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Plant Reproduction • Angiosperms represent an evolutionary innovation with their production of flowers and fruits • Flowers are modified stems that house the gametophyte generation of angiosperms • Flower organs are believed to be evolved from leaves; these organs appear as distinct whorls of parts • A complete flower has four distinct whorls of parts • An incomplete flower lacks one or more of the whorls gynoecium androecium Enhance reproductive success corolla calyx Develops into fruit Develop into seeds Floral Symmetry • Primitive flowers exhibit radial symmetry • Advanced flowers exhibit bilateral symmetry http://www.flickr.com/photos/mimbrava/22802180/ Pollination • Plant sexual life cycles are characterized by an alternation of generations • In angiosperms, the gametophyte generation is very small and completely enclosed within the tissues of the parent sporophyte • Female gametophyte = embryo sac • Male gametophytes = pollen grains • Pollination is the process by which pollen is placed on the stigma Pollination • Self-pollination – pollen from a flower’s anther pollinates stigma of the same flower • Cross-pollination – pollen from anther of one flower pollinates another flower’s stigma Pollination • When pollen reaches the stigma, it germinates and a pollen tube grows down, carrying the sperm nuclei to the embryo sac • After double fertilization, development of the endosperm and the embryo begins • The seed matures within the ripening fruit Double fertilization • 2 sperm cells fertilize 2 cells in the ovary • 1 sperm fertilizes the egg zygote • Other fuses with 2 polar nuclei endosperm Pollination • Some angiosperms are wind-pollinated – A characteristic of early seed plants – Gymnosperms and some Angiosperms (grasses, birches, oaks) – Pollen only travels short distances (100 meters) Pollination • Successful pollination in many angiosperms depends on a regular attraction of pollinators • Bees attracted to flowers by color • Flowers visited by butterflies frequently have a flat ‘landing platform’ • Flowers visited by moths are often white or pale, heavily scented • Flowers visited by birds often are red • Monkeys are attracted to orange and yellow fruits Asexual Reproduction • Asexual reproduction produces a genetically identical individual (only mitotic cell divisions occur; no fusion of sperm and egg) • Differs from self-pollination (sexual reproduction, although genetic diversity is limited) Asexual Reproduction in Plants • No alternation of generations • New plants are cloned from other parts of the adult plant • Called “vegetative reproduction” – Rhizomes – underground stems – Stolons or runners – long, slender stems that run along surface of soil – Fragmentation – adventitious leaves or roots (reproductive leaves and “suckers”, respectively) Adventitious plantlets Plant Life Spans • Perennial plants – continue to grow year after year – Majority of vascular plants are perennial • Annual plants – grow, reproduce, and die within a single year – Annuals flower, produce fruits and seed (and then die) – Many crop plants are annuals Fertilization in Animals • Asexual reproduction – Fission – separation of parent into two or more offspring of equal size (single-celled organisms) – Budding – new individuals split off from existing ones – Fragmentation and regeneration – the breaking of the body into several pieces; some or all of which develop into new adults • Budding in Cnidaria • Fragmentation in Planaria Fertilization in Animals • Parthenogenesis – a specialized form of asexual reproduction – Females produce offspring from unfertilized eggs – Some species are exclusively parthenogenic (all females) – Others switch between parthenogenesis and sexual reproduction (e.g., Cladocera, Honeybees) Fertilization • Hermaphroditism - A specialized form of sexual reproduction – One individual possesses both testes and ovaries; can produce sperm and eggs – Some can self-fertilize (e.g., Ctenophores) – Some hermaphroditic individuals can change sex • Protogyny – “female first” • Protandry – “male first” Sexual Reproduction • Most animals reproduce sexually • Involves fusion of two haploid gametes (sperm and egg) to form a diploid zygote • Increases genetic variability • Advantageous when environmental conditions are unstable or change often • Two modes of fertilization in animals – External – Internal Sexual Reproduction • External fertilization – eggs are shed by female and fertilized by the male in the environment – Moist environments – No parental care; large numbers of zygotes with low survival – Requires synchronization • Due to environmental cues or pheromones External fertilization • Most amphibians and bony fishes have external fertilization Sexual Reproduction • Internal fertilization – egg is fertilized within the female’s body • Evolved in terrestrial vertebrates • Requires cooperative behavior leading to mating • Requires sophisticated reproductive systems with copulatory organs • Fewer zygotes with increased survival; protection of the embryo and parental care Internal Fertilization • Vertebrates that practice internal fertilization exhibit three distinct strategies for embryonic and fetal development – Oviparity – fertilized eggs are deposited outside of body; some fish, most reptiles, all birds, some mammals – Ovoviviparity – fertilized eggs are retained within female; embryos receive nutrition from yolk; some fish, some reptiles – Viviparity – embryos develop within female; receive nutrition from mother; most cartilaginous fish, all mammals