Considering its significance to life on Earth – it literally made every single human – sexual reproduction remains something of an enigma. The great majority of plants and animals use it to produce young, and on the face of it the advantages are obvious: offspring produced sexually contain a mixture of genes inherited from both parents. If mates are chosen wisely, weaker genes can be paired with better ones, creating a vigorous variety that improves survival chances. But exactly how such a system evolved is a mystery. The first sexual organisms would be easily outcompeted by the extant asexual ones, who passed on all their genes at every generation and were able to reproduce without the need to find a mate.
Nevertheless, sexual reproduction is the dominant strategy among animals and plants. That has meant most life forms are divided into two types, or sexes: male or female. Sexual reproduction requires that the sexes work together to produce young.
Before the advent of sexual reproduction, all life forms would have reproduced asexually. That means all offspring have just one parent, and all individuals are capable of reproducing by themselves. Asexual reproduction is quick and efficient and allows a single individual alone to populate an empty habitat. The simplest method of asexual reproduction is binary fission – or put more simply, splitting in two.
Only single-celled organisms can breed by binary fission: it is used by all bacteria and other prokaryotes plus many unicellular eukaryotes. The process involves a cell division similar to mitosis (see here): The organism’s genetic material is duplicated, the cell doubles in volume, and then divides into two new cells. The concept of parent and offspring breaks down at this point. One cell could be regarded as the original, but in general the parent cell is said to produce two daughter cells. In optimal conditions, a bacterium can perform a binary fission every 20 minutes, and so one cell grows into 5 sextillion (5 x 1021) in 24 hours.
Binary fission is the simplest form of reproduction, used by single-celled organisms that grow in number by dividing in two, to make two identical copies.
The daughter cells of a binary fission contain 100 per cent of the genes of their parent cell, creating minimal variation between one bacterium and the next. The advantages of rapid reproduction must be weighed against this lack of variety – the population can grow at an exponential rate but any attack may result in a mass die-off of similar proportions: if one bacterium is killed by the threat then so will all the rest. To counter this problem, bacteria have evolved a form of genetic transfer called conjugation.
This involves a donor bacterium transferring DNA to a recipient. Only a small portion of the donor’s genome is transferred, in the form of a plasmid, or loop of DNA. The donor connects to a recipient via a pilus, a hairlike extension of the cell membrane that pulls the cells together so they can form a temporary connection. Conjugation only takes place when the recipient does not contain the plasmid already. This ensures that the process always results in the spread of genes.
Conjugation is sometimes called bacterial sex. It involves a small ring of DNA being transferred from one cell to an unrelated neighbour.
While binary fission results in two identical daughters, another form of asexual reproduction produces a clear distinction between parent and daughter. This process, known as budding, is not limited to unicellular organisms: simple multicellular organisms, such as corals, flatworms and sponges do it as well. As the name suggests, budding does not simply involve a parent dividing in two: instead, the offspring develops as an outgrowth or bud on the parent’s body.
When it has reached a large enough size to live independently, the bud breaks off. This daughter is smaller than its parent, and will continue to grow to reach a mature size before it starts producing buds of its own. Understanding how animals can grow an entirely new body in this fashion may have implications for using human stem cells (see here) to heal injuries. Fragmentation is another alternative form of asexual reproduction, used by worms and some starfish – the parent breaks into several pieces, each of which grows to a full size.
A hydroid, a relative of jellyfish, reproduces by simply growing and releasing a section, or bud, of its body into the water. This then becomes a new individual.
Asexual reproduction involves a simple replication of genetic material that is passed to the next generation in its entirety. Sexual reproduction requires an offspring to receive genetic material from both parents. Mendel’s Law of Independent Assortment tells us that all alleles are passed on independently of each other, so this precludes the idea that a male parent provides one half of the alleles while the female provides the other half. Instead, both sexes provide a full set of alleles, and the offspring’s cells therefore contain a double set.
This concept is summed up by the principle of ‘ploidy’. An asexual organism is monoploid, which means it has one set of alleles in its cells. Sexual organisms are diploid; they have two sets of alleles, but for the purpose of sexual reproduction, the double sets are segregated again into single sets. The result of this segregation is a sex cell, or gamete, containing just one set of alleles. Perhaps confusingly, gametes are sometimes described as haploid, implying half the usual (diploid) number of alleles.
The haploid number is the number of chromosomes needed to carry a full set of genes. Most body cells are diploid, meaning they have two full sets of chromosomes.
The male gamete, or sex cell, is the sperm. Typically, it is a highly mobile cell, propelled by a single flagellum. A surprising number of organisms use this kind of actively swimming ‘motile’ sex cell – not only animals, but also mosses, ferns and some coniferous plants. Flowering plants and fungi produce non-motile sperm – in the case of plants they are encased in structures like pollen grains. These still move, but must rely on alternative means of transport (see here).
The difference between the sexes is encapsulated in a comparison of a sperm with its opposite number, the ovum or egg. A sperm can travel Herculean distances if required. However, it carries only a tiny cargo – a haploid set of genetic material. When it meets the egg, that load is transferred inside, and the sperm’s job is complete. This is the defining contribution of the male sex to reproduction, and it means males can produce vast quantities of sex cells at minimal cost in terms of biological resources and energy.
The sperm cell is remarkably similar across the animal kingdom – even some simple plants produce the same kind of swimming cell.
Also known as an egg cell, the ovum is the female gamete. It could hardly be more different from the male sperm – a sperm is around 0.05 mm long, including its long tail, while a human ovum is pretty typical at around 0.1 mm wide and just about visible to the naked eye. The cells found in bird and reptile eggs are enormously larger than this.
The greater size indicates the purpose of the ovum. Like the sperm, it is haploid and contains half a full set of genes in its nucleus. The sperm’s genetic load passes to the ovum, and, the ovum also carries all the nutrients and cellular equipment required to power the growth of a new individual. This material is stored in the cell’s voluminous cytoplasm, termed specifically the ooplasm, but often better known as the egg’s yolk. As in any cell, the ooplasm is surrounded by a membrane, but there are further layers around the cell that provide protection and are there to receive the successful sperm – and ensure that no extra rivals get in.
The egg cell, or ovum, contains the material needed to become the first cell of a new organism once it receives chromosomes from a sperm.
Gametes are the only haploid cells in an animal body, and therefore can only be produced by a special kind of cell division called meiosis. This converts one diploid cell into not two, but four haploid cells. Meiosis occurs in the gonads, or sex organs. The female gonad is almost universally termed the ovary, while the male one goes by various names (testis, in the case of humans).
The same spindle machinery used in mitosis to separate genetic material (see here) is at work in meiosis, but with one crucial difference: meiosis is really two divisions. The first division makes two haploid cells. It does this by grouping the chromosomes into homologues – pairs of chromosomes that carry the same alleles, each one originally inherited from either parent. The first division separates the homologous pairs, while the second division pulls the chromosomes (doubled into chromatids) apart as in mitosis. The result is four daughter cells with half the number of chromosomes of the parent cell.
Sex cells are produced by cell division called meiosis. It differs from mitosis because it reduces the number of chromosomes in cells by half.
Meiosis results in the chromosomes inherited from one parent being thoroughly shuffled with the set inherited from the other. This is done at the level of the chromosome during the first meiotic division. The homologous pairs are split randomly, so paternal and maternal chromosomes can end up together in the cells that result. However, there is a further shuffling of genes that takes place between homologous chromosomes in a process called chromosomal crossover.
Crossover occurs when homologous pairs are lined up ready for the first division of meiosis. At this stage the chromosomes are made up of two identical chromatids, and chromatids from adjacent chromosomes become entwined. Where they cross over, chunks of chromosome are cut and swapped with the neighbour. This results in the chromatids on each chromosome – once identical – now carrying different genes. In the end, the four haploid cells produced by meiosis will contain a unique version of each chromosome.
Crossing over during meiosis results in chromosomes from parents mixing their genes to make unique combinations that are then passed on to offspring.
The formation of gametes is only the first stage in the creation of offspring by sexual reproduction. For a new individual to be formed, a male and female gamete must fuse in a process called either conception or fertilization.
For conception to take place, the gametes need to come together at the same place at the same time. Humans make use of internal fertilization via the tried and tested method of copulation (many other animals do this as well with a great variety of techniques). Fish, frogs and many invertebrates rely on external fertilization, where sperm and eggs are mixed together outside the body. Higher plants have a passive transfer of gametes in the process of pollination (see here)
At the cellular level, one sperm fertilizes one egg. The sperm burrows through the ovum’s outer layers and gives up its chromosomes, making the cell diploid. It is now a ‘zygote’, the first cell of a new, unique individual.
Sperm compete to reach the egg first and burrow inside. Only one will make it.
The embryo is the earliest stage of an organism’s development. It typically involves a period of rapid growth leading to the organism becoming able to live independently – at which point it hatches out or is born. A different approach is used by plants, where the embryo is contained in the seed. The main growth and development of the plant does not begin until after the seed germinates, sprouting into an independently living individual.
All embryos begin with a single cell, known as the zygote. This is the diploid product of the fusion of two haploid sex cells. Using energy stored in the yolk or ooplasm, the zygote divides by mitosis, rapidly forming a ball of cells. In animals this ball is called the blastula, and from here the cells begin to differentiate into the different layers and tissue types that will make up the eventual animal body. Plant embryos contain an embryonic stem called the hypocotyl, a root or radical, and one or two nutrient-packed embryonic leaves, called cotyledons.
The concept of ‘stem cells’ is becoming a familiar one, promoted as an exciting new medical tool with the potential to rebuild damaged and diseased body parts. This is possible because all bodies are constructed from stem cells in the first place. Any complex organism is composed of many different cell types that are specialized to perform particular jobs. Once specialized, a cell and its descendants cannot be deployed to another role. Only a stem cell is able to change its function.
The zygote is said to be a ‘totipotent’ stem cell. That means it is able to specialize into any cell type – and to produce more totipotent stem cells. The embryo grows from these totipotent cells, which specialize through successive levels to produce the many cell types in the body. A fully grown adult body also contains stem cells. These are said to be ‘pluripotent’, meaning they cannot be used to build a new embryo, but they can develop into almost any cell type already in the body.
A mature body grows from a ball of stem cells, each one able to develop into any body part.
A multicellular body is a mass of genetically identical cells that are cooperating with each other. Each cell is differentiated in some way, meaning it performs a specific role in the body by deploying a particular set of its genes. There are three main cell types in a body: germ cells, somatic cells and stem cells. Stem cells create the other two types, germ cells develop into gametes, and somatic cells make up everything else.
Somatic cells arise from a cascade of stem cell divisions. A totipotent cell divides into pluripotent cells, which in turn produce ‘multipotent’ cells. These have the potential to become one of an entire class of related somatic cells – different kinds of blood cell, for example. There may be more stages where the potency of a cell diminishes further, until it arrives at a specific type of somatic cell (a red blood cell, for example). Somatic cells are generally incapable of dividing themselves (liver cells are an exception), and so new ones can only be produced by the action of stem cells.
In biological terms, tissues are one way of understanding the different systems at work in a body. A tissue is a group of cells that all originate from the same source – a particular kind of stem cell – and which are all using the same genetic instructions to carry out a particular job in the body. Examples would include the muscles, the lining of the gut and the vessels that run through a plant’s stem and leaves.
Ignoring simple organisms like sponges, nearly all animal tissues are derived from three layers of cells that form right at the beginning of an embryo’s development. (Plants also have a three-layer development, although it is unrelated.) The ectoderm, or outer layer of cells, develops into nervous tissue, including the brain, skin, teeth, hairs and sweat glands, etc. The mesoderm, or middle layer, becomes the connective tissues, such as bone, blood vessels, cartilage and muscle. Finally, the endoderm, the inner layer, forms the internal organs, such as the lungs, digestive tract and liver.
Tissues are studied using microscopically thin slices.
An organ is the next stage up in complexity from a tissue. Put simply it is a collection of distinct tissues that are massed together to carry out a particular core function in the body. By this definition, a plant’s organs would be its roots, stem, leaves and flowers, each comprising a collection of different tissues. In humans, we often refer to the vital organs – the brain, heart, lungs, liver and kidneys – without which sustained life becomes impossible. All animals have some kind of analogous organ performing the same role as each of our vital ones. (For example, fish have gills instead of lungs, while insects excrete not with kidneys but via organs called malpighian tubules.)
As well as the so-called vital organs, a body has many others – the nose, eyes, various glands and, of course, gonads. It is sometimes more helpful to understand individual organs as core components of wider body systems, such as the nervous system, digestive system, circulatory (blood) system and so on.
Most of the human’s vital organs are contained in the torso, but they are all under the control of impulses from the brain.
The life cycles of the majority of animal life involve ‘oviparity’. Oviparous organisms develop entirely outside of the body of the mother: the most obvious examples are birds or reptiles that lay their eggs after they have been fertilized internally. The embryo does not really start to grow until it leaves the mother.
A simpler, more primitive version of oviparity is used by fish, frogs and aquatic invertebrates. It involves external fertilization where the female releases her eggs and the male then times the release of his sperm to mix with them as effectively as possible. The fertilized eggs may be simply allowed to float away, or be left stuck in a safe place. However, one or both parents often offer some kind of protection. The external fertilization system makes the male the last parent on the scene – the female can slip away and leave the male ‘holding the babies’, so to speak. As a result, male fish and frogs are frequently the primary carers of young, the opposite scenario to oviparous organisms that use internal fertilization.
The development of an oviparous embryo takes place entirely within a self-contained vessel called an egg. The terminology gets confused here: The egg in question is composed mainly of the constituents of the ovum, but it now contains the zygote. Most reptiles and their evolutionary descendents (including the monotreme mammals, such as the duck-billed platypus) lay what we might easily recognize as an egg – a tough shell containing a yolk. The shell has formed around the egg cell, or ovum, and in effect the whole thing is still one giant cell, vast in comparison to regular body cells. The shell makes the egg waterproof, so it can retain its yolky fuel supply in arid land habitats. However, the shell is permeable to air – the embryo inside needs oxygen to get in – and so shelled eggs cannot survive underwater. The opposite is generally true of unshelled eggs laid by the great majority of oviparous animals. Insects coat their eggs in a waxy sheath to stop them drying out, but in general these eggs have to be underwater, or at least kept moist, for successful development.
The alternative to laying eggs is ‘viviparity’, where the embryo develops inside the mother and is born without the protection of an egg, at a comparatively advanced stage of development. Mammals are the masters of viviparity, although scorpions, sharks and a few lizards do it as well, though not in exactly the same way.
In fact, there is an important distinction between viviparity and ovoviviparity, a halfway house between egg laying and live birth in which eggs are retained inside the mother for safekeeping but receive no direct nourishment from her. Sometimes the eggs hatch inside the mother, but the young remain inside, and may eat their brothers and sisters. This kind of cannibalistic viviparity is seen in large sharks. Another source of nutrition for the young is oophagy, where they are fed on a supply of infertile eggs produced by the mother’s ovaries. The final kind of viviparity involves nutrition supplied from the mother’s body via a placenta or similar organ, as seen in humans.
The period of time when an embyro is developing inside the mother of a viviparous animal is called the gestation. In mammals, we use the word ‘pregnancy’, but this describes the state of the mother – gestation refers to the activity of the embryo. Non-mammalian creatures that carry young are seldom called pregnant. Instead, they are said to be ‘gravid’.
An embryo gestates inside a space called the uterus. This is generally an enlarged section of the oviduct, the tube connecting the ovary to the genital opening. The embryo inside is supplied with nutrients from the mother. Scorpions do this by developing outgrowths from the uterus, called diverticula, that connect to the mother’s intestines, collecting nutrients that secrete through the uterus wall. Most mammals connect the embryo to the mother’s blood supply by a placenta. This organ develops from the blastula alongside the embryo. The uterus of marsupials is too small for a working placenta, so their young complete their gestation inside an external pouch.
The 40 weeks of human gestation see a tiny ball of cells grow into a body capable of surviving outside the mother.
A single pregnancy may involve multiple gestations, and result in more than one offspring being born. In smaller mammals this is the norm, with the female releasing several eggs at once from the ovary. Virginia opossums give birth to 50 or more babies at once – although the mother can only suckle a maximum of 13, so most die straight away. The offspring in such a litter are fraternal, meaning they are born at the same time but developed from different zygotes. This means they are no more closely related than brothers and sisters born at a different time. In human terms, two babies born this way are fraternal twins – although they might be brother and sister!
However, it is also possible for multiple births to arise from a single fertilized egg – a single zygote. For example, armadillo mothers habitually produce identical quadruplets from a single zygote that splits, or cleaves, early in its development. When this happens in humans it creates identical twins, which are genetically identical and therefore always have the same sex.
Seen in all female placental mammals, the oestrus cycle’s purpose is to prepare the uterus for receiving a fertilized egg and supporting the ensuing embryo. In the first week of development, the zygote and blastula float inside the uterus. After that period, having reached a larger size, the ball of cells implants in the wall of the uterus, where it builds a placental connection to the mother’s blood supply.
The oestrus cycle ensures that the egg ripens and emerges from the ovary – an event called ovulation – at just the right time so that when it arrives in the uterus, the lining has thickened up ready to receive it. If the egg has met a sperm on its journey to the uterus, the resulting zygote will pause the cycle and keep everything ready for the arrival of the embryo. If fertilization does not take place, the egg decays and the lining of the uterus is shed in a period of menstrual bleeding. Then the cycle begins again with the ovary ripening a new egg and the uterine lining thickening once more.
Deriving from the Greek for ‘virgin birth’, parthenogenesis is a form of asexual reproduction seen in plants, many invertebrates, fish, amphibians and reptiles. A few freak occurrences have been noted in birds but it has never been recorded in mammals. Parthenogenisis uses the mechanism of sexual reproduction, but the young are produced without the need to fertilize the eggs. This is possible when the meiosis process that produces eggs (see here) only produces two daughter cells – both diploid. The precise steps vary, but at some point pairs of haploid cells merge back into a single diploid one.
Some species can only reproduce by parthenogenesis. All members of the species are necessarily female. However, other species breed parthenogenetically to exploit good conditions, but revert to sexual reproduction at other times. In some systems, a male’s sperm (or pollen) is still needed to stimulate the female’s egg, but the sperm’s genes are not passed on.
An aphid gives birth to a daughter, who already has more identical granddaughters developing inside her body.
There are sometimes misconceptions about the word hermaphrodite, and many people think that hermaphrodites can reproduce without the need for sex. This confusion probably stems from the fact that some hermaphrodites are simply able to have sex with themselves. An hermaphrodite is an organism that has both male and female gonads. This is largely the norm for flowering plants but is also the case for some animals.
Snails, slugs and earthworms are examples of simultaneous hermaphrodites – meaning they have both sex organs at the same time, and some (but not all) are able to fertilize their own eggs with their own sperm. Other animals are sequential hermaphrodites, starting out life as one sex and changing to another as they get older and bigger. Clownfish (Amphiprioninae family) begin life as males and become females in later life – which may be why Finding Nemo II has never been commissioned.
A pair of leopard slugs copulating by twisting their penises together.
Pollination is the method of gamete transfer used by higher plants – conifers and flowering plants, which all produce seeds. The male sex cells are housed inside a pollen grain, and it is this body that makes the journey from one plant to the next. Conifers rely on the wind to blow their microscopic pollen grains out of their male cones and into a receptive female cone nearby. Many flowering plants, such as grass and oak trees, rely on wind pollination: their flowers are frequently long, wispy and inconspicuous, built to catch the breeze not the eye. Flowers that rely on insects or other animals to transfer pollen are bright, scented and laden with nectar to attract pollinators.
Once it reaches another flower, the pollen is collected on a tall, sticky receptor called the stigma. To gets its sex cells into the ova, the pollen burrows down to reach the ovary. After fertilization, each ovum grows into an embryo housed within a seed casing, and usually some kind of surrounding fruit that develops from the ovary and remnants of the flower.
In animals, the only cells that are haploid (with half the normal compliment of chromosomes) are the reproductive gametes, but the same is not true of plants. Instead, plant bodies alternate between a diploid body and a haploid one in a phenomenon known as the ‘alternation of generations’. The diploid form is known as the sporophyte, while the haploid structure is the gametophyte.
In higher plants, gametophytes are small and totally dependent on the sporophyte: the male pollen grain and the female ovule (an egg container deep within the flower). But in ‘lower’ plants, such as moss and ferns, the two generations form larger structures. In mosses the gametophyte is the dominant form, while in ferns – the ancestors of ‘higher’ plants – the sporophyte is the main structure. Meiosis within the sporophyte (see here) releases haploid spores that grow into a gametophyte, within which sex cells – sperm and eggs – are produced. In heavy rains, the sperm can then swim to neighbouring plants, fertilizing eggs that grow into the next generation of sporophytes.
The familiar shape of ferns is just one of two body forms grown by these plants.