Ambistomy

Ambistomy, called mole salamanders in English-speaking countries, are endemic to North America , where they are distributed from southern Canada and southeastern Alaska to Mexico . Ambistomes are known for their larval stage - axolotl ( Ambystoma mexicanum ), which was widely used as a laboratory animal in various studies, after which it got to aquarists. Other ambistomes - tiger ( A. tigrinum , A. mavortium ) - are the most common amphibians in many American states, and they are also sometimes kept as pets.

Adults living on land have ambistomes with a wide head, small eyes, a dense stocky body with noticeable bony grooves, thin limbs and a tail that is rounded in cross section. Many species are spectacularly colored: with bright varied shapes and colors (from blue specks to large yellow ribbons) spots on a dark background. Terrestrial adults spend most of their lives under the forest canopy under leaf litter or in burrows that they dig or occupy left by other animals. A number of northern species in the same burrows winter. They live alone and feed on various invertebrates. Adults return to water only in a short period of reproduction, choosing for this the same reservoirs where they were born at one time. Most often this occurs in early spring, but a number of species reproduce in the fall, for example, annular ambistoma ( A. annulatum ) and marble ( A. opacum ).

All types of oviparous eggs, enclosed in several dozens, and sometimes hundreds of pieces in separate bags, are laid in standing or slow-flowing reservoirs, only a marble ambistoma places eggs in various soil depressions on land, which are then quickly filled with water in autumn rains. Aquatic larvae are similar in proportion and composition to adult individuals. They are distinguished from 3 pairs of external gills with 4 pairs of gill slits behind the head. On the gills are located scarlet from a multitude of blood-filled capillaries, filiform gill petals. In addition, from the base of the head to the end of the tail from the dorsal side and from the end of the tail to the cloaca from the abdominal larva, high skin folds are stretched, forming the caudal fin. The tail usually ends with a tail thread. Limbs are present from the moment of birth of the larva with 4 fingers on the front and 5 on the back. The eyes of the larvae are devoid of eyelids and are “un bulged”, “fish”. The general color is usually dim and plain. They swim, bending the body like fish. The larvae of some species (especially the southern populations of tiger ambistomes and related species) are able to grow to adult sizes without undergoing metamorphosis . In the process of metamorphosis, gills and fin folds disappear, the animal sheds, the skin begins to acquire a typical coloring for adults, and eyelids appear in the eyes. The lungs finally develop, preparing the animal for a completely terrestrial existence.

Tailed amphibians of small or medium size. The skin is smooth. There are movable eyelids . Fertilization is internal. Representatives of the family are characterized by biconcave (amphitic) vertebrae, the absence of an angular bone of the skull, transverse arrangement of palatine teeth. The absence of grooves between the nostrils and the edge of the upper jaw distinguishes ambistome from other North American salamanders - pulmonary ( Plethodontidae ). Diploid chromosome set - 28

More detailed morphological features of the family are taken from Duellman and Trueb (1986), Larson (1991) and Larson and Dimmick (1993):

Skull

• paired premaxillary bones (ossa premaxillaria) isolated, do not merge into a single bone;
• nasal bones (ossa nasalia) are paired, symmetrical, each ossifies from one, laterally located focus, a long posterior process of the premaxillary passes between them and completely separates them;
• maxillary bones (ossa maxillaria) are well developed;
• the presence of paired and symmetric cloisonne maxillary bones (ossa septomaxillaria);
• lack of lacrimal bones (ossa lacrimaria);
• lack of squamous bones (ossa quadratojugularia);
• pterygoids (ossa pterygoidea) are presented;
• an opening of the internal carotid artery is present on parasphenoid in some species;
• angular bone (os angulare) merged with the mandibular (mandibula);
• the column (columella) and operculum are present as separate ear bones, separated from the auditory capsule in some species or the column is fused with the capsule in others;
• the opener teeth are transverse; they are replaced starting from the back of the opener;
• teeth with a distinctive crown and enamel;
• the front surface of the muscle that raises the lower jaw (musculus levator mandibulae) includes elements that are extra-occipital in origin.

Inner Ear

• the basilar complex is presented;
• amphibious recess (recessus amphibiorum) in the inner ear is oriented horizontally;
• the auditory vesicle (saccus oticus) is vascularized and filled with calcium salts;
• amphibious perilymphatic canal (canalis perioticus) without fibrous tissue;
• peri lymphatic cistern (cysterna periotica) large.

Torso and limb skeleton

• scapula and coracoid fused to form scapulocoracoid;
• vertebral bodies are amphicel;
• ribs with two heads;
• openings of the spinal nerves are in the neural arches of all vertebrae, with the exception of the spinal nerves extending between the atlas and the first trunk, first trunk and second trunk vertebrae;
• the anterior glomeruli of the kidneys are reduced or absent.

The existence of large larvae in populations makes a number of species an amphibian fully or partially neotenic . Adult individuals in such species do not leave water bodies, retain gills and fin folds, although their lungs also develop, serving as an additional respiratory organ. They reach maturity without undergoing metamorphosis.

Neotenic populations and ambistorant species were originally discovered in the mountainous regions of the United States and on the Central Plateau in Mexico. The conditions conducive to the occurrence of neoteny are significant altitude, the absence of aquatic predators and arid conditions outside water bodies. Most neotenic populations belong to a complex of tiger ambistoma species - Ambystoma tigrinum , A. velasci , A. mavortium and related species.

Fully neotenic species of ambists are called axolotls - A. mexicanum , A. taylori , A. andersoni and A. dumerilii . Neotenics retain an increased ability to regenerate , characteristic of young larvae, and can restore lost limbs, tail and almost any internal organ. (see also Tailed Amphibians - Regeneration)

Ambistomaceae belong to those amphibian groups whose systematics have been substantially revised over the past decades. By the beginning of the 1980s, the family consisted of 35 species and 4 genera - Ambystoma , Rhyacosiredon , Dicamptodon and Rhyacotriton , however, the use of molecular genetic methods in taxonomic studies amended both the classification of the entire group and within the genera. These changes are described below.

Dicamptodon and Rhyacotriton

Pacific ambistomes (genus Dicamptodon ) were formerly included in the family of ambistomes, but now they prefer to separate them out. However, the results of molecular studies indicate their undoubted kinship with the Ambystoma - Rhyacosiredon species (the modern family of ambistomes), which suggests a good monophyletic group (Larson, 1991; Larson and Dimmick, 1993). The genus Rhyacotriton , included in the family in the past, is also related to the listed groups, although this relationship is distant.

Species of Rhyacosiredon

In the past, a group of these species was isolated in a separate genus Rhyacosiredon , common in Central America. But the cladistic analysis of the relationship between the species of Rhyacosiredon and the species of the genus Ambystoma makes the latter group paraphyletic, since some representatives of Ambystoma turned out to be closer to the species of Rhyacosiredon than to other Ambystoma . In accordance with the principles of cladistics , the genus Ambystoma should be broken up, with some of the species included in Rhyacosiredon , but most taxonomists have chosen to preserve Ambystoma by including the species Rhyacosiredon .

A complex of species of tiger ambists

Previously, all these species were considered as one, called the tiger ambistoma Ambystoma tigrinum with an area from Canada to Mexico. Outwardly, all tiger ambistomes are quite similar to each other, have a large head, small eyes and a thick body. Perhaps their structure is the most primitive among the ambistomes. They are also the largest species in the family.

The lifestyle and life cycles of all tiger ambists are also the same. However, it is worth considering the populations most distant from each other, as it becomes clear that we are dealing with albeit close, but different species, thus forming a complex. The problem is that their characters significantly overlap, hybridization between them is possible, which further erodes interspecific boundaries. Despite the differences in the color and morphology of the larvae, it was very difficult to differentiate subspecies based only on morphological characters, not to mention giving a specific status to individual populations. Confusion was also facilitated by the presence of neotenic populations adjacent to normal populations in which large larvae were found.

Nevertheless, initially they tried to solve the problem of significant differences between individuals from distant populations by isolating several subspecies within A. tigrinum . Recently, the status of one of the subspecies, the striped tiger ambistoma ( A. tigrinum mavortium ), was raised to a species, A. mavortium , which includes populations of the tiger ambistoma from the western and central US states. In turn, within this new species, a number of subspecies are also distinguished, which in the future are quite capable of acquiring species status. The California tiger ambistoma was also distinguished as a separate species, A. californiense , and, judging by the data of molecular genetic studies, it is indeed the most isolated from other representatives of the ambistome family. Again, the results of molecular genetic analysis (1997) showed that the Mexican tiger ambistoma should be recognized as an independent species - A. velasci .

Finally, all opinions about the close relationship between the Mexican axolotl and the tiger ambistome are now recognized as erroneous, since these two species were found to be geographically separated by the ranges of many other species of ambistos. Instead, it is assumed that the closest relative of axolotl is precisely the Mexican tiger ambistoma, which also reproduces in the lakes where axolotls live. Perhaps the Mexican tiger ambistoma is the ancestor of a whole group of neotenic species, and, in turn, can itself be a paraphyletic species, and therefore, should be broken down into new ones.

A complex of species Ambystoma laterale-jeffersonianum and interspecific hybrids

The most remarkable result of frequent hybridization between ambisto species in the areas of cohabitation is observed in the laterale-jeffersonianum complex . These two closely related species were separated in the Ice Age, but in the inter-Middle Ages their ranges overlapped, and they could easily interbreed. Jefferson’s ambistoma ( A. jeffersonianum ) and blue-spotted ambistoma ( A. laterale ) were periodically combined into one species due to the existence of these hybrid populations, or were again regarded as independent species.

The point in the debate was the molecular genetic analysis of their isolated populations, proving the validity of the species. Interspecific crossbreeding led to the appearance of two unique, completely consisting of females, ambisto populations. These populations are triploid , as are similar hybrids in some lizards. Silvery ambistoma ( A. platineum ) is actually one of such hybrids of A. jeffersonianum species - laterale . Being represented by individuals of the same sex, silver ambistomes are completely unable to reproduce normally. They reproduce as interspecific hybridization: A. platineum females mate with A. laterale males. But parthenogenesis actually takes place. During the formation of the egg, reduction division does not occur, triploidy absolutely prevents this. Spermatozoa of A. laterale , merging with the egg, only start its development, without transferring its chromosome set to the future embryo, and the laid eggs contain genetic clones of the mother's body. This form of parthenogenesis is called gynogenesis .

It turns out that the entire population of silver ambistoma contains clone organisms and does not exchange genetic materials with A. laterale . Paradoxical as it may seem at first glance, the population is reproductively isolated from the latter species, and therefore can be considered as an independent species. Thanks to hybridization, another exclusively “female” population arose - the Tremblay ambistoma ( A. tremblayi ). Her reproduction also occurs through gynogenesis, but the females are already crossed with males A. jeffersonianum . Individuals of both populations look like a blue-spotted ambisto.

In the tissues of some species (yellow-spotted ambistoma Ambystoma maculatum , etc.), cells of the algae Oophila amblystomatis live. These algae are present under the shell of eggs, in the embryos themselves and even in adults. Inside the amphibian cells where the algae settled, the latter are surrounded by mitochondria. These algae stain eggs and embryos in green. For some reason, the vertebral immune system does not respond to these algae. ^[2]

At the moment, the family includes a single genus with 33 species.

• Genus Ambistoma ( Ambystoma Tschudi, 1838 )

• Mountain Ambistoma ( Ambystoma altamirani Dugès, 1895 ) ( Rhyacosiredon altamirani )
• Spotted Ambistoma ( Ambystoma amblycephalum Taylor, 1940 )
• Chunky Ambistoma ( Ambystoma andersoni Krebs and Brandon, 1984 )
• Ringed Ambistoma ( Ambystoma annulatum Cope, 1886 )
• Ambystoma barbouri Kraus and Petranka, 1989
• Bishop's Ambistoma ( Ambystoma bishopi Goin, 1950 )
• Brilliant Ambistoma ( Ambystoma bombypellum Taylor, 1940 )
• California Tiger Ambistoma ( Ambystoma californiense Gray, 1853 ) - Ambystoma californiense
• Reticulated Ambistoma ( Ambystoma cingulatum Cope, 1868 )
• Ambystoma dumerilii Dugès, 1870
• Speckled Ambistoma ( Ambystoma flavipiperatum Dixon, 1963 )
• Brown Ambistoma ( Ambystoma gracile Baird, 1859 )
• Olive Ambistoma ( Ambystoma granulosum Taylor, 1944 )
• Jefferson's Ambistoma ( Ambystoma jeffersonianum Green, 1827 )
• Blue Spotted Ambistoma ( Ambystoma laterale Hallowell, 1856 )
• Shy Ambistoma ( Ambystoma leorae Taylor, 1943 ) ( Rhyacosiredon leorae )
• Edible Ambistoma ( Ambystoma lermaense Taylor, 1940 )
• Silver Blue Ambistoma ( Ambystoma mabeei Bishop, 1928 )
• Long-toed Ambistoma ( Ambystoma macrodactylum Baird, 1850 )
• Yellow Spotted Ambistoma ( Ambystoma maculatum Shaw, 1802 )
• Ambystoma mavortium baird , 1850
• Mexican Ambistoma ( Ambystoma mexicanum Shaw and Nodder, 1798 )
• Marble Ambistoma ( Ambystoma opacum Gravenhorst, 1807 )
• River Ambistoma ( Ambystoma ordinarium Taylor, 1940 )
• Silvery ambistoma ( Ambystoma platineum Cope, 1868 )
• Ambystoma rivulare Taylor, 1940 ( Rhyacosiredon rivularis )
• Pink Ambistoma ( Ambystoma rosaceum Taylor, 1941 )
• Ambystoma silvense Webb, 2004
• Ambystoma subsalsum Taylor, 1943
• Mole-shaped Ambistoma ( Ambystoma talpoideum Holbrook, 1838 ) ( Mole-shaped Ambistoma )
• Taylor's Ambistoma ( Ambystoma taylori Brandon, Maruska and Rumph, 1982 )
• Short-headed Ambistoma ( Ambystoma texanum Matthes, 1855 )
• Tiger Ambistoma ( Ambystoma tigrinum Green, 1825 )
• Ambistoma tremblayi Comeau, 1943
• Mexican Tiger Ambistoma ( Ambystoma velasci Dugès, 1888 )

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• Larson, A. and WW Dimmick. 1993. Phylogenetic relationships of the salamander families: A analysis of congruence among morphological and molecular characters. Herpetological Monographs 7: 77-93.
• Larson, A., DW Weisrock, and KH Kozak. 2003. Phylogenetic systematics of salamanders (Amphibia: Urodela), a review. pp. 31–108 in Reproductive Biology and Phylogeny of Urodela (DM Sever, ed.) Science Publishers, Inc., Enfield (NH), USA.

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• Names used in Russian literature
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