Modes of Reproduction PDF

Modes of reproduction Modes of reproduction 1. Asexual reproduction It is a process in which new organism is produced from a single parent without the involvement of gametes or sex cells. It is two types a. Vegetative reproduction b. Apomixis  Vegetative reproduction – 2. Artificial Propagat...

Modes of reproduction Modes of reproduction 1. Asexual reproduction It is a process in which new organism is produced from a single parent without the involvement of gametes or sex cells. It is two types a. Vegetative reproduction b. Apomixis  Vegetative reproduction – 2. Artificial Propagation of Plants – When many plants are grown from one plant using man-made methods, it is called artificial propagation. There are three common methods of artificial propagation of plants. They are: – i) Cuttings – ii) Layering – iii) Grafting and Tissue culture Advantages – The new plant will have exact features as that of parent plant. – Fruit trees grown by grafting bear fruit much earlier. – Plants need less attention in their early years. – Can get seedless plants. Apomixis It refers to the formation of the plant from a seed without fertilization or normal sexual reproduction. However, embryo may develop from an un-fertilised egg cell or from a cell other than the egg cell within the embryo sac or from the cell outside the embryo sac. Significance ▪ Identical to parent ▪ Easy to propagate ▪ More adaptable in a particular environment ▪ Highly uniform ▪ Help in fixing heterosis Sexual reproduction – This process of reproduction involves the fusion of male and female gametes and formation of seed. – Sexually reproducing crop plants produce a special structure, called flower which bears the essential male and female reproductive parts which produce male and female gametes – It completes in two steps – 1. sporogenesis – 2. gametogenesis – Sporogenesis: – Production of microspores and megaspores is known as sporogenesis. – In anthers there are pollen sacs which contain numerous PMC (Pollen mother cell) undergoing meiosis, produces microspores or pollen, the process is called micro-sporogenesis. – Inside the ovary the ovules are present in which the MMC (Megaspore mother cell) undergoes meiosis and produces 4 megaspores out of which 3 degenerate and one survives, this process is called mega-sporogenesis. – Gametogenesis: – During maturation of pollen, the microspore nucleus divides mitotically to produce generative and vegetative nucleus. The generative nucleus then divides to form male gametes or sperms. The pollen along with pollen tube and sperms is called micro-gametophyte and the production of sperms is known as micro-gametogenesis. – The nucleus of functional megaspore divides mitotically three consecutive times to produce eight nuclei, which get arranged in the embryo sac. The embryo sac contains generally one egg cell, two synergids, two polar nuclei and three antipodals. All these cells are haploid and this process is called as mega-gametogenesis. – Fertilisation: – Fusion of one of the two sperms with the egg cell, to produce a diploid zygote, is known as fertilisation, and the fusion of the remaining sperm with the secondary nucleus leading to the formation of primary endosperm nucleus known as double fertilisation or triple fusion. – The zygote divides mitotically to produce diploid embryo. The primary endosperm nucleus produces endosperm by mitotic division. Modes of pollination  Modes of pollination – Pollination is transfer of pollen grains from anthers to stigma. Pollination is brought about by various agencies like air (anemophily), water (hydrophily), insects (entomophily) and animals (zoophily). – There are two modes of pollination 1. Self pollination- Transfer of pollen from an anther to stigma of same flower or another flower from the same plant is called “self pollination” or autogamy 2. Cross pollination- Transfer of pollen grains of one plant to the flower of another plant is called cross pollination. The resultants fertilization is known as cross fertilization or allogamy 3. Often cross-pollinated plants- the occurrence of cross pollination ranges from 5 per cent to 30 per cent eg cotton, tobacco  Mechanism promoting self pollination 1. Bisexuality : The male and female reproductive units are present in the same flower. E.g., Hibiscus, Wheat, Paddy, etc. 2. Homogamy: Here anther and stigma of bisexual flower mature at the same time. E.g. Green peas, etc. 3. Cleistogamy : Flowers do not open at all in case of bisexual flowers ensures self pollination. E.g., Lettuce, Legumes, etc. 4. Flower structure: In some species, stigmas are surrounded by anthers in such a way that self pollination is ensured eg tomato and brinjal. In some legumes, the stamens and stigma are enclosed by the petals in such a way that self pollination is ensured. Examples are greengram, blackgram, soybean, chickpea and pea. Chasmogamy: In some species, flower open but only after pollination has taken place eg rice, wheat Mechanism promoting cross pollination 1. Dicliny or unisexuality - This is of two types: viz. i) monoecy and ii) dioecy. – When male and female flowers are separate or present in same inflorescence but present in the same plants, it is known as monoecy. Eg maize – When staminate and pistillate flowers are present on different plants, it is called dioecy. It includes papaya, date palm, spinach, hemp and asparagus. 2. Dichogamy- It refers to maturation of anthers and stigma of the same plant at different times. It is of two types: i) protogyny and ii) protandry. – When pistil matures before anthers, it is called protogyny such as in pearl millet. – When anthers mature before pistil, it is known as protandry. It is found in maize, sugarbeet and several other species. 3. Heterostyly. When styles and filaments in a flower are of different lengths, it is called heterostyly such as linseed. 4. Herkogamy- Hinderance to self-pollination due to some physical barriers such as presence of hyline membrane around the anther. Such membrane does not allow the dehiscence of pollen and prevents self-pollination such as in alfalfa. 5. Male sterility- an inability to produce or to release functional pollen and is the result of failure of formation or development of functional stamens, microspores or gametes. 6. Self incompatibility: The inability of fertile pollens to fertilize the same flower is referred to as self incompatibility. It prevents self-pollination and promotes cross pollination. Self incompatibility is found in several crop species like Brassica, Radish, Nicotiana, and many grass species. Male sterility Male sterility Male sterility is a condition in which pollen is absent or non- functional. There are three types of male sterility. 1. Genetic male sterility 2. Cytoplasmic male sterility 3. Cytoptasmic genetic male sterility Features of Male Sterility – Prevents self pollination and permits cross pollination. – Leads to heterozygosity – Female gametes function normally – In nature, occur due to spontaneous mutations – Can be induced artificially Manifestations of Male Sterility – Absence or malformation of male organs. – Failure to develop normal microsporogenous tissue- anther – Abnormal microsporogenesis (deformed or inviable pollen) – Abnormal pollen maturation – Non dehiscent anthers – Barriers other than incompatibility preventing pollen from reaching ovule Genetic male sterility – It is governed by nuclear genes, without any influence of cytoplasm. Generally, this type of male sterility is due to recessive allele – It is usually governed by a single recessive gene ms, but dominant gene governing male sterility are also known Eg. Safflower. – The male sterility alleles may rise spontaneously or it can be induced artificially and is found in several crops viz. Pigeon pea, castor, tomato, barley, cotton, etc. – A male sterile line may be maintained by crossing it with heterozygous male fertile plant, such a mating produces 1:1 male sterile and male fertile plants.  Genetic Male Sterility Nuclear gene Nuclear gene msms ssssss S Cytoplasm X MSMS ssssss S/F Cytoplasm Male sterile Male fertile Nuclear gene F1 MSms Cytoplasm ssssss S Male fertile Thermo sensitive genetic male sterility Plants are sterile when temperature exceeds 32 0c/ 24 0c (day/night) and become fertile when the temp. is below 24 0c/ 18 0c (day/night). Photoperiod sensitive genetic male sterility Within temperature, complete sterility is obtained in rice plants grown under long-day condition (day length more than 13 hr 45 min), but under short conditions almost normal fertility is obtained. 2. Cytoplasmic Male Sterility – it is governed by cytoplasmic DNA and it shows maternal inheritance. – Eg Ogura CMS in Brassica – Progenies would always be male sterile since the cytoplasm comes from female gamete only.  Cytoplasmic Male Sterility: Nuclear gene Nuclear gene rr ssssss S Cytoplasm X rr ssssss F Cytoplasm Male sterile Male fertile Nuclear gene F1 rr Cytoplasm ssssss S Male sterile Transfer of cytoplasmic male sterility into new strain CMS may be transferred easily to a given strain by using that strain as a pollinator (recurrent parent) in the successive generation of backcross programme. After 6-7 backcrosses the nuclear genotype of male sterile line would be almost identical to that of the recurrent pollinator strain. The male sterile line is maintained by crossing it with pollinator strain used as a recurrent parent in backcross, since the nuclear genotype of the pollinator is identical with that of the new male sterile line. Such a male fertile line is known as maintainer line or ‘B’ line and ‘male sterile line is also known as ‘A ‘ line. Cytoplasmic-Genetic Male Sterility – In this system, male sterility is expressed by homozygous recessive gene in presence of sterile cytoplasm – fertility is restored by dominant allele in presence of fertile as well as sterile cytoplasm. – In this system, a variety having sterile cytoplasm (S) and recessive homozygous (rr) genes, is used as seed parent and is known as 'A' line. – In maintainer line ‘B‘, gene present are recessive (rr), but the cytoplasm is male fertile (F), – whereas, in restorer line R, dominant gene (RR) are present, but the cytoplasm may be fertile or sterile. – A-line: MS line is called as A line. This is sterile due to the genes in cytoplasm (mitochondrial DNA). – B-line: This is isogenic (genotypically identical except one gene) to MS line except fertility. It maintains A line. It means if you want seed of A line, you have to cross it with B line otherwise A line will be no more as it has no active male parts. – R line: This line has restorer genes in nucleus to restore fertility of A line. R line is entirely different to that A and B line. R line has very high SCA (Specific Combining Ability) effects. It is used to produce hybrid seeds – A x A = No seed – B x B = B line seed – R x R = R line seed – A x B = A line seed – A x R = Hybrid seed 4. Chemical induced MS The chemical which induces male sterility artificially is called as male gametocide. It is rapid method, but the sterility is non- heritable. Some of the male gametocides used are gibberellins (rice, maize) and Maleic hydrazide (wheat, onion). 5. Transgenic MS When the male sterility is induced by the techniques of genetic engineering eg Barnase Barstar in brassica Self-incompatibility (SI) – It is defined as “inability of the plant producing functional gametes to set seed upon self-pollination” – It is caused by a genetically controlled physiological hindrance to self- fertilization. 1) Heteromorphic System: In this system, flowers of different incompatibility groups are different in morphology. E.g In Primula there are two types of flowers. Pin and thrum. Pin flowers have long style and short stamens. While thrum flowers have short styles and long stamens. This situation is referred as distyly. Tristyly is known in some plant species. E.g Lythrum in such case, the style of a flower may be either short, long or of medium length. Distyly – Found in Primula Pin (ss) x Pin (ss) Incompatible Thrum (Ss) x Pin (ss) 1 thrum , 1 pin Pin (ss) x Thrum (Ss) 1 thrum , 1 pin Thrum (Ss) x Thrum Incompatible (Ss)  Tristyly – Lythrum  2) Homomorphic System: In the Homomorphic system incompatibility is not associated with morphological differences among flowers. Two types of homomorphic system 1. Gametophytic control 2. Sporophytic control  Self Incompatibility Gametophyti Sporophyti c c the ability of the pollen to function is the incompatibility characteristics of determined by its own genotype and the pollen are determined by the plant not the plant that produces it. (sporophyte) that produces it. occurs in species such as red clover, white It occurs in species such as broccoli, radish, clover, and yellow sweet clover and kale. They exhibit no dominance. The sporophytic system differs from the gametophytic system in that the S allele exhibits dominance. The incompatible pollen is inhibited in the The pollen germination inhibited on style. stigma only Pollen is incompatible (recognized by the recipient pistil as self-pollen) when its parent expresses both (S1S2) or one (S1Sx or S2Sx) of the S haplotypes expressed by the diploid recipient pistil (S1S2).  Genetic consequences Self-pollination leads to the production of plants with less genetic diversity, since genetic material from the same plant is used to form gametes, and eventually, Homozygosity the zygote. In contrast, cross-pollination—or out-crossing—leads to greater genetic diversity because the microgametophyte and megagametophyte are derived from Heterozygosity different plants.  QUALITATIVE AND QUANTITATIVE TRAITS Sr. no. Particulars Qualitative Traits Quantitative Traits 1 Gene involved One or few Several 2 Effect of individual gene Large & detectable Small and undetectable 3 Variation Discontinuous Continuous 4 Grouping into distinct classes Possible Not possible 5 Effect of environment Little High 6 Metric Measurement Not possible Possible 7 Analysis Based on frequencies and ratios Means, variance, covariances 8 Stability High Low to medium 9 e.g., Colour, shape, surface of plant parts Yield , seed number  Polygenic inheritance In 1910, a Swedish geneticist, Nilsson-Ehle provided a classic demonstration of polygenic inheritance It may be explained by making three basic assumptions: 1.Many genes determine the quantitative trait. 2.These genes lack dominance. 3.The action of the genes are additive.  Genetic variance R A Fisher divided variance into three components 1. Additive variance 2. Dominance variance 3. Epistatic variance  Formula for genetic variance VP = VG +VE +VGE where VP = total phenotypic variance of the segregating population, VG = genetic variance, VE = environmental variance, and VGE = variance associated with the genetic and environmental interaction.  Heritability The extent of contribution of genotype to the phenotypic variance for a trait in a population is called heritability It is expressed as ratio of genetic variance to phenotypic variance for the trait H = Vg/Vp It is of two types 1. Broad sense 2. Narrow sense  1 Broad sense heritability. Heritability estimated using the total genetic variance (VG) is called broad sense heritability. It is the ratio of genotypic variance to total or phenotypic variance. It is expressed mathematically as: H = VG/VP 2 Narrow sense heritability. Because the additive component of genetic variance determines the response to selection, the narrow sense heritability estimate is more useful to plant breeders than the broad sense estimate. It is estimated as: H2 = VA/VP  Narrow sense heritability is estimated from additive genetic variance it plays important role in the selection of elite genotypes from the segregating populations, whereas broad sense heritability estimates are more useful in selecting superior lines from the homozygous materials.  Genetic Advance Improvement in the mean genotypic value of selected families over that of the base population is known as genetic advance under selection (Gs) It is the measure of genetic gain under selection. The success of genetic advance under selection depends on three main factors: 1. Genetic Variability 2. Heritability 3. Selection Intensity  Computation of genetic advance Knowledge of genetic gain and selection differential is essential before dealing with the computation of genetic advance. Genetic gain:- The difference between the mean phenotypic values of the progeny of selected plants and the base or parental population is known as genetic gain. It is denoted by R. R = XP – XO Where, XP – Mean phenotypic values of the progeny of selected plants XO – Mean of base population Selection in self pollinated crops Methods of breeding SP crops 1. Introduction 2. Selection a) Pure line selection b) Mass selection 3. Hybridization and selection i) Inter varietal a) Pedigree Method b) Bulk Method c) Single Seed Descent Method ii) Interspecific hybridization 4. Back cross method 5. Multiline varieties Pure line theory – Proposed by Johannsen in 1903 based on his studies with French bean (Phaseolus vulgaris) variety Princess – There was variation for seed size in commercial seed lot – Larger seeds produced larger seeds and vice versa – He further studied 19 lines, each line was a progeny of a single seed Conclusions – Variation within a pureline was non heritable and due to environment – Variation between purelines are due to genetic variation and thus heritable – Continuous inbreeding (selfing) leads to homozygosity. What is a pure line? – Pure line is the progeny of a single homozygous plant of a self-pollinated species – All the plants in a pure line have same genotype Sources of variation – Mechanical mixture – Natural hybridisation – Chromosomal aberrations – Mutation  Pure line selection In pure-line selection, a large number of plants are selected from a self-pollinated crop and are harvested individually; individual plant progenies from them are evaluated, and the best progeny is released as a pure-line variety.  Features of Pure-line Selection 1. Homogeneous 2. Non-Heritable Variation 3. Highly Uniform 4. Selection is Ineffective 5. Narrow Adaptation 6. More Prone to New Diseases 7. Isolation of Pure-Lines 8. Sources of Variation Advantages 1. It is a rapid breeding method. 2. The method is inexpensive to conduct. 3. The base population can be a landrace. The population size selected is variable and can be small or large, depending on the objective. 4. The cultivar developed by this method has great “eye appeal” because of the high uniformity. 5. It is applicable to improving traits of low heritability, because selection is based on progeny performance. 6. Mass selection may include some inferior pure lines. 7. In pure-line selection, only the best pure line is selected for maximum genetic advance. Disadvantages 1. The purity of the cultivar may be altered through admixture, natural crossing with other cultivars, and mutations. Such off-type plants should be rogued out to maintain cultivar purity. 2. The cultivar has a narrow genetic base and hence is susceptible to devastation from adverse environmental factors, because of uniform response. 3. A new genotype is not created. Rather, improvement is limited to the isolation of the most desirable or best genotype from a mixed population. 4. The method promotes genetic erosion because most superior pure lines are identified and multiplied to the exclusion of other genetic variants. 5. Progeny rows take up more resources (time, space, funds). Mass selection Mass selection is an example of selection from a biologically variable population in which differences are genetic in origin. The purpose of mass selection is population improvement through increasing the gene frequencies of desirable genes. Selection is based on plant phenotype and one generation per cycle is needed. Mass selection is imposed once or multiple times (recurrent mass selection). The improvement is limited to the genetic variability that existed in the original populations (i.e., new variability is not generated during the breeding process). Mass selection Features of Mass Selection The main features of varieties developed by mass selection in self and cross pollinated species are given below: 1. Application Mass selection is applicable to both self and cross pollinated species. 2. Genetic Constitution In self pollinated crops, a mass selected variety is homozygous but heterogeneous, because it is a mixture of several pure-lines. 3. Adaptation Mass selected varieties have wide adaptation and are more stable against environmental changes due to heterogeneity which provides better buffering capacity. In other words, mass selected varieties have broader genetic base than pure lines. 4. Genetic Variation 5. Selection Selection is e ective in case of mass selected varieties of self pollinated crops due to presence of heritable varieties. 6. Quality A variety developed by mass selection is less uniform in the quality of seed than pure-lines due to presence of heritable variation. 7. Resistance Mass selected varieties are less prone to the attack of new diseases due to genetic diversity. In other words, they are more resistant or tolerant to new diseases.  1 It is rapid, simple, and straightforward 2 It is inexpensive to conduct. 3 The cultivar is phenotypically fairly uniform even though it is a mixture of pure lines, hence making it genetically broad-based, adaptable, and stable. Disadvantages 1. To be most effective, the traits of interest should have high heritability. 2. Because selection is based on phenotypic values, optimal selection is achieved if it is conducted in a uniform environment. 3. Phenotypic uniformity is less than in cultivars produced by pure-line selection.