origin and maintenance of the sexes

Question 1

what type of selection do you need to go from ancestral isogamy to anisogamy?

Answer 1

disruptive selection on size

Question 2

why is there not much known about the evolution of separate sexes?

Answer 2

because there is not fossil record. We need to rely on mathematical models

Question 3

what type of selection did the early explanations (sperm limitation) rely on? why are they flawed?

Answer 3

Group selection- anisogamy should evolve to maximise efficient fertilisation events. BUT, individual selection should be stronger. (Williams, 1966)

Question 4

What did the paper by (Wiese et al, 1978) show?

Answer 4

showed compelling evidence for inducible anisogamy.
experimental, phylogenetically
controlled inducible anisogamy from
isogamous, dioecious chladymonas sp.

Question 5

who wrote the model for gamete competition?

Answer 5

Parker et al, 1972

Question 6

In PBS, what were the two opposing selection pressures?

Answer 6

2 opposing selection pressures: small
gametes, gamete productivity benefits,
large gametes, zygote fitness

Question 7

what were the starting conditions of the PBS model?

Answer 7

isogamous, no sexual dimorphism, zygote size determines zygote fitness

Question 8

what is the equation for zygote fitness?

Answer 8

=wz=mz^k

Question 9

what are the three possible outcomes of the PBS model?

Answer 9

k is small: directional selection for small
gemetes. k is large: directional selection for
large gametes. k is intermediate: disruptive
selection on gamete size

Question 10

PBS showed their original model would work for two genetic systems. Who confirmed this in game theory models and who confirmed it for diploid inheritance?

Answer 10

game theory–> (Maynard Smith,1978,1982|)

diploid inheritance–> (Charlesworth, 1978)

Question 11

what was the problem with the outcomes of PBS initially?

Answer 11

assumed no minimum viable zygote size for zygote fitness. graph has linear positive association. (Maire et al, 2001) - a minimum viable zygote size would be more realistic,
especially in metazoans.

Question 12

what is still unresolved from PBS?

Answer 12

why any sexual species are isogamous

& the relationship between zygote size and zygote fitness has not been shown

Question 13

what predictions did Bulmer and Parker (2002) make about the relationship between zygote and gamete size?

Answer 13

added to the model, gamete survival
until fertilization depends on gamete size. when optimal
zygote size is of the same order of magnitude as optimal
gamete size, isogamy evolves.

Question 14

how did Bulmer and Parker (2002) explain the evolution of anisogamy in larger animals?

Answer 14

bulmer and parker argue that the difference in z + g
size becomes bigger in more complex and larger
organisms restoring the PBS prediction of
anisogamy in larger animals.

Question 15

who wrote the sperm limitation theory?

Answer 15

Kalmus, 1932

Question 16

What did sperm limitation theory predict?

Answer 16

this model assumed that natural selection would lead to gamete sizes that result in the largest population-wide number of successful fertilisations (Dusenberry, 2000)

Question 17

which group of spawners is sperm limitation theory particularly relevant to?

Answer 17

broadcast spawners

Question 18

is there any evidence of sperm limitation?

Answer 18

evidence to support gamete limitation in three
congeneric broadcast spawning species of sea urchins.
species with larger eggs always achieved a higher rate of
fertilization (Levitan, 1992)

Question 19

what was the rationale behind sperm limitation theory?

Answer 19

if gametes were limited so that some remained
unfertilized, selection would favour traits that
increased encounter liklihood.

Question 20

why is sperm limitation viewed as evolutionarily unstable?

Answer 20

a selfish mutant in a group could have a large selective advantage by not acting for the collective good
(maynard smith 1982)

Question 21

why is gamete limitation theory challenging gamete competition theory?

Answer 21

gamete limitation by itself can lead to the divergence of gamete sizes even under selection at the individual level (Iyer & Roughgarden, 2008)

Question 22

who showed that that gamete competition and gamete limitation are the ends of a continuum of selective pressures, and they can act separately or together depending on the conditions?

Answer 22

Lehtonenen & Kokko (2010)

Question 23

Why does theory suggest that gamete limitation could only be the dominant force in the origin of the sexes very specific circumstances?

Answer 23

the presence on average of just one competitor can makes the ‘selfish’ evolutionary force of gamete competition stronger than the ‘cooperative’ force of gamete limitation even if gamete limitation is very acute (approaching 100% of eggs remaining unfertilised). (Parker & Lehtonen, 2014)

Question 24

why is conflict between sexes caused over control of cytoplasmic inheritance?

Answer 24

you get nuclear genes from mum and dad, but you get mtDNA from mum only. since nuclear genes ‘dont care’ which mtDNA is present, selection will favour
ejection by one party

Question 25

what does control of CI theory say is the way to avoid conflict between haplotypes?

Answer 25

one way to avoid conflict between
mt haplotypes is to ensure there’s
only one present. sperm evolved to eliminate
conflict between mt genomes

Question 26

what has happened to many mitochondrial genes?

Answer 26

many mt genes have moved from
mitchondria to the nucleus. this
removes conflict over gender

Question 27

what are the problems with CI?

Answer 27

issue: needs to evolve 'kill your
own cytoplasm' mechanism 
issue: we do get paternal CI - e.g. in
rapeseed plants, Brassica napus
(Erickson & Kemble, 1990

Question 28

why do scientists think that most organelles are inherited maternally?

Answer 28

The predominance of the maternal mode has been suggested to be a result of a higher mutational load in the paternal gamete. Uniparental inheritance evolved from relaxed organelle inheritance patterns because it avoids the spread of selfish cytoplasmic elements. (control of CI). (Greiner at al, 2014)

Question 29

uniparental inheritance of mitochondria has also been argued to not have the potential to drive the evolution of the sexes… why?

Answer 29

In a model which considers UPI in a BPI population, as UPI increases in the population its relative fitness advantage diminishes in a frequency-dependent manner. Only when two mating types exist beforehand can associated UPI mutants spread to fixation under the pressure of high mitochondrial mutation rate, large mitochondrial population size and selfish mutants.(Hadjivasiliou et al, 2013)

Question 30

the degree of sex chromosome differences among taxa is huge. can you give some examples?

Answer 30

Spectrum of homomorphic to heteromorphic sex chromosomes, from a single sex-determining locus, as seen in pufferfish, a small differentiated region (strawberry and emu), most of the sex chromosomes apart from short recombining regions (humans), to the entire sex chromosome pair, as seen in Drosophila.(Batchrog et al, 2014)

Question 31

what is profound about marine algae in terms of its gametes?

Answer 31

some marine green algae are isogamous, either having two large gametes or two small gametes. some are profoundly anisogamous, and some are only slightly anisogamous. This doesn’t appear to be related to zygote survival, as all types of systems exist together with the same fitness, in the same environment. the evolutionary trajectories of populations have been shown to depend on random mutations and precise ecological conditions at the time of evolutionary divergence of the different mating types (Togashi et al, 2012)

Question 32

who said this :the presence on average of just one competitor can makes the ‘selfish’ evolutionary force of gamete competition stronger than the ‘cooperative’ force of gamete limitation even if gamete limitation is very acute (approaching 100% of eggs remaining unfertilised)?

Answer 32

(Parker and Lehtonen, 2014)

Question 33

who created a model considering UPI in a BPI population?

Answer 33

(Hadjivasiliou et al, 2013)

Question 34

what are sex roles?

Answer 34

definitions of sex roles tend to focus on sexual
dimorphism in either: (i) the degree of within-sex reproductive competition; (ii) how discriminating individuals
are during pair formation; or (iii) the extent to which they
exhibit parental care after mating (but sex roles can also exist in hermaphroditic species)

Question 35

what do some people think drives sex roles?

Answer 35

sex roles arise entirely from
chance, or from sex differences in environment-driven
‘habits of life’, such as encounter rates, mortality schedules
and re-mating rates, externally imposed, not from anisogamy (Ah-King & Nylin, 2010)

Question 36

why do (Scharer et al, 2012) disagree with externally imposed processes driving sex roles?

Answer 36

Were this to be true,sex roles would be variable to the extent that females and males (as defined by anisogamy) would be no more or less likely to have any one sex role
sex specificity of selection, coupled with historical
contingency has led to the diversity of differences
between the sexes in sex roles that are seen in nature

Question 37

what is muller’s ratchet in terms of CI?

Answer 37

However, on evolutionary timescales, uniparentally inherited organelles are susceptible to mutational meltdown (Muller’s ratchet)