Reproduction

Reproduction

What are the mechanisms of reproduction?

The range of biological processes by which new individual organisms are produced from their parents allows for the continuity of the species, as organisms don’t live forever.

Types of reproduction

-          Asexual

-          Sexual

Asexual reproduction

Asexual reproduction is the simplest way to reproduce. It involves producing identical offspring from just ONE parent. It produces new individuals by mitosis (not in prokaryotes), in which each cell receives a copy of every chromosome of the parent cell. The offspring are clones - there is no variation; they are genetically identical unless genetic mutations occur.

Methods of Asexual Reproduction

It is most common in unicellular organisms because there is no cell specialisation, reproductive organs, or germ cells to produce gametes. Many multicellular organisms can also reproduce asexually. They arise from body cells, somatic cells.

Binary fission - splitting 1 cell into 2 cells of equal size. Identical to parent cell, e.g bacteria and protists.

Budding - outgrowths from a parent, each identical but smaller than parent, e.g protists and fungi

Fragmentation of body parts, similar to fission, but happens in multicellular organisms. The organism breaks into 2 or more parts, each regenerates the missing pieces to form a new complete individual, e.g flatworms.

Spore formation-  reproductive cells that are able to develop into an adult without the fusion with a second cell.g fungi (note: spores can also be involved in sexual reproduction)

Vegetative propagation in plants doesn’t include the production of seeds and spores, instead specialised tissue can grow into a new plant. E.g many plants, including flowering plants

Parthenogenesis - A type of cloning in some females, formation of egg in the absence of fertilisation - ‘virgin birth’. E.g. some reptiles, birds, insects (bees, wasps)

Advantages and disadvantages of asexual reproduction

Advantages

-          Efficient form of reproduction

-          Amount of time and energy to produce offspring is minimal

-          Population sizes can increase rapidly in optimal environments

-          There is no need to find a sexual partner

-          Offspring are genetically identical to the parent call, so they are well suited to a stable environment

Disadvantages

-          Rapid population growth can lead to overcrowding and increased competition for resources

-          The lack of genetic variation in a population can cause death of the entire population if conditions change (e.g a disease pathogen arrives or a severe drought) because they are not adaptable to new environmental conditions

Sexual reproduction

-          Involves the meeting of sex cells - gametes, which carry genetic information from both parents to the offspring. As a results they contain a mix of parental genes, there is variation to the parent and other offspring.

-          Variation leads to genetic diversity which is an advantage for continuity of the species. Some individuals may have random variations which may make them better suited to a new and/or changing environmental conditions - selective advantage - survival of the fittest.

-          Every species has a characteristic number of chromosomes, e.g humans have 46, camels 70 and tomatoes 24. Note, the number of chromosomes does not always reflect complexity of the organism.

-          Each species has 2 sets of chromosomes arranged in homologous pairs.

-          Chromosome number is constant for each species and doesn’t change from one generation to the next.

-          To prevent the doubling of chromosomes in each generation Meiosis occurs. This is a mechanism to ensure only ½ the number of chromosomes is passed on.

-          Meiosis is a type of cell division that takes place in the reproductive organs, e.g in plants and animals. It produces sex cells - gametes, (e.g egg and sperm).

-          Each gamete has only half the number of chromosomes - Haploid. (e.g 23 in humans)

Advantages

-          Fertilisation is less risky and the young are more likely to survive

-          Unfavourable (deleterious) genetic variation is eliminated from the population more efficiently

-          Generates genetic variation through recombination during meiosis and selects for beneficial genetic variation more efficiently

-          Populations are better able to adapt to and survive changing environmental conditions

-          Improves long-term evolutionary potential of populations

Disadvantages

-          Slower reproductive rate – fewer offspring are produced over a long period of time

-          Mates have to be found and accepted as suitable. Finding and competing for a mate can be risky and energetically costly

-          Recombination during meiosis can break apart beneficial genomic combinations and introduce deleterious variation to populations

-          Potential for spread of sexually transmitted diseases throughout population

-          Energetically costly; gamete production, mating, gestation and rearing young requires a lot of ongoing energy input from the parent

Sexual reprodution – Mitosis

Mitosis

A process of cell duplication, or reproduction, during which one cell gives rise to two genetically identical cells.

P – Prophase

M – Metaphase
A – Anaphase
T – Telophase

Sexual reproduction

Mating

Pairing between two opposite sex for sexual reproduction

Copulation

Union of sex organs

Fertilisation

Fusion between gametes (Sperm + Ova)

Diploid and Haploid

-          Diploid and haploid refer to the number of sets of chromosomes within a cell.

-          Most cells have 2 sets of chromosomes- these are somatic cells (body or non-reproductive cells). They are referred to as diploid (2n), e.g humans have 46 or 23 pairs of chromosomes.

-          Sex cells have 1 set of chromosomes - Haploid e.g in humans 23 chromosomes.

-          Chromosome numbers are restored in the process of fertilisation, 1 set of chromosomes is received from the mother (maternal chromosomes) the other from the father (paternal chromosomes).

-          The fusion of egg and sperm (haploid gametes) creates a zygote, which is a Diploid cell.

-          The diploid zygote divides by mitosis to become an embryo.

Mechanisms for sexual reproduction in animals

Sexual reproduction in animals

Members of the animal kingdom have an amazing diversity of reproductive strategies, often involving complex behavioural, physiological and structural adaptations for attracting mates, mating and protecting and nurturing developing offspring

Reproductive strategies

-          Unisexual - there are separate male and female individuals (advantage -variation).

-           Most animals are unisexual. However some are bisexual or hermaphrodites - individual has both male and female reproductive organs. This is advantageous for species with low population density or are non-motile (can’t move like coral). The disadvantage is that it requires a lot of energy to grow and maintain 2 sets of reproductive organs. Also when self fertilisation occurs, the gametes carry fewer combinations of gametes - less variation within the species.

If these strategies are advantages within their particular environment they can increase the reproductive success

-          Type of fertilisation - internal and external

-          Number of gametes produced

-          Where the young develop - inside or outside the body

-          Nature of parental care

Fertilisation – Internal and External

Fertilization is the process that occurs when a sperm cell from a male and an egg cell from a female join to form a zygote. In order for fertilisation to occur, sexually mature adults need to be brought together so that male gametes can swim to the female gametes. This is called mating.

Some stimuli that trigger mating are:

-          Change in weather

-          Abundance of food

-          Changing water levels

-          Phases of the moon

Internal and External Fertilisation

Based on the fact that life originated in the ocean; fertilization requires a water body in which to occur either in an ocean, sea, lake, river, pond, etc, or by using the internal water of a parents body.

External fertilisation

-          Occurs in most invertebrates and some vertebrates (fish and amphibians)

-          Takes place outside the body

-          Is a characteristic of most aquatic organisms as their environment prevents the gametes from drying out. It also occurs in moist terrestrial environments.

-          Gametes are released directly into the water, where fertilisation occurs -

-          The fertilised eggs develop into adults

-          Chances of fertilisation are increased by

-          Cyclical reproductive behaviours

-          Synchronised timing of release of gametes (sperm and eggs) - spawning events

-          Many millions of gametes are released

-          Courtship behaviours

Advantages and disadvantages of external fertilisation

Advantages

-          Usually more rapid and prolific

-          Female can continue to reproduce without pausing while the first young develop

-          Parents do not expend energy for gestation and caring for the young

-          Young are widely dispersed, reducing competition with the parent and with each other

Disadvantages

-          More gametes need to be produced

-          No control over the gametes once released

-          Decreased chance of successful fertilisation. This can improved by synchronised release of gametes (e.g spawning events) Young usually need to fend for themselves immediately

-          Must take place in an aquatic environment. Gametes and zygotes are exposed to predation and disease

Examples

-          Staghorn coral

-          Amphibians

-          Bony fish

Internal Fertilisation & parental care

-          Occurs in some invertebrates ( insects and snails) and most vertebrates (reptiles, mammals and birds)

-          Occurs inside the body of female and involves the mate attraction and copulation (sexual intercourse), which require energy investment.

-          Tend to be organisms adapted to terrestrial environments.

-          The internal environment protects the gametes from dehydration and loss to external elements.

-          It protects the fertilised eggs and developing young from predation.

As a consequence of the above:

-          Fewer eggs are required for the survival of sufficient offspring

-          The egg may develop a shell and be laid in the external environment to continue its development OR continue development inside the body.

Advantages

-          Can take place on dry land

-          Less gametes have to be produced

-          More likely to be successful as gametes are positioned close together in female reproductive tract

-          Gametes and zygotes are protected from predation and disease

-          Developing young are fed and protected increasing their chance of survival

Disadvantages

-          Usually slower with fewer progeny

-          Mating rituals and practice are more complex to get to the point of copulation

-          Potential for spread of STD throughout population

-          Energetically costly; requires a lot of energy input from the parent, especially the female

-          Parental care of young may be lengthy and demanding

Oviparous, Viviparous, and ovoviviparous

Oviparous produces eggs that develop and hatch outside the maternal body. An example of these animals include frogs, snakes, lizards, fish, etc

Viviparous produces living young instead of eggs from within the body in the manner of nearly all mammals, many reptiles, and a few fishes. An example of these animals include dolphins, whales, humans, lions, tigers, elephants and dogs

Ovoviviparous produces eggs that are hatched within the body, so that the young are born alive but without placental attachment, as certain reptiles or fishes. An example of these animals include sharks, rays, snakes, fish and insects

Examples of internal reproduction

-          Reptiles

-          Birds

-          Mammals (Includes the 3 subclasses: Monotremes, Marsupials and Eutherians/Placentals)

Comparison of internal and external fertilisation in animals

Pregnancy and birth in mammals

-          For all types of mammals sexual reproduction produces variation in offspring promoting continuity of the species.

-          There are 3 types of mammals, classified based on their different reproductive strategies:

-          Placentals

-          Marsupials

-          Monotremes

-          The differences are based on the extent of fetal development & how the fetus is nourished during development.

Monotremes

-          Oviparous, after internal fertilisation the female  lay leathery eggs. The ‘puggle’ develops inside this egg until it is hatched. The puggle is then protected by the mother.

-          Hatchlings obtain milk from mother’s mammary glands

-          e.g platypus (eggs in nest) and echidna (eggs in pouch).

Marsupials

-          Viviparous - Develop internally for a short time after fertilisation then move to a pouch to continue embryonic development

-          Drink milk from mammary glands in the pouch.

-          E.g kangaroos and koalas

-          Marsupials such as Kangaroos can have up to 3 offspring at different stages of development –

-          one out of the pouch but still drinking

-          One in the pouch attached to a nipple

-          One fertilised ovum in the uterus

-          The development of the youngest is triggered by the the second youngest detaching from the nipple - Embryonic diapause - this delay in development a strategy to increase chance of survival.

Placentals

-          Following internal fertilisation embryonic development continues internally in an organ called the uterus, which nurtures and protects the embryo.

-          Once the embryo implants into the uterus a placenta is formed, connecting the young to the bloodstream of the mother, supplying it with requirements for life and removing waste.

-          The mother gives birth to live - Viviparous - young that are mature and have a greater chance of survival

-          They give birth to one to a few offspring and invest a large amount of energy into parental care, increasing the chance of survival

-          E.g humans, dingoes, cows, etc.

Mechanisms for reproductive success in mammals

-          Internal fertilisation to increase likelihood that gametes will meet (occurs in all subclasses)

-          Implantation of the embryo in the uterine wall, with internal development of the embryo to increase chances of survival (placentals & marsupials)

-          Pregnancy to allow the developing young to be protected from the external environment, have a constant supply of nutrients and complete a gestation period (short in marsupials; Prolonged in placentals, whose young are well developed when born)

Female and male reproductive system

The primary male sex organs are the testes that produce sperm

The primary female sex organ are the ovaries that produce eggs (ova)

Secondary sex organs include other glands and organs involved in mating and reproduction. Sperm are made continuously after puberty and are produced in the millions. Immature eggs (oocytes) are present in female ovaries before they are born. After puberty ova mature and are released every 28 days (approx) until menopause halts ovulation. It is normal to have variation in the cycle.

Hormones

All stages of sexual reproduction are timed and synchronised by hormones that coordinate the reproductive cycle to ensure greater reproductive success and continuity of the species. These include:

-          Gamete formation

-          Courtship behaviour

-          Breeding seasons (some mammals)

-          Pregnancy

-          Birth

Hormones and glands

Hormones are chemical substances that act as messengers in the body. They are produced in endocrine glands. The pituitary gland is attached to the base of the brain. It is the master gland that regulates all other glands by secreting hormones that inhibit or stimulate other glands in the body. Sex hormones - affect the growth and/or functioning of the reproductive organs or the development of secondary sex characteristics. They are produced in the ovaries, testes, pituitary and adrenal glands (above kidney). Sex hormones trigger the onset of puberty.

3 types of hormones

-          Androgens

-          Oestrogens

-          Progestogens

Androgens

-          Androgens: male hormones.

-          Control the development and functioning of male sex organs and secondary sex characteristics, such as deepening of the voice, increased hair, size of muscles and bones.

-          The Testes secrete the androgen testosterone, which plays a role in spermatogenesis (sperm production). Note - ovaries also produce smaller amounts of this hormone.

Oestrogens

-          Oestrogens: main group of female hormones.

-          Controls ovarian function and therefore fertility.

-          Control the development of female reproductive system and secondary sex characteristics - enlarged breasts, pubic hair and widening of the hips.

-          Responsible for onset of oestrus ‘heat’ in seasonal breeders.

-          Occur in both male and female, but higher levels in females.

-          In males, along with testosterone, matures sperm

Oestrogen is made by the placenta. Oestrogen stimulates ovulation, aids blood flow to offspring, aids organ development and stimulates progestogen.  

Progestogens

-          2nd group of female hormones

-          Progesterone is the most common type and plays a role in pregnancy.

-          Stimulates milk secretion – lactation

-          Drop in this level initiates menstruation

Progestogen is made by the ovaries and then the placenta. Progestogen stimulates the thickening of the endometrium (uterus lining), aids with placentas function and relaxes uterus, helps mothers immune system tolerate infant.

Female reproduction

Oestrogen and progesterone, produced by the ovaries and controlled by hormones of the pituitary, they regulate:

-          Ovarian cycle

-          Menstrual cycle

-          Maintenance of pregnancy

-          Preparation for and maintenance of lactation

Ovarian cycle

-          Females are born with all the eggs (ova), they will produce in their lifetime.

-          Ova are immature and party developed in the ovaries

-          During puberty the ovarian and menstrual cycle begin - menarche.

-          Follicles form around the egg resulting in the dominant ovary

-          Hormones trigger the development and maturation of the ova each month until menopause is reached.

-          The cycle repeats approx every 28 days.

Ovulation in humans  

The ovarian cycle is controlled by the several hormones:

-          The control of this comes from the Hypothalamus which releases the hormone GnRH

-          GnRH triggers the anterior pituitary gland to release

-          Follicle stimulating hormone (FSH)

-          Luteinising hormone (LH)

-          FSH - responsible for follicles to grow in the ovaries

-          LH -  High levels cause egg to burst out of mature follicle - Ovulation - and cause the remnants of the burst to form a corpus luteum (produces progesterone and oestrogen). Also stimulates secretion of testosterone.

-          To assist the regulation of the ovarian cycle , various hormones provide feedback to the brain

2 phases of ovulation

-          Follicular

-          Luteal

Follicular - The large dominant egg moves to the surface of the ovary. A surge in LH which results in the bursting of the follicle and the release of the egg  -  ovulation. The egg is released into the fallopian tube. If sperm is present fertilisation may occur. Ovulation usually occurs mid cycle. Ova is only viable for 12-24 hours.

Luteal - is usually 14 days, occurs after ovulation, in the second half of the cycle. The burst follicle enlarges and secretes the hormone progesterone which acts on the uterus, preparing it for pregnancy.

Female organs

Fallopian tubes – An important passageway for an egg and a sperm to meet and for a fertilised embryo to make its way to your uterus

Uterus – Nurtures the fertilised ovum

Fimbriae – Place a newly released egg from your ovaries into your fallopian tubes

Ovary – Produce and store your eggs (ovum) and make hormones that control your menstrual cycle and pregnancy

Endometrium – Preparation for implantation, maintenance of pregnancy if implantation occurs and menstruation in the absence of pregnancy

Cervix – Allows fluid, such as menstrual blood to pass from the uterus into the vagina

Vagina – Provides a passageway for blood and mucosal tissue from the uterus to leave the body during menstrual period

Male organs

Bladder - a reservoir and active excretory organ for urine

Seminal vesicle - two small glands that store and produce the majority of the fluid that makes up semen. During ejaculation, the fluid from the seminal vesicles is expelled into the ejaculatory duct where it can then move on to mix with sperm and other reproductive fluids.

Prostate - the production of a fluid that, together with sperm cells from the testicles and fluids from other glands, makes up semen. The muscles of the prostate also ensure that the semen is forcefully pressed into the urethra and then expelled outwards during ejaculation

Vas deferens - transports mature sperm to the urethra in preparation for ejaculation.

Testicle - The testes are responsible for making sperm and are also involved in producing a hormone called testosterone.

Epididymis - The main function of the epididymis is to store the sperms for maturation and transport it to vas deferens.

Penis - Used for urination and sexual intercourse.

Urethra - the function of the male urethra is to allow passage of urine and semen. The urethra connects the distal portions of the urinary system, such as the urinary bladder, to the external environment and allows for urine excretion from the body.

Menstrual cycle

-          Accompanies the ovulation cycle

-          Starts with a menstrual period - menses which last about 4 days

-          This involves the endometrium (lining of the uterus) breaking down tearing away which results in bleeding.

-          The 1st day of the period is the 1st day of the follicular phase.

-          A new lining of the uterus forms over 5-12 days - pre ovulation phase.

Pregnancy

Fertilization to Implantation: The journey begins with the fertilized egg, which divides rapidly and eventually implants in the uterine wall.

Cellular Development: Early cell differentiation occurs, forming the foundations of vital organs and systems.

Organ Formation: Around three weeks, the heart begins to beat. Various organs, including the brain, kidneys, and liver, start developing.

Facial Features and Limbs: By the first trimester, the face and limbs form, becoming recognizable. Cells grow at a remarkable pace, shaping the fetus's appearance.

Bone and Muscle Formation: Skeleton and muscle structures begin to develop, with noticeable movements around 18-20 weeks.

Third Trimester: Final organ maturation and growth happen, preparing the fetus for life outside the womb.

Fertilisation

Is the fusion of 2 haploid gametes (egg and sperm) to form a single diploid zygote cell.

-          The male inserts the penis into the female vagina and a muscular contraction (ejaculation) pushed semen from the urethra into the vagina.

-          The sperm swim, using the movement of their flagella, through the cervix into the uterus and onto the fallopian tube - oviduct.

-          Fertilisation occurs in the oviduct when one sperm penetrates into the egg - Recall this is dependent on the ovulation cycle.

Sperm can last up to 5 days in the reproductive tract, but 5 days in more typical

4 steps of fertilisation

1. The sperm uses enzymes to dissolve and penetrate the protective layer of the egg to reach the cell membrane

2. Molecules on the sperm surface bind to receptors (specialised proteins) on the egg’s cell membrane to ensure that a sperm of the same species fertilises the egg, then the nucleus of the sperm enters the cytoplasm of the egg

3. Changes at the surface of the egg occur to prevent the entry of multiple sperm cucile into the egg

4. Fusion of the haploid egg and sperm nuclei result in a diploid zygote cell - fertilised egg.

Implantation

After fertilisation, the zygote continues to travel down the oviduct. The process of embryonic development has already begin with a stage called cleavage (rapid cell division by mitosis. This continues as it travel down the oviduct. Day 4-5 it is referred to as a morula. When the embryo reaches the uterus it is known as a blastocyst (small ball of cells. Mitotic division continue and cells begin to differentiate. Implantation occurs at about day 8-9. The outer layer  of cells send out finger like projections into the wall of the uterus and this area develops into the placenta. The inner mass of cells gives rise to the embryo. The umbilical cord attaches the placenta to the fetus, which will remain attached until after the birth.

The zygote is the first stage of development. It is a diploid cell resulting from fusion of haploid egg and sperm. The embryo is the second stage of development. The fetus is the third stage of development, following the embryonic stage when all the major structures of the adult mammal have begun their development.

Pregnancy and Birth

Placental mammals require complex hormonal coordination to ensure the development of the embryo and foetus. Some hormones promote pregnancy and others terminate the pregnancy or promote the birthing process:

Pregnancy occurs when the embryo implants into the uterine wall. It is controlled by the secretion of progesterone by the corpus luteum in the 1st 3 months and then later by the placenta.

Birth

The chemical regulators of the birthing process include oxytocin, oestrogen, progesterone and prostaglandin

-          After 9 months, the baby is fully grown and stretches the walls of the uterus – placing a strain on both mother and infant

-          This stress induces the release of chemicals which trigger a rise in the levels of oestrogen (estriol in particular)

-          Oestrogen prepares the smooth muscle of the uterus for hormonal stimulation by increasing its sensitivity to oxytocin

-          Now that the uterus is primed for childbirth, the brain triggers the release of oxytocin from the posterior pituitary gland

-          Oxytocin stimulates the uterine muscles to contract, initiating the birthing process

-          The foetus responds to this uterine contraction by releasing prostaglandins, which triggers further uterine contraction - it also inhibits progesterone secretion - There is a drop in progesterone prior to birth.

-          As the uterine contractions trigger the release of chemicals that cause further contractions, a positive feedback loop ensues

-          Contractions will stop when labour is complete and the baby is birthed (no more stretching of the uterine wall)

Note: When the mother is close to giving birth Relaxin is released, it helps dilate (open) the cervix to allow the passage of the foetus. It also widens the pubic bone and relaxes pelvic ligaments

Summary of hormones involved in pregnancy and birth in mammals

Asexual reproduction in animals

-          All animals are multicellular and most can move around to find mates. However many marine organisms can’t, e.g coral polyps, sponges and barnacles. The ability to move about makes sexual reproduction more common.

-          When mates are not available or rapid identical reproduction is an advantage asexual methods are used by some animals. Sometimes species alternate between asexual and sexual reproduction, gaining advantages from both modes of reproduction while avoiding the disadvantages.

Budding in animals

-          When conditions are favourable the cells of the parent divide by mitosis and grow into a multicellular outgrowth which divides into a smaller but identical individual or bud. This bud detaches from the parent and grows into an adult.

-          Advantage - if there is not environmental change the offspring will always be adapted to the environment. Disadvantage if there is environmental change or disease.

-          E.g jellyfish, hydra and grooved brain coral

-          Image beside shows Hydra budding by producing multicellular outgrowth from the side of the parents body

Other methods of asexual reproduction in animals

-          Fragmentation

-          Fission

-          Parthenogenesis