evolution - Polymorphism in number of chromosomes?

The answer to this question, saying that Down Syndrome - a trisomy of human chromosome 21 - is caused by de novo mutation (rather than resulting from standing variation) made me think about polymorphisms in the number of autosomes (not so much for sex chromosomes because of dosage compensation). The reason for this question is that I would never have thought that an aneuploidy, in theory, could be a segregating trait because of meiotic barriers.

I found evidence of ploidy polymorphisms in plants (especially their hybrids) that occurs when plants with different karyotypes hybridise [see, for example, Vandenhout et al. (1995) and Husband (2014)]. Additionally, I also found that this also applies to some aquatic vertebrates, however, 'fish' are also known for karyotype diversity (see Zhou et al. (2007) and Zhao et al. (2016)).

In rather sharp contrast to that, mammals are (almost) strictly diploid and most mammal species are characterised by fixed chromosome numbers, even though exceptions exist, i.e in mice and other rodents that exhibit intraspecific variation in diploid numbers - these are created by Robertsonian translovations [Graphodatsky et al. (2011)].

However, all of these are diploid and variation always needs to be expressed as a chromosome number of the form $2n$ because of meiosis. The karyotype of a human with trisomie 21 cannot be expressed in that form as this person has $2n +1$ chromosomes. All cases of aneuploidiy in autosomes I am aware of cause disease. My (related) questions now are:

(1) Are there described cases of non-deleterious aneuploidies in autosomes?

Those cannot be segregating in the populations as aneuploidies are not inherited to children, so more importantly:

(2) Are there indirectly segregating aneuploidies in autosomes (i.e. some sort of heritable trisomy that e.g. result from a trisomy followed by chromosomal rearrangement)?

If so, do any of these also exist in mammals, particularly in humans?

Answer

Good questions, but you make a claim that isn't necessarily true:

However, all of these are diploid and variation always needs to be expressed as a chromosome number of the form $2n$ because of meiosis. The karyotype of a human with trisomy 21 cannot be expressed in that form as this person has $2n+1$ chromosomes.

The emphasized text isn't necessarily true - there are certainly more involved ways to describe the karyotype of a given individual or cell line. For instance, take the GM12878 cell line, likely the most well-defined cell line in terms of genetics, epigenetics, and chromatin biology today. It's karyotype is listed as:

46,XX[23].arr[hg19] 9p13.1(38,787,480-40,911,212)x3

This is a fairly common way of representing changes known as copy number variation, wherein only a small region of a given chromosome is duplicated or deleted. For this cell line, which relatively closely mimics lymphoblastic B cells, the genome is nearly normal (46 chromosomes, XX) with only a small amplification on chromosome 9 (9p13.1(38,787,480-40,911,212)x3). This is a mild example, but smaller copy number variations like this are quite common and can amplify or delete both large chunks or very focal regions of genomes.

But to answer your first question, no, there are no known non-deleterious trisomies in autosomes. Trisomy 13 (Patau's syndrome) and trisomy 18 (Edward's syndrome) children can live to birth, but die within the first few months of life. Unsurprisingly, these chromosomes are also the smallest chromosomes with respect to the number of transcripts that they encode 1. This suggests that the additional amount of genetic material determines the severity of the defects, and this trend also holds true in mice. From a figure caption of the cited paper (which is likely behind a paywall for those without institutional access):

A correlation between degree of aneuploidy and organismal fitness in humans and >mice. (A) The number of known human transcripts/chromosome in humans. Trisomies >below the line develop to birth. Trisomies above the line are embryonic lethal. Only those chromosomes containing the least amount of transcripts survive birth. (B) In mice, survival of the embryo is inversely correlated with the size of the chromosome that is present in three copies. Linear regression analysis fits the data with an R2 of 0.71.

I was unable to find any examples for your second question. Cancer cells can get very messed up in terms of copy number, but I've never seen anything about heritable aneuploidy.