Evolution of sex chromosomes Flashcards
bird sex chromosomes
ZW females, ZZ males
evolutionary strata
successive spread of non-recombining regions of sex chromosomes, leads to a smaller recombining region. seen in mammals and birds
Y chromosome degeneration across mammals
seems to be degenerating across mammals- however mayve not as fast as we thought, e.g. only a couple of gene losses between chimps and humans
models of Y degeneration
muller’s ratchet- ‘best’ genes become irriversibly lost if they are lost due to a lack of recombination
selective sweeps- a haplotype just moves through the population due to being the ‘best’- based on advantageous mutation rate
background selection- based on deleterious mutation rate, removing stuff from the chromosome
dosage compensation
upregulation of the X chromosome in males, e.g. see it in Drosophila- or inactivating some Xs in females
alternative origin of Y chromosomes theory
Y chromosome acquisition through segregation of male-specific genes onto a new chromosome, not homologous to X
Y degeneration and sex chromosome turnovers
Y degeneration may drive sex chromosome turnovers- once Y degenerates, there is fitness loss, therefore need to switch to a new region for sex-specific traits
what is sex chromosome turnover
a sex determining gene or region either evolving or moving in a region away from the normal sex chromosome
example of plant sex systems
hermaphroditic- common in angiosperms
gyno/androdioecy- hermaphrodite and F/M
dioecy- separate M/F
example of a mutation which can change plant sexual systems
recessive male-sterility genes can create a gynodioecy system (females when both alleles are for male-sterility, otherwise hermaphroditic)
two-factor sex determination system
e.g. female suppressor genes and male promoter genes- need both to be on to create a male
need F supressor off and M promoter on to get hermaphrodite
M promoter off F suppressor on - neuter
in asparagus
example of a single factor sex determination system
master switch gene, if shut off creates a particular sex
e.g. populus alba- turning off the gene makes it male
how can dioecy evolve from monoecy?
either hormonal factors (changed regulation of sex related genes), or mutations in sec determination systems, e.g. resulting in single factor systems
Y degeneration in animals vs plants
may be slower
example of Y degeneration in plants
could see it when studying Silene- able to track the differences between X and Y chromosomes in terms of the number of synonymous vs deleterious mutations
