Reproduction Flashcards
What is reproduction
Process by which animals increase in number
Animals grow and develop until they reach maturity
Asexual reproduction
Effectively cloning of tissues to produce a novel individual
Replication of cells takes place but these are clones of the original cells and these get organised into a smaller replica of the original animal, e.g. budding in hydra
Fragmentation
involves development of a new animal from a fragment of an original – annelids, flatworms and echinoderms
Which organisms reproduce by binary fission
Cnidarians (jellyfish, sea anemones, corals and hydroids) and tunicates (sea squirts)
Parthenogenesis
asexual reproduction where unfertilised eggs develop naturally into adults without input from males (obligate parthogenesis)
Observed in hymenopterans, aphids, water fleas and some lizards
Egg cells are produced by mitosis so are diploid rather than haploid
Cyclical parthenogenesis
life-cycle that alternates between asexual and sexual reproduction
advantage of cyclical parthenogenesis is that the population can be quickly increased by asexual reproduction so as to exploit prevailing conditions, such as a good food supply.
Facultative parthenogenesis
arises when there is low frequency or absence of males for a long period for a species that normally reproduces sexually
Observed in crayfish and all vertebrate groups, except mammals
Triggers for spontaneous development are unclear – may involve environmental ‘shocks’
Sexual reproduction
involves the fusion of two specialised haploid cells – gametes produced by meiosis in male and female animals
Male gametes are spermatozoa produced in a testis
Female gametes are ova produced in an ovary
External fertilisation
Aquatic animals often release their gametes into the water (spawning) and fertilisation is external
This is a risky strategy so spawning is often synchronised to maximise the chances of fertilisation
Synchronised spawning can be triggered by environmental factors such as temperature, photoperiod, lunar periodicity or tidal cycles
What triggers synchronised spawning
environmental factors such as temperature, photoperiod, lunar periodicity or tidal cycles
Internal fertilisation
involves the introduction of the spermatozoa into the female’s body so that the zygote is formed in the oviduct
Internal fertilisation is obligate for those animals that retain their offspring as they develop
Oviparous
animals release their eggs that have little or no embryonic development
Fertilisation can be external or internal
Embryonic development takes place outside of the female’s body
Ovo-viviparity
involves the retention of large fertilised eggs in the female’s body and embryonic development takes place internally and is fuelled by the contents of the egg.
Viviparity
involves the retention of small fertilised eggs in the female’s body and embryonic development takes place internally and is fuelled by an inter-change of nutrients from the maternal blood
Semelparity
once a lifetime reproduction
Salmon spawn once before death
High risk if environmental conditions are not stable
High risk if life span of species is < 1 year
Iteroparous
more than one-time investment in reproduction before death
Increases reproductive success in variable environment
Can be a single, extended mating season or multiple mating seasons
Male reproductive anatomy
based around paired testes and vas deferens that connects to the urethra at the prostate gland, which then exits through an intromittent phallus
Therefore, mammals have moved from a common cloaca because there is a separate anus
Spermatogenesis
takes place in the testes
In particular within the seminiferous tubules of the testes
Essentially sperm start their life as germ cells on the outer rim of the seminiferous tubule.
They move ‘inwards’ towards the lumen as they develop
Female reproductive anatomy
based on paired ovaries that are connected to a uterus via Fallopian tubes
Uterus is the oviduct and is a fluid-filled muscular tube that is partially ciliated
The uterus is linked to the outside via a cervix and vagina
Ovulation releases the ovum into the body cavity and cilia waft it to the opening of the infundibulum and into the Fallopian tube
Oogenesis
formation of the haploid ovum (female sex cell) within the ovaries within a follicle
Vitellogenin
In many taxa yolk deposition is based around the production of vitellogenin by the liver. Vitellogenin is commonly found in vertebrates (except mammals) and it provides amino acids, lipids and ions. In the amphibian egg (shown) vitellogenin provides almost all of the protein and all of the lipid in the yolk.
The amount of yolk is crucial – small amounts only provide enough energy to have a brief period of embryonic development before the larva has to be free-living (fish and amphibians) or the embryo is provided with an external source of nutrition (mammals).
Vitellogenesis in reptiles and birds
Yolk is the most abundant material stored in the ooplasm
In eggs with a macroscopic ovum, e.g. birds, there is an enlargement of the oocyte by deposition of yolk in large amounts
Oviparous vertebrates synthesise the lipoprotein VLDL in maternal liver and transported via the blood
VLDL:
60-70% of solids in fowl egg yolk
30% in Crocodylus porosus yolk
Absent in lizard eggs (?)
Copulation
effectively the end result of courtship behaviour that typically bonds the male and female. This courtship bond can last a lifetime or be transitory. For those species with internal fertilisation copulation is essential to introduce the spermatozoa into the female’s reproductive tract.
Fertilisation
process involves the binding of the sperm head to the surface of the ovum and the breakdown of the ovum plasmalemma by the acrosome reaction
Stages of embryogenesis
Zygote
Cleavage
Blastoderm
Gastrulation
Segmentation
Organogenesis
Holoblastic cleavage
entire egg divides during cleavage
small egg with little yolk
Daughter cells get progressively smaller
Meroblastic cleavage
partial cleavage
Large eggs with large egg reserve
Larger cells at vegetal pole
Cells that divide are concentrated at the animal pole
Cytoplasm forms a “polar cap” at the site of nucleus following fertilisation – this begins to divide
Cleavage in amphibians
Anuran cleavage pattern
- asymmetrical cleavage — more like the holoblastic pattern
Metamorphosis
A biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal’s body structure through cell growth and differentiation.
Generally, illustrated in two quite disparate groups – insects and amphibians.
Insect metamorphosis
Grasshoppers, bugs and cockroaches, lay eggs that hatch into neonate that has some resemblance to the adult. Over a series of moults the nymphs increasingly resemble the adult (imago)
Butterflies, flies, beetles, lay eggs that hatch into neonate that bear little resemblance to the adult. The larva undergoes moult that largely keeps the same form but the final moult is into a pupa. In this stage tissues are reorganised into an adult that is very different to the larval form
Genetic sex determination
Associated with heteromorphic (sex) chromosomes
XX/XY – male = heteromorphic sex
ZZ/ZW – female = heteromorphic sex
Temperature dependent sex determination in reptiles
Gender of the offspring is not determined by heteromorphic chromosomes
Zygote at conception has no fixed gender and the incubation environment determines the sex of an individual
Types of sex determination
Genetic
Temperature dependent
Can you get high temperature induced female crocodiles?
low and high temperature females and males at intermediate temperatures
What are females sex cells called?
Ova
What is the difference between semelparity and iteroparity?
Semelparity- once a lifetime reproduction
Iteroparity- more than once
Do birds lay fertilised eggs that undergo holoblastic cleavage?
No- meroblastic cleavage
What is gastrulation?
proliferation of cells and their migration into the blastocoel to form a multi-layered embryo
Eutherian mammal ova
very small and contain almost no yolky cytoplasm so development is impossible unless the developing embryo implants into the uterine wall and so can gain additional nutrients
What is vitellogenin
commonly found in vertebrates (except mammals) and it provides amino acids, lipids and ions
What type of ova undergo holoblastic cleavage
Small cells
