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LT18 Sex-specific Selection
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Sex-specific selection - sexual dimorphism
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Sex-specific selection: The evolution of males and femalesThe Evolution of Anisogamy – why are there sperm and eggs?
Isogamy – gametes equal in size: usually both motile (one sex – most unicellular
organisms)
- 2 or more mating types with, no morphological difference (gametes, progeny)
- Physalarum polycephalum (slime mould) >500 mating types
- Schizophyllum commune (hairy mushroom) >20 000 mating types
- Mating types to maximise outbreeding like mating types cannot successfully
fertilise
Anisogamy – 2 sizes of gamete
- 2 sexes – many unicellular and all multicellular organisms
- May not be major differences, but still can be distinguished
Oogamy: form of anisogamy with large non-motile gametes (ova), small motile
gamete (sperm)
- 2 sexes- most multicellular organisms
- Do the transitions always occur through these steps or jumps?
Anisogamy is the fundamental sexual dimorphism
Why does anisogamy evolve?
Ensuring collisions (Kalmus, 132) – get more fusions between + and – gametes if one
type is divided maximally to give the maximum number of gametes (group selection)
not a lot of support, rare type of selection
Cell parasites (Hurst 1990) – reduced transmission of cytoplasmic replicators if
sperm transmit no cytoplasm to zygote (not enough evidence to support/debunk it)
Sperm limitation (Levitan, 1996) – ova get larger since bigger ova bigger targets and
this ensures higher probability of fertilisation when sperm are limited (some
externally-fertilising invertebrates)
Disruptive selection
- Males and females originate by disruptive selection (Parker, Baker and Smith,
1972) – broadcast spawner release
gametes into the water (eg. corals)
- Each parent has equal amount of
resource, R to allocate to gametes
- Gametes can be any size m, depending
on how resource is divided up
- Can produce many small gametes or few
large ones – size-number trade-off
n=R/m
, - Gamete fusion is random with zygote size S= m 1 + m2
- Survival or fitness f, of zygote increases with its size S – typically larger the
zygote, larger the fitness
1. In a primitive marine ancestor, individuals produce a range of gamete sizes,
fusion between pairs of gametes is at random in the sea
2. Each parent has fixed budget for reproduction, so size-number trade-off,
number of gametes produced is inversely proportional to size
3. Success of zygote increases with size or provuisioning which equals the sum of
the sizes of 2 fusing gametes (Parker et al., 1972)
How do sexual dimorphisms arise?
Contradictory selection pressures between males and females
Offspring fitness is tied to egg volume, fertilisation competition selects for more,
smaller, faster sperm
- Size is tied to fitness egg volume tied
to offspring survival
- Speed of sperm inversely correlated
with sperm size (smaller the better)
- Gamete sizes diverge
Single –celled organisms: no relationship
between zygote size and fitness,
suggested that with the advent of
multicellularity, anisogamy could evolve
Empirical supporting disruptive selection leading to origin of 2 sexes, though the
present mode of reproduction in Volvocales does not immediately fit assumptions of
the model
How is anisogamy maintained? What prevents loss of anisogamy and reversion to
isogamy?
As sperm competition reduces, what will
prevent sperm getting larger to contribute
to cytoplasm to zygote
Oftern there is a second transition from
external to internal fertilisation
Suppose that N ejaculates compete for each
set of eggs
- Some argue that there is no such system
(where there is no sperm competition)
, - If sperm competition is greatly reduced
(how is anisogamy maintained in internal
fertilisers)
- Suppose p = risk that a given ejaculate
faces sperm competition (it will always
occur to some degree) (Parker, 1982)
Conclusions
No part of sperm should relate to helping to provision of zygote
All part of sperm form and function should relate to acquiring fusions
- Seen in Drosophila birfurca 1 sperm = 6cm, 20x time the length of male fly to
fill up space in female reproductive tract to block other sperms
- Diving beetle sperm cluster in long chains to help navigate the female
reproductive tract
The basis of sexual dimorphisms how are secondary (somatic) sexual
dimorphisms achieved
Establishment of separate sexes creates possibility of sex-specific fitness optima,
conflict over reproductive interests which leads to sex-specific selection pressures,
sexual conflict, sexual dimorphism
Due to gamete asymmetry, different sexes have different needs to produce
gametes all traits due to disruptive selection
The fitness optima for sexes can vary females maybe larger in egg-laying species,
smaller males higher mobility/flexibility, can also differ for phenotypes such as
gene expression, transcription, metabolism
Dimorphism of many traits correlates to testosterone levels
- Down-regulates many genes
- In fish, testosterone increases, in females helps to grow bigger, produce and
spawn more eggs
Axis of sexual dimorphisms
Reproductive tract only (intermediate) reproductive tract and some somatic
differences reproductive tract extreme somatic differences often with survival
differences (angler fish – males are several orders of magnitude smaller than the
female, and at some point parasitise female)
How do secondary sexual dimorphisms arise?
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