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Female gametophyte

This document summarizes the development of the female gametophyte in angiosperms. It begins with an introduction to the megaspore mother cell and its development into the female gametophyte through meiosis. There are three main types of female gametophytes - monosporic, bisporic, and tetrasporic - which are distinguished by the number of megaspores involved. A monosporic gametophyte develops from a single megaspore, while bisporic and tetrasporic involve two and four megaspores, respectively. Examples of each type are provided, including the common Polygonum type of monosporic gametophyte and the Allium and

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FEMALE GAMETOPHYTE • PRESENTED BY:- GOSWAMI KHUSHBOOBEN.B M.SC (BOTANY) SEM:- 3 PAPER CBO:- 404 DEPARTMENT OF LIFE SCIENCES, H.N.G.U.PATAN.
CONTENTS • INTRODUCTION • THE FEMALE GAMETOPHYTE • TYPES OF FEMALE GAMETOPHYTE • MONOSPORIC • BISPORIC • TETRASPORIC
INTRODUCTION • The megaspore makes the begning of the femalegametophyte generatation. The nucleus of the megaspore divides and developes into the female gametophyte. • The female gametophyte of angiosperms is very much reduced and totally dependent for its nutrition upon the tissue of the sporophyte.
THE FEMALE GAMETOPHYTE • The angiosperm female gametophyte is critical for plant reproduction. It is contains the egg cell and central cell that become fertilized and give rise to the embryo and endosperm of the seed,respectively. • female gametophyte development begins early in ovule development with the formation of a diploid megaspore mother cell that undergoes meiosis.
FIGURE
DEVELOPMENT OF FEMALE GAMETOPHYTE • The female gametophyte first arise as a tiny protuberance from the placenta, in the cavity of the overy. • As it mature,the ovule consists of a nutritive cellular mass known as nucellus, protected by two integuments, and attached to the placenta by a funiculus.
• Even at an early stage, one cell becomes evident in the nucellus, which is known as the megaspore mother cell(MMC). • MMC is diploid (2n). • The MMC then increases in size, and divides twice (meiotically) to from 4 megaspore as ‘linear tetrad’. • The first division is a reduction division. • The resultant megaspores are haploid (n). • Out of the four megaspores, three degenerate, forming caps, while one remains functional. • This functionl megaspore is haploid (n).
• The functional megaspore, then grows rapidally, by absorbing nutrition from nucellus, and forms the embryo sac. • The process of formation of embryo sac is known as megasporogenesis. • The embryo sac consists of one nucleus. • The nucleus divided into two daughter nuclei, which move towards the poles. • The pole of the embryo sac, closer to the chalaza, is the chalazal end,and the one closer to the microphyle is the micropylar end.
• Each daughter nuclei divides again, and result in four nuclei. • Each nuclei divides again, for the last time, and result in eight nuclei. • One nuclei from each end, come to the centre, and fuse to from a diploid secondary nucleus. • The nuclei at the chalazal end, get surronded by cytoplasm, and form three distinct, haploid, antipodal cells. • Similarly, the nuclei at the micropylar end, form the egg apparatus, consisting of tow haploid synergids on either side of a large, central, haploid egg cell.
• The types of female gametophytes are distinuished by the number of megaspores involved in the formation of the embryo sac. • Based on this criteria, there are three types of female gametophytes: • MONOSPORIC • BISPORIC • TETRASPORIC • Each type has more than one subtype. • Some of the common ones are discussed further.
1) MONOSPORIC • A monosporic embryo sac is derived from a single haploid megaspore. • All the nuclei in the embyo sac are genetically idwntical, as they develop from the same megaspore. • There are tow subtypes of monosporic female gametophytes. • There are the, 1) Polygonum type 2) oenothera type.
POLIGONUM • It was first described by stresberger in 1879. here, embryo sac is formed by the megaspore at the chazal end, and the rest of the tetrad degenerats. • The embryo sac is 8 nucleated, three antipodal cell are located at the chazal end, a central diploid secondary nucleus exists, along with a micropylar egg apparatus, consisting of two synergids, and an egg cell. • This type of ES is most common in angiosperm.
OENOTHERA • This type of ES was derived from the megaspore of the micropylar end of the spore tetrad. • The ES comprise of and egg apparatus and a uninucleate central cell. • This type of ES a characteristic of onagraceae family.
2) BISPORIC • In the plant bearing bisporic ES the first division is accomplished by wall formation so that a dyad is formed. • One of the dyad cell undergoes the sec division, whereas the other one degenerates. • Both the megaspore contribute in the formation of mature bisporic ES. • Each megaspore nucleus undergoes two mitotic division, forming 8 nuclei. • The final organization of the embryo sac is similar to the polygonum type.
• In bisporic type, the ES is derived from 2 meitoic products, so the nuclei belong to two different genetic constitutions. • Four nuclei are different from the other four. • Bisporic ES is 2 types: 1) Allium 2) Endymion. • The ES is derived from the chalazal dyad, and is similar to that of polygonum type. • The mature ES consists of one binuclate central cell, three antipodal cells at the chalazal end, and an egg apparatus at the micropylar end.
1) ALLIUM • the mature ES consits of 1 binucleate central cell, three antipodal cells at the chalazal end, and an egg apparatus at the micropylar end. 2) ENDYMION • The mature ES consits of 1 binucleate central cell, three antipodal cells at the chalazal end, and and an egg apparatus at the micropylar end.
3) TETRASPORIC • A tetrasporic ES, is more heterogeneois, than a bisporic ES because, all the 4 megaspores are involved in the formation of ES and are genetically different. • There are 7 types of tetrasporic ES.
1) FRITILLARIA • After the nuclear fusion , both the nuclei of the coenomegaspore (1 haploid + 1 triploid), divide twice, forming four nuclei at each pore. • The mture ES, comprises of an egg apparatus of three haploid cell, three triploid antipodal cells, and a central cell with tow polar nuclei, one haploid and one triploid
REFERENCES • www. Slideshare.com • A text book of botany angiosperm by B.P.Pandey
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