Reproduction
Reproduction is the biological process that turns one organism, or two, into offspring. Every living thing on Earth descends from a chain of it stretching back roughly 3.5 billion years, to the last universal ancestor of all present life. The puzzle is not that it happens. The puzzle is why it happens the way it does. Biologists call sexual reproduction a major puzzle, and for good reason. It carries a two-fold cost: only 50 percent of organisms reproduce, and each parent passes on only 50 percent of its genes. So why do most animals and plants bother with it, when a single organism can simply copy itself? Why would evolution favor fewer offspring over more? What does sex buy a species that cloning cannot? And could life exist at all without reproduction? These are the questions this documentary follows, from binary fission in bacteria to mouse pups born from two fathers.
Bacteria divide via binary fission, splitting cleanly into two. That is asexual reproduction in its starkest form: one organism creating a genetically similar or identical copy of itself, with no genetic material from any other. It is not limited to single-celled life. Cloning an organism is a form of it. The cloning Hydras, invertebrates of the order Hydroidea, reproduce by budding, and so do yeasts, growing a copy off their own bodies. The ant species Mycocepurus smithii is thought to reproduce entirely by asexual means. Most plants can reproduce asexually too. Parthenogenesis is one route, the growth and development of an embryo or seed without fertilization. It occurs naturally in water fleas, aphids, some bees and parasitic wasps, in some reptiles and fish, and very rarely in domestic birds. In lower plants the same phenomenon goes by another name, apomixis. Viruses sit at the strange edge of this picture, hijacking host cells to produce more viruses. Many organisms keep both options open: hydra, yeast, and jellyfish can reproduce asexually yet also reproduce sexually when the moment calls for it.
Gametes are specialized reproductive cells carrying half the number of chromosomes found in somatic cells. They are made by meiosis, and a sperm cell fertilizing an egg cell of the same species produces a fertilized zygote whose traits come from both parents. Most organisms form two different types of gamete. In these anisogamous species, the male produces sperm or microspores and the female produces ova or megaspores. Isogamous species break that neat division. Their gametes are similar or identical in form, and because they look alike they generally cannot be classified as male or female. The green alga Chlamydomonas reinhardtii has gametes labeled simply plus and minus. Stranger still, some fungi and the ciliate Paramecium aurelia have more than two sexes, called mating types. Bryophytes show how unequal the two phases of sexual life can be. The larger, commonly seen organisms are haploid and produce gametes; the diploid stage is relatively small and short-lived. The advantage of diploidy, heterosis, exists only in that brief diploid generation, yet bryophytes keep sex anyway. That hints sexual reproduction offers something beyond heterosis: genetic recombination, a wider range of expressed traits, and a population better able to survive environmental variation.
Allogamy is the fertilization of flowers through cross-pollination, when a flower's ovum is fertilized by spermatozoa from the pollen of a different plant's flower. Pollen reaches the female gamete through the pollen tube, carried by pollen vectors or by abiotic carriers such as wind. Also known as cross fertilization, it stands opposite the methods of self-fertilization. Autogamy is one of those methods, occurring in hermaphroditic organisms where both fused gametes come from the same individual. Many vascular plants do this, as do some foraminiferans and some ciliates. The term sometimes describes self-pollination within the same flower, distinguished from geitonogamy, the transfer of pollen to a different flower on the same plant. The masking of deleterious alleles is part of why these arrangements matter. In organisms that alternate between haploid and diploid phases, where recombination occurs freely, that masking is believed to favor the evolution of a dominant diploid phase.
Mitosis and meiosis are the two types of cell division, and they keep different company. Mitosis occurs in somatic cells; meiosis occurs in gametes. Mitosis ends with twice the number of original cells, each carrying the same chromosome count as the parent cell. Meiosis takes a longer road. A diploid cell duplicates itself, then undergoes two divisions across two phases, meiosis I and meiosis II, moving from tetraploid to diploid to haploid. The result is four cells, each with half the chromosomes of the parent. Animals, including mammals, run this machinery inside their gonads, the testicles in males and the ovaries in females. Sperm are produced by spermatogenesis, eggs by oogenesis. During gametogenesis in mammals, numerous genes encoding DNA repair proteins show enhanced or specialized expression. Male germ cells in the testes carry out special repair during meiosis, including homologous recombinational repair and non-homologous end joining. Oocytes wait in the primordial follicle in a non-growing, prophase-arrested state, yet they too can perform highly efficient homologous recombinational repair of double-strand breaks. That protection of the genome is the quiet job of meiosis that gets passed to the next generation.
Scientific research is investigating same-sex procreation, offspring with equal genetic contributions from two females or two males. The obvious approaches, drawing a growing amount of activity, are female sperm and male eggs. The work has moved in recorded steps. In 2004, by altering the function of a few genes involved with imprinting, Japanese scientists combined two mouse eggs to produce daughter mice. In 2010, American scientists used genetically manipulated stem cells to produce viable mouse offspring carrying genetic contributions from two fathers. In 2018, Chinese scientists created 29 female mice from two mothers, but could not produce viable offspring from two fathers. The barrier finally gave way in 2023, when Japanese scientists created mouse pups from two fathers that grew into adulthood. Researchers note there is little chance these techniques would be applied to humans in the near future.
George C. Williams reached for lottery tickets to explain why so many species pay the high cost of sex. Asexual reproduction, he argued, produces little or no genetic variety, like buying many tickets that all share the same number, which limits the chance of producing surviving offspring. Sexual reproduction was like buying fewer tickets with a greater variety of numbers, and therefore a greater chance of success. The lottery principle is less accepted these days, because asexual reproduction turns out to be more prevalent in unstable environments, the opposite of what the analogy predicts. The numbers behind these strategies are stark. A rabbit, mature after 8 months, can produce 10-30 offspring per year; a fruit fly, mature after 10-14 days, can produce up to 900. These are the poles of r-selection, many offspring, and K-selection, few. The human and the northern gannet sit far at the K end, slow to reach maturity and sparing with offspring, able to pour resources into each one. Species also differ in rhythm. Polycyclic animals reproduce intermittently across their lives. Semelparous organisms reproduce only once, like annual grain crops and certain salmon, spider, bamboo and century plant species, often dying shortly after. Iteroparous organisms, like perennial plants, reproduce in successive cycles and survive many seasons. Some hedge their fertility further: honey bees and fruit flies retain sperm in a process called sperm storage.
Abiogenesis is the biological study of how the origin of life produced reproducing organisms from non-reproducing elements. Whether or not there were several independent abiogenetic events, biologists believe the last universal ancestor to all present life lived about 3.5 billion years ago. Scientists have speculated about making life non-reproductively in the laboratory, and several have produced simple viruses from entirely non-living materials. But viruses are often regarded as not alive: nothing more than a bit of RNA or DNA in a protein capsule, with no metabolism, able to replicate only by hijacking a cell's machinery. A truly living organism with no ancestors is a far harder task, yet a synthetic genome has been transferred into an existing bacterium, replacing its native DNA and producing a new M. mycoides organism. Debate followed over whether that cell counts as completely synthetic, since the chemically synthesized genome was an almost 1:1 copy of a natural one, placed in a naturally occurring recipient cell. The Craig Venter Institute keeps the term synthetic bacterial cell, while clarifying it does not consider this creating life from scratch, but rather creating new life out of already existing life using synthetic DNA. Venter plans to patent his experimental cells, stating that they are pretty clearly human inventions. The researchers propose stretching the boundaries between life and machines until the two overlap to yield truly programmable organisms, a goal they call relatively close in reach and cheap compared to the effort needed to place man on the Moon.
Common questions
What are the two forms of reproduction in biology?
The two forms of reproduction are asexual and sexual. In asexual reproduction, one organism creates a genetically similar or identical copy of itself without another organism. Sexual reproduction combines the genetic material of two organisms through meiosis and the fusion of gametes.
Why is sexual reproduction considered a puzzle for biologists?
Sexual reproduction is a puzzle because it carries a two-fold cost: only 50 percent of organisms reproduce, and each organism passes on only 50 percent of its genes. It also requires far more energy than asexual reproduction and diverts organisms from other pursuits.
What is the difference between mitosis and meiosis in reproduction?
Mitosis occurs in somatic cells and produces twice the number of original cells, each with the same chromosome count as the parent. Meiosis occurs in gametes and produces four cells through two divisions, each with half the chromosomes of the parent.
How do K-selection and r-selection reproductive strategies differ?
K-selection produces few offspring with more resources devoted to each, as seen in the human and the northern gannet. R-selection produces many offspring, as seen in a fruit fly that can produce up to 900 offspring per year, though most do not survive to adulthood.
Have scientists created mice from two fathers?
In 2023, Japanese scientists created mouse pups from two fathers that grew into adulthood. Earlier, in 2010, American scientists used genetically manipulated stem cells to produce viable mouse offspring carrying genetic contributions from two fathers.
What is the lottery principle in sexual reproduction?
The lottery principle is George C. Williams's analogy that asexual reproduction is like buying many tickets with the same number, while sexual reproduction is like buying fewer tickets with a greater variety of numbers and a greater chance of success. It is less accepted today because asexual reproduction is more prevalent in unstable environments, the opposite of what it predicts.