Reproduction & Development

• Asexual reproduction is creation of offspring without the fusion of egg and sperm
• Sexual reproduction is the creation of an offspring by fusion of a male gamete and female gamete to form a zygote
• Many invertebrates reproduce asexually by fission; the separation of a parent into two or more individuals of about the same size
• In budding, new individuals arise from outgrowths of existing ones
• Fragmentation is breaking of the body into pieces, some or all of which develop into adults. It must be accompanied by regeneration, regrowth of lost body parts
• Parthenogenesis is the development of a new individual from an unfertilized egg
• Sexual reproduction results in genetic recombination, increasing variation, increasing the rate of adaptation, and eliminating harmful genes
• Reproductive cycles are controlled by hormones and environmental cues
• Hermaphroditism is a solution for organisms that seldom encounter a mate in which each individual has both male and female reproductive systems and can self-fertilize
• Sex reversals may happen when the ratio of males to females in a population is too low
• Some simple systems do not have gonads, but gametes form from undifferentiated tissue
• Gonads are organs that produce gametes
• Sperm is stored in spermatheca during copulation
• Gametogenesis is the production of gametes by meiosis and differs in females and males
• Spermatogenesis is the production of mature sperm
• Oogenesis is production of mature eggs
• Sperm form in seminiferous tubules, highly coiled tubes surrounded by connective tissue. Leydig cells produce hormones and are scattered between the tubules. Sertoli cells provide nutrients.
• The follicle contains the partially developed egg (oocyte) surrounded by support cells. Ovulation is when the egg cell is expelled from the follicle. The remaining tissue grows within the ovary forming the corpus luteum. It secretes hormones that help to maintain pregnancy. It degenerates if the egg isn't fertilized.
• Spermatogenesis differs from oogenesis in that: only 1 egg forms from each cycle of meiosis vs 4 sperm, oogenesis ceases late in life, and oogenesis has long interruptions rather than the continuous production that occurs in spermatogenesis
• Fertilization is the union of egg and sperm
• In external fertilization, eggs shed by the female are fertilized by sperm in the external, usually wet, environment
• In internal fertilisation, sperm are deposited in or near the female reproductive tract, and fertilisation occurs within the tract. It requires behavioral interactions and compatible copulatory organs. All fertilisation requires critical timing, often mediated by environmental cues, pheromones, and/or courtship behaviour.
• Species with external fertilisation generally produce more gametes than species with internal fertilization
• Species with internal fertilization provide greater protection of the embryos and more parental care
• The embryos of some terrestrial animals develop in amniote eggs with protective layers
• Sexual selection is a form of natural selection where the selection is focused on relative success at attracting mates. It is driven in part by levels of parental investment and competition within and between the sexes
• Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction
• Amongst vertebrates, the clearest dimorphism is between gamete size which explains why behavioural sex differences exist
• Bateman's gradient indicates the strength of sexual selection. The steeper the gradient, the stronger the sexual selection. This has lead to evolution of different mating strategies. It is also linked to parental investment
• Monogamy is relatively rare among animals. Some animals have evolved mechanisms to decrease the chance of their mate mating with another individual
• Speciation is how a new kind of plant or animal species is created
• Polyploidy is the heritable condition of possessing more than two complete sets of chromosomes
• Polyploidy typically results in instant speciation—the new polyploid may be immediately isolated reproductively from its parent or parents; this process greatly increases biodiversity and provides new genetic material for evolution
• Model organisms are species that are representative of a larger group and easily studied
• Development is determined by the zygote's genome and molecules in the egg called cytoplasmic determinants
• Cell differentiation is the specialization of cells in structure and function
• Morphogenesis is the process by which an animal takes shape
• Fertilisation brings the haploid nuclei of sperm and egg together, forming a diploid zygote. The contact between the sperm and the egg's surface initiates metabolic reactions in the egg that trigger the onset of embryonic development
• The acrosomal reaction is triggered when the sperm meets the egg. The acrosome at the tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg. The acrosomal process extends and proteins from the tip bind with receptors in the egg membrane. Gamete contact and / or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy.
• The Cortical Reaction is also initiated by the fusion of the egg and sperm. This reaction induces a rise in Calcium ions that stimulates cortical granules to release their contents outside the egg's cytoplasm. These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy.