Variants in the Reproductive Process

Apomixis Substitute for sexual reproduction Development of an embryo w/o sex Identified in over 300 species across 37 families. Not rare but relatively uncommon

Reproductive Behavior Obligate Apomict: a plant which reproduces only by asexual reproduction. Facultative Apomict: a plant which has the potential to reproduce either sexually or asexually. Both process may occur simultaneously or one may be predominant. Within a genus or species, all reproduction strategies may be possible

Apomixis Must bypass two different processes: Meiosis: no reduction of chromosome # Fertilization: development of embryo w/o fusion of sperm and egg

Stimulus for Embryo Development Autonomous: requires no further stimulation - embryo and endosperm may be developed before flowers open Pseudogamy (False-fertilization): development of 2n gametophyte requires pollination and/or pollen tube growth. Sometimes a sperm nucleus fuses with polar nuclei, but never with egg

Genetics of Apomicts Apomixis is genetically controlled in most cases studied Control is complex (multigenic) and recessive to sexual reproduction Polyploidy may affect apomictic reproduction Hybridization tends to increase apomixis

Use of Apomixis in Breeding Production of exact replicas of diploid Fix hybrid vigor Fix heterozygous status

Breeding and Pollination Systems

Plant Breeding Systems General Issues to consider: A flower is an adapted leaf specialized for sex. Its function is to bring the gametes together thus forming zygotes with new genetic combinations A species’ survival is dependent on the production of offspring adapted to a particular environment. Plants have evolved a variety of adaptations that either encourage outbreeding (cross-pollination) or inbreeding (self-pollination or selfing). Each breeding system has genetic ramifications

Basic Terms: Outbreeding: Sexual reproduction between individuals (cross-pollination). (It involves two individual plants) Inbreeding: Sexual reproduction within an individual (self-pollination). (It involves one individual plant) Sexual Reproduction: Meiosis and fertilization. The sexual process is a mechanism to bring about gene recombination. Recombination is the chief source of hereditary variation and provides the raw materials for species to adapt to changing environmental conditions.

Outbreeder or Inbreeder?? Often one can tell just by looking at a flower whether it cross-pollinates or self-pollinates Trait Incompatibility Flower # Flower size Flower color Nectories Scent Nector guides Anther position Pollen # Style position Stigma OUTBREEDER INBREEDER self-incompatible self-compatible many flowers few flowers large flowers small flowers bright colors mono-colored nectaries present nectaries absent scented flowers unscented flowers nectar guides present nectar guides absent anthers far from stigma anthers close to stigma many pollen grains fewer pollen grains style exserted from flower style included in flower stigmatic area well-defined stigmatic area poorly-defined

Outbreeding (Cross-pollination) Advantages: Increases genetic variability Strong evolutionary potential Adaptation to changing conditions Successful long-term Disadvantages: Can destroy well-adapted genotypes Relies on effective cross-pollination, seed dispersal and establishment

Inbreeding (Self-pollination) Advantages: Preserves well-adapted genotypes Insures seed set in the absence of pollinators Single colonizing individual possible Disadvantages: Decreases (or maintains) genetic variability Evolutionary dead-end Cannot adapt to changing environmental conditions Successful short-term