Origin of endosperm
Lineal angiosperms possess seeds having smaller embryos and plentiful endosperm, and the ancestral growth of angiosperms shows a pattern towards faunal species having adult seeds with less or nil endosperm. In more acquired angiosperms, the egg inhabits most of the seed and under-developed endosperm can be seen before the embryonic seed develops.
Double fertilization process
Endosperm develops when two sperm cells fuse with female embryo sac (female gametophyte). One nuclei undergoes the process of fertilization with the egg and forms a zygote, while the other sperm nuclei merges with a cell having two nuclei to from an endospermic cell possessing a nucleus called the triple fusion nucleus. The cell that takes birth in this process is called the endosperm.
Around seventy percent of the flowering plant species possess endospermic cells that have more than two set of homologous chromosomes (polyploid). Mostly, these possess 3 sets of chromosomes i.e. show triploidy, but sometimes show varied difference showing diploid (2n) to 15n. One of the angiosperms, Nuphar polysepala, possesses endosperm that has two sets of homologous chromosomes that result from the merging of a pollen nuclei with one of the mother nuclei. It is similar case for other basal flowering plant species. Earlier the growth of flowering species lineages, species reproduced by duplicating. These species generated 7-celled / 8 nucleate female gametes, and endosperms with set of three homologous chromosomes with a 2:1 maternalistic to paternalistic genomic ratio (Yan et al, 2014).
There are three categories of Endosperm development (Yi et al, 2015)
Nuclear endosperm development – This is also known as liquid endosperm. The term ‘nuclear endosperm’ is that primary endosperm in which cell wall formation takes place after free – nuclear separation. For e.g. Coconut water.
Cellular endosperm development – Cell wall growth coincides with separation of nucleus. Acoracea shows cellular endosperm formation. E.g. meat of Coconut.
Helobial endosperm development – Cell wall is intermediate cellular and nuclear pattern.
Figure 2. Types of endosperm
The evolution theory of endosperm is not very clear. Two major theories exist:
- The process of double fertilization earlier produced 2 similar and self-sufficient fertilized egg (“twins”). Later, these eggs performed different responsibilities, one becoming an adult specie and another just providing support. Early endosperm had two sets of homologous chromosomes (diploid), like the seed egg. Some seed bearing plants not enclosed in an ovary (gymnosperms) like Ephedra (genus), produces twin eggs by the process of double fertilization. Any one of these embryos has the capability to accommodate in the seed, but one usually grows further when the other gets aborted. Also, most of the basal flowering species consist of the 4-celled egg sac and generate endosperms with two sets of homologous chromosomes (diploid).
- Endosperm is the hereditary remain of the real gametophyte located in seed bearing plants (gymnosperms), quite alike the multi-celled gametophyte located in the gymnosperms.
- In such case, accession of the additional nuclei from the sperm cell exhibits a stage of evolution. This nucleus offers control over growth of endosperm to the parental species. Showing triploidy or polyploidy are later steps of evolution of this “primary gametophyte”.