Week Three Flashcards
What are the benefits of mating types?
Having a type prevents self-fertilization.
- -> Lower probability of inbreeding
- -> Helps assure the diversity benefits of genetic exchange.
- -> Hurst argues that the advantage is that endosymbionts are not shared when gametes fuse.
What are the drawbacks to mating types?
Only 1/2 the individuals encountered are potential sex partners.
Hurst argues that selection should favor continuous production of new mating types, especially if mate-searching is a cost.
What is the fungal life cycle?
Fruiting body (mushroom) is dikaryotic, fertilization produces diploid zygotes.
Meiosis of zygotes produces haploid spores
Spores germinate and mycelia start to develop.
Fusion of two hyphae of compatible mating types
Growth of dikaryotic mycelium (two nuclei).
Do organisms that only exchange nuclear material have more than two mating types?
Scizophyllum commune has over 280000 known mating types
- -> two loci control mating type
- Loci MTA, MTB have over 300 and over 90 known alleles respectively.
- Probability of inercompatibility with a random S. commune from anywhere is the world: 99.98%.
What is Isogamy?
The state where producers of gametes make the same-sized gametes, on average [there can still be variation in gamete size] is assumed to be the primitive state.
Complex sexual processes.
Sexual reproduction is often occasional or episodic in these organisms, with vegetative asexual growth being the dominant visible form of life.
What is pseudoanisogamy?
The situation where gamete producers vary in the size of gamete they produce, but the gametes are not discriminating about who they fuse with (big + big, small + small, big + small)
Potentially a pivotal step towards anisogamy.
Sexual reproduction is often occasional or episodic in these organisms, with vegetative asexual growth being the dominant visible form of life.
What is Parker’s Sexual Cascade: Part 1 ?
Meiosis recombination syngamy –>(sex creates selection to disperse gametes)
Isogamous unicellular eukaryotes –> (gamete size creates survival advantage (for gamete or zygote)
Anisogamous unicellular eukaryotes –> (multicellularity selects for bigger eggs and ovaries; # competition selects for big testes)
What are the four factors important in the Evolution of Separate sexes?
Who are the authors who posit them?
- Investment/Dead Beat Dads: One gamete-producer skimps on quality in favor of quantity and mobility. (PARKER)
- Investment/ Survivor: One gamete producer gains by making better-surviving gametes or zygotes through higher investment. (PARKER).
- Safe sex/ Parasites: Uniparental inheritance of cytoplasm reduces spread of STDs (HURST).
- Safe sex / Cyto-nuclear conflict: Mitochondria and plastids have selfish genetic interests that interfere with the nucleus. Uniparental inheritance, again. (HAMILTON AND HURST).
What is effect of disruptive selection on gametes?
Fitness increases at the extremes of gamete size and investment in offspring!
-Smaller sperm are selected for, while larger eggs are selected for.
Anything in the middle is an immediate reduction in fitness.
Why is the allocation of cytoplasm controlled during fertilization?
Organelles have their own selfish transmission interests.
Mitochondria and other organelles from sperm are typically excluded from the fertilized egg.
- Simplest way to ensure that only one parent supplies the cytoplasm in syngamy (fusion) is to have one sex designated to do so.
What are the two selection pressures when endosymbionts conflict with nucleus in sex?
- There is selection to help host in host-host competition and clonal reproduction, by providing energy for the cell (Nice helpful mitochondria/ plastids).
- Selection to invade and take over new hosts when cytoplasm is shared during sex (Many, nasty mitochondria/plastids).
What is the best case scenario for a reproducing cell, in terms of endosymbionts?
What are the benefits?
A cell that could exchange nuclear genes but also exclude invasion by foreign organelles would be optimal.
- Protect/nourish less virulent, cooperative endosymbionts.
- Retain the benefits of nuclear sex.
In what ways are Mitochondria enslaved to hosts?
Selection for cooperation between mt and nucleus naturally leads to loss of bacteria-specific functions and genetic coding in mitochondria.
- -> mt have simplified genomes, import many proteins.
- -> They have evolved to be either obligate mutualists or slaves.
What does the ‘cooperator’ eukaryote select for? (3)
- The cooperator to invest more in gametes, if size of gamete is correlated with survival of the zygote.
- May select for a cheaper, stripped down gamete type (AKA sperm) to capitalize on the cooperator.
- Mechanisms of strict cytoplasm-exclusion.
Which species are more likely to be isogamous, anisogamous unicellular, or anisogamous or multicellular?
Many modern protists are either isogamous or anisogamous unicellular.
Most modern non-motile organisms have large ovaries and testes; therefore anisogamous multicellular eukaryotes.
Which sex is likely to be the targets of selfish endosymbionts?
Males because cytoplasm is maternally-inherited,
WHat is Cytoplasmic Male sterility in plants?
Many plants are simultaneous hermaphrodites.
–> Mitochondria are passed down from ovules/seeds
Mitochondria that do not cooperate in pollen production are widespread in plants, feminizing hermaphrodites.
- -> Spread if turning off male function leads to more or better ovules
- -> May also help by reducing self-fertilization (inbreeding)
What is Mito-Nuclear Conflict?
The CMS mitochondria are in direct conflict with the nucleus.
- -> Evolution of nuclear restorer genes that regenerate male function
- -> unlike most nuclear drivers, there are many different CMS mitotypes and restorer genes
What is doubly Uniparental inheritance?
In some bivalves (marine and freshwater molluscs) there are special M-type (male) mitochondria and normal F-type.
- -> Females are all F-type
- -> Males preserve M-type mitochondria in the TESTIS tissue
Suggests a tradeoff between somatic ad spermatic function of mitochondria.
MT that make good sperm may not be best in the body tissues.
–>
What is the effect of bacterium Wolbachia on Drosophilia?
Sterilize uninfected females when they mate with infected males.
–> Effect is prolonged even though sperm do not transmit the bacteria.
How does Wolbachia spread?
The mechanism of Wolbachia induced cytoplasmic incompatibility can be likened to a poison/antidote system.
–> Wolbachia in egg can correct mods made by sperm stem cells.
Females carrying the antidote can mate with all males in the population (infected)
- Uninfected females experience ever greater levels of sterility as the infection spreads.
What are feminizers? Which system does this favor?
Wolbachia spp. and several other microbes known to convert males into females in several insects and a wide variety of crustaceans.
- Found in ZW sex determination systems (female heterogametic)
- Bacteria mimic gene expression of genes that inhibit development of androgenic gland.
What are the different modes of killing for endosymbionts?
Late Killers: Preferential proliferation in male hosts.
–> Kill host and migrate to female hosts (horizontal transmission) where cytoplasm is passed on (vertical transmission)
Early Killers: Kill developing male embryos
- -> gain from resource reallocation to female embryos
- -> female cannibalism of male embryos
Can male-killers drive extinction?
As males become rare and rarer they become more valuable.
- Any resistance or avoidance gene that arises to allow male survival will be highly selectively favourable.
- This makes it less likely that the male-killer will exterminate species.
