The evolution of sex Flashcards
Determination of sex at the gamete level
Gametes can be identical (isogamy)
Often they are not (anisogamy) (sperm and egg) and the type with the smaller cell is the male
Give types of asexual reproduction
*Binary fission (bacteria, protists & some unicellular fungi)
*Budding (baker’s yeast, hydra, anemone)
*Vegetative reproduction (plants- rhizome, runner, tubers and bulb)
*Spores (some fungi, some algae)
*Fragmentation (lichens, annelids, sea stars, plants)
*Parthenogenesis-Unfertilised eggs develop into new organisms e.g haploid-diploid sex determination in bees. (rotifers, insects, reptiles, amphibians)
Why can sex be costly?
usually requires two separate sexes
usually need to spend time finding partner (or gamete)
may need to fight other members of your own sex
need to persuade partner to mate (courtship)
need to produce gametes, most of which are wasted
risk catching diseases
own alleles are diluted 50% with those of partner
What is the the two-fold cost of sex?
- Sexual females have half as many daughters as do asexual females
- If a sexual and asexual female produce the same number of offspring, the asexual population grows at twice the rate
Why is sex still common?
More of a variety of life forms are sexual
experiments show that sexual lineages usually outcompete asexual lineages in the long term e.g mealybugs
What are the potential advantages of sex?
Explain the alleles
Explain the overall advantages
leads to unique combinations of alleles:
alleles segregate independently into gametes
fertilization combines alleles from different lineages
crossing over shuffles alleles between chromosomes
Three possible advantages:
generates genetically diverse offspring
eliminates costly mutations quickly
allows beneficial alleles to combine
explain Muller’s ratchet
Muller’s ratchet describes the accumulation of mutations (mostly derimental) in asexual individuals.
Asexual vs sexual reproduction
*Asexual reproduction
Requires less energy
Not costly non-reproducing sex
Quicker
Offspring are clones of the parents
*Sexual reproduction
Requires more time and energy
Two-fold cost of producing males
Offspring are genetically diverse
Mutations are more easily purged
Beneficial mutations can combine more easily
Are there macroscopic differences between male and female?
Can you use Sex determination systems?
No one rule to determine sex for each species
Not the same in all organisms
chromosomal system doesn’t work some insects don’t have specialised sex chromosomes
environmental sex determination
some are temperature-dependent e.g turtles
What systems effect the determination of sex?
sex chromosomes
hormone sensitivity
stress
social
temperature
parasitic microbes (wolbachia)
hermaphodites
Reproductive parasites: Wolbachia
Wolbachia are only able to be passed on in the maternal line therefore they have evolved different ways to change the sex of the host by;
feminizing hormone systems
killing males
sperm egg incompatibilities
cytoplasm incompatibility
Give an example of an Infection-induced sex determination by wolbachia
Armadillidium vulgare (woodlouse)
isopod crustacean
androgenic gland
feminisation by Wolbachia
degradation of gland
suppression of ‘male’ gene
Give examples of hermaphrodites
common garden snail have bothe part
common in plants
clown fish can switch between sexes in their lifespan
Can you use differences between gametes to determine sex?
explain gametes
cells that ‘fuse’ = fertilization
morphologically different:
Heterogamy (anisogamy)
larger produced by female (egg/ovum)
smaller produced by male
(sperm/spermatozoan)
carry one set of chromosomes (haploid)
gametes contain mix of alleles from parents due to independent assortment and recombination
however
gametes can also look the same and be isogamy
so not always good for determining
inheritance from one parent?
Mitochondrial DNA (mtDNA)
This is the only consistent way to determine between the sexes
Uniparental (non-Mendelian) inheritance
mitochondria are maternally inherited.
usually
