POLYPLOIDY IN MAMMALS: THE FIRST EVIDENCE OF A TETRAPLOID RODENT

Instituto de Ecología y Evolución, Universidad austral de Chile, Casilla 567, Valdivia, CHILE

Karyotypic and genome-size data provide strong evidence for the first known polyploid mammal, the rodent Tympanoctomys barrerae (Hystricognathi, Octodontidae) from Argentina. This desert specialist has a totally biarmed, 102-chromosome karyotype with pericentromeric heterochromatin and a XY sex determination system. The genome size of T. barrerae is 16.8 pg DNA, doubling the estimates of all other octodontids, and strikingly contrasting with the 6-9 pg DNA recorded for the hystricognath rodents, and for most mammals. The exceedingly large sperm head of T. barrerae and its gametic genome size (9.2 pg DNA) agree with somatic data. Consistent with the fact that polyploidy is expected to increase cell size, exceedingly large liver cells as compared to allied taxa were also recorded. Since diploid number is not exactly twice that of any closely related species living today, chromosome elimination (i.e., in the sex-chromosomes and in the pair with the secondary constriction) is presumed to have occurred during the evolution of this lineage. Although polyploidy in animals is believed to disrupt the balance of X chromosome relative to gene products normally maintained by dosage compensation, disomy for the sex chromosomes in T. barrerae is thought to be either sufficient or the only possibility for a functional tetraploid eutherian to exist. Since fertility barriers likely isolated this tetraploid lineage from its ancestors, instantaneous speciation, although rare is possible in mammals, and a role for doubling series variation in genome size to trigger evolutionary novelties is suggested by this unique tetraploid rodent. Funded by Fondecyt 1970710 and the Fulbright Commission.