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Modes of reproduction

Modes of reproduction


Deependra Dhakal

November 07, 2020


  1. Modes of reproduction and pollination control Modes of reproduction and

    pollination control Modes Modes Types of cultivars Types of cultivars Annuals and perennials Annuals and perennials Reproductive sterility Reproductive sterility Pollination Pollination Incompatibility Incompatibility Male sterility Male sterility Deependra Dhakal Deependra Dhakal 2019/04/06 (updated: 2020-10-05) 2019/04/06 (updated: 2020-10-05) 1 / 48 1 / 48
  2. Modes of reproduction Modes of reproduction 2 / 48 2

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  3. Sexual reproduction Involves fusion of male and female gametes Gametes

    may be derived either from two different parents or from a single parent. Sexual reproduction is reliant on the process of meiosis. Involves Megaspores within the ovule of the pistil Microspores within the stamen Typical meiotic division of a female diploid species will result in formation of four haploid megaspores (Megasporogenesis). With analogy: Microsporogenesis Hence fertilization involves fusion of haploid male gamete with haploid female gamete Male gametophyte generation is a tiny pollen tube and three haploid nuclei (microgametophyte) Female gametophyte is a single multinucleated cell, also called the embryo sac (megagametophyte, aka embryo sac). 3 / 48
  4. Fertilization in plants 4 / 48

  5. Embryo sac and pollen formation 5 / 48

  6. Autogamy/Self-pollination Mechanisms that promote autogamy Cleistogamy (flower fails to open)/Chasmogamy

    (opening of flower after pollination) Proximity of anther Mechanisms that prevent autogamy Self incompatibility, male sterility and dichogamy 6 / 48
  7. Self incompatibility (lack of self-fruitfulness) A condition in which the

    pollen from a flower is not receptive on the stigma of the same flower, and hence incapable of setting seed. Both pollen and ovule development are normal and viable. It is caused by a genetically controlled physiological hindrance to self-fertilization. Self incompatibility is widespread in nature, occurring in families such as Poaceae, Cruciferae, Compositae, and Rosaceae. The incompatibility reaction is genetically conditioned by a locus designated S, with multiple alleles that can number over 100 in some species such as Trifolium pretense. Unlike monoecy and dioecy, all plants produce seed in self-incompatible species. There are two well known systems: Homomorphic Heteromorphic 7 / 48
  8. Heteromorphic Caused by heterostyly Pin (long styles and short anthers)

    and thrum (long anthers and short styles; e.g., primula) Pin conditioned by genotype ss, and thrum by Ss. Cross ss x ss as well as Ss x Ss is incompatible But ss x Ss or SS x ss is compatible The condition is called distyly 8 / 48
  9. Homomorphic Homomorphic 9 / 48 9 / 48

  10. Gametophytic incompatibility (originally called the oppositional factor system) The ability

    of the pollen to function is determined by its own genotype and not the plant that produces it. This form of SI is more widespread (e.g. red clover, white clover and yellow sweet clover) Controlled by a series of alleles at a single locus ( S1, S2, S3 ... Sn ) The alleles of the incompatibility gene(s) act individually in the style. Alleles exhibit no dominance Incompatible pollen is inhibited in the style. The pistil is diploid hence contains two incompatibility alleles (e.g., S1 S3, S3 S4). Reactions occur if identical alleles in both pollen and style are encountered. Only heterozygotes for S alleles are produced in this system. 10 / 48
  11. Sporophytic incompatibility The incompatibility characteristics of the pollen are determined

    by the plant (sporophyte) that produces it. It occurs in species such as broccoli, radish, and kale. The sporophytic system differs from the gametophytic system in that the S allele exhibits dominance. Also, it may have individual action in both pollen and style, making this incompatibility system complex. The dominance is determined by the pollen parent. Incompatible pollen may be inhibited on the stigma surface. For example, a plant with genotype S1 S2 where S1 is dominant to S2, will produce pollen that will function like S1. Furthermore, S1 pollen will be rejected by an S1 style but received by an S2 style. Hence, homozygotes of S alleles are possible 11 / 48
  12. Incompatibility reactions of different mating combinations in a mono- and

    digenic cross. Cross Male parent Female parent Pollen gamete Gametophytic Reaction Gametophytic Progenies Sporophytic Pollen functional type Sporophytic Reaction Sporophytic Progenies 1 S1S2 S1S2 S1 Incompatible - S1 Incompatible - 1 S1S2 S1S2 S2 Incompatible - S1 Incompatible - 2 S1S2 S2S3 S1 Compatible S1S2, S1S3 S1 Compatible S1S2, S1S3 2 S1S2 S2S3 S2 Incompatible - S1 Compatible S2S2, S2S3 3 S1S2Z1Z2 S1S2Z1Z2 S1Z1 Incompatible S1Z1 Incompatible 3 S1S2Z1Z2 S1S2Z1Z2 S1Z2 Incompatible S1Z1 Incompatible 3 S1S2Z1Z2 S1S2Z1Z2 S2Z1 Incompatible S1Z1 Incompatible 3 S1S2Z1Z2 S1S2Z1Z2 S2Z2 Incompatible S1Z1 Incompatible 4 S1S2Z1Z3 S1S2Z1Z2 S1Z1 Incompatible S1Z1 Incompatible 4 S1S2Z1Z3 S1S2Z1Z2 S1Z3 Compatible S1Z1 Incompatible 4 S1S2Z1Z3 S1S2Z1Z2 S2Z1 Incompatible S1Z1 Incompatible 4 S1S2Z1Z3 S1S2Z1Z2 S2Z3 Compatible S1Z1 Incompatible 5 S3S4Z3Z4 S1S2Z1Z2 S3Z3 Compatible S3Z3 Compatible 5 S3S4Z3Z4 S1S2Z1Z2 S3Z4 Compatible S3Z3 Compatible Assumptions: Suppose there are incompatibility conditioning genes S and Z Two genes could have alleles S1, S2, S3, S4… and Z1, Z2, Z3, Z4,… Under SSI system, dominance relation holds: S1>S2>S3 and Z1>Z2>Z3. 12 / 48
  13. Incompatibility reactions (...continued) Cross Male parent Female parent Pollen gamete

    Gametophytic Reaction Gametophytic Progenies Sporophytic Pollen functional type Sporophytic Reaction Sporophytic Progenies 5 S3S4Z3Z4 S1S2Z1Z2 S4Z3 Compatible S3Z3 Compatible 5 S3S4Z3Z4 S1S2Z1Z2 S4Z4 Compatible S3Z3 Compatible 6 S1S3Z1Z4 S1S2Z1Z2 S1Z1 Incompatible S1Z1 Incompatible 6 S1S3Z1Z4 S1S2Z1Z2 S1Z4 Compatible S1Z1 Incompatible 6 S1S3Z1Z4 S1S2Z1Z2 S3Z1 Compatible S1Z1 Incompatible 6 S1S3Z1Z4 S1S2Z1Z2 S3Z4 Incompatible S1Z1 Incompatible 7 S1S3Z2Z3 S1S2Z1Z2 S1Z2 Incompatible S1Z2 Incompatible 7 S1S3Z2Z3 S1S2Z1Z2 S1Z3 Compatible S1Z2 Incompatible 7 S1S3Z2Z3 S1S2Z1Z2 S3Z2 Compatible S1Z2 Incompatible 7 S1S3Z2Z3 S1S2Z1Z2 S3Z3 Compatible S1Z2 Incompatible 8 S1S3Z2Z3 S1S2Z1Z3 S1Z2 Compatible S1Z2 Compatible 8 S1S3Z2Z3 S1S2Z1Z3 S1Z3 Incompatible S1Z2 Compatible 8 S1S3Z2Z3 S1S2Z1Z3 S3Z2 Compatible S1Z2 Compatible 8 S1S3Z2Z3 S1S2Z1Z3 S3Z3 Compatible S1Z2 Compatible 13 / 48
  14. Expression of SI 1. The germination of the pollen may

    be decreased (e.g., in broccoli); Removing the stigma allows normal pollen germination. 2. Pollen germination is normal but pollen tube growth is inhibited in the style (e.g., tobacco). 3. The incompatibility reaction occurs after fertilization (e.g., in Gesteria). This mechanism is rare. 14 / 48
  15. Implications of SI Presence of this system may be exploited

    to facilitate some breeding methods. Self-incompatibility promotes heterozygosity. Consequently, selfing self-incompatible plants can create significant variability from which a breeder can select superior recombinants. Overcoming: Removal of the stigma surface Early pollination Lowering the temperature Self-incompatibility may be used to breed for F1 hybrids, synthetics, triploids. First, however, homozygous lines must be developed. Sporophytic incompatibility is widely used in breeding of cabbage and other Brassica species. The single cross hybrids are more uniforms and easier to produce. The top cross is commonly used. A single self-incompatible parent is used as female and is open- pollinated by a desirable cultivar as pollen source. 15 / 48
  16. A S1S1 AB S1S2 B S2S2 Production of single cross

    hybrid using inbred lines and sporophytic self incompatibility 16 / 48
  17. A S1S1 AB S1S2 B S2S2 C S3S3 CD S3S4

    D S4S4 ABCD S1S3, S1S4, S2S3, S2S4 Production of double cross hybrid using inbred lines and sporophytic self incompatibility 17 / 48
  18. A S1S1 AB S1S2 B S2S2 C S3S3 C S3S3

    ABC S1S3, S2S3 D S4S4 DE S4S5 E S5S5 F S6S6 F S6S6 DEF S4S6, S5S6 ABCDEF S1S4, S1S6, S1S5 S2S4, S2S6, S2S5 S3S4, S3S6, S3S5 Production of triple cross hybrid using inbred lines and sporophytic self incompatibility 18 / 48
  19. Male sterility Male sterility 19 / 48 19 / 48

  20. Male sterility is a condition in plants whereby the anthers

    or pollen are non-functional. Male sterility also enforces cross-pollination. as absence of or extreme scarcity of pollen, severe malformation or absence of flowers or stamens, or failure of pollen to dehisce. Similarly, it can be exploited as a tool to eliminate the need for emasculation for producing hybrid seed. 20 / 48
  21. Kinds of sterility based on the origin of the abnormality

    1. True male sterility - This is due to unisexual flowers that lack male sex organs (dioecy and monoecy), or bisexual flowers with abnormal or non-functional microspores (leading to pollen abortion). 2. Functional male sterility - The anthers fail to release their contents even though the pollen is fertile. 3. Induced male sterility - Plant breeders may use chemicals to induce sterility. 21 / 48
  22. True male sterility systems True male sterility systems 22 /

    48 22 / 48
  23. Genetic male sterility Genetic (nuclear, genic) male sterility is widespread

    in plants. The gene for sterility has been found in species including barley, cotton, soybean, tomato, potato, and lima bean. It is believed that nearly all diploid and polyploidy plant species have at least one male sterility locus. May be manifested as pollen abortion (pistillody) or abnormal anther development. Genetic male sterility is often conditioned by a single recessive nuclear gene, ms, the dominant allele, Ms, conditioning normal anther and pollen development. In alfalfa, however, two independently inherited genes have been reported The expression of the gene may vary with the environment. But to be useful, the system must be stable 23 / 48
  24. Maintainance of GMS system The breeder cannot produce and maintain

    a pure population of male sterile plants. The genetically male sterile types ( msms) can be propagated by crossing them with a heterozygous pollen source ( Msms). What is the result of cross ? Breeders will always harvest 50% male sterile plants by harvesting only the male sterile plants. How to identify sterile from non sterile ? Bright green hypocotyls in broccoli, leaf shape of potato and green stem in tomato 24 / 48
  25. Genetic male sterility as used in practical breeding 25 /

  26. Cytoplasmic male sterility Sometimes, male sterility is controlled by the

    cytoplasm (mitochondrial gene) but may be influenced by nuclear genes. A cytoplasm without sterility genes is described as normal (N) cytoplasm, while a cytoplasm that causes male sterility is called a sterile ( s) cytoplasm or said to have cytoplasmic male sterility (CMS). Transmitted through the egg only (maternal factor). Has been found in species including corn, sorghum, sugar beet, carrot, and flax. The condition has been induced in species such as sorghum by transferring nuclear chromosomes into a foreign cytoplasm. Has real advantages in breeding ornamental species because all the offspring is male sterile, hence allowing them to remain fruitless. 26 / 48
  27. Cytoplasmic-genetic male sterility Cytoplasmic male sterility may be modified by

    the presence of fertility-restoring genes in the nucleus. CMS is rendered ineffective when the dominant allele for the fertility-restoring gene ( Rf) occurs, making the anthers able to produce normal pollen. CMS is transmitted only through the egg, but fertility can be restored by Rf genes in the nucleus. Three kinds of progeny are possible following a cross, depending on the genotype of the pollen source. The resulting progenies assume that the fertility gene will be responsible for fertility restoration. 27 / 48
  28. The three systems of cytoplasmic genetic male sterility. The three

    factors involved in CMS are the normal cytoplasm (N), the male sterile cytoplasm (S), and the fertility restorer (Rf, rf). 28 / 48
  29. Hybrid seed production using CMS requires three types of genotypes:

    cytoplasmic sterile female lines with male sterile cytoplasm but with no restorer genes (called A lines; maintained by crossing with isogeneic cytoplasmic male fertile line) 'male-fertile' female lines (called B lines or maintainer lines; maintained by selfing) with normal cytoplasm and no restorer genes male fertile lines (called R lines; maintained by selfing) with no cytoplasmic male sterility genes and which are homozygous for a dominant restorer gene (i.e. normal ( n) cytoplasm, RfRf) Hybrid production using CGMS 29 / 48
  30. Dichogamy (mechanism enforcing cross pollination) Dichogamy is the maturing of

    pistils and stamens of a flower at different times. Protogyny (stigma is receptive before the anther is mature to release the pollen) Protandry (pollen is released from the anther before the female is receptive). 30 / 48
  31. Asexual reproduction Asexual reproduction 31 / 48 31 / 48

  32. Production of offsprings that are genetically identical to the mother

    plant, and plants that are produced this way are called clones. Two methods of asexual reproduction Reproduction through plant parts Reproduction through apomixis 32 / 48
  33. Reproduction through plant parts A bulb is a modified shoot

    consisting of a very much shortened stem enclosed by fleshy leaves (e.g. a tulip or an onion). A corm is a swollen stem base bearing buds in the axils of scale-like remains of leaves from the previous year's growth (e.g. gladiolus). A cutting is an artificially detached part of a plant used as a means of vegetative propagation. A rhizome is an underground stem with buds in the axils of reduced leaves (e.g. mint or couch grass). A stolon is a horizontally growing stem that roots at nodes (e.g. strawberry runners). A tuber is a swollen stem that grows beneath the soil surface bearing buds (e.g. potato). 33 / 48
  34. Reproduction by apomixis Asexual production of plant seeds can occur

    in obligate and facultative apomicts. In facultative apomicts most seeds are asexually produced, since sexual reproduction can occur. Apomixis arises by following mechanisms (based on which cells are responsible for producing an embryo) Androgenesis (from the sperm nucleus of a pollen grain) Apospory (from somatic ovary cells) Diplospory (from 2n megaspore mother cell) Parthenogenesis (Egg cell without fertilization) In many cases, pollination must occur (pseudogamy) if viable apomictic seeds are to be formed. 34 / 48
  35. Reproductive options in plants Reproductive options in plants 35 /

    48 35 / 48
  36. Hermaphrodity versus unisexuality Hermaphrodites have both male and female sexual

    organs, and hence may be capable of self fertilization. On the other hand, unisexuals, having one kind of sexual organ, are compelled to cross-fertilize. Each mode of reproduction has genetic consequences. Hermaphrodity promotes a reduction in genetic variability, whereas unisexuality, through cross-fertilization, promotes genetic variability. 36 / 48
  37. Self-pollination versus cross-pollination Hermaphrodites that are self-fertile may be self

    pollinated or cross-pollinated. In terms of pollen donation, a species may be autogamous (pollen comes from the same flower - selfing) or allogamous (pollen comes from a different flower). There are finer differences in these types. For example, there may be differences between the time of pollen shed and stigma receptivity. Self-fertilization versus cross-fertilization Just because a flower is successfully pollinated does not necessarily mean fertilization would occur. The mechanism of self-incompatibility causes some species to reject pollen from their own flowers, thereby promoting outcrossing. 37 / 48
  38. Sexuality versus asexuality Sexually reproducing species are capable of providing

    seed through sexual means. Asexuality manifests in one of two ways - vegetative reproduction (in which no seed is produced) or agamospermy (in which seed is produced). 38 / 48
  39. Alternation of generation in owering plants Alternation of generations 39

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  40. Hybrid Open pollinated 40 / 48

  41. Types of cultivar Types of cultivar 41 / 48 41

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  42. Cultivar types include pure-lines, hybrids, clones, open-pollinated populations, composite-crosses, synthetics

    and multilines. It is difficult, if not impossible to develop a pure-line cultivar of a crop species like potato ( Solanum tuberosum ) as it is mainly reproduced vegetatively, and has many deleterious (or lethal) recessive alleles. Similarly, pea ( Pisum sativum ) is almost an obligate self-pollinator and so it would be difficult to develop hybrid pea, if nothing else seed production is likely to be expensive. 42 / 48
  43. Pure-line cultivars Pure-line cultivars are homozygous, or near-- homozygous, lines.

    Can be produced most readily in naturally self-pollinating species. It is generally accepted that it is normally one in which the line is homozygous for the vast majority of its loci (usually 90% or more) Commonly developed by inbreeding the species through continuous selfing of hybrid generated from crossing for 6-7 generations to the point where line is considered to be "commercially true breeding" Alternatively, doubled haploid lines may to generated 43 / 48
  44. Show 6 entries Search: Showing 1 to 6 of 20

    entries Previous 1 2 3 4 Next Commonly self pollinated crops Common name Scientific name 1 Barley Hordeum vulgare 2 Chickpea Cicer arietinum 3 Clover Trifolium spp. 4 Common bean Phaseolus vulgaris 5 Cotton Gossypium spp. 6 Cowpea Vigna unguiculata 44 / 48
  45. Heterogenous populations comprised of different plants which are genetically non-

    identical. Also heterozygous Almost exclusively from cross pollinating species Stable for character of interest E.g.: Onions, rye, herbage grass, non-hybrid sweetcorn, sugar beet and oil palm Open pollinated crops Open pollinated cultivars 45 / 48
  46. Hybrid Hybrid cultivars have different leves of homogeneity but, importantly

    are highly heterozygous. Exploits the phenomenon of heterosis Variants exist: Single cross Double cross Three way cross Is one of the most complex of breeding methods Inbred line development Test cross comparison Selection and hybridization 46 / 48
  47. Clonal cultivars Clonal cultivars are genetically uniform but tend to

    be highly heterozygous Uniformity of plant types is maintained through vegetative rather than sexual reproduction. Cultivars are vegetatively propagated by asexual reproduction (cloning) including cuttings, tubers, bulbs, rhizomes, grafts A cultivar can also be classified as a clone if it is propagated through obligate apomixis (e.g. buffelgrass) Genetic constitution of cloned selection remains 'fixed' e.g.: Potatoes, Bananas, Peaches, Apples, Cassava, Sugarcane, Strawberries, Blueberries and Chrysanthemums. 47 / 48
  48. Synthetics A synthetic cultivar development follows procedure similar to open

    pollinated population improvement, although a synthetic cultivar cannot be propagated by open-pollination without changing the genetic make-up of the population. It has been commercial seed companies that have been responsible for breeding almost all synthetic varieties. By 1980 there were 17 public-sector alfalfa breeders, but now there are more than three times (52) the number of private breeders developing synthetic lines. A synthetic cultivar must be reconstructed from its parental lines or clones. Synthetics have also been used almost exclusively in the development of alfalfa, forage grass and forage legume cultivars, maize, etc. The breeding method used for the development of synthetic cultivars is dependent on the ability to develop either homozygous lines from a species or to propagate parental lines clonally. In the case of maize, for example, synthetic cultivars are developed using inbred lines as a three-stage process: develop a number of inbred lines; progeny-test the inbred lines for general combining ability; identify the 'best' parents and intercross these to produce the synthetic cultivar. This process is almost identical to the procedure used for developing hybrid cultivars, and only differs in the last stage where many more parents will be included in a synthetic than in a hybrid cultivar. 48 / 48