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BIOL/ BMSC 114: Asexual vs. sexual reproduction

Asexual and sexual reproduction
• Two modes of animal reproduction: asexual and sexual reproduction. – In most cases asexual reproduction relies on mitotic cell division. – Sexual reproduction is the creation of offspring by fusion of haploid gametes to form a zygote. • Gametes are formed by meiosis. • Sexual reproduction directly reduces your contribution to the next generation!

Phil Lester, KK413
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Asexual reproduction
• Asexual reproduction has many advantages: – All your wonderful genes are contributed to the next generation (no dilution!). – It enables animals living in isolation to reproduce. – It can create numerous offspring in a short amount of time. • Theoretically it is most advantageous in stable, favourable environments because it perpetuates successful genotypes precisely.

Types of asexual reproduction: Fission.
• Many invertebrates reproduce asexually by fission. – The separation of a parent into two or more individuals of roughly equal size.

Phylum Cnidaria– sacs with central digestive system.
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Types of asexual reproduction: Budding.
• A new individual splitting off from an existing one. – For example certain species in the Phylum Cnidaria: new individuals grow out from the body of a parent. – May detach or remain joined to the parent.
Phylum Cnidaria– hydras can reproduce sexually when conditions are unfavourable.
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Types of asexual reproduction: Fragmentation.
• The breaking of the body into several separate pieces. – Some or all of which may develop into new adults. – Always associated with regeneration e.g. crown-ofthorns starfish.

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Types of asexual reproduction: Gemmules.
• Some invertebrates release specialized cells that can grow into new individuals. – For example the gemmules of Porifera that are formed when certain cells migrate together within the sponge– then surrounded by a protective coat.

Parthenogenesis
• Parthenogenesis is the process by which an unfertilized egg develops into (often) haploid adult. • Parthenogenesis plays a role in the social organization of species of bees, wasps, and ants. • Male honeybees are haploid and female honeybees are diploid.
Haploid= one set of chromosomes. Diploid= two sets of chromosomes; one set from each parent.
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Phylum Cnidaria– sacs perforated with holes.

Haplodiploidy
• Queen ants or bees able to control the release of sperm. • Males develop from unfertilized eggs (one set of chromosomes= haploid). • Females develop from fertilized eggs (two sets of chromosomes= diploid). – Eggs develop into new queens or workers.

Strange ants, conditionally using sexual reproduction
Pearcy et al. (2004) Conditional use of sex and parthenogenesis for worker and queen production in ants. Science 306: 1780-1783. • Central question: why don’t more organisms reproduce asexually? • In a few ant species, females (workers or queens) can be produced from unfertilized eggs. – Can also develop from fertilized eggs.

The ant species Cataglyphis cursor of dry European forests.
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Strange ants, conditionally using sexual reproduction
Pearcy et al. (2004) Conditional use of sex and parthenogenesis for worker and queen production in ants. Science 306: 1780-1783. • Collected 38 colonies of ants in Southern France. • Looked at genetic diversity and relatedness of workers and queens within a nest.

Strange ants, conditionally using sexual reproduction
Lots Relatedness to the queen Less Lots Relatedness to the queen Less

Queens

Workers

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Strange ants, conditionally using sexual reproduction
Lots
% reproduction by parthenogenesis

Strange ants, conditionally using sexual reproduction
Pearcy et al. (2004) Conditional use of sex and parthenogenesis for worker and queen production in ants. Science 306: 1780-1783. • Workers have more genetic variation than queens. – Though queens do have a little, but probably need less than the workers.

Queens

None Lots
% reproduction by parthenogenesis

Workers

• Workers may need the genetic variation much more than the queens. • Maximum benefits of both systems: queens selectively use parthenogenesis to pass on most of their genes but still get the benefits of sexual reproduction.

None
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Variation in reproductive cycles within animals
• Animals may be solely asexual or sexual. • Or they may alternate between the two modes depending on environmental conditions. • Daphnia reproduce by parthenogenesis under favourable conditions and sexually during times of environmental stress.

Parthenogenesis in vertebrates
• Several genera of fishes, amphibians and lizards reproduce by parthenogenesis – Involves a doubling of chromosomes after meiosis to create diploid “zygotes”. • Whiptail lizards: No males, but during courtship one female imitates male courtship behaviour.

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Parthenogenesis in vertebrates
• Each female whiptail lizard takes turns “being a male”. • Female behaviour occurs when levels of female sex hormones are high. • Ovulation more likely to occur if mounted during critical time.

Humans - A future without men?
• Bryan Sykes hypothesis: – With the declining sperm count in men, and the continual atrophy of the Y chromosome within 5,000 generations (approximately 125,000 years) the male of the human species will become extinct. • One option for the survival of humanity is unisex reproduction by females. – female eggs fertilised by the nuclear X chromosomes of another female and implanted using in vitro fertilisation.
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Hermaphroditism
• Sexual reproduction in sessile or burrowing animals or parasites who may have difficulty encountering a member of the opposite sex. • Hermaphroditism: one individual is functional as both a male and a female. Some self-fertilize. • Each individual receives and donates sperm.

Flatworms “Penis Fencing”
• Class Tuberellaria consist mainly of free-living (nonparasitic) marine flatworms

Two flatworms “penis fencing”

Fig not in book

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Intersexuality (hermaphroditism) in humans
• The term "hermaphrodite" has historically been used to describe people with ambiguous genitalia or biological sex (better term= intersexual). • ~1 percent of live births exhibit some degree of sexual ambiguity – between 0.1% and 0.2% of live births are ambiguous enough to become the subject of specialist medical attention,

Intersexuality (hermaphroditism) in humans
• There are dozens of named medical conditions that may lead to intersex anatomy. • Rather than just XX and XY, in fact there are other possible combinations such as as XO, XXX, XXY, XYY, XO/XY, XX male, XY female. • Sex chromosomal mosaicism can cause "true hermaphroditism", the presence of both testicular and ovarian tissue in one individual.
Intersex of Northern Pintail Anas acuta

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Sequential Hermaphroditism
• An individual reverses its sex during its lifetime. • Sometimes the individual is a male first, then a female: for other species visa versa. • Bluehead wrasse in the Carribean.

Why sex? Why not asexual?
• The “Red Queen Hypothesis”: – According to the Red Queen hypothesis, sexual reproduction persists because it enables many species to rapidly evolve new genetic defences against parasites that attempt to live off them. • Experiments with a fish called topminnow in Mexico.

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Complex reproductive systems in many phyla
• The least complex reproductive systems lack gonads. – e.g. Polychaete worms lack gonads. • Eggs and sperm develop from undifferentiated cells lining the coelom. • Some reproductive systems, such as that seen in parasitic flatworms, can be very complex.
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Macroparasite example: other liver flukes

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Mechanisms of sexual reproduction
• Internal fertilization: sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract. – Internal fertilization requires sophisticated reproductive systems. • External fertilization: eggs are shed by the female and fertilized by the male in the environment.

External fertilization
• Requires an environment without desiccation or heat stress e.g. aquatic environments. • Timing is crucial. – many corals living on the Great Barrier Reef spawn about four to five days after the full moon in October or November.

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External fertilization– “Masting”

Internal fertilization
• Species with internal fertilization usually produce fewer zygotes but provide more parental protection than species with external fertilization. • Internal fertilization usually results in the production of fewer zygotes than does internal fertilization. • However, the survival rate is lower for external fertilization than it is for internal fertilization.

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Sperm competition
• Multiple mating frequently occurs in insects. • In evolutionary terms, it is best to try to ensure your genes are the ones contributing to the next generation. • Dragonflies…
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Sperm competition
• Combination of multiple matings and sperm storage has resulted in “sperm competition”, either by: – “Sperm displacement” where a male pushes other males sperm to the back of the spermetheca, so “last-in-first-out”. – “Sperm removal” where a male removes other sperm prior to depositing his own.

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