WHY SEX? -SEXUAL REPRODUCTION-TECHNICAL POINT OF VIEW FOR DISCUSSION

The sexual cycle
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Sex is Very Costly

  • Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required
  • Intimate contact provides route for infection by parasites (AIDS, syphillis, etc.)
  • Genetic costs: in sex you pass on only half your genes to your children
  • Males are an expensive luxury- in most species they contribute little to rearing offspring

But There are Some Advantages

  • More genetic diversity: more potential for survival of species when environmental conditions change
    • Shuffling of genes in meiosis
    • Crossing-over in meiosis
    • Fertilization: combines genes from 2 separate individuals
  • DNA back-up and repair
    • Asexual organisms don’t have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged
    • Sexual mechanisms , especially recombination, are used to repair damaged DNA- the undamged chromosome acts as a template and eventually both chromosomes end up with the correct gene

Reproduction Without Sex and Sex Without Reproduction Both Occur in Nature

  • Sex is the transfer of genes from one cell to another and in microorganisms this often occurs without cell division, so that there is no reproduction
  • Many species can reproduce without sex
    • Most single-cell organisms
    • Some multcellular organisms

There Are Many Successful Asexual Species

  • Single cells reproduce whenever they divide
  • Some multicellular organisms reproduce by budding or branching from parent (Hydra, sponges, sea anenomes)
  • Some higher organisms produce eggs that develop into new individuals without fertilization ; called parthenogenesis (virgin birth)
    • Aphids in summer (they become sexual in the fall)
    • Whiptail lizards
    • Some salamanders
  • Offspring of asexual organisms are clones of the parent (genetically identical)

Sexual Reproduction Has Been Adopted by Most Higher Organisms

  • Almost all organisms with eukaryotic cells undergo sexual reproduction
  • The sexual lifestyle:
    • Diploid cells: 2 sets of chromosomes
    • Meiosis: a type of cell division that produces reproductive cells that are haploid (1 set of chromosomes); usually there are 2 types of reproductive cells, sperms and eggs
    • Fertilization: combination of a sperm and egg to produce a new diploid cell (zygote)
    • Development of the zygote into a new individual

There Are a Large Number of Natural Sexual Strategies

  • Hermaphrodites have both sexes on same individual (many flowers,¬†earthworms, snails, some fish); in earthworms and snails when 2 individuals mate each fertilizes the other
  • Alternate sexual and asexual stages (gall wasps, aphids)
  • Fertilization may be internal or external
  • Development of embryo may be internal or external

Sex Determination Can be Genetic or Environmental

  • In many species sex is determine solely by the chromosomes- birds and mammals
  • In other species environmental factors are important
    • In turtles sex is determined by temperature of egg development
    • In some species environmental factors change the sex at different stages of life
      • Some marine worms change sex (from male to female) when they get larger
      • Coral reef fishes (wrasse) : if the male fish dies the largest female in the group changes into a new male

In Humans (and Other Mammals) Sex is Determined Genetically

  • We have 46 chromosomes, 23 pairs
  • 22 pairs are called somatic chromosomes
  • The 23rd pair consists of two chromosomes, the X and Y, that are somewhat different from each other
  • The X and Y determine sex: a person who is XX is female; a person who is XY is male
    • The X chromosome is required for life, the Y is not
    • The sex of a child is always determined by the father- he can make both X and Y sperm
    • The Y chromosome is small and degenerate but it has a gene controlling the production of testosterone- if this hormone is present the embryo develops into a male

Sex Organs Develop from a Unisex Gonad

  • At 5-6 weeks the developing embryo, whether male or female has a pair of unspecialized gonads and 2 paired ducts, the Wolffian and Mullerian, which connect to the future urethra
  • Differentiation into male or female starts at about the 7th week
  • If the embryo has a Y chromosome testosterone is produced and male gonads develop
  • If there is no Y chromosome the embryo develops into a female
  • Male development:
    • Wolffian duct -> vas deferens and epididymis
    • Mullerian duct -> degenerates
    • Unisex gonads -> testes
    • Labioscrotal swellings -> scrotum
    • Genital tubercle -> penis
    • Testes descend into scrotum in 7th month
  • Female development
    • Mullerian duct -> oviduct and uterus
    • Wolffian duct -> degenerates
    • Unisex gonads -> ovaries
    • Labioscrotal swellings -> labia majora
    • Genital tubercle -> clitoris

Phenotypic Sex May Differ from Genetic and Gonadal Sex

  • Hermaphrodites have both male and female sex organs
    • Double fertilization can produce some XY cells in an XX individual
    • An XY person who is insensitive to testosterone (receptor defect) will develop some female characteristics
    • Mutation may block production of testosterone in XY individual
  • Some individuals have unusual numbers of X and Y chromosomes: usually male if there is at least one Y chromosome

Both Sexes Form Gametes by Meiosis

  • Some definitions:
    • Haploid cell = cell with 1 set of chromosomes
    • Diploid cell = cell with 2 sets of chromosomes (1 from each parent)
    • Gamete = haploid reproductive cell (sperm or egg)
    • Zygote = diploid cell produced by fusion of an egg and sperm during fertilization
  • Meiosis is a type of cell division designed to produce gametes for sexual reproduction:
    • DNA duplicates
      This is a vector version of :Image:MajorEvents...
      Image via Wikipedia
    • 1st cell division produces 2 cells with single sets of duplicated chromosomes
    • 2nd cell division separates the duplicates, resulting in 4 haploid gametes

Production of Haploid Gametes in Meiosis:

  • Note that the first deivision separates the homologues and that the second division separates the sister chromatids
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