Abrotrichini, also known as the Andean clade or southern Andean clade, is a tribe

Tribe (biology)

In biology, a tribe is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.Some examples include the tribes: Canini, Acalypheae, Hominini, Bombini, and Antidesmeae.-See also:* Biological classification* Rank...

 of rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s in the subfamily Sigmodontinae

Sigmodontinae

The subfamily Sigmodontinae is one of the most diverse groups of mammals. It includes New World rats and mice, with at least 376 species. Many authorities include the Neotominae and Tylomyinae as part of a larger definition of Sigmodontinae. When those genera are included, the species count...

. It includes about fifteen species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 in five genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

, distributed in South America from southern Peru to southernmost South America, including the Patagonia

Patagonia

Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...

n steppes. The earliest known fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s are from the Pliocene

Pliocene

The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

 of Argentina.

Taxonomy

Abrotrichines were universally placed in the tribe Akodontini

Akodontini

Akodontini is the second most speciose tribe of the subfamily Sigmodontinae. It includes at least 106 living species in 19 genera and is distributed mainly in the southern half of South America, with only two genera extending into Guyana and Venezuela . It also includes genera previously placed in...

 until the 1990s, and some were even classified within the genus Akodon

Akodon

Akodon is a genus consisting of South American grass mice. They mostly occur south of the Amazon Basin and along the Andes north to Venezuela, but are absent from much of the basin itself, the far south of the continent, and the lowlands west of the Andes. Akodon is one of the most species-rich...

. Allozyme

Allozyme

Variant forms of an enzyme that are coded by different alleles at the same locus are called allozymes. These are opposed to isozymes, which are enzymes that perform the same function, but which are coded by genes located at different loci....

 studies in the early 1990s first provided evidence for their distinction from Akodontini, and in 1999 a study analyzing sequences of the mitochondrial

Mitochondrial DNA

Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...

 cytochrome b

Cytochrome b

Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.- Function :...

 gene found further evidence for the distinction between Akodontini and this group and proposed the name Abrotrichini for the latter. The name Abrotrichini remained formally unavailable under the International Code of Zoological Nomenclature

International Code of Zoological Nomenclature

The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...

, however, because their proposal had been conditional. Thus, the clade remained without a valid name and for this reason it was included in Akodontini in the 2005 third edition of Mammal Species of the World

Mammal Species of the World

Mammal Species of the World, now in its 3rd edition, is a standard reference work in zoology giving descriptions and bibliographic data for the known species of mammals.An updated Third Edition of Mammal Species of the World was published late in 2005:...

. Other phylogenetic studies, which also incorporated nuclear

Nuclear DNA

Nuclear DNA, nuclear deoxyribonucleic acid , is DNA contained within a nucleus of eukaryotic organisms. In mammals and vertebrates, nuclear DNA encodes more of the genome than the mitochondrial DNA and is composed of information inherited from two parents, one male, and one female, rather than...

 genes, affirmed the distinction between Akodontini and the new group. In 2007, Guillermo D'Elía and coworkers published a full diagnosis of the tribe Abrotrichini, validating the name.

Abrotrichini includes five genera—Abrothrix

Abrothrix

Abrothrix is a genus of rodent in the tribe Abrotrichini of family Cricetidae.It contains the following living species:*Abrothrix andinus*Abrothrix hershkovitzi*Abrothrix illuteus*Abrothrix jelskii*Abrothrix lanosus...

, Chelemys

Chelemys

Chelemys is a genus of South American rodents in the tribe Abrotrichini of family Cricetidae. Three species—Chelemys delfini, Chelemys macronyx, and Chelemys megalonyx—are known, all found in central and southern Chile and Argentina....

, Geoxus, Notiomys, and Pearsonomys—which fall into two major subgroups, one including only Abrothrix and one including the four other genera. Abrothrix has formerly been included in Akodon and includes about nine species, among which are the northernmost abrotrichine, Abrothrix jelskii as well as far southern forms such as Abrothrix lanosus. Within the remaining group, Chelemys is sister to the remaining genera; it includes three species found in central and southern Chile and nearby Argentina. The remaining three genera, each of which includes a single species, are closely related and share fossorial

Fossorial

A fossorial organism is one that is adapted to digging and life underground such as the badger, the naked mole rat, and the mole salamanders Ambystomatidae...

 (digging) habits. Geoxus occurs in central and southern Chile and nearby Argentina, Notiomys in southern Argentina, and Pearsonomys in central Chile.

The tribe is part of the clade Oryzomyalia, which includes most of the subfamily Sigmodontinae

Sigmodontinae

The subfamily Sigmodontinae is one of the most diverse groups of mammals. It includes New World rats and mice, with at least 376 species. Many authorities include the Neotominae and Tylomyinae as part of a larger definition of Sigmodontinae. When those genera are included, the species count...

. Within Oryzomyalia, some studies have recovered Wiedomys

Wiedomys

Wiedomys is a genus of South American rodents in the family Cricetidae.It contains the following species:* Cerrado Red-nosed Mouse * Red-nosed Mouse...

as the closest relative of Abrotrichini, but the basal relationships among the components of Oryzomyalia remain elusive. Sigmodontinae encompasses hundreds of species found throughout South America and into North America. It is one of several subfamilies recognized in the family Cricetidae

Cricetidae

The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice...

, which includes many more species, mainly from Eurasia and North America.

Description

Abrotrichines are small to medium-sized sigmodontine rodents with long and soft, usually gray or brown fur, a short, hairy tail and large, strong feet with well-developed claws. In the skull

Skull

The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

, the snout is long, the interorbital region

Interorbital region

The interorbital region of the skull is located between the eyes, anterior to the braincase. The form of the interorbital region may exhibit significant variation between taxonomic groups....

 hourglass-shaped, and the braincase rounded. The palate

Palate

The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...

 is long (extending back beyond the third molar

Molar (tooth)

Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....

s). There are no grooves in the upper incisor

Incisor

Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...

s and the molars are not hypsodont

Hypsodont

Hypsodont dentition is characterized by high-crowned teeth and enamel which extends past the gum line. This provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cows, horses and deer; all animals that feed on gritty, fibrous material. The opposite...

 (high-crowned). The molars lack many accessory features, in particularly the third upper molar. There are 13 thoracic (chest) vertebrae with associated rib

Rib

In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

s, 6 lumbar

Lumbar vertebrae

The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...

 vertebrae, and 18 to 29 caudal (tail) vertebrae.

A karyotype

Karyotype

A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...

 of 52 chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s (2n = 52), present in several species, has been suggested as a synapomorphy

Synapomorphy

In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...

 of the tribe, but while this possibility is yet to be tested, Pearsonomys is now known to have 2n = 56 and some Abrothrix olivaceus have 2n = 44.

Literature cited

• D'Elía, G. 2003. Phylogenetics of Sigmodontinae (Rodentia, Muroidea, Cricetidae), with special reference to the akodont group, and with additional comments on historical biogeography (subscription only). Cladistics 19:307–323.
• D'Elía, G., González, E.M. and Pardiñas, U.F.J. 2003. Phylogenetic analysis of sigmodontine rodents (Muroidea), with special reference to the akodont genus Deltamys (subscription only). Mammalian Biology 68:351–364.
• D'Elía, G., Luna, L., González, E.M. and Patterson, B.D. 2006. On the sigmodontine radiation (Rodentia, Cricetidae): An appraisal of the phylogenetic position of Rhagomys (subscription only). Molecular Phylogenetics and Evolution 38:558–564.
• D'Elía, G., Pardiñas, U.F.J., Teta, P. and Patton, J.L. 2007. Definition and diagnosis of a new tribe of sigmodontine rodents (Cricetidae: Sigmodontinae), and a revised classification of the subfamily. Gayana 71(2):187–194.
• Musser, G.G. and Carleton, M.D. 2005. Superfamily Muroidea. Pp. 894–1531 in Wilson, D.E. and Reeder, D.M. (eds.). Mammal Species of the World: a taxonomic and geographic reference. 3rd ed. Baltimore: The Johns Hopkins University Press, 2 vols., 2142 pp. ISBN 978-0-8018-8221-0
• Smith, M.F. & Patton, J.L. 1999. Phylogenetic relationships and the radiation of sigmodontine rodents in South America: evidence from cytochrome b (subscription only). Journal of Mammalian Evolution 6:89–128.