LT18 Sex-specific Selection

Sex-specific selection: The evolution of males and females

The 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?