The mole salamanders (

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

''Ambystoma'') are a group of advanced salamanders endemic to North America. The group has become famous due to the study of the

axolotl The axolotl (; from ) (''Ambystoma mexicanum'') is a neoteny, paedomorphic salamander, one that Sexual maturity, matures without undergoing metamorphosis into the terrestrial adult form; adults remain Aquatic animal, fully aquatic with obvio ...

(''A. mexicanum'') in research on

paedomorphosis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the Physiology, physiological, or Somatic (biology), somatic, development of an organism, typically an animal. Neoteny i ...

, and the

tiger salamander The tiger salamander (''Ambystoma tigrinum'') is a species of mole salamander and one of the largest terrestrial salamanders in North America. Description These salamanders usually grow to a length of with a lifespan of around 12–15 years ...

(''A. tigrinum, A. mavortium'') which is often sold as a pet, and is the official amphibian of four US states.

General description

Terrestrial Terrestrial refers to things related to land or the planet Earth, as opposed to extraterrestrial. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on o ...

mole salamanders are identified by having wide, protruding eyes, prominent

costal groove The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great vessels ...

s, and thick arms. Most have vivid patterning on dark backgrounds, with marks ranging from deep blue spots to large yellow bars depending on the species. Terrestrial adults spend most of their lives underground in

burrow file:Chipmunk-burrow (exits).jpg, An eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of Animal lo ...

s, either of their own making or abandoned by other animals. Some northern species may

hibernate Hibernation is a state of minimal activity and metabolic reduction entered by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It is most ...

in these

s throughout the winter. They live alone and feed on any available

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

. Adults spend little time in the water, only returning to the ponds of their birth to breed. All mole salamanders are

oviparous Oviparous animals are animals that reproduce by depositing fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings kno ...

and lay large eggs in clumps in the water. Their fully aquatic

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e are branchiate, with three pairs of external

gills A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

behind their heads and above their

gill slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In c ...

s. Larvae have large

caudal Caudal may refer to: Anatomy * Caudal (anatomical term) (from Latin ''cauda''; tail), used to describe how close something is to the trailing end of an organism * Caudal artery, the portion of the dorsal aorta of a vertebrate that passes into th ...

fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. F ...

s, which extend from the back of their heads to their tails and to their

cloaca A cloaca ( ), : cloacae ( or ), or vent, is the rear orifice that serves as the only opening for the digestive (rectum), reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles, birds, cartilagin ...

e. Larvae grow limbs soon after hatching, with four toes on the fore arms, and five toes on the hind legs. Their eyes are wide-set and lack true eyelids. The larvae of some species (especially those in the south, and tiger salamanders) can reach their adult size before undergoing

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...

. During metamorphosis, the gills of the larvae disappear, as do the fins. Their tails, skin, and limbs become thicker, and the eyes develop lids. Their lungs become fully developed, allowing for a fully terrestrial existence. Some species of mole salamanders (as well as populations of normally terrestrial species) are

neotenic Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny in modern humans is more signif ...

(retaining their larval form into adulthood). The most famous example is the

. They cannot produce

thyroxine Thyroxine, also known as T4, is a hormone produced by the thyroid gland. It is the primary form of thyroid hormone found in the blood and acts as a prohormone of the more active thyroid hormone, triiodothyronine (T3). Thyroxine and its acti ...

, so their only means of metamorphosis is mainly through the outside injection of it. This usually shortens the lifespan of the salamander.

Tiger salamander complex

Morphologically, tiger salamanders (''Ambystoma tigrinum'' complex) have large heads, small eyes, and thick bodies. This basic morphology is similar across most mole salamanders (genus ''Ambystoma''), though tiger salamanders are among the largest of the mole salamanders, and have relatively large larvae. Tiger salamanders inhabit a wide variety of ecosystems across North America. Given this geographic diversity, subpopulations of tiger salamanders exhibit morphological and behavioral diversity. Whether subpopulations constitute independent species or subspecies within the ''Ambystoma tigrinum'' complex, as well as the driving forces behind diversification, remains an active area of research as of 2024. Several subspecies within the ''Ambystoma tigrinum'' complex have been reclassified as an independent species. For example: # ''Ambystoma mavortium'' (

barred tiger salamander The barred tiger salamander or western tiger salamander (''Ambystoma mavortium'') is a species of mole salamander that lives in lower western Canada, the western United States and northern Mexico. Description The barred tiger salamander typical ...

) comprises former subspecies ''A. t. diaboli'', ''A. t. mavortium'', ''A. t. melanostictum'', ''A. t. nebulosum'', and ''A. t. stebbinsi.'' # ''Ambystoma californiense'' (

California tiger salamander The California tiger salamander (''Ambystoma californiense'') is a vulnerable amphibian native to California. It is a mole salamander. Previously considered to be a subspecies of the tiger salamander (''A. tigrinum)'', the California tiger sala ...

) # ''Ambystoma velasci'' (

Plateau tiger salamander The plateau tiger salamander or Mexican tiger salamander (''Ambystoma velasci'') is a species of mole salamander in the family Ambystomatidae. Description The plateau tiger salamander exhibits facultative paedomorphosis. The adult coloration ...

), which may be paraphyletic and shares habitats with

(''A. mexicanum'')

Hybrid all-female populations

Unisexual (all-female) populations of ambystomatid salamanders are widely distributed across the Great Lakes region and northeastern North America. The females require sperm from a co‑occurring, related species to fertilize their eggs and initiate development. Usually the eggs then discard the sperm genome and develop asexually (i.e.,

gynogenesis Gynogenesis, a form of parthenogenesis, is a system of asexual reproduction that requires the presence of sperm without the actual contribution of its DNA for completion. The paternal DNA dissolves or is destroyed before it can fuse with the egg. T ...

, with premeiotic doubling); however, they may incorporate the genome from the sperm into the resulting offspring. Sperm incorporation commonly takes the form of genome addition (resulting in

ploidy Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

elevation in the offspring), or genome replacement, wherein one of the maternal genomes is discarded. This unique mode of reproduction has been termed

kleptogenesis In biology, a klepton (abbreviated kl.) and synklepton (abbreviated sk.) is a species that requires input from another biological taxon (normally from a species which is closely related to the kleptonic species) to complete its reproductive cycle. ...

by Bogart and colleagues. This is in contrast to

hybridogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

, where the maternal genomes are passed hemiclonally and the paternal genome is discarded every generation before the egg matures and reacquired from the sperm of another species. The nuclear DNA of the unisexuals generally comprises genomes from up to five species: the

blue-spotted salamander The blue-spotted salamander (''Ambystoma laterale'') is a mole salamander native to the Great Lakes states and northeastern United States, and parts of Ontario and Quebec in Canada. Their range is known to extend to James Bay to the north, and ...

(''A. laterale''),

Jefferson salamander The Jefferson salamander (''Ambystoma jeffersonianum'') is a mole salamander native to the northeastern United States, southern and central Ontario, and southwestern Quebec. It was named after Jefferson College in Pennsylvania. It is typically ...

(''A. jeffersonianum''), small-mouthed salamander (''A. texanum''),

streamside salamander The streamside salamander (''Ambystoma barbouri'') is a species of mole salamander from North America, occurring in several Midwestern states of the US. Description The streamside salamander is a medium-sized amystomatid salamander. It typica ...

(''A. barbouri''), and tiger salamander (''A. tigrinum''), denoted respectively as L, J, Tx, B, and Ti. This flexibility results in a large number of possible nuclear biotypes (genome combinations) in the unisexuals. For example, an LJJ individual would be a triploid with one ''A. laterale'' genome and two ''A. jeffersonianum'' genomes, while an LTxJTi individual would be a tetraploid with genomes from four species. Because they have hybrid genomes, unisexual salamanders are a cryptic species with morphology similar to coexisting species. For example, LLJs look like blue-spotted salamanders and LJJs look like Jefferson salamanders. Silvery salamanders LJJ (''A. platineum''), Tremblay's salamanders LLJ (''A. tremblayi''), and Kelly's Island salamanders LTxTx and LTxTi (''A. nothagenes'') were initially described as species. Species names were later dropped for all unisexual salamanders because of the complexity of their genomes. The offspring of a single mother may have different genome complements; for example, a single egg mass may have both LLJJ and LJJ larvae. Despite the complexity of the nuclear genome, all unisexuals form a

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

group based on their

mitochondrial DNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondrion, mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the D ...

. The maternal ancestor of the unisexual ambystomatids was most closely related to the

, with the original hybridization likely occurring 2.4~3.9 million years ago, making it the oldest known lineage of all-female vertebrates. The hybridization was most probably with an ''A. laterale''. All known unisexuals have at least one ''A. laterale'' genome and this is thought to be essential for unisexuality. However, the ''A. laterale'' genome has been replaced several times, independently, in each of the lineages by matings with ''A. laterale''.

Limb regeneration

''Ambystoma mexicanum'', a

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...

with exceptional regenerative capabilities is one of the principal models for studying limb regeneration.García-Lepe UO, Cruz-Ramírez A, Bermúdez-Cruz RM. DNA repair during regeneration in Ambystoma mexicanum. Dev Dyn. 2021 Jun;250(6):788-799. doi: 10.1002/dvdy.276. Epub 2020 Dec 17. PMID 33295131 Limb regeneration involves the propagation of a mass of low differentiated and highly proliferative cells termed the

blastema A blastema (Greek ''βλάστημα'', "offspring") is a mass of cells capable of growth and regeneration into organs or body parts. The changing definition of the word "blastema" has been reviewed by Holland (2021). A broad survey of how blast ...

.García-Lepe UO, Torres-Dimas E, Espinal-Centeno A, Cruz-Ramírez A, Bermúdez-Cruz RM. Evidence of requirement for homologous-mediated DNA repair during Ambystoma mexicanum limb regeneration. Dev Dyn. 2022 Jun;251(6):1035-1053. doi: 10.1002/dvdy.455. Epub 2022 Jan 25. PMID 35040539 During limb regeneration, blastema cells experience DNA double-strand breaks and thus require

homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...

, a form of

DNA repair DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...

that deals with double-strand breaks.

Taxonomy

''Rhyacosiredon'' was previously considered a separate genus within the family Ambystomatidae. However, cladistic analysis of the mole salamanders found the existence of ''Rhyacosiredon'' makes ''Ambystoma''

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, since the species are more closely related to some ''Ambystoma'' species than those species are to others in ''Ambystoma''. The stream-type morphology of these salamanders (which includes larvae and

neotenes ''Neotenes'' is a genus of moths belonging to the family Tortricidae. Species *'' Neotenes astromontana'' Diakonoff, 1972 *'' Neotenes canescens'' (Diakonoff, 1954) See also *List of Tortricidae genera This is a taxonomy of the moth family Tor ...

with short gills and thicker

gular fold A gular fold is a feature of the body of lizards and many other reptiles. It is a granular fold found on the ventral throat In vertebrate anatomy, the throat is the front part of the neck, internally positioned in front of the vertebrae. It con ...

s) may have led to their misclassification as a different genus. The genus name ''Ambystoma'' was given by

Johann Jakob von Tschudi Johann Jakob von Tschudi (25 July 1818 – 8 October 1889) was a Switzerland, Swiss naturalist, explorer, and diplomat. He is known for his travels in South America, his scientific contributions to zoology and anthropology, and his diplomatic se ...

in 1839. Tschudi did not provide a derivation for the name, and many thought that he intended the name ''Amblystoma'', "blunt-mouth." Occasionally, old specimens and documents use the name ''Amblystoma''. Writing in 1907,

Leonhard Stejneger Leonhard Hess Stejneger (30 October 1851 – 28 February 1943) was a Norwegian-born American ornithologist, herpetologist and zoologist. Stejneger specialized in vertebrate natural history studies. He gained his greatest reputation with repti ...

offered a derivation of ''Ambystoma'' based on a contraction of a Greek phrase meaning "to cram into the mouth", but others have not found this explanation convincing. In the absence of clear evidence that Tschudi committed a

lapsus In philology, a lapsus (Latin for "lapse, slip, error") is an involuntary mistake made while writing or speaking. Investigations In 1895 an investigation into verbal slips was undertaken by a philologist and a psychologist, Rudolf Meringer and ...

, the name given in 1839 stands.

Species

The genus ''Ambystoma'' contains 32

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, listed below, the newest being ''A. bishopi''. Some species are

, others are

, and some species have established populations of both neotenic and terrestrial forms. In addition, two groups of unisexual hybrid populations are sometimes named under their own species: *

Silvery salamander The silvery salamander (''Ambystoma platineum'') is a hybrid species of mole salamander from the United States of America and Canada. It is usually between long and slender, with many small silvery-blue spots on its back and sides. It is brownis ...

(''A. platineum'') *

Tremblay's salamander Tremblay's salamander (''Ambystoma tremblayi'') is a member of the family Ambystomidae from the United States of America and Canada. Reaching between , the salamander is long and slender with many bluish-white markings. It is dark gray to gray-b ...

(''A. tremblayi'')

References

*http://amphibiaweb.org/lists/Ambystomatidae.shtml

External links

Tree of Life: AmbystomatidaeIUCN redlist of threatened Ambystomatidae
{{Authority control Vertebrate parthenogenesis