Invertebrate mating systems 1

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Invertebrate mating systems (1)

• Sexual Selection the Evolution of Sex

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Onthophagus taurus
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Onthophagus taurus
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Weapon dimorphism
Weapon diversity
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Onthophagus taurus

• Fighters and sneakers
• Two alternative mating tactics

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The competition doesnt end here
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Complex genitalia and sperm competition
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Why bother with Sex?Asexual Reproduction Works
Just Fine

• Many invertebrate groups have stages of asexual
  reproduction within their life-cycle.
• Some species have entire populations consisting
  only of asexually reproducing females.
• Some invertebrate classes (e.g. Phylum Rotifera
  Classes Bdelloidea Monogonata) entirely asexual

4 Bdelloid rotifers
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Why bother with Sex?The costs of sex!

• Recombination successful genotypes can be lost
  and costly homozygous genotypes could be created.
• Meiosis you only pass on half of your genes.
• Costs of mating Time, energy, risk of
  predation, STDs, sexual conflict

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Why bother with Sex?The continual production of
new variant offspring with diverse genotypes.

• Group selection arguments
• The Lottery Hypothesis
• The Coevolution /
• Red Queen Hypotheses

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Group Selection

• Sexual reproduction ensures a continuing level of
  genetic diversity, and the future survival of the
  species. If some environmental change or disease
  wipes out one genotype, others may be resistant.
• PROBLEM This idea supposes that individuals act
  with great altruism and foresight. However,
  selfish individuals that had a larger immediate
  reproductive output would tend to outcompete them.

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Lottery Hypothesis

• G.C. Williams likened sexual reproduction to a
  lottery. You are less likely to win the prize if
  you buy many tickets with the same set of
  numbers.
• At least some of a diverse group of offspring
  might be resistant to a future environmental
  change.

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Aphids Order Hemiptera Superfamily Aphidoidea
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Life-Cycle of AphidsOrder Hemiptera
Superfamily Aphidoidea
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Hydra (Phylum Cnidaria Class Hydrozoa)
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Liver flukes
Fasciola hepatica
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The Coevolution Hypothesis

• Sexual Reproduction is favoured by the
  evolutionary interaction between a species and
  the other species that it seeks to exploit, or
  which exploits it.
• Parasite / Host
• Predator / Prey
• Herbivore / Plant

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The coevolution hypothesisMore snails and worms
Potamopyrgus antipodarum
Microphallus spp.
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The Red-Queen theory
Here, we have to run faster and faster, in
order to keep exactly where we are
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Consequences of Sex Anisogamy

• The evolution of two sexes
• Not always the case (Protozoa)
• Two different types of gamete
• The essence of femaleness is to produce a
  relatively smaller number of larger,
  well-provisioned gametes.
• The essence of maleness is to produce a huge
  number of small mobile gametes that basically
  only contribute DNA to the offspring

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Anisogamy and Sexual Selection

• Female Gametes are a scarcer resource
• Therefore
• We expect that males will be in competition for
  access to females.
• We expect that females will be choosy about the
  quality of males.
• However, many examples of female competition and
  male choosiness

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Sexual Selection

• Selection for traits that are solely concerned
  with increasing mating success
• Example Glow-worms and Fireflies
• Bioluminescence in Glow-worms is under natural
  selection, as its function is to attract prey
• Bioluminescence in Fireflies is under sexual
  selection, as its function in males is to attract
  females

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Sexual Selection

• What traits can be under sexual selection?
• Morphological structures
• Behaviours
• Signals Visual, Acoustic and Chemical

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Sexual Selection

• Intra-sexual selection
• Male/Male Competition
• Female/Female Competition
• Inter-sexual Selection
• Female Mate Choice
• Male Mate Choice

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Male/Male CompetitionFiddler Crabs Family
Ocypodidae Uca spp
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Female/Female CompetitionMormon CricketsOrder
Orthoptera Anabrus simplex

• Role reversal
• Also male choice
• Nuptial Feeding
• Occurs when males are scarce
• Occurs when females are food-limited

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Nuptial (Courtship) Feeding
Amblycorypha parvipennis
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Female choiceGiant Cuttlefish Sepia apama
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Male choice St Andrews Cross SpiderArgiope
keyserlingi
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Male mate choice inArgiope keyserlingi
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Intra- Inter-sexual selection Stalk-eyed
FliesOrder Diptera Cyrtodiopsis spp.
Both intra- and intersexual selection can
sometimes act on the same traits e.g. the width
of the eye-stalks in Cyrtodiopsis
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Next Lecture Invertebrate Mating Systems
Monogamy to Promiscuity