Modes of Reproduction: Modes of Reproduction Sexual reproduction involves union of gametes Sperm and egg produced via sporogenesis Asexual reproduction bypasses fertilization Some plants combine both strategies Differences between animals and plants Movement vs. stasis…..pollen can move Developmental fate vs. meristematic ‘plasticity’ Totipotency Alternation of generations and endosperm development Embryo to adult vs. embryo-dormant-adult 1


Slide2: Megasporogenesis Formation of 4 haploid (n) megaspores from meiotic division of the 2n megaspore mother cell Microsporogenesis Formation of 4 haploid (n) microspores from meiotic division of the 2n microspore mother cell Meiotic Stages-Reductional Division Meiosis I is first division of MMC, each of the two cells divides again in Meiosis II to form 4 cells Mitotic Stages-Equational Division No pairing or crossing over of chromosomes Terms and Concepts 2


Slide3: Male Each of 4 microspores develops into a pollen grain Each microspore undergoes a mitotic division and forms a generative and vegetative nucleus The generative nucleus divides mitotically to form 2 sperm Female 3 of 4 microspores disintegrate 4th megaspore undergoes 3 mitotic divisions to form the 8 nucleate female gametophyte, or embryo sac Fertilization Egg cell unites with sperm from pollen to form a 2n zygote Mitotic division of zygote results in embryo (seed) Two polar nuclei and second sperm form 3n endosperm Overview of Gamete Formation and Fertilization 3


Slide4: Male Female Microspore Mother Cell 2n Meiosis Microspores Mitosis of Nucleus (2x) Pollen Grain Tube and generative nucleus 2 sperm and tube nucleus Stigma Style SEED Megaspore Mother Cell 2n Megaspores Mitosis of Nucleus (3x) 8-nucleate female gametophyte 3 degenerate All functional Pollen tube Double Fertilization 2 polar nuclei + sperm = 3n endosperm Egg + sperm = zygote Antipodals Polar nuclei Egg 4


Slide5: Endosperm is triploid tissue Of great importance in human diet More genotypes possible in endosperm AA x aa results in endosperm AAa Reciprocal cross would have Aaa endosperm Zygote Aa in either case Xenia effect: immediate effect of pollen parent on female seed parent Implications for seed crops Endosperm Genotypes 5


Slide6: Individual flowers Hermaphrodite = bisexual Pistillate = female Staminate = male Individual plants and plant populations Hermaphrodite = bisexual Monoecious = male and female Dioecious = male or female Gynoecious = female Androecious = male Gynomonoecious = bisexual and female Andromonoecious = bisexual and male Trimonoecious / trioecious = bisexual and male and female Cross and Self-Pollination 6


Slide7: Mechanism Description Facilitates self-pollination Cleistogamy flowers self before opening Homogamy synchronous maturation of m & female Facilitates cross-pollination Chasmogamy open flowers can cross-pollinate Dichogamy stigma and stamen mature differentially Protogyny stigma receptive before anthesis Protandry anthesis precedes stigma receptivity Incompatibility sexual crosses fail with like plants Sterility nonfunctional sex organs / gametes Heterostyly modification of floral parts Floral Modifications in Hermaphrodites adapted from Dellaporta and Calderon-Urrea, 1993, Plant Cell 5:1241 7


Slide8: Sex Expression in Cucumber The m allele conditions andromonoecy The F allele conditions femaleness The A locus conditions maleness The De gene enhances pistillate flowers M-F- plants are gynoecious M-ff plants are monoecious mmF- plants are hermaphroditic Mmff plants are andromonoecious Standard cultivars monoecious, gynoecious Methionine SAM ACC Ethylene Femaleness F Cobalt Silver Nitrate, GA 8


Slide9: Characteristic Selfer Crosser Genotypes of sporophyte Homozygous Heterozygous Genotypes of single plant Same Different Progeny of single plant Homogeneous Heterogeneous Progeny of F1 plant Heterogeneous Heterogeneous Self-incompatibility None Frequent Inbreeding depression Usually none Usual Populations and Reproductive Mode adapted from R.J. Lambert, Univ. Illinois 9


Slide10: Evolution of Self-Pollination Inbreeding thought to arise from outbreeding Wheat, tomato, pea are derived inbreeders High immediate fitness with inbreeding? Take advantage of niche environment Showy flowers not useful in many legumes But this facilitates some cross pollination Even low levels of crossing may be very important Inbreeders may have some flexibility More devices in nature to prevent inbreeding 10


Slide11: Some Examples Selfers Barley, oat, rice, sorghum, wheat, pea, bean, peanut, soybean, apricot, nectarine, peach, citrus, cotton, eggplant, lettuce, pepper, tobacco, tomato, parsnip, endive Crossers Maize, rye, apple, avocado, banana, cherry, fig, grape, mango, olive, pear, plum, alfalfa, almond, pecan, walnut, beet carrot, artichoke, onion, cucumber, pumpkin, spinach, squash 11


Slide12: Vegetative reproduction and apomictic reproduction are means of asexual reproduction Vegetative: stolons, rhizomes, buds, shoots, cuttings, etc. Apomixis: production of seed without fusion of gametes Facultative apomicts typical “Procreation without recreation” (S. Peloquin) Megaspore mother cell divides meiotically, but embryo sac aborts Somatic cells in ovule divide mitotically to form embryo sac Diplospory: embryo sac from MMC via mitosis Apospory: embryo sac from nucellus cells Adventitious embryony: diploid ovule cell forms embryo mitotically Asexual Reproduction and Apomixis 12


Slide13: Sexual Reproduction Vegetative Apomixis Allogamy Autogamy stolon Protandry cleistogamy cutting Protogyny time of pollen shed rhizome Monoecy perfect flowers bulb Dioecy tiller Exserted style bulbil Self-incompatibility graft Male sterility bud Recurrent Non-Recurrent Gametophytic Adventitious Embryony embryo sac cell Nucellus or integument leads to 1n leads to 2n sporophyte sporophyte Diplospory / Aposproy Parthenogenesis / Apogamety 13


Slide14: Gametophytic Apomixis Adventitious Embryony Alternation of Generations No Alternation of Generations Diplospory Apospory MMC forms embryo somatic cells Sac mitotically in ovule form embryo sac Parthenogenesis Apogamety Egg cell Non egg cell 2n Gametophyte 2n Sporophyte Development of embryosac Development of embryo 14


Slide15: Floral Biology Stigma Ovary Nectaries Anthers Petals 15


Slide16: Pollen Flow and Gene Dispersal Wind pollination, insect pollination, self-pollination Outcrossing percentages vary in facultative selfers Selfing rates vary in facultative crossers Faba bean is partially allogamous and is pollinated by honeybees and several different kinds of non-social ground nesting apoids Average cross-pollination is 50% Pollination involves ‘tripping’, where pollinators cause release of style and rupture the stigmatic surface Spontaneous disruption of membrane causes selfing Pollinator situation very dependent upon environment Case Study 16


Slide17: Pollen Flow and Gene Dispersal Position of flower in bee visitation sequence studied in Faba bean Caged plants with single Bombus terrestris individuals used 17 bees visited 1261 flowers, forming 2812 seeds Seeds analyzed for hybridity 21.3% and 17.5% outcrossing for first 5 and 10 flowers visited 78%, 13.4%, and 8.5% of pods contained 1,2, and 3 hybrid seeds Unaffected by floral node After first visits to recipient flowers, crossing loses efficiency High frequency of ‘alternate foraging’ needed Placement of parental lines is crucial for crossing success Carre et al., 1998, Crop Science 38:322-325 Case Study 17


Slide18: Case Study Average Pollen Dispersal Transgenic canola now a reality on world market Herbicide-resistant strains of canola available Efforts to limit uncontrolled escape of genes to wild/weedy plants Or, crop itself could become a weed outside of crop production Or, transgene could be transferred to another nearby field of crop Or, a traits like herbicide resistance is transferred to weeds Pollen dispersal of a single plant measured 50% of pollen fell within 3 m; probability of pollination after that decreased slowly along a negative exponential of distance Lavigne et al., 1998, TAG 96:886-896 18


Slide19: Case Study Case Studies in Reproduction Single dominant gene control of apomixis in Tripsacum, due to displospory, mapped as cluster of genes (Grimanelli et al., 1998, Heredity, 80:33-39) Apomicts identified in Malus spp. when single-gene dominant conditioned red pigmentation in seedlings is absent (Ur-Rahman et al., 1997, TAG, 95:1080-1083) Self-incompatibility in Indian mango, Floridian mango is self-fertile. Outcrossing rates in self-fertile mango increased dramatically as fruit matured- selective abscission of selfed progeny (Dag et al., 1998, JASHS 123:618-622) 19


Slide20: Case Study Case Studies in Reproduction Maintenance of hybrids or other genotypes difficult to propagate sexually can be conducted via apomixis Kentucky bluegrass (Poa pratensis) is a major turfgrass species It is a facultative aposporic apomict, but can be sexual as well Apomixis in this species requires fertilization of polar nuclei for endosperm development, but progenies are of maternal origin only. This process is known as Pseudogamy Parthenogenesis in Poa pratensis likely controlled by a single dominant gene. Sexual x apomictic matings support a single dominant gene model that is simplex in polyploid Poa (Barcaccia et al., 1998, TAG) 20


Slide21: Case Study Utilizing Apomixis in Breeding Current efforts to transfer apomixis genes to sexual species Major cereal grasses have been targeted ‘Fixation’ of hybridity a major commercial target Vielle Calzada et al. (Science 274, 1996): ‘Asexual Revolution’ “The harnessing of apomixis will lead to large increases in agricultural production” Apomixis seemingly under simple genetic control Mutants have arisen which result in apomixis However the process may not be simple And efforts so far have not resulted in transfer of apomixis 21