Sex Flashcards
define sex
recombination of genes from separate sources into a single organism.
in EK, what complications does sex involve?
- alternation between diploid and haploid (often just gametes) life stages.
- reduction of genetic info by meiosis
- crossing over during meiosis
- fusion of haploid gametes with those of another individual of the opposite sex (syngamy).
what 4 problems does sexual reproduction present which don’t occur in asexual?
- the 2 fold cost - asexual females can produce twice as many offspring as sexual females, and pass on 100% DNA. therefore, sex is 2x as costly.
- meiosis is costly as it is prone to errors in replication.
- finding a mate/competition is costly.
- Linked co-adapted genes are separated, eg heterozygous for having sickled RBC is advantageous if in Malaria prone zones. heterozygosity can be removed by meiosis.
how did diploidy evolve?
Perhaps to provide a replacement for defective genes. Firstly, there may have been 2 identical sets of chromosomes, one to act as a back up copy to cover for mutations, and DNA could be repaired using the counterpart molecule.
also, could expose deleterious mutations so can be removed from the lineages.
Why is diploidy expensive?
more DNA to copy and repair. therefore, as a coping strategy in bad conditions, can switch to haploidy w/o crossing over. (Reduction division).
why may crossing over have evolved?
May be a by product mechanism of double stranded repair. promoted variation.
why may syngamy have evolved?
to restore heterozygosity after double strand repair and crossing over.
Would crossing over or syngamy have evolved first
- If x-ing over first, then the 2 chromosomes would have been identical, so doesn’t create much variation (but still good for DNA repair)
- If syngamy came first, x-ing over (between homologous, but not identical chromosomes) increases variation
what is a classic theory as to why sex is maintained?
what is a problem of this theory?
Fisher 1930 - speeds up evolution by bringing good genes together. if only asexual, unlikely that two good mutations will spread to fixation, whereas with sex, beneficial mutations will be brought together immediately.
However - it is only a benefit in a large pop, with mutations of small benefit, as in small populations when mutations give large benefits they spread v rapidly. therefore sex doesn’t actually speed things up.
describe muller’s ratchet theory
at a given time,there are individuals with 0 (most fit), 1 (2nd fittest), 2 etc mildly deleterious mutations. in a small pop, by chance those with 0 mutations may die, fail to breed or mutate. makes the best individuals those with 1 mutation. the ‘ratchet’ has clicked round and pop has lower fitness. this can lead to extinctions, which may be why asexual species are so rare.
how does sex counteract Muller’s ratchet effect?
Purges deleterious recessives so sex can restore a perfect genome. deleterious recessives are eliminated from the pop in offspring which die.
why must there be short term advantages to sex
what are they?
some organisms can do both sexual and asexual rep.
- variable offspring allows exploitation of spatially unpredictable environments.
- can evade parasites better - red queen hypothesis, Hamilton 1980.
- purges synergistic genes.
describe some evidence for hamilton’s red queen hypothesis
Curt Lively found that, in a population of freshwater snail that could self-fertilise facultatively, it was more likely to self-fertilise in ponds where parasites were rare, but outbreed in pond where parasites were more common.
what is a mating type?
different to sex
eg angiosperms have incompatibility systems and 2 sexes.
eg Stylonychia has 48 mating type, each one can mate with an of 47 others but not its own.
mechanism to avoid inbreeding.
what is a classic theory for the evo of different sexes
Parker et al 1972 - GAME THEORY
determines ESS - evolutionary stable strategy
How has game theory resulted in large and small gametes forming?
- the fitness of a zygote probably increases with size as it has more resources etc, but the benefits of being larger dont keep increasing.
- if two large gametes fuse = big zygote.
- mutation causes small gametes, can produce many at same cost as one large gamete. large+small gamete = zygote big enough to have moderate fitness.
- strong selection of small to fuse with big gametes.
- weak selection of big gametes to avoid small ones.
-selection assymetry allows small gametes to invade pop. ESS is bimodal distribution of gamete size.
benefits of being big or small are freq dependent.
What is hamilton and hurst’s theory of why different sexes evolved?
1992
based on genomic conflict
endosymbiotic bacteria, mit and ch have own DNA, and when cells fuse, comp btw these organelles for precedence in cytoplasm eg in chlmydomonas chloroplasts attack each others DNA with restriction endonucleases.
cytoplasmic conflict is costly to nuclear genes as they dont benefit from resources used on cytoplasmic competition.
therefore in order for a gene to spread, must suppress its own cytoplasmic genes and fuse with an unsuppressed cytoplasm. Polymorphism of supressor types = gamete types.
how does cytoplasmic suppression occur?
- unilateral destruction of cytoplasmic genes prior to fusion
- or make gametes so small there is no room for cytoplasmic genes
evidence in ciliates which have no true sexes - isogametes, only pass nucear DNA down a tube. Peritritch ciliates have secondarily evolved full fusion and have 2 sexes.
what are some consequences of having 2 sexes?
- different selection pressures on m and f
- can be beneficial to allocate resources unequally to m and f offspring - sex ratio adjustment
what are 4 different methods of sex determination?
genetic:
1. heterogamety (2x sex chr)
2. multifactor (>2 sex chr)
3. haplodiploidy
4. Environmental factors eg temp, size etc
species could use a combination of factors. Clear evidence for this is seen in humans (and other animals) that are chromosomally one sex but phenotypically the other.
describe male and female heterogamety
Ususally male are heterogametic (XY), but some sp of birds, butterflies, reptiles and fish have females heterogametic (females ZW, males ZZ).
in drosophila - ratio of X chr to autosomes determines sex. high ratio = female.
why would heterogamety have evolved?
if the best sex to be doesnt change throughout life.
however this is unpredictable at birth so is a gamble.
not known why male heterogamety evolved over female. no obvious phenotypic constraint. might not matter as it is an ESS.
what is haplodiploidy?
sex determination whereby a fertilised egg is female, and unfertilised is male.
female stores sperm and can choose to release it or not as eggs are laid.
what is sequential hermaphroditism
individual changes sex during their lifetime
clearly not genetic determination
example of temp dependent sex determination
mississippi alligator
female lays eggs and covers with vegetation. incubation temp determines sex. cool = female
variation between reptile groups.
how do size and social factors influence sex
eg slipper limpet
live in stacks, f larger on bottom, males on top.
change sex as they grow, if largest f removed, l m changes sex.
why change sex?
Charnov 1982 = theory of sex allocation
if rep value increases with another factor eg size, but this increase is greater in males than females. when small, females have greater value but when large, males more valuable. there is a crossover point, meaning higher lifetime fitness if start as f and change to male. eg Blue headed wrasse
word for starting as female and changing to male?
vice versa
Protogyny (f-> M)
protandry (M->F)
why dont all animals change sex
required: long reproductive lifespan
strong dependence of rep value on size/age/condition
sex specific experience must be relatively unimportant
why may being both sexes at once be a good strategy?
advantage in low density - even if self sterile, can mate with everyone.
similar benefits to asexuality - self fertilisation is good when genetic similarity to parents is good, but most simultaneous hermaphrodites have mechanisms to avoid self fertilisation.
