Sex Determination and Sex Chromosomes

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Sex Determination and Sex Chromosomes

• Chapter 7

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Outline

• Primary vs. secondary sexual differentiation
• Unisexual vs. bisexual (herphaphrodites)
• Model organisms (Chlamydomonas (Green algae), Zea
  Mays (plant), C. elegans, Protenor (insect),
  Lygaeus turicus (insect), Drosophila

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Sexual differentiation

• Autosomes vs Sex chromosomes
• Heterogametic sex (2 types of gametes)
• Homogametic sex (1 type of gamete)
• Males are not always heterogametic sex - females
  are heterogametic in birds, moths, some fish,
  reptiles and amphibians

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Sexual differentiation

• Primary Sexual Differentiation
• involves gonads where gametes are produced
• Secondary Sexual Differentiation
• overall appearance of the organism
• Unisexual Dioecious Gonochoric
• contains only male or only female reproductive
  organs
• Bisexual Monoecious Hermaphroditic
• contains both male and female reproductive organs

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Model organisms sexual differentiation

• Chlamydomonas (Green algae)
• Most of life in haploid stage/only infrequent
  periods of sexual reproduction
• Asexually reproduce daughter cells by mitotic
  division
• Unfavorable conditions (such as nitrogen
  depletion) function as gametes to produce zygotes
• Diploid Zygotes withstand unfavorable
  conditions

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Sexual differentiation in Chlamydomonas

• Meiosis occurs then returns to the haploid state
• Isogametes gametes are indistinguishable
  (isogamous is the species producing the
  isogametes)
• When forming zygotes, only and - isogametes
  mate
• Chemical difference between cells but no
  morphological difference

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Only and - isogametes mate
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Sexual differentiation Plants

• Life cycle of plants alternate between haploid
  gametophyte and diploid sporophyte
• Meiosis and fertilization link the two phases

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Sexual differentiation Zea mays

• Stamen - Male
• Produce diploid microspore mother cells
• Each mother cell undergoes meiosis to produce 4
  haploid microspores
• Each haploid microspore develops into a mature
  male microgametophyte (pollen grain)

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Sexual differentiation Zea mays

• Pistil (Female)
• Produce diploid megaspore
• After meiosis, only 1 haploid megaspore survives
• Megaspore divides 3 times producing a total of 8
  nuclei in one embryo sac

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8 nuclei in one embryo sac

• Two nuclei in the middle endosperm nuclei
• Micropyle end (where two sperm nuclei enter) 3
  nuclei remain/oocyte nucleus and 2 synergids
• Antipodal nuclei the three nuclei opposite from
  micropyle end

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Double fertilization

• One sperm unites with the haploid oocyte nucleus
  diploid zygote nucleus
• One sperm unites with the two endosperm nuclei
  triploid endosperm nucleus

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Sexual differentiation

• Each ear of corn contains as many as 1000 of
  these structures
• Each develops into a single kernel
• If allowed to germinate will give rise to a new
  plant sporophyte

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Sexual differentiation C. elegans

• Contains 959 cells
• 2 sexual phenotypes
• Males testis X (No Y chromosome)
• Hermaphrodite testis and ovaries, XX
• Majority of organisms are hermaphrodites only
  about 1 males

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Sexual differentiation C. elegans

• Hermaphrodite testis and ovaries
• During larval stage, testis produce sperm
  (stored)
• Ovaries produced but no oogenesis until adult
  stage
• Able to self fertilize
• If hermaphrodite mates with male ½
  hermaphrodite, ½ males

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Life cycle of C. elegans
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Sexual Differentiation in Protenor (insect)

• 1906 Edmund Wilson found female somatic cells
  contained 14 chromosomes, including 2 X sex
  chromosomes
• Gametes from female contains 7 chromosomes,
  including 1 X chromosome
• Gametes from the male contains 6 chromosomes
  without an X or 7 chromosomes with an X

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Sexual Differentiation

• Fertilization by X bearing sperm results in
  female offspring
• Fertilization by X deficient sperm results in
  male offspring
• XX/XO or Protenor mode of sex determination

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Insect Lygaeus turicus

• Insect has 14 chromosomes
• 12 autosomes and 2 X chromosomes Female
• 12 autosomes and 1 X 1 Y - Male
• Females produce only X chromosomes
• Males produce X and Y chromosomes
• XX/XY mode of sex determination

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Sex determination in Drosophila

• Males are XY and females are XX
• The Y chromosome is not involved in sex
  determination
• The Y chromosome lacks male-determining factors
  but does contain information essential to male
  fertility

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Sex is determined in Drosophila by the ratio of X
chromosomes to the number of autosomes

• A ratio of 1.0 leads to fertile females
• A ratio greater than 1.0 results in a metafemale
  that is infertile
• A ratio of 0.5 leads to a male
• a ratio of 0.33 leads to an infertile metamale
• A ratio intermediate between 0.5 and 1.0 leads to
  flies that express both male and female
  morphology and are called intersexes.