Embryology of Angiosperms M.Sc. Botany Semester-II

Embryology of Angiosperms

M.Sc. Botany Semester-II/ B.Sc. Botany

Semester-IV

Development and Types of Female Gametophyte or Embryo Sac

Compiled by: Dr. Ankit Samuel Singh Department of Botany Ewing Christian College

Structure of Embryo Sac or Female gametophyte  Typically the female gametophyte or embryo sac commonly present in angiosperms is a 7 celled structure.  The large cell present in the centre of embryo sac is called central cell, with two polar nuclei, they fuses to form a secondary nucleus.  The micropylar end of embryo sac is occupied by egg apparatus, which comprises of an egg and two synergids.  The chalazal end of embryo sac, have 3 antipodal cells.  The cells of egg apparatus as well as antipodals are haploid and uninucleate; while the central cell is binucleate.  The occurrence of egg is universal and it was always haploid. The antipodal cells are almost always present except in Oenothera type, but there number and ploidy is variable.  The development of the embryo sac starts with the elongation of the functional megaspore.

Megagametogenesis: The process of development of Embryo Sac or Female Gametophyte from functional megaspore is called Megagametogenesis.

Types of Embryo Sac or Female Gametophyte Development

Tetrasporic Type: (a)With no nuclear fusion:

(b)Nuclear fusion occurs: After the meiosis-II, 3 megaspore nuclei fuses to form triploid nucleus at chalazal end of coenomegaspore and the 4th nucleus, which is present at the micropylar end remains haploid.

Bambacioni Effect: Bambacioni (1928) first reported the fusion of the spindles of the 3 chalazal nuclei in coenomegaspore, to form a triploid nucleus in Fritillaria and Lilium. Therefore the phenomenon is called Bambacioni Effect.

Chrysanthemum cinerariaefolium Type of Embryo Sac: A special type of tetrasporic embryo sac was described by Martinoli (1939), which is called as Chrysanthemum cinerariaefolium type. After the meiosis-II, the 4 nuclei of coenomegaspore are arranged in 1+2+1 fashion, 1 at chalazal end, 1 at micropylar end and 2 in the centre of the cell. Based on the behaviour of the central nuclei, embryo sac shows two variations, which are as follows:  In type A the 2 nuclei, present in the centre of the cell remains associated but do not fuse and divide.  In type B the 2 nuclei, present in the centre of the cell, fused to form a diploid nucleus. They further divides by 2 mitotic divisions to form 4 diploid nuclei.

Type A:

Type B:

The various components of embryo sac or female gametophyte are as follows: (1)Synergids: Synergids are present at the micropylar end of embryo sac; they are elongated and pointed or hooked towards the micropyle. The wall of synergid is incomplete; which only covers the one third of the cell. The wall thins towards the chalazal end and finally disappears; due to this the chalazal one third of the cell, lacks a wall. An important structure is present, in the upper part of cell at the micropylar end of each synergid which is known as filiform apparatus. The electron microscopic studies suggested that the filiform apparatus is a mass of finger like projections, of the wall into the cytoplasm. It is made up of substances like cellulose, hemicellulose, pectin, proteins and callose. The embryo sac which contains 2 synergids, one of the synergid cell degenerates before or soon after the entry of pollen tube inside the embryo sac. The other synergid cell also known as persistent synergid degenerates shortly after receiving the pollen tube discharge.

The cytoplasm close to, filiform apparatus is rich in secretory organelles. It is suggested that the transport of substances into and out of the synergid cell is facilitated by filiform apparatus. The seat for pollen tube discharge, in the embryo sac is also formed by one of the synergid cell.

Synergid haustoria: A most exclusive synergid haustoria showed by Quinchamalium chilense, a member of family Santalaceae. The synergids are visibly large in this species. The haustorium arises as finger like projection, which penetrates the micropyle.

(2)Egg: The egg apparatus consist of 3 cells (an egg and 2 synergids) they are arranged in a triangular fashion. The 2 synergids and the central cell show common walls with egg. Small or large amount of starch may contained by egg, which is consumed during fertilization or early embryo development.

(3)Antipodals: The antipodals shows many variations amongst all the cells of embryo sac. Typically before or soon after fertilization, they degenerate without any considerable enlargement. They are rich in ascorbic acid, oxidases, starch, lipid and proteins. Low concentrations of RNA and polysaccharides are also found in it. These cells perform a nutritive role and associated with the nutrition of embryo sac. They may contain large quantities of starch, lipids and proteins, which can also be utilized by the developing endosperm and embryo. Antipodal cell also shows Haustorial behaviour in some plants like Grindelia, Haplopappus etc.

(4)Central cell: The central cell is the biggest cell of embryo sac and also the mother cell of endosperm. The vacuole in the central cell is responsible for the swelling of embryo sac after the last nuclear division. The nuclei present in the centre of this cell are called polar nuclei, which fuses before or during double fertilization and forms secondary nucleus. It contains sufficient amount of reserve food material which is available for use during fertilization and development of endosperm in early stages.

Nutrition of embryo sac: Nucellus surrounds the embryo sac, from all the sides and it is the lane for nutrients in the embryo sac. The chalazal end of ovule is the main pathway of food material supply into the embryo sac and the common vascular channel of ovule nutrient supply, terminates at the base of integuments.

Naked embryo sac: The embryo sac is located inside the ovule in most of the angiosperms, which is enclosed by thick nucellus and integuments. But in some genera as Torenia, Galium, Thesium and Utricularia etc. the embryo sac comes out through the micropyle, which is known as naked embryo sac.

Embryo sac haustoria: Like synergids and antipodal cell haustoria, the whole embryo sac may also grow beyond the tissue of ovule and behaves or acts, as haustorium example, Utricularia flexuosa. References:

Bhojwani, S.S., Bhatnagar, S. P. and Dantu, P.K. (2015) The Embryology of Angiosperms 6th Edition, Vikas Publishing House Pvt. Ltd, New Delhi.

Maheshwari, P. (1950) An Introduction to the Embryology of Angiosperms McGraw-Hill, New York.

Johri, B. M. (1984) Edited, Embryology of Angiosperms, Springer-Verlag, Berlin, Heidelberg, New York, Tokyo.

Dwivedi, J. N. (1990) Embryology of Angiosperm, Rastogi Publications, Meerut.

Reddy, S. M. (2004) Edited, University Botany-3, New Age International Publishers, New Delhi.