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Sex Determination and Sex Chromosomes

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... XX. Majority of organisms are hermaphrodites; only about 1% males ... Hermaphrodite testis and ovaries. During larval stage, testis produce sperm (stored) ... – PowerPoint PPT presentation

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Title: Sex Determination and Sex Chromosomes


1
Sex Determination and Sex Chromosomes
  • Chapter 7

2
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

3
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

4
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

5
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

6
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7
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

8
Only and - isogametes mate
9
Sexual differentiation Plants
  • Life cycle of plants alternate between haploid
    gametophyte and diploid sporophyte
  • Meiosis and fertilization link the two phases

10
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)

11
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

12
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

13
Double fertilization
  • One sperm unites with the haploid oocyte nucleus
    diploid zygote nucleus
  • One sperm unites with the two endosperm nuclei
    triploid endosperm nucleus

14
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

15
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

16
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

17
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18
Life cycle of C. elegans
19
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

20
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

21
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

22
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

23
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.
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