Sex Determination and Sex Chromosomes

1
Chapter 5

• Sex Determination and Sex Chromosomes

2
Sexual Differentiation

• Successful mating between organisms relies on
  sexual differentiation in organisms
• see even at bacterial and algal levels
• evolutionarily higher organisms differences
  between sexes is more noticeable
• X and Y are dissimilar chromosomes
  heteromorphic chromosomes
• characterize one sex or the other sex
  chromosomes
• gene ultimately serve as basis of sex
  determination

3
Life Cycles Depend on Sexual Differentiation

• Primary sexual differentiation only gonads,
  gametes produced
• Secondary differentiation overall appearance of
  organism mammary glands and external genitalia
• Unisexual, dioecious and gonochoric only male
  OR female reproductive organs
• Bisexual, monoecious and hermaphroditic both
  male AND female reproductive organs
• Intersex intermediate sexual differentiation -
  sterile

4
Chlamydomonas Green Algae

• Only infrequent periods of sexual reproduction
• Spend most of the time as haploid asexual
  reproduction (mitosis)
• In unfavorable nutrient conditions, certain
  daughter cells can act as a gamete, fertilization
  yields diploid zygote that can withstand the
  unfavorable environment
• When environment improves, undergo meiosis to
  return to haploid, vegetative cells

5
Mating Types

• Very little visible difference between haploid
  and diploid cells
• 2 cells undergoing fertilization are
  indistinguishable isogametes and species is
  isogamous
• mt mates only with mt- cells
• Following fertilization and meiosis 4 haploid
  cells 2 mt and 2mt-
• mt and mt- differ chemically even though look
  morphologically similar, really different at the
  chemical level
• primitive sex differentiation

6
Maize (Zea mays)

• Monoecious sporophyte phase and morphological
  structures predominate during life cycle
• male (tassel) and female (ear) are present on
  same plant

7
Corn Reproduction

• Tassels are the stamens that make diploid
  microspore mother cells that undergo meiosis to
  create microspores that then becomes the
  microgametophyte which is a pollen grain
• 2 sperm nuclei per pollen grain
• Pistol of the sporophyte (ear) has megaspore and
  meiosis produces only 1 of 4 megaspores surviving
  which then undergoes mitosis to make 8 haploid
  nuclei in embryo sac
• 2 nuclei fuse to make the endosperm nuclei
• Sperm enters at micropyle end of embryo sac, 3
  nuclei remain oocyte and 2 synergids, other 3
  are clustered at the opposite end of the embryo
  sac

8
Corn Reproduction (cont)

• Pollen makes contact with the silk (stigma) and
  makes an extensive pollen tube growing toward
  the embryo sac
• Undergo double fertilization
• 2 sperm nuclei enter the embryo sac, one sperm
  unites with haploid oocyte and the other with the
  2 nuclei in the endosperm
• creates a diploid zygote and triploid endosperm
  nucleus which becomes the kernel of corn, makes a
  new sporophyte plant

9
Mutant Corn

• Discovery of mutant genes that disrupt normal
  tassel and pistol formation supports the notion
  that normal products of gene play a role in sex
  determination
• Mutant genes caused sex reversal
• tassel seed (ts) interfere with tassel production
  and induce female structures so functionally a
  female
• silkless (sk) and barren stalk (ba) interfere
  with pistil and make only functioning male
  reproductive organs
• Certain cells are determined to be male or female
  structures

10
C. elegans

• Used in genetic studies of development
• 959 cells in adult and can trace back to specific
  embryonic origin study behavioral modifications
  and aging studies

11
C. elegans Reproduction

• 2 sexual phenotypes
• males with testes and hermaphrodites that contain
  both sexes (testes and ovaries)
• Larvae make testes that make sperm which is
  stored and ovaries when adult 3 days later, egg
  fertilized by the stored sperm (self-fertilized)
• Most organisms are hermaphrodites,
• As adult males mate with hermaphrodites get 50
  males and 50 hermaphrodites
• maleness determined by genes on X and autosomal
  chromosomes, no Y chromosomes
• males have 1 X and hermaphrodites have 2 X
• sex thought to be ratio of x to sets of autosomes
  
• 1.0 hermaphrodite
• 0.5 male

12
X and Y Linked to Sex Determination

• In a group of insects saw 14 chromosomes and 2
  happen to be X, during meiosis cells with 6
  chromosomes and 1 X
• In male somatic cells saw 13 chromosomes, in
  sperm had 6 chromosomes plus an X and some with
  only 6 chromosomes
• Egg x sperm with X female
• Egg x sperm without X male
• Called XX/XO or Protenor mode of sex
  determination
• random distribution of X in sperm

13
Hemiptera

• Have 14 chromosomes in both sexes, females have 2
  Xs and males have 1 X and 1 heterochromosome, Y
  chromosome
• Females have one gamete type 6 chromosomes 1
  X
• Males have 2 gamete types 1 with 6 chromosomes
  and 1 X and 1 with 6 chromosomes and 1 Y
• Make equal numbers of progeny
• Called Lygaeus mode or XX/XY mode of sex
  determination

14
New Terms

• Heterogametic males, 2 different sex
  chromosomes used to determine sex
• Homogametic females, 1 sex chromosome in 2
  copies
• Birds, moths, butterflies and other organisms are
  in the inverse
• males are homogametic ZZ
• females are heterogametic ZW