Organisms That Reproduce Asexually

Asexual reproduction encompasses the forms of reproduction that involve a single parent, and lead to the creation of a genetically identical progeny. Although, such reproductive processes are observed primarily in unicellular organisms, a wide array of multicellular organisms also exhibit asexual reproduction. The current article provides an elaborate account of the same.
Reproduction is one of the distinguishing features of living organisms, and is essential for the survival of any species. Such perpetuation of species is achieved through two fundamental modes - sexual and asexual. Sexual reproduction involves the formation of a zygote, through the fusion of gametes from two parents. The zygote develops into an embryo, and grows into an offspring. On the other hand, asexual reproduction involves the formation of a genetically identical offspring from a single individual.

The primitive life forms predominantly exhibit asexual reproduction. The ability to exist in two genders evolved with the changing environment. With the increase in complexity of the newly evolved life forms, the dominance of asexual reproduction decreased. Nevertheless, each kingdom encompasses a set of strictly asexual organisms, as well as a set of organisms characterized by the presence of both sexual and asexual phases.

What is Asexual Reproduction?

Asexual reproduction is a reproductive process devoid of gamete formation and fertilization, and mostly proceeds via mitosis. The resultant offspring is a clone of the parent organism due to the absence of genetic exchange. Budding, fission, fragmentation, vegetative propagation, etc. are the various mechanisms of asexual reproduction. Such reproductive cycles complete in less time, as compared to sexual reproduction, which serves as an evolutionary advantage. However, owing to the lack of genetic variation, the ability to adapt to changing environment is limited. In asexual organisms, genetic variations can occur only through random mutations. Being so, they are dependent on favorable mutations to cope with environmental changes.

List of Organisms that Reproduce Asexually

Asexual reproduction is the primary mode of reproduction in archaea, bacteria, and protists. Fungi and plants show a combination of sexual and asexual modes of reproduction. The primary mode of reproduction in animals is sexual reproduction, but certain animals have retained the ability to reproduce asexually. Some organisms where asexual reproduction is the only means for perpetuation, as well as those with a dominant asexual phase have been enlisted below.

Kingdom Archaebacteria

All archaebacteria reproduce asexually through budding, fission, or fragmentation processes. The commonly known ones include:

» Methanogens: Methanococcus jannaschii, Methanocalculus pumilus
» Thermophiles: Pyrolobus fumarii, Sulfolobus solfataricus
» Halophiles: Halococcus thailandensis, Halobacterium salinarum
» Psychrophiles (Cryophiles): Arthrobacter chlorophenolicus, A. crystallopoietes

Kingdom Eubacteria

Although most of the bacteria reproduce asexually, in certain bacteria, genetic exchange occurs through bacterial conjugation and transformation processes.

► Strictly Asexual
» Bacilli: Bacillus licheniformis, Staphylococcus aureus
» Nitrifying bacteria: Nitrosomonas europaea, Nitrococcus mobilis
» Purple sulfur bacteria: Thiocapsa purpurea, Alkalilimnicola ehrlichii
» Purple non-sulfur bacteria: Rhodospirillum rubrum, Rhodocyclus tenuis
» Cyanobacteria: Oscillatoria princeps, Nostoc commune
» Spirochaetes: Treponema pallidum, Leptospira interrogans
» Others: Prosthecochloris aestuarii, Clostridium tetani, Rickettsia rickettsii, Zymomonas mobilis

► Asexual and Sexual
Escherichia coli, Shigella dysenteriae, Chromatium okenii, Chlorobium tepidum, Vibrio cholerae

Kingdom Protista

This is the kingdom that first showed a distinct presence of gametes, and an alternation in the sexual and asexual forms. In protists, reproduction occurs through budding, fragmentation or sexual and asexual spores. However, the asexual phase dominates the life cycle of such protists.

► Strictly Asexual
» Fungus-like protists: Dictyostelium discoideum, Labyrinthula terrestris
» Plant-like protists: Euglena sanguinea
» Animal-like protists: Amoeba proteus

► Asexual and Sexual
» Fungus-like protists: Phytophthora infestans, Pythium insidiosum, Physarum polycephalum, Didymium iridis
» Plant-like protists: Pfiesteria piscicida, Navicula lanceolata, Rhoicosphenia curvata, Dinobryon divergens
» Animal-like protists: Leishmania donovani, Toxoplasma gondii, Cyclospora cayetanensis, Paramecium aurelia

Kingdom Fungi

Fungal reproduction occurs through sexual and asexual spores. The asexual spores germinate and give rise to hyphae in the presence of moisture. It is the deuteromycetes (Fungi imperfecti) that reproduce solely through asexual mechanisms, and belong to the phylum Ascomycota or Basidiomycota.

► Strictly Asexual
» Ascomycetes: Aspergillus niger, Penicillium roqueforti
» Basidiomycetes: Tilachlidiopsis racemosa

► Asexual and Sexual
» Chytridiomycetes: Batrachochytrium dendrobatidis, Allomyces macrogynus
» Ascomycetes: Saccharomyces cerevisiae (Budding yeast), Schizosaccharomyces pombe (Fission yeast)
» Basidiomycetes: Puccinia graminis, Heterobasidion annosum (Spiniger meineckellus)
» Zygomycetes: Rhizopus oryzae, Mucor mucedo

Kingdom Plantae

Asexual reproduction in plants occurs through vegetative propagation and apomixis. Very few plant species reproduce strictly through asexual modes. A majority of them exhibit an alternation of both sexual and asexual reproductive phases.

► Strictly Asexual
Vittaria appalachiana (Appalachian shoestring fern), Dahlia imperialis (Bell tree dahlia)

► Asexual and Sexual
» Algae: Sargassum muticum, Chlamydomonas reinhardtii
» Mosses: Campylopus introflexus, Acrophyllum dentatum, Philonotis tenuis
» Ferns: Dicksonia squarrosa, Asplenium flabellifolium, Hypolepis ambigua
» Gymnosperms: Cycas revoluta, Pinus oocarpa, Juniperus horizontalis, Sequoia sempervirens
» Angiosperms: Solanum tuberosum (Potato), Fragaria virginiana (Strawberry), Taraxacum officinale (Dandelion)

Kingdom Animalia

Animals, the most evolved organisms, perpetuate primarily through sexual reproduction. However, a few of the species, mostly aquatic invertebrates, exhibit the presence of asexual reproduction as well.

► Strictly Asexual

► Asexual and Sexual
» Sponges: Racekiela ryderi, Spongilla lacustris
» Corals: Zoopilus echinatus, Diaseris fragilis
» Sea anemones: Epiactis prolifera, Anthopleura elegantissima
» Starfish (Sea stars): Coscinasterias tenuispina, Stephanasterias albula
» Ringed worms: Sabellastarte spectabilis, Lumbriculus variegatus
» Flat worms: Dugesia dorotocephala, Pseudoceros pardalis
» Insects: Dahlica fennicella (Bag worm moth), Timema morongensis
» Reptiles: Cnemidophorus neomexicanus (New Mexico whiptail lizard)
» Sharks: Instances of asexual reproduction in hammerhead and bonnethead sharks have been identified recently.

Types of Asexual Reproduction

► Budding: This process involves the formation of small outgrowths, called buds, on the parent organism. The bud enlarges, and then separates from the parent after a particular stage of maturity is attained. The separated bud develops into a new organism.
Commonly exhibited in yeasts, bacteria and hydra.

► Fission: In this mode of reproduction, the parent organism grows in size and divides into two or more organisms. Binary fission implies the splitting of parent organism into two new organisms, whereas multiple fission implies a division into more than two daughter organisms.
Commonly exhibited in archaea, bacteria, yeasts and protists.

► Gemmulation: It is the process of formation of an internal bud comprising a specialized mass of cells. The internal buds then germinate under favorable conditions, and develop into a mature organism.
Commonly exhibited in sponges.

► Fragmentation: This mode is characterized by the development of an entire organism from a fragment of the parent organism. The parent body breaks into two or more parts which develop into new organisms.
Commonly exhibited in flatworms, sea anemones, mosses and ferns.

► Sporulation: This form refers to the formation of asexual spores, followed by their release and dispersal. These spores germinate and develop into a new organism.
Commonly exhibited in algae and fungi.

► Parthenogenesis: It is the unique process wherein a female gamete develops into an embryo without fertilization with the male gamete. However, an exceptional fact about this process is that the formation of female gametes can occur through mitosis or meiosis, and hence the resultant offspring may or may not be genetically identical to the parent organism.
Commonly exhibited in aquatic invertebrates, arthropods and reptiles.

► Apomixis: It is a process specific to plants, and refers to the development of seeds without fertilization. Such seed development may be through parthenogenesis or through embryo development from ovarian cells.
Only exhibited in plants.

► Vegetative propagation: The various forms of asexual reproduction in plants is collectively termed vegetative propagation/reproduction. It involves the formation of specialized structures like bulbils, stolons (rhizomes), bulbs, suckers, etc. In certain plant species vegetative reproduction occurs through the formation of buds on leaves, stems or roots.
Only exhibited in plants.

Asexual reproduction is the primary mode of reproduction in lower life forms like bacteria, algae, protists, etc. On the other hand, higher organisms either show only sexual reproduction or a combination of both sexual and asexual reproductive processes. Each mode of reproduction has its own set of unique features that provide an evolutionary advantage to the organism.