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Mystery among rodents

Apparently identical species differ in the number of chromosomes

How is it that animals that are physically the same can have different numbers of chromosomes? To understand this peculiarity of Brazilian wildlife, Maria José de Jesus Silva, of the Biosciences Institute of the University of São Paulo (USP), landed at the University of California, Berkeley in the United States, where she will do her post-doctorate. Her research includes a new species of rodent that she discovered in an expedition to the Mato Grosso, in 1997 – the Akodon sp.n. It is 10 to 12 centimeters long and it is, as she herself concluded, the wild rodent with the fewest chromosomes yet described: just 10. Her finding deserves attention because there is no morphological difference between it and the standard Akodon, until then the mouse with the fewest chromosomes (from 14 to 16). These are the so-called cryptic species, only differentiated by the cariotype  , the set of chromosomes.

In Berkeley, in order to understand the evolution of the chromosomes of these animals, she will work using a supplementary approach, analysis of mitochondrial DNA, found in the mitochondria, one of the compartments of cells. Chromosomal, molecular, and bio-geographical data are essential to establish evolutionary theories”, she says.

There are similar phenomena in other groups of animals. In mammals, the species with the fewest chromosomes is the Indian muntjac deer (Muntiacus muntjak vaginalis): the females have six chromosomes and males seven. Comparison with the closest species, the Chinese muntjac (Muntiacus reevesi), which has 46 chromosomes, suggests that rearrangements and mergers may have taken place during the course of chromosomal evolution. In ants, the number of chromosomes ranges from 2 to 94. While theories are sought to explain the size of the variations, the discoveries pile up. A short while ago, another new species of rodent, of the genus Oryzomys, was discovered in the Baturité Ridge, in Ceará. It has 76 chromosomes.

The project
Cytogenetic and Molecular Studies in Oryzomys Rodents (Sigmodontinae, Rodentia) of the Atlantic and Amazon Forests (nº 99/08156-0); Type Support for research project; Coordinator Maria José de Jesus Silva – IB-USP; Investment R$ 29,760