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As discussed in Why is polyploidy lethal for some organisms while for others is not?, polyploidy is normally lethal in mammals.

However, two species of Octodontidae (South American rodents), are tetraploid due to a recent doubling of all chromosomes:

See the paper discussing T. barrerae, the first of these tetraploid rodents discovered.

According to the Wikipedia article, both are believed descended from the same species, Octomys mimax (or possibly now-extinct close relatives thereof), which has 2x = 2n = 56 chromosomes, half those of T. barrerae.

What was special about Octomys that allows it to survive polyploidy, unlike most mammals?

Interesting:

The sperm head of Tympanoctomys is is by far the largest naturally occurring in mammals, and its size is causally related to the double genome size it has to accomodate.

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Actually, that is not what is discussed in the question you linked to. The following is a quote from the very comprehensive accepted answer (emphasis mine):

Polyploidy arises easily in both animals and plants, but reproductive strategies might prevent it from propagating in certain circumstances, rather than any reduction in fitness resulting from the genome duplication.

In fact, try rereading that answer and the references therein, it answers your general question.

There is a popular theory (mostly accepted) that there have been whole genome duplication events (WGDs) in the vertebrate ancestry. If true, it means that all mammals are the descendents of polyploid ancestors. For a very nice review of WGDs in vertebrate evolution, see here [1]. Octomys is simply the only known mammal with a more recent WGD event. As for what makes it special (taken from [2]):

Our data demonstrate that parental-specific silencing of at least one gene and normal X chromosomal dosage mechanism are conserved in the tetraploid genome. We hypothesize a concerted action of genetic and epigenetic mechanisms during the process of functional diploidization of this tetraploid genome.

1) Van de Peer Y, Maere S, Meyer A., The evolutionary significance of ancient genome duplications., Nat Rev Genet. 2009 Oct;10(10):725-32.

2) Bacquet C, et al., Epigenetic processes in a tetraploid mammal, Mamm Genome. 2008 Jun;19(6):439-47.

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  • $\begingroup$ I would say in practice that it is more complex than this. Trichrisomy (extra chromosome) is often (not always) fatal or causes significant negative phenotype (Downs syndrome for instance) in humans. Genome duplication has occurred several times in the course of evolution, but it is still very rare in animals. Plants are much more resilient to polyploidy than animals. $\endgroup$
    – shigeta
    Oct 3, 2012 at 13:10
  • $\begingroup$ @shigeta, true but the main difference here is that polyploidy at the species level (such as WGD) is very different to individual aberrations such as Downs'. Individuals are very sensitive to changes in ploidy, however, changes in gene or even chromosome copy number have occurred multiple times in multiple lineages. Clearly they were accompanied by compensatory changes elsewhere to allow the species to deal with the change in ploidity. I think it is not reasonable to compare trichrisomy or Down's to WGD events. $\endgroup$
    – terdon
    Oct 3, 2012 at 13:43
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    $\begingroup$ the point is taken that while its not disputed that occasionally animals tolerate genome duplication, its still not clear to me that the whole genome duplication is tolerated by all animals. there may be something particularly permissive in these rodents given the frequency seen in that branch... $\endgroup$
    – shigeta
    Oct 3, 2012 at 15:53

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