me walk you through it.
The diversity of interspecific pollen-pistil incongruity in Nicotiana
As I mentioned, this one is quite in-depth when it comes to technical aspects. But it's exactly what I was looking for. Basically, the researchers investigate what happens to the pollen tubes when attempting to make three different hybrids. For all three hybrids,
N. tabacum functions as the female parent. Three other species within
Nicotiana provide the pollen. They are
N. repanda,
N. rustica and
N. trigonophylla. For this current thread we are mainly interested in the cross with
N. rustica. That being said, it's also quite interesting to see how the behaviour of pollen is different in different species. So we'll have a look at all of them.
The first given from the article is that the pollen tubes, which act as the conduit to transport the male gamete from the pollen to the female ovary, are unilaterally inhibited by the pistils of
N. tabacum. So where the pollen of
N. tabacum pollen on a
N. tabacum pistil would send the gamete straight into the ovary, this is hindered when the pollen is not from
N. tabacum but from one of the other mentioned species. This is what makes it difficult to create hybrids.
The authors refer to another article which I will come to in a later post, that holds that there are many combinations of hybrids in which pollen tubes only reach the ovary in one direction of the cross, but not its reciprocal. We had heard this before. This is what many report to be the case in
N. rustica x
N. tabacum versus the other way around,
N. tabacum x
N. rustica. In this current article we will find out why this is the case.
So this research revolves around measurements of pollen tubes. Pollen tube morphology and elongation patterns. To have material for comparison they start with looking at self-fertilization. After that they go for the hybrids. It's too bad that these aren't colour shots, but they're good enough to make things out.
It might take a while to adjust to these microscope photographs. They are all pollen tubes, and they should be straight lines, headed for the ovary. When self fertilized,
N. tabacum,
N. repanda,
N. rustica and
N. trigonophylla all portray straight lines on the pictures. The hybrid pollen tubes do not. For our focus here, picture G is the most interesting as it shows
N. tabacum x
N. rustica. The direction of the tubes is winding sort of a winding trend. This is not good, because it limits the length in which the tube can stretch itself. It needs a minimum length in order to reach the ovary. There is another figure in this paper which shows a longitudinal cross section of a pollen tube of the
N. tabacum x
N. rustica hybrid and it shows a pollen tube elongating backward. You don't reach ovaries that way, that's for sure.
So let's have a little look at the graphical trajectory of pollen tubes.
As described, the first graph shows the average length of pollen tubes in self fertilizing species. As you can see,
N. tabacum pollen tubes travel furthest.
N. repanda finishes a good second. It takes a further distance to reach the
N. tabacum ovary. So when we're looking at the behaviour of pollen tubes in the cross fertilizations we see that on average, the pollen tubes of other species do not reach the required length. We also see that they do behave different compared to fertilizing one of their own species. For the first 12 hours
N. rustica pollen tubes elongate even faster than
N. tabacum fertilizing itself. However, after reaching 23 mm on average, there wasn't much growth detected. Some pollen would swell, circle about, or even move backward.
The authors give each style of inhibiting a label. For
N. rustica to fertilize
N. tabacum, they call it the Winding type because of the form of movement by the pollen tubes.
N. repanda is the stigma type, as these pollen remained on the stigma.
N. trigomophylla is the delay type. But there is yet another thing.
They look at the pollen tubes in further detail, namely, pollination at different ages of the female flower pistil. They divide these stages into four. For all three inhibiting types it became apparent that the inhibition is strengthened during maturation of the pistil. In other words, the older the flower, the less changes of pollen tubes reaching the ovary. In
N. tabacum x
N. rustica, pollen tubes were found in every ovary when crossed at stage 1 of the female flower stage. Then, with maturation of the flower, crossing results in fewer ovaries containing pollen. So immature pistils of
N. tabacum are less inhibitory to pollen from
N. rustica. The same is true for the other varieties replacing the male parent. This is interesting stuff, as for crossing in the field we need to work with very young pistils, as to make sure that the flower has to prevent self pollination. It seems that it raises the chance of success, if anything.
Then there's a last little experiment which is where the authors look at pollen fertilizing F[SUB]1[/SUB] hybrids. However, as an F[SUB]1[/SUB] hybrids they are using
N. rustica x N. tabacum, not the
N. tabacum x N. rustica. That last hybrid would have been more interesting to experiment on, as we could maybe have predicted whether it is easier to pollinate the hybrid than it is to make the hybrid in the first place.