lecture 26 - animal reproduction Flashcards
What are the differences between asexual and sexual reproduction?
asexual -> offspring inherits all genetic material from a single parent ->clone- mitosis (not always) -> meiotic parthogenesis is an exception, does not produce a clone
sexual -> offspring inherits genetic material from two parents -> genetically unique individual
(meiosis->fertilisation->mitosis)
All animals reproduce, and several different ways of reproduction have evolved. The main two categories of reproduction are asexual and sexual reproduction. Asexual reproduction is the production of genetically identical cells or individuals called clones (in most cases of asexual reproduction), whereas sexual reproduction occurs by combining a complete set of genetic information from two individuals to make a new genetically unique individual. Many animals can reproduce both asexually and sexually (most animals that reproduce asexually can also reproduce sexually).
Describe the asexual forms of animal reproduction
- Budding - forming a bud, or protrusion, that eventually breaks off to form a new organism. The cells of the bud are formed by mitosis. An example of a budding animal is Hydra (they can also reproduce sexually).
- Fragmentation - splitting of one organism into pieces, each of which develops into a new individual
Example: Sea stars
Fragmentation can be done intentionally or due to damage (e.g. by predators). - Parthenogenesis - females produce eggs that are not fertilised but divide by mitosis and develop into new individuals.
Example: Komodo dragon
Eggs are usually the product of meiosis and are haploid. In some organisms, these develop into haploid adults. In other organisms, DNA content doubles during development diploid adult.
In some organisms, the egg cell does not undergo meiosis and divides by mitosis to form a diploid adult.
In Komodo dragons, the egg is formed by meiosis and the DNA content doubled. Offspring is not genetically identical to parent, but all genes are inherited from a single parent.
Describe seuxal animal reproduction
- Involves meiosis and fertilisation
Types of gametes
Male: spermatozoa (sperm)
Female: ova (eggs)
Sexual reproduction involves formation and fusion of gametes. The larger, female gametes are called ova, and the smaller, male gametes spermatozoa. The fusion of two gametes results in a zygote. The zygote divides by mitosis and develops into an embryo.
Meiosis: form of cell division that halves the number of chromosomes
Fertilisation: involves fusion of specialized cells to restore the original chromosomal content
(Meiosis evolved in ancestors of modern eukaryotes sexual reproduction only occurs in eukaryotes)
What are the costs of sexual reproduction?
- Finding and attracting mates takes time and energy
Example: male bowerbirds build and decorate homes to attract mates
Sexual reproduction has some disadvantages over asexual reproduction, including time and energy required to find and attract mates.
Some species (e.g. bowerbird) build elaborate homes to attract mates, others invest a lot of energy in growing colourful plumage or carrying out mating rituals. For some species with a low population density, finding a mate can be difficult.
- Slower increase in population size
In asexual reproduction, all offspring can produce more offspring, allowing for exponential population growth if conditions are favorable. In sexually reproducing organisms, only females produce new offspring. The consequence is that an asexual group can have a much greater rate of population growth compared to that of a sexually reproducing one.
What are the genetic consequences of sexual reproduction?
fusion of two genetically unique gametes + random segregation of homologous chromosomes + recombination between homologous chromosomes = genetically unique individual
In contrast to asexual reproduction, which in most cases produces organisms that are genetically identical to their parent, sexual reproduction leads to the generation of genetically unique individuals because of recombination between homologous chromosomes, random segregation of homologous chromosomes, and fusion of two genetically unique gametes.
What are the advantages of genetic variation?
The observation that most animals can reproduce sexually suggests that sexual reproduction offers advantages over asexual reproduction.
Hypotheses for the advantages of sexual reproduction over asexual reproduction:
- It allows organisms to adapt faster than asexual ones because rare beneficial mutations that arise in different organisms can be brought together, increasing the overall fitness of the population.
- It allows a population to purge itself of harmful mutations more quickly than could a population of asexual individuals.
- It may be a mechanism of parasite defence—the Red Queen hypothesis. (The Red Queen said to Alice in Lewis Carroll’s ‘Through the Looking-Glass’ in her explanation of the nature of Looking-Glass Land: ‘Now, here, you see, it takes all the running you can do, to keep in the same place’
These different explanations for the prevalence of sexual reproduction are not mutually exclusive. They could work together, or vary in importance for different organisms.
Describe the phylogenetic distribution of asexual species
Exclusive asexual reproduction seems to be an evolutionary dead end, and asexual populations are typically found at the tips of phylogenetic trees (species that only reproduce asexually are not very old in evolutionary terms). This suggests that at least in the long-term, sexual reproduction is advantageous.
Describe how some organisms reproduce both sexually and asexually
Example: Daphnia
Type of reproduction depends on environmental conditions
Most organisms that reproduce asexually are also capable of reproducing sexually.
The freshwater crustacean Daphnia reproduce asexually by parthenogenesis in the spring, when food is plentiful, perhaps because asexual reproduction is a rapid form of reproduction that allows the organism to take advantage of abundant resources. The parthenogenesis is ameiotic (progeny develops from diploid egg). Development of females or males is under environmental control.
Later in the season, when conditions are less favorable because of increased crowding and decreased food and temperature, Daphnia reproduce sexually, mating to produce a zygote that starts to form an embryo.
At an early stage, development is arrested and metabolism slows dramatically. The embryo forms a thick-walled structure, called a cyst, which enables it to resist freezing and drying and therefore last through the winter.
It is unclear why some animals (e.g. mammals) have lost the ability to produce asexually. One reason might be that imprinting (differential expression of some genes depending on if they were inherited from father or mother) has given an evolutionary advantage, but was incompatible with asexual reproduction.
Describe how external fertilisation takes place
To increase chances of fertilisation:
Large gamete number
Release of gametes in same location and at the same time
Animals with external fertilisation are usually r-strategists
Eggs and sperm require a wet environment to survive. In aquatic organisms like fish and amphibians, eggs and sperm can be released directly into the water in external fertilisation.
External fertilisation is a chance affair: sperm must meet egg in a large environment. Animals that release gametes externally therefore have developed strategies to increase the probability of fertilisation. One strategy involves releasing large numbers of gametes. Externally fertilising animals may also come close together physically to improve chances that the sperm will fertilise the eggs.
What are r-strategists and K-strategists?
r-strategists:
Production of large numbers of offspring without provision of a lot of parental care
Evolve in unstable, changing, unpredictable environments
K-strategists:
Production of few offspring and provision of a lot of parental care
Evolve in stable, unchanging, predictable environments with limited resources
(r and K originate from equations that describe the population growth of different species. r=reproductive capacity, K=carrying capacity of the environment)
While animals with external fertilisation are usually r-strategists, animals with internal fertilisation are usually K-strategists. The two strategies represent two ends of a continuum of reproductive strategies.
Describe internal fertilisation
Land-dwelling animals need to prevent their gametes from drying out. Internal fertilisation is an adaptation to living on land, but is not restricted to land-dwelling animals.
Animals with internal fertilisation can be oviparous or viviparous.
Oviparous animals: lay eggs, very little development occurs inside the mother. Nutrients for developing embryo are provided in the yolk. Vertebrate eggs contain several extraembryonic membranes:
Yolk sac: contains yolk.
Allantois: encloses a space where metabolic wastes collect.
Chorion: surrounds the entire embryo along with its yolk and allantoic sac.
Amnion: fluid-filled cavity that allows embryo to develop in a watery environment
The embryo is further protected in many species by a hard shell that prevents drying out and facilitates gas exchange.
Viviparous animals give birth to live young. In placental mammals, the chorion and allantois fuse to form a placenta.
