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polyembryony.pptx

This seminar presentation discusses polyembryony, which is the occurrence of more than one embryo in a seed. It was first reported in orange seeds in 1719. Polyembryony is common in conifers and some dicots and monocots. There are two types of polyembryony - true and false. True polyembryony results from embryos developing within or projecting into a single embryo sac, through cleavage or adventitiously. False polyembryony results from multiple embryo sacs arising from the same or different megaspore mother cells or nucellar cells. Causes of polyembryony include necrohormone and hybridization theories. Polyembryony has importance for plant

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PATNA UNIVERSITY DEPARTMENT OF BOTANY A SEMINAR PRESENTATION ON “ POLYEMBRYONY” SUBMITTED TO: SUBMITTED BY: Dr. MEENAKSHI SINGH ALKA KUMARI HOD, DEPARTMENT OF BOTANY ROLL NO.- 04 PATNA UNIVERSITY M.Sc. BOTANY SEMESTER –II SESSION: 2020-22
 Introduction • Polyembryony can be defined as the occurrence of more than one embryo in a seed. • The first case of polyembryony was reported in certain orange seeds by Antonie van Leeuwenhoek(1719). • Polyembryony is quite common among conifers (Gymnosperms) but many species of both dicotyledons and monocotyledons exhibits this phenomenon.
 Classification • Ernst and Schnarf classified the polyembryony into two types: 1. True polyembryony 2. False polyembryony • True polyembryony may be sub-divided into two types: a. Cleavage polyembryony b. Adventive polyembryony
1. True polyembryony • The production of embryos within or by projecting into a single embryo sac is termed as true polyembryony. Fig: Cleavage polyembryony
a) Cleavage polyembryony • Where the embryos arise within an embryo sac either by a cleavage of the egg or from the synergids , antipodals or endosperms.
b) Adventives polyembryony • Where the embryos arise from the tissue lying outside the embryo sac. • The cells of the nucellus or the integuments but generally they come to lie within the embryo sac.
2. False polyembryony • Multiple embryo sacs in an ovule may arise from: a) Derivatives of the same megaspore mother cell b) Derivatives of two or more megaspore mother cells c) Nucellar cells
 Causes of polyembryony • Many theories have been proposed to explain the occurrence of polyembryony by different workers. • Some of the important theories are as follows: A) Necrohormone theory: • Haberlandt(1921,1922) proposed the “Necrohormone theory”. He regarded the degenerating cells of the nucellus as source of stimulus for the adjacent cells to divide and form adventive embryos.
B) Hybridization theory • According to this theory the occurrence of multiple embryos is due to hybridization. • The recombination of genes takes place during the process of hybridization, forming a single unit that gives rise to the multiple embryos.
 Importance of polyembryony • Plant breeding and horticulture. • Nucellar embryos are supposed to be free from disease. • Propagation of the fruit tree, such as Citrus and Mango. • The application of adventive embryos is also important for providing genetically uniform seedlings of fruit trees. • Can be used for the development of homozygous diploid
 Disadvantages of Polyembryony • It constitutes problem in crop improvement because it makes the identification of hybrids, particularly in crosses involving taxonomically closely related parents. • Loss of vigour of zygote embryo due to suppression by the nucellar embryos during seed development. • Germination of monoembryonic seed is low due to the competition between the embryos in a single seed. • It reduces the variability since nucellar seed will have the same genotype as the parental genotypes.
polyembryony.pptx