Question: What is the single fertilized egg cell called in botany?
About the article "Botany", section "Epigenetics".

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In algae, the egg cell is often called oosphere.  
Egg or eggs may refer to:  Egg (biology), an organic vessel in which an embryo first begins to develop  Egg cell (ovum), in anisogamous organisms, the larger, non-motile gamete, also called the female gamete  * Zygote (fertilized egg), the cell formed by the fusion of gametes  Egg (food), the egg of a fowl, fish, or reptile, consumed as food  
In the presence of a large amount of yolk in the fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage.
Two major types of meroblastic cleavage are discoidal and superficial.
Discoidal In discoidal cleavage, the cleavage furrows do not penetrate the yolk.
The embryo forms a disc of cells, called a blastodisc, on top of the yolk.
Discoidal cleavage is commonly found in monotremes, birds, reptiles, and fish that have telolecithal egg cells (egg cells with the yolk concentrated at one end).
Superficial In superficial cleavage, mitosis occurs but not cytokinesis, resulting in a polynuclear cell.
With the yolk positioned in the center of the egg cell, the nuclei migrate to the periphery of the egg, and the plasma membrane grows inward, partitioning the nuclei into individual cells.
Superficial cleavage occurs in arthropods that have centrolecithal egg cells (egg cells with the yolk located in the center of the cell).  
Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two nuclei.
Before karyogamy, each haploid cell has one complete copy of the organism's genome.
In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process known as plasmogamy.
Once within the joined cell membrane, the nuclei are referred to as pronuclei.
Once the cell membranes, cytoplasm, and pronuclei fuse together, the resulting single cell is diploid, containing two copies of the genome.
This diploid cell, called a zygote or zygospore can then enter meiosis (a process of chromosome duplication, recombination, and division, to produce four new haploid cells), or continue to divide by mitosis.
Mammalian fertilization uses a comparable process to combine haploid sperm and egg cells (gametes) to create a diploid fertilized egg.
The term karyogamy comes from the Greek "karuo-" (from "karuon") meaning "nut" and "-gamos" meaning "marriage". 
Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg (in humans, identical twins).
Polyembryony occurs regularly in many plants and animals.
The nine banded armadillo, for instance, usually gives birth to four identical young.
Polyembryony is best known among parasitoid insects of the order Hymenoptera, families Encyrtidae, Dryinidae, Platygasteridae and Braconidae.
The term is also used in botany to describe the phenomenon of two seedlings emerging from one seed.
In plants, polyembryony often gives rise to the enigma of a single offspring.
The mechanism underlying this phenonomenon is programmed cell death (PCD) which removes all but one embryo.
A more striking example of the use of polyembryony as a competitive reproductive tool is found in the parasitoid Hymenoptera family Encyrtidae.
The progeny of the splitting embryo develop into at least two forms, those that will develop into adults and those that become a type of soldier, called precocious larvae.
These latter larvae patrol the host and kill any other parasitoids they find with the exception of their siblings, usually sisters.