Julius B. Lucks/Meetings and Notes/SMBE2007/nei lecture

Deborah Charlesworth (Edinburgh): The evolution of sex chromosomes - similarities and differences between plants and animals

Tue Jun 26 06:58:50 EDT 2007

Words to Look Up

• dioecy
  • dioecious
• PAML

Talk

Dr Nei's work on evol of sex chromosomes

• how scelection can act on a modifier that reduces recomb, including modifiers on sex chrom themselves
• why recomb lost? - sex chromosomes don't recombine
  • if sex determined by 2 or more genes - must inheret them together
• how is recomb lost?
• what are consequences of loss of recomb?
  • Y gene poor - Y and X homologs and Y has lost
  • is genetic degeneration caused by fact that Y heterozygous?
• 'Theory shows that sex chromosomes should evolve low recombination'

Why Plant sex chromosomes?

• test the theory
• in plants can study how separate sexes evolved
• plant sex chrom evol young
• evol indep many times over
  • tell which features found when sex chrom evolve
  • maybe more than 100 times
  • Darwin noticed
• degeneration processes not affected by prior existence of sex chromosomes
• Darwin noticed - male and female flowers can look very similar - evolved recently and haven't lost redundancy
• Human Y about 1/3 of X - recombine in very small portion of autosomal region that are homologous
• Silene dioica - 230 Mb Y
• Papaya - most sex chrom pseudo-autosomal, and only small portion of Y is male-specific
  • very young sex chromosome

Why Recomb Lost?

• requires several genes
  • at least 2 - male and female suppressor genes - one on each chrom (so that don't end up with both and be neuter)
• argument for 2 or more evol steps
• sex determining loci must be initially be linked for separate sexes to evolve
• higher allocation to male function will generally lower female functions
  • holly - male (many flowers, female few)
  • something about huge pumpkins