BIO372 : Biological & Ecological Implications of Sex : Sex Determination

I. Chromosomal

(Review article by Werren and Beukeboom (Annual Reviews of Ecology and Systematics 29: 233): Abstract only)

1. Sex chromosomes
   • Mammals : Sex determining factor resides on the Y chromosome
     • Gonadal sex, anatomical sex, brain sex
     • Chromosomal mistakes, e.g., nondisjunction revealed by a karyotype. Do this exercise by clicking on Patient Histories at the bottom of the page. Pick Patient B.
       • Kleinfelter's and Turner's syndromes
       • Triplo-X syndrome
   • Birds, butterflies and moths, some fish: males are the homogametic sex
   • Fruit flies: Drosophila: Ratio of sex chromosomes to autosomes with dosage compensation
   • Haplodiploidy in ants, bees, and wasps: females develop from fertilized eggs (contain 2 sets of chromosomes); males develop from unfertilized eggs (contain only 1 set of chromosomes)
   • Grasshoppers (Romalea): females have two X-chromosomes but males have one X-chromosome and no Y-chromosome
   • Lychnis (a plant): Y-chromosome is larger than the X
2. A different kind of male: a third sex or a special strategy?
   • Red-sided garter snakes
   • Bluegills

II. Temperature-dependent sex determination

Leopard gecko, crocodiles, and snapping turtles Low and high temperatures result in females Intermediate temperatures result in males Estrogen overrides temperature Turtles Cool temperatures result in males Warm temperatures result in females Alligators Cool temperatures result in females Warm temperatures result in males

III. Behavior-dependent sex determination

1. Sequential hermaphrodites: Sequence depends on ecological conditions
   • Large females lay more eggs; small males produce lots of sperm
   • Large males compete better for females and for territories
2. Simultaneous hermaphrodites
   • Fish
   • Earthworms
3. Potential mates
   • Bonellia

IV. Parthenogenesis

1. Whiptail lizards
2. Aphids
3. Daphnia

V. Sex ratios

1. Birds and mammals
2. Hymenoptera: ants, wasps, bees
3. Aphids: parthenogenesis plus sexual reproduction with males
4. Rotifers: no males?
5. Humans
   • Even sex ratio at conception
   • Male biased sex ratio at birth
   • Even sex ratio at first age of reproduction
   • Female biased sex ratio in older years
6. Theory
   • Theory 1
   • Theory 2

VI. Alternation of generations

1. Cycle of asexual and sexual forms
2. Coelenterates
3. Moss
4. Fern: Diagram 1, Diagram 2, Diagram 3, Diagram 4
5. Flowering plants: Diagram 1, Diagram 2, Diagram 3
6. Chlamydomonas

VII. Imprinting: dosage compensation in mammals

1. Mammals need both parents, but frogs don't
2. Methylation
3. DNA competition

When you have completed this lesson, go on to Review Questions