lecture 14 Flashcards
Sexual reproduction
characteristic of eukaryotes, and involves reproduction with meiosis and syngamy (fusion of gametes)
Sexual reproduction results in new genotypes due to…
- independent assortment of chromosomes
- crossing over
Sex in bacteria & viruses refers to what?
exchange of DNA
how we define sex
Parthenogenetic (asexual) reproduction
“virgin birth”
not requiring exchange and mating with, with other individuals and fusion of gametes,
Has evolved in dependently many times and in many different ways
facultative parthenogenesis
not always parthenogenetic/asexual
able to do both
obligate parthenogenesis
always parthenogenetic/asexual
Example: Amazon mollies (Poecilia formosa)
reproduce parthenogenetically, but require sperm from males of closely-related species to trigger development
females product asexual clones of eachother but need male sperm to trigger the development
parthenogens are…
highly successful and abundant (and many pests, weeds, invasives)
Case study: A high incidence of parthenogenesis
in agricultural pests
Wanted to count for every different family of insects, what percent of species are Parthenon genetic and then of every of every
pest species in that family, what percent are Parthenon genetic
Case study: A high incidence of parthenogenesis
in agricultural pests
Figure
- each data point is a different family of insects from N. America
- for one data point (look at graph): ~25% of the species in this family are parthenogens, but 80% of the species in this family that are pests are parthenogens
Case study: A high incidence of parthenogenesis
in agricultural pests
results
if parthenogenesis is as frequent in pest species as in non- pest species, then a straight line with a slope of 1 is expected
* but instead we see all the families of different insects are way above the one to one to the left so they are over represented
* A higher percentage of them are of the ones that are pests-> A higher percentage of Parthenon genetic
Even though parthenogenetic lineages are found across the tree of life, they are long-lived/short-lived over evolutionary time scales
short-lived
they are found on the tips of evolutionary trees
Parthenogenesis is self-destructive for scaled reptiles MO Moreira, C Fonseca, D Rojas (2021) Biology Letters 17, 20210006
- scaled reptiles (Squamata) are the only vertebrates that repeatedly evolved parthenogenesis (39 described true parthenogens)
- performed a phylogenetic analysis and mapped mode of reproduction on the tree
Results of MO Moreira, C Fonseca, D Rojas experiment
asexual reproduction parthenogenesis on the phylogenetic tree
on the tips, always evolved and arising but never last
why?
they tend to go extinict faster than sexual reproducing ones
What are the benefits of parthenogenetic (asexual) reproduction?
what prof said:
- faster rate of growth (two fold cost of sex
- reproductive assurance
- no need to invest in expensive traits involved in sexual reproduction
- avoid STIs and other risks and costs of mating
- preserves successful genotypes
The two-fold cost of sex
- If a sexually-reproducing female has 50% sons, then half of her resources have gone into offspring that can’t bear any offspring themselves!
- An asexual mutant will double in frequency every generation
the benefits of sexual reproduction and the disadvantages of parthenogenetic
- Two ways to make new genotypes:
- new mutations
- new combinations (recombination, reassortment)
- Recombination brings beneficial mutations together
- Recombination breaks associations with deleterious mutations
Recombination brings beneficial mutations together: increases the rate of adaptation
- Takes longer in asexuals (much faster in sexual reproduction) for independent beneficial mutations to come together in the same individual genotype
- Asexual lineages with different beneficial mutations end up competing with each other, this is called clonal interference
The Red Queen hypothesis & how it relates to the paradox of sex
always running just to stand still
really strong selection to change geneone just to not go anywhere
usually changing involves pathogens so changing just to keep up with someone else that is changing
asexual is great until they have to adapt to other factors in the environment, this is where sexual reproduction is better
Snails, sex versus parthenogenesis, & resistance to parasites
new zeland mud snails evolution and their parasites
some lakes had more asexual clones than others also sexually reproduce
Potamopyrgus antipodarum
New Zealand mud snails:
A mixture of obligate sexuals and obligate asexuals
Microphallus sp. trematode parasites
get into the mud snails and seralize them-> negative effects on fitness-> get eaten by ducks where parasites do not damge
Evidence from a New Zealand snail for the maintenance of sex by parasitism
Sexual snails are more abundant when parasitism is high
proportion of sexual reproduction
(females just make clones of themselves so
males would only be involved in sexual reproduction)
Infection Dynamics in Coexisting Sexual and Asexual Host Populations: Support for the Red Queen Hypothesis
sexuals gets infected less than asexuals
this is bc sexuals can change and shift genonetypes faster and adapt
2000 paper about snails
no sexual clones, just looking at the Parthenogenetic
2000 paper about snails
what did they do (sympatric)?
first half is he’s basically taken different clones
of snails and exposed them to the parasite that they would encounter in their native lake at that time
- has a bunch of common and rare clones and exposed them to the parasites
2000 paper about snails
results (sympatric)?
- The common asexual clones get infected and a very high rate (more than 80% or sometimes 100%)
- the parasite is locked onto the common one
- the ones that are rare are still asexual but the parasite isn’t locked in on them and it’s only infecting them at about 60%
So the common ones will eventually become more and more sterile and the rare ones will become more common and then the parasite is gonna adapt to be able to infect it
When there is the parasites sexual ones are better because they can shuffle and evolve faster
2000 paper about snails
what did they do (allopatric)?
They took the same clones and exposed them to parasites from a different lake
2000 paper about snails
results (allopatric)?
The infection is much, much lower, so less than 20%
shows that it isn’t something inherent about those a sexual clones
- the common ones are not always acceptable to parasites
what happens when there are mixed infections
two different flu strains come together in the same host can come together and mix/reassort
Viruses (and other pathogens & microbes) can also recombine
Recombination breaks associations with what mutations
deleterious
example of unlinked
AB AB
Ab Ab
aB aB
ab ab
example of linked
AB AB
AB AB
ab ab
ab ab
Linkage disequilibrium (LD)
- Non-random association of alleles at different loci (combinations of alleles that are more or less common than expected)
- Linkage disequilibrium can be caused by many things, including:
- reduced recombination due to physical linkage (most common reason)
- inbreeding
- natural selection
Genetic hitchhiking
When selection favours an allele at one locus, alleles at nearby linked loci may also increase in frequency, even if they are neutral
- important in the concept of beneficial mutations
- Beneficial mutation arises and increases in frequency and spreads through the population and replaces and out competes other alleles at the same locus
- The neighbors (linked sites) that are not beneficial, also increase in frequency
- Overtime you get recombination that breaks up association with the beneficial mutation
“Ruby in the rubbish”
when beneficial mutations are stuck in linkage with deleterious ones
ruby is beneficial mutations and rubbish is baggage of the deleterious mutations that
are stuck to the benefit (neighbor)
“Muller’s ratchet”
deleterious mutations will accumulate in small asexual populations
keep advancing in steps and there is not turning back
- asexual reproduction (no reshuffling)
- small popualtion size (no genetic drift)
Muller’s ratchet
- in small asexual populations, the class of individuals with the lowest # of deleterious mutations can be lost by chance (& never recovered!)
- this process continues, leading to an irreversible decline in fitness
my notes:
- once you have deleterious no getting away from it
- small pop-> can completely lose individual bc of deleterious mutation
- this is why sexual reproduction is important bc it if it gets a deleterious mutation it can get rid of it not doomed
Sex speeds adaptation by altering the dynamics of molecular evolution
Michael J. McDonald1,2, Daniel P. Rice1,2 & Michael M. Desai1,2,3
EXPERIMENTAL EVOLUTION
Over 1000 generations, experimentally evolved yeast strains that only differed as to whether they a) only reproduced asexually, or b) underwent sexual reproduction every 90 generations
wanted to make sure comparisons were as similar as possible
Something about freezing and comparing?
Michael J. McDonald1,2, Daniel P. Rice1,2 & Michael M. Desai1,2,3 results
Higher fitness increase in sexual than asexual lines
evolved lines all did better
Deleterious mutations become fixed in asexual lines, but never in sexual ones
- they can isolated what mutations were the ones that became fixed and figure out if they are beneficial or deleterious
- deleterious become fixed
-> neighbors that get pulled along
- lots of beneficial become fixed
- both beneficial and deleterious are dynamic
- deleterious never make it (become fixed) bc sexual reproduction gets rid of deleterious
Different types of mutations get fixed in sexual than asexual lines (Why?)
difference in the types of mutation that got fixed in the sexual compared to the asexual
biggest difference: fixed
- no synonymous that became fixed in sexual
- almost no intergenic became fixed in sexual
remember the fixed are the ones that made it and become 100%
Reproductive modes with combinations of sex & asex
- Cyclical parthenogenesis (alternating cycles of sex & asex)
- best of both worlds
- Selfing(with rare outcrossing)
The amount of recombination also varies a great deal within genomes
Some parts of the genome exhibit little or no recombination (they are asexual), with huge consequences
what part of our genome has no recombination
- mitochondria
- y chromosomes
- do not recombine have virtually no genes (eg, human Y encodes only ~30 distinct proteins) are littered with noncoding DNA
- much lower polymorphism
- dont have anyone to combine with
- pic not included - centromeres and thomeres
- chloroplasts
Case study: A high incidence of parthenogenesis
in agricultural pests
results
if parthenogenesis is as frequent in pest species as in non- pest species, then a straight line with a slope of 1 is expected
* but instead we see all the families of different insects are way above the one to one to the left so they are over represented
* A higher percentage of them are of the ones that are pests-> A higher percentage of Parthenon genetic
