page_title: Transandinomys talamancae
section_title: Karyotype
hierarchical_section_title: Transandinomys talamancae / Description / Karyotype
caption_attribution_description: English: Upper (left) and lower (right) toothrow of Oryzomys talamancae (now Transandinomys talamancae). This is specimen USNM 179601, a subadult female from Cana, Panama.
context_page_description: Transandinomys talamancae is a rodent in the genus Transandinomys that occurs from Costa Rica to southwestern Ecuador and northern Venezuela. Its habitat consists of lowland forests up to 1,500 m above sea level. With a body mass of 38 to 74 g, it is a medium-sized rice rat. The fur is soft and is reddish to brownish on the upperparts and white to buff on the underparts. The tail is dark brown above and lighter below and the ears and feet are long. The vibrissae are very long. In the skull, the rostrum is long and the braincase is low. The number of chromosomes varies from 34 to 54.
  The species was first described in 1891 by Joel Asaph Allen and thereafter a variety of names, now considered synonyms, were applied to local populations. It was lumped into a widespread species "Oryzomys capito" from the 1960s until the 1980s and the current allocation of synonyms dates from 1998. It was placed in the genus Oryzomys until 2006, as Oryzomys talamancae, but is not closely related to the type species of that genus and was therefore moved to a separate genus Transandinomys in 2006.
context_section_description: The karyotype in T. talamancae is variable. Samples from two different localities in Venezuela have 34 chromosomes and a fundamental number of 64 chromosomal arms (2n = 34, FN = 64). Four specimens from another Venezuelan locality each have a different karyotype, with the number of chromosomes ranging from 40 to 42 and the fundamental number from 66 to perhaps 67. The autosomes (non-sex chromosomes) of the 2n = 34 karyotype all have two major arms, but the 2n = 40–42 karyotypes include several acrocentric autosomes, which only have one major arm. The 2n = 34 karyotype includes two large submetacentric pairs, which have two long arms but one distinctly longer than the other, and one pair of subtelocentric chromosomes, with a long and a much shorter arm, but the 2n = 40–42 karyotypes lack the submetacentrics and have another pair of subtelocentrics. Both Robertsonian translocations (fusions of the long arm of one chromosome with the long arm of another and the short arm of the one with the short arm of the other) and pericentric inversions (reversals of part of a chromosome that includes the centromere) are needed to explain the difference between the two groups. Musser and colleagues, in discussing these karyotypes, assumed that the 2n = 40–42 sample was from within a hybrid zone between two karyotypic morphs.
  The karyotype of an Ecuadorean sample from north of the Gulf of Guayaquil is similar to that of Venezuelan animals at 2n = 36, FN = 60; it includes four acrocentric and two subtelocentric pairs and no submetacentrics. In contrast, a sample from south of the Gulf had 2n = 54, FN = 60, including 23 pairs of acrocentrics and four pairs of metacentrics (with two equally long arms). Musser and colleagues termed the difference between the two Ecuadorian forms "impressive" and noted that further research was needed to understand the karyotypic differentiation within the species more fully. Both T. bolivaris and H. alfaroi have more chromosomes and arms, at 2n = 58, FN = 80 and 2n = 60–62, FN = 100–104 respectively. Hylaeamys megacephalus has 2n = 54, FN = 58–62 and the similar Hylaeamys perenensis has 2n = 52, FN = 62; these karyotypes resemble that of southern Ecuadorean T. talamancae.
caption: Upper molars (left) and lower molars (right) of a specimen from Cana, Panama[76]
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