Term 2 Lecture 13: The reproductive system and spermatogenesis Flashcards
Asexual or clonal reproduction
-Fission, single celled organisms split in two
- Budding, as seen in hydra, new individuals are formed by budding. In hydra the buds detach and become independent. In other organisms such as sponges they remain attached
- Fragmentation e.g. a worm cut in two becomes two worms
- Parthogenesis (aka virgin birth) - some animals produce offspring via eggs but without fertilisation. Offspring may be haploid or diploid depending on the species and living conditions. Aphids, bees and wasps can do parthogenesis. Water fleas lay two kinds of egg one that needs fertilisation and one that does not.
When there is stress in the environment fertilisation provides gene mixing for natural selection which is advantageous.
Snails also lay eggs that can develop without fertilisation.
Komodo dragons lay unfertilised eggs that develop into offspring in good conditions. When conditions are not so good they find a mate and lay fertilised eggs.
Sexual reproduction generates diversity
Due to genetic variation via fusion of gametes produced by meiosis.
Different reproductive strategies enhance evolutionary fitness in animals
Strategies:
- synchronise release of gametes
- increase chance of fertilisation
- allow competition to determine sexual selection
- determined by competition
Fertilisation requires an internal or external aquatic medium
For sperm to be able to swim to the egg.
The egg has attractants/ pheremones to guide sperm to it - which is particularly important in external fertilisation e.g. frog and fish eggs
Risks of external fertilisation
- predators
-water disturbance by wind, tide or current
as there is a risk of loss of gametes more eggs must be produced
In some species of bee (internal)
When they mate the males gonads are ripped off blocking the reproductive tract so that other bees cannot mate with that female
A species of fruit fly (internal)
Produces only one sperm with a tail 3x the length of its body. Presumably it is coiled in the male, once inserted into the female it blocks her reproductive tract so that she cannot mate with other males.
In dragonflies (internal)
The end of a dragonfly penis is spoon shaped to scoop out competing sperm from other males whilst they deposit their own
Frogs mate by amplexus ( external)
When a male clings to a female he causes her to release eggs which he then fertilises
Internal fertilisation
Gametes are far more likely to meet than in external fertilisation
3 modes:
oviparous - egg birth
viviparous - live birth
ovoviviparous - live birth from eggs hatched inside mother
Oviparous: Spiders, reptiles and birds
Spiders, reptiles and birds - internal fertilisation then they lay eggs that develop and young hatch out.
Birds will sit on their eggs to incubate them.
Turtles bury their eggs in warm sand.
(See oviparous egg diagram)
Oviparious: Monotremes
Echidna and platypus lay fertilised eggs that develop and hatch however they then feed their young with milk from their milk glands after they hatch.
(See oviparous egg diagram)
Viviparous: placental mammals and marsupials
-embryo retained in the mother
- have a uterus which develops a placenta to nourish the embryo
Marsupials have a poorly developed uterus/placenta so their babies are born at a very rudimentary stage of development, when born the babies climb directly into their mothers marsupial pouch (Aka marsupium) and attach to a nipple to feed and develop - technically externally.
Ovoviviparous: some fish, lizards, amphibians and snakes
Fertilised eggs are retained in the body so the embryos develop inside the eggs and hatch inside the mothers body before being born.
Some baby sharks eat each other in the womb before birth
Mating systems
-Monogamy <10% of mammals
-Promiscuity
-Polyandry (rare) one female with many males e.g. mole rats
-Polygyny one male many females is seen in 80% of mammals and leads to intense male competition
Female choice leads to sexual dimorphism - difference in male size, colour etc. to attract a mate and increase mating success.
In some cases females are larger to be able to carry eggs
Hermaphroditism is a variation on sexual reproduction
two types:
Simultaneous hermaphroditism:
Both mature egg and sperm producing tissue is present in the same individual at the same time.
Occurs in slugs and snails.
Often they do not fertilise themselves and their eggs and sperm can develop at different times.
This basically means that any two individuals can mate
Sequential hermaphroditism:
Individual will be protogynous - first a female or
protoandrous - first a male
Occurs in various fish species e.g. clown fish
