Which symbol is used to denote endospermic chromosome number?
Answers
First of all, we need to clarify that the origin of the Endosperm in Gymnosperms and Angiosperms is different. In Angiosperms, double fertilization occurs: one sperm nucleus fertilizes (n) the egg (n) and forms the embryo (2n) and the other sperm nucleus (n) fuses with the two polar nuclei (n each= n+n) in the large central cell of the female gametophyte, producing a triploid cell (3n). This triploid cell is going to develop into a real endosperm.
In contrast to this, Gymnosperms don’t have double fertilization. There are two sperm nuclei as well, but one them degenerates. In consequence, there is no a REAL endosperm. However, there has to be a nutritive tissue because it is necessary for the growth and germination of the embryo. So the real question is how does this tissue form?
Let’s take a look at the typical life cycle of a pine:
Let’s briefly describe it:
We have both ‘’female’’ cones (ovulate) and ‘’masculine’’ cones (pollen). In the ovulate cones, there are ovules, which consist of: A diploid cell called megasporocyte inside a mega sporangium (2n as well), everything covered by an integument (These 3 elements are what we call the Ovule). The megasporocyte undergoes meiosis and produces 4 haploid megaspores, only one of them surviving (n). This megaspore develops into the female gametophyte that contains two or tree archegonia; each one will form an egg. See how archegonium has reduced to a microscopic structure when in mosses and ferns it was much bigger.
In the pollen cones, there are cells called microsporocytes (2n) inside microsporangia. These microsporocytes undergo meiosis, producing haploid microspores (n). Each microspore develops into a pollen grain (n) containing the male gametophyte.
The pollen grain consists of 4 cells: two prothallial cells, a generative cell, and a tube cell. The generative cell and the tube cell are involved in reproduction, the other two degenerate soon. The generative cell of the four-celled microgametophyte undergoes division, giving rise to two kinds of cells—a sterile cell and a spermatogenous cell. Subsequently, before the pollen tube reaches the megagametophyte, the spermatogenous cell divides to produce two sperm nuclei. One sperm nucleus unites with the egg nucleus, and the other degenerates. *In Angiosperms the situation is different: inside the pollen grain 2 sperm nuclei form: one nucleus develops the pollen tube and the other nucleus divides in two, forming again 2 sperm nuclei. When eggs inside archegonia are mature, the 2 sperm nuclei are discharged in the pollen tube. One nucleus fertilizes the egg and the other fertilizes the fused polar cells of the endosperm (2n)*
You are probably wondering where is the nutritive tissue that is going to feed the embryo. Well, remember how the female gametophyte contained many cells, but only the eggs were ‘’used’’ for the fertilization? The food reserves are then this feminine gametophyte tissue (n) or you could call it the primary endosperm (different to the one in angiosperms).
To finish, it is important to notice something. In angiosperms, the only way that the endosperm is formed is if fertilization occurred previously; nonetheless, in Gymnosperms, the feminine gametophyte is formed even if there is no fertilization whatsoever. This means that in Gymnosperms, all the energy invested in the generation of the feminine gametophyte could be wasted if no fertilization occurs.