Genetic background of sex determination; From sex determining gene to sex chromosome Flashcards
ploidy number in meoisis and fertilization
haploid (n)
diploid (2n)
evolution of sex
asexual rep sexual rep production of haploid gametes fusion-diploid zygote parts of evolution: origin of sexual reproduction evolution and maintenance
what is sex
cyclic process between haploid and diploid type of cell
connection between cell types via meiosis generating the gamates and fertilization which will generate somatic cells
Asexual rep- who does it
lower ranked organisms, unicellular, plants
commons types of asexual reproduction in animals
- fission (amoeba, plasmodium)
- budding (hydra, sponges, corals)
- regeneration (ex. worms, echinoderms)
- fragmentation (sponges)
- Parthenogenesis (honey bees, ants, wasps)
- cloning (artifical method)
only known vertebrate genus where partheogenesis occurs
cnemidophorus uniparents- only females
limited, reduced genetic diversity is typical
How does a purely asexual lineage persist for millions of years in the face of changing environments and evolving parasites?
during evolution, their genes become very different, diversified
bdelloid homologous chromosomes have diverged to the point that most genes have only one functional copy
now they are locked into asexuality
Advantages of asexuality
Avoids the two-fold cost of producing males.
No need to locate mates, an advantage at low density.
Maintains coadapted gene complexes, an advantage in stable environments.
Disadvantages of asexuality
Deleterious mutation accumulation (Muller’s Ratchet) in small populations.
Time delay in acquiring optimal multilocus genotypes in changing environments.
Slow rate of evolution allows sexually reproducing antagonists (parasites, competitors, and predators) to get the upper hand.
Selective sweeps can eradicate all variation from a population.-any newly introduced variations would be harmful, will be eliminated/eradicated
why have sex
recombination allows genetic variation
two fold cost of sex
sexual reproduction= either 1 male or 1 female produced (ignore twins, triplets, etc)
asexual reproduction- double amount of females is produced then during sexual reproduction
spectrum of mutations
is enormous, ranging from chromosomal rearrangements (translocations and inversions) and duplications to insertion and excisions of transposable elements to single base substitutions, insertions, and deletions
vast majority of mutations appear to be…
deletions
slightly deleterious mutations are far more common than lethals
the input of slightly deleterious new mutation decreases population mean fitness by 1-2% each generation
Mullers rachet
An asexual genome cannot produce offspring better than itself, except by rare back mutation.
The ratchet advances when the best class leaves no offspring, or if all of its offspring have acquired new deleterious mutations.
A mutational meltdown begins when the mutation load is so great that the populations is unable to replace itself.
why sex is beneficial in relation to mullers ratchet
Recombination/sex is directly beneficial by purging deleterious mutations
Idea is that deleterious mutations affect the success of parthenogens - each generation new mutations increase the average genome contamination
Hypotheses for the maintenance of sex (recombination)
carried out by the meiotic process and recombination of the homologous chromosomes
“big benefit of sex’
Recombination provides a mechanism for genomic repair, eliminating deleterious mutations
parents can produce offspring that have higher fitness genotypes than themselves
DNA repair hypothesis
Repair is only possible when a suitable template exists - homologues provide the template & repair involves crossing over
What’s less clear is whether DNA repair is the principle function of sex
2 problems with the DNA Repair Hypothesis
- Crossing over is absent in some circumstances (e.g., it is often absent in one sex), yet there is no obvious increase in mortality associated with repair
- DNA repair should be greatest in environments where damage is highest, e.g., at high altitudes. However, parthenogenesis predominates in these habitats
Fisher- Muller Hypothesis
Sex may facilitate response to environmental change by generating new gene combinations allowing populations to track a dynamic environment
This is because adaptive favorable mutations can be combined horizontally through a population
aka recombination spreads beneficial genes rapidly through a population
Genetic Hitchhiking - long one sorry
newly arising beneficial mutations can occur on a chromosome that also contains a deletion
owing to recombination, beneficial alleles can fix on the X chromosome without dragging along linked deleterious mutations
on a non-recombining y chromosome, however, the fixation of the beneficial mutation will simultaneously fix the linked deleterious mutation
genetic hitchhiking requires that the selective advantage of the beneficial mutation outweights the effect of the linked deleterious allele - so that y chromosome containing the beneficial mutation has a non selective advantage
long term benefit of sex
muller’s ratchet
population experiences greater genetic load of accumulated mutations
sex/recombin can help prevent mullers ratchet from turning, by allowing new combinations of alleles
flip side: beneficial combinations created as well
Short term benefits of sex
red queen hypothesis
in the arms between hosts and parasites, there is constant, strong selection for new gene combinations
what factors contributed to the rapid diversification of eukaryotic lineages
increased atmospheric O2 concentration- switch to aerobic respiration/
global climate change- major ice age around 2.7 BYA/
evolution of sexual rep.
