Trends in Life on Earth Reproduction http://intranet.cbhslewisham.nsw.edu.au:82/science/concepta.asp?conID=13&resource=1 Retrieved 4 June 2011

Trends in Reproduction 1. Asexual to Sexual Reproduction 2. External to Internal Fertilisation 3. Increased care for embryos & young

Asexual & Sexual Sexual Reproduction Asexual Reproduction Involves mitosis Two daughter cells genetically identical to the parent Offspring with the same number of chromosomes as the parent Examples: spores of fungi, binary fission and budding in bacteria Sexual Reproduction Involves meiosis Four daughter cells genetically different to the parents Gametes with half the number of chromosomes as the parent Examples: flowering plants, most animals 1. Asexual to Sexual Reproduction

Asexual vs. Sexual Advantages Disadvantages Does not require another individual to reproduce Little variation between organisms (unlikely to survive change) Variation in offspring (greater chance of survival if change occurs) Requires water (for the sperm to travel) Can reproduce successfully over many generations in stable conditions Sheer chance which gametes are involved (maybe the best won’t be fertilised) Gametes must be produced, ready and meet at the same time 1. Asexual to Sexual Reproduction

Overcoming the Disadvantages If the trend is to reproduce sexually (to increase variation in species), how do organisms overcome the disadvantages of sexual reproduction? Environmental factors stimulate the production and release of gametes in males and females (Eg. phases of the moon, changing water levels, warmer weather, abundance of food, etc.) Courtship behaviours ensure members of a species mate within their species and to know the other is ready to meet (Eg. bowerbird) For male gametes (of animals) to swim- external fertilisation in the water (frogs/fish) and internal fertilisation on the land (mammals) occurs 1. Asexual to Sexual Reproduction

How do plants sexually reproduce? Flowers are the reproductive organs of angiosperm plants, parts are: Sepals: protect a budding flower Petals: surround the male & female reproductive organs Stamen: male reproductive organ that produces the male gamete (pollen); consists of the anther and the filament Pistil: female reproductive organ that produces the female gamete (egg); consists of the stigma, style and ovary The gametes must meet! 1. Asexual to Sexual Reproduction

How do animals sexually reproduce? Males produce a haploid gamete called sperm; they usually have tails so that they can swim towards the female gamete. Females produce a haploid gamete called an ova (egg); they are usually much larger than sperm cells and contain food stores for the developing embryo. The gametes must meet! 1. Asexual to Sexual Reproduction

How do the gametes meet? The haploid gametes must be brought together in a process called fertilisation for reproduction to be successful. Animals: the male gamete (sperm) is released, when it reaches the female gamete (ova) ‘fertilisation’ occurs. Plants: the male gamete (pollen) is released, when it reaches the female gamete (ova) ‘pollination’ occurs. Fertilisation can occur: Inside (internal fertilisation) or Outside (external fertilisation) of the organism’s body. 2. External to Internal Fertilisation

External Fertilisation Gametes are shed into the water, fertilisation occurs & zygotes develop outside of the body Common in aquatic animals: Fish Amphibians Coral polyps 2. External to Internal Fertilisation

External Fertilisation Advantages Disadvantages Many fertilisations can take place at once; thus a species can rapidly colonise a large body of water After the gametes are released, the organism is unable to control them; can’t ensure fertilisation will occur Large numbers of gametes are wasted (meaning the energy put into their production is lost) 2. External to Internal Fertilisation

Overcoming the Disadvantages How do organisms overcome the disadvantages of external fertilisation? Produce many, many gametes to increase the chances of successful fertilisation (Eg. Sea urchin) Synchronised timing of the production and release of gametes (Eg. Palolo worm in Pacific Ocean shed posterior parts in the Moon’s last quarter in Oct and Nov & fertilisation occurs) Courtship and mating behaviours to bring males and females close together when gametes are released (eg. frogs - male clasps on females back and when she releases eggs the male releases sperm) 2. External to Internal Fertilisation

Internal Fertilisation Gametes of a male are released into a female The fertilisation and the development of zygotes occur inside the body of the female. Common in: land animals (mammals, birds & reptiles) & flowering plants 2. External to Internal Fertilisation

Internal Fertilisation Advantages Disadvantages Females produce less gametes (save energy for offspring that will survive) Must bring the sexes together for the transfer to occur Increases the chance of successful fertilisation Plants rely on insects, wind and birds for fertilisation (cross-pollination) to occur Doesn’t require water in the environment for fertilisation to occur Requires a large amount of energy for the female to ‘raise’ the embryo 2. External to Internal Fertilisation

Animals overcome the disadvantages If the trend is toward internal fertilisation (to colonise the land), how do animals overcome the disadvantages of internal fertilisation? Courtship and mating behaviours to bring males and females in contact for fertilisation to occur (Eg. Bowerbird with elaborate nests and blue ‘gifts’) Have an organ for gamete transfer (Eg. presence of penis in reptiles to transfer semen to the reproductive tract of females or the cloaca in birds) Some animals don’t carry the embryo until it is fully developed (Eg. birds & reptiles lay eggs) Some animals carry the embryo until it is fully developed have fewer young (Eg. mammals) 2. External to Internal Fertilisation

Plants overcome the disadvantages If the trend is towards internal fertilisation (to colonise the land), how do plants overcome the disadvantages of internal fertilisation? Some plants self-pollinate (Eg. dandelions & pea plants) Plants that cross-pollinate using the wind may have: Anthers hang outside the flower to expose pollen to the wind Stigmas are feathery to increase the surface area (increase chance of contact with pollen) Plants that cross-pollinate using vectors (insects, birds or mammals) may have: Sticky stigmas so that the pollen on vectors sticks to the stigmas Brightly coloured flowers or anthers (red, orange or yellow for birds and white or yellow dots/lines that reflect ultraviolet radiation for insects) to attract vectors Sticky pollen so that it sticks to vectors to be carried to another plant Sweet stamen to attract vectors Scents or mimicry to attract vectors Have a ‘platform’ on which the insect can land 2. External to Internal Fertilisation

Development of the Embryo & Young For those zygotes that have been fertilised externally their chances of survival are limited because: Their parents play no role in their development Environmental conditions impact their survival For those gametes that have been fertilised internally, the development of the embryo can occur: Externally Germinating seeds Laying eggs in water (Invertebrates, Fish, Amphibians) Laying shelled eggs on land (Reptiles, Birds, Monotremes) Internally Undeveloped Young (Marsupials) Developed Young (Placentals & Unshelled eggs within body) 3. Increased care for embryos & young

Plants The embryo forms within a seed (that provides food and protects the embryo) and the ovary becomes a fruit or nut. The seeds are dispersed from the parent plant to a place where it is most likely to germinate and survive. Modes of seed dispersal: Animals (within/carried by) Wind (carried by) Water (carried by) Self (rolling/catapulting) 3. Increased care for embryos & young

Animals Laying Eggs (Unshelled) Invertebrates (eg. insects & sea horses) Many are often laid in a position that ensures a ready food source Most are abandoned after being laid Male seahorse guards their young in a brood pouch 3. Increased care for embryos & young

Animals Laying Shelled Eggs on Land In reptiles, birds, monotremes & many insects Shells Protect the embryo Allow gaseous exchange Prevent desiccation (so that organisms don’t have to return to the water to reproduce) Contain food (in the yolk sac) 3. Increased care for embryos & young

Animals Laying Shelled Eggs on Land Level of parental care after hatching varies: Reptiles: turtles build a special burrow or nest to give protection and stable environment but abandon after laying Birds: build nests, keep eggs warm & protected and feed and guard chicks after they hatch Monotremes: feed and care for their young for months

Animals Internal Development Ensures the developing young have: Protection A supply of nutrients A constant temperature A constant chemical environment The organisms will have fewer young, but these few young have a great chance of survival. 3. Increased care for embryos & young

Animals Undeveloped Young (Eg. Red Kangaroo) Fertilised eggs only develop when environmental conditions are good (eg. no drought) Females retain fertilised eggs in their uteruses; young are born immature (hairless and less than 1g) after 33 days Young use tiny limbs and claws to pull itself into its mother’s pouch without assistance from the mother & attach to the mother’s teat Young stays in mother’s pouch for 235 days until it weighs 4-5 kg; when it leaves another embryo commences the cycle Young remains with mother for another 4 months but feeds from another teat (different kind of milk) 3. Increased care for embryos & young

Animals Developed Young Ovoviviparity Some reptiles (blue- tongued lizards), some fish and many insects The fertilised egg continues to develop inside the mother in an unshelled egg (protection & nutrients). The young are born alive and most fend for themselves immediately. 3. Increased care for embryos & young

Animals Developed Young Viviparity Eg. placental mammals Young develop within the mother’s uterus (protection, constant temperature, constant chemical environment) The young receives nutrients via the placenta After birth the young continues to receive nutrients from suckling and are protected by the mother 3. Increased care for embryos & young

External to Internal Fertilisation Trends of Life on Earth Trend 1 Trend 2 Trend 3 Less Care to More Care External to Internal Fertilisation Asexual to Sexual From Simple to Complex