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 calyx corolla Enhance reproductive success androecium gynoecium

Develop into seeds Develops into fruit

Floral Symmetry Primitive flowers exhibit radial symmetry Advanced flowers exhibit bilateral symmetry

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