Reproduction

Aconitum napellus reproduces mainly sexually by the process of double fertilization, but it also has the potential to reproduce asexually through such means as fragmentation (the separation of parent plant into parts that develop into their own whole plants), or apomixes (the creation of seeds without pollination or fertilization, using a diploid cell from one parent plant to make an embryo). Double fertilization can happen either through self-fertilization, the fertilization of the plant by its own pollen, or cross-fertilization, which is the fertilization by a different plant’s pollen. Although, some plants have mechanisms in place that make it harder to for successful self-fertilization so that cross-fertilization is favored and there is more genetic recombination within the species. Fruiting is also an important part of this plant's reproductive process. Aconitum napellus makes fruit for increased seed dispersal.

Since double fertilization is the most popular means by which this plant produces let’s go into more detail on how that happens. We are going to go through double fertilization in a step-by-step process, but first let’s review some background information on the structures that we are going to need to understand beforehand.

Double fertilization is a form of sexual reproduction that is exhibited in flowering plants. The nine main structures we are going to focus in on are these:

1. The pistil: the flower pistil is essentially the girl part of a flower. It can often be found in the very middle of the flower, and it consists primarily of a long hollow stalk with pollen receptors at the end (1A), and an ovule tucked away near the bottom.

2. The pollen: pollen is for all essential purposes a protective transportation device for the male gametes of the plant. Pollen is often dispersed into the air, or carried by various organisms, known as pollinators, from one flowering plant to the next.

3. The pollen tube: the pollen tube is what the pollen grows into once it attaches to one of the pollen receptors of the pistil. This structure is what eventually brings the sperm nuclei stored inside the pollen to the egg of the plant where fertilization occurs.

4. The generative cell: this is the cell that is directly transported through the pollen tube to the ovule. An important thing to notice about this is that this cell has TWO sperm nuclei. This is a fact that will come in later.

5. The egg: this is the female gamete, and it is the middle of the three cells beneath the center cell.

6. The synergids: these are the two cells on either side of the cell that will eventually become the embryo. Both of the synergids actually contain a haploid nucleus inside them, just like the egg, but they play a very different role in the reproduction process.

7. The polar nuclei: these two nuclei are important in the formation of the endosperm, an important part of the seed that provides nourishment to the plant embryo as it first starts to grow, and they reside together in one larger cell that is at the center of the entire structure (central cell).

8. The three antipodial cells: these cells are also haploid cells, but they sit above the cell with polar nuclei rather than below it. They are not extremely important in anything we are going to go over, but it is still important to recognize that they are there.

9. The ovule: this is the general place where fertilization occurs. It is essentially a structure incorporating all of the important reproductive female cells in the flower. This entire structure is what eventually becomes the seed after double fertilization is completed.

Now let’s go through a brief step-by-step summary of double-fertilization and how it works!

Step One: Pollination
 The first thing that must happen for double-fertilization to occur is that pollen must reach the pollen receptors of the pistil. In Aconitum napellus this generally happens through the help of a pollinator, the most common of which for this species of flower is one we are all very familiar with, the common bumblebee. Below is a perfect video showing this mutualistic relationship between the two species. For information on how the flower has adapted to hold preference toward this particular pollinator click on the link here.



Step Two: Growth of the pollen tube

Once the pollen is received at the pollen receptor, it has to travel the relatively large distance to the actual ovule of the flower. This is done through the growth of a pollen tube down the pistil to transport the generative cell with the two vital sperm nuclei inside it to the egg for fertilization. The sperm tube can take a couple days to grow, so this is often the longest part of the fertilization process. The tube grows down until it is under the ovule as shown in the diagram to the right.

Step Three: Merging of the sperm cell nuclei with one of the synergids

Depending on what side of the ovule the sperm tube ended up growing to, it will eventually penetrate into the bottom of the ovule and fuse its generative cell with the two sperm nuclei into either the right or left synergids. The synergid cell is then temporarily triploid as it has three haploid nuclei in it at one time. The important thing to recognize is that the nuclei do not fuse together. Here is a picture of that step below:


Step Four: Movement of two nuclei: one to the polar body cell, and one to the ovary

In this step the two extra nuclei that just passed into the synergid move to different cells. One of the nuclei pass through the cell into the egg cell, and the other passes up into the center cell with the two polar nuclei and creates a new triploid cell (3n). Diagram above.

Step Five: The egg and sperm nuclei fuse

Finally, the two nuclei of the parent plants are able to fuse together, creating a diploid zygote that will eventually become the new adult plant. A diagram is above.

Step 6: The formation of the endosperm/seed. As the sperm and egg are fusing the other parts of the ovule develop into their own special roles to help make a seed. The triploid cell with the sperm nucleus and two polar body nuclei is what develops into the endosperm of the seed. To hear more about the endosperm click on the link here.

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