Evolution of Sex Chromosomes

1
Evolution of Sex Chromosomes

• Nicole Milliner
• Biology
• Spring 2005
• Bellarmine University

2

• 44 autosomes
• 2 sex chromosomes
• Male XY
• Females - XX

3
Sex Determination

• Reptiles temp of egg incubation
• Warm blooded animals arose no longer
• Today genetic signal, master regulatory
  response to signal, double switch gene

4
Evolution of sex chromosome

• Derived from autosomes
• 300 mya
• One autosome mutated and aquired SRY gene
• SRY bearing became Y
• SRY deficient became X

5
Evolution cont.

• Recombination btwn X and Y stopped
• Recombination suppressed by
• Inversion
• Control function resisting pairing of chromosome

6
Dr. Page and Lahn

• Differentiation initiated one stratum at a time
• Identify min and max age of strata
• Proposed pathway gene shared btwn X and Y
  evoloved into X-linked gene
• X chromo product of pathway

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Inversion

• Autosome with inversion is eliminated -
  evolutionary genetic drift
• Impossible for sex chromosomes since determine
  sex
• Sex chromosomes driven apart
• Inverted chromosomes added to DNA that could not
  align and recombine

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Results of Inversion

• Y degenerating and shrinking
• X retain integrity and size continued to
  recombine

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Genes on Chromosomes

• X chromosome thousands of genes
• Y chromosome just a few dozen
• Both share 19 genes
• Of 19 genes
• concentrated on tip of short arm on X
• Scattered on Y
• Order not consistent btwn chromosomes

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Dioecious Plants

• Evolution of sex chromosomes understood by
  looking at molecular aspects of dioecious plants
• White campion 1 0f 3 dioecious plants w/ same
  XY system as mammals
• Help to explain evolutionary forces that shape Y
  chromosome

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Studies of Y chromosome

• 2 evolutionary themes
• Decay of gene function on Y chromosome
• Selective retention and amplification on the Y

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References

• Charlesworth, Deborah and Brian Charlesworth.
  Sex Choromosomes Evolution of the Weird and
  Wonderful. Current Biology Vol 15 No 4.
• Evolution of the human sex chromosome. Human
  Molecular Genetics. http//www.ncbi.nlm.nhi.gov/bo
  oks/bv.fcgi?ridhmg.section.1708
• Filatov, D. Sticklebacks view of sex chromosome
  evolution. Hereditary (2005) 94, pg 275-276.
  www.nature.com/hdy
• M, Nicolas, Marcis G, et al. Evolution of Sex
  Chromosomes The Case of the White Campion. PLoS
  Biology Jan 2005 Vol. 3 No 1. www.plosbiology.org
  
• Negrutiu, Ioan, et al. Dioecious Plants A Key
  to the Early Events of Sex Chromosome Evolution.
  Plant Physiology Dec. 2001 Vol. 127 pg 1418-1424.
  
• Page, David C. Germ Cells and Sex Chromosomes in
  Mammals. Howard Hughes Medical Institute 2005.
  http//www.hhmi.org/research/investigators/page.ht
  ml
• Parmet, Sharon. Research Looks at What Drove Sex
  Chromosomes Apart. University of Chicago
  Chronicle Nov. 18, 1999 Vol. 17 No5. Medical
  Center Public Affairs. http//chronicle.uchicag
  o.edu/99118/chromosomes.shtml
• Studies Expand Understanding of X Chromosome.
  Press Release National Institute of Health.
  March 16, 2005. http//www.nhi.gov/news/pr/mar200
  5/nhgri-16.htm
• Tracing the Evolution of Sex Chromosomes.
  Whitehead Press Release 1999 Evolution of Sex
  Chromosomes. http//www.wi.mit.edu/nap/1999_press_
  99_dpevolution.html