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This is an extension of this question about What limits chromosomal length?.

I am wondering what could be the specific reasons behind the number of chromosomes an organism carries. In other words, what drives the number of chromosomes and is there anything "stabilizing" the number of chromosomes.

Of course I am not talking about multiple copies of the same chromosome, i.e. trisomy, but rather the reasons behind the partition of the genetic information into a defined number of chromosomes.

From the question I referred to, chromosomes have an upper length limit which is based on their physical size, i.e basically they must be shorter than half of the spindle arm axis and they probably also have a lower limit. Yet it is unclear to me if any non-random equilibrium exists between chromosome number and length other than just being bound by an upper and lower limit in size.

Also let's restrict this question to diploid organisms to avoid organisms such as the Oxytricha trifallax which has 16,000 chromosomes but is also ampliploid or plants such as wheat which can be tetraploid.

I tried to search in the literature and couldn't find a satisfying answer. Also I had in mind the example of the Hela cells which have completely shuffled chromosomes yet still exactly 46.

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cagliari2005
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    With more chromosomes, the cell would need more number of centromeres and telomeres (this unnecessarily increases the size of the genome). Also the metaphase checkpoint has to be more stringent to prevent aneuploidy. This is just a guess though. In general there would be a higher metabolic cost in maintaining a very large number of small chromosomes compared to fewer but longer chromosomes. Additionally meiotic crossover would be relatively difficult with smaller chromosomes – WYSIWYG Jun 12 '15 at 08:16

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Practically, there is an obvious barrier to changes in chromosome number: haploid cells produced by diploids must contain exactly one copy of each chromosome (or chromosome section, or locus). Otherwise over- or under- expression of genes can lead to dosage problems. Across all of diploids some large groups show strong conservation of number of chromosomes but others seem fairly free to change the number.

An example of conservation are the Drosophilids (fruit flies) where for approximately 100 million years they have maintained five or six chromosomes (search for Muller elements). Incidentally, with a huge amount of intra-chromosomal shuffling of genes "Drosophila 12 Genomes Consortium", 2007, Nature.

An extreme example of changes in chromosome number can be seen between similar species of Muntjac deer . Where in a relatively short time period (of the order of a million years or less (?) - my guess), the number of chromosomes has reduced from 46 to six (Wurster, D. H., Benirschke, K., Jun. 1970). Someone may correct me but I think amongst vertebrates, mammals show particularly fast evolution of chromosome number relative to, say, birds (though more birds also have micro-chromosomes...).

The above changes (or lack of change in Drosophilids) appear to be independent of genome size - so that was a long way round to saying "It's not genome size".

It could be repetitive DNA. The centrosomes (where mitotic spindles attach) of mammals are full of repetitive DNA and for this reason have been hard to sequence accurately (Graphodatsky, A. S., Trifonov, V. A., Stanyon, R., 2011. ). These repeats evolve (in sequence and copy number) and may promote more or fewer spindle attachments. Drosophilids (in general) have fewer repeats but I don't know if this is a diploid-wide pattern [Research anyone?].

Dave Gerrard
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  • Very interesting, thanks! Do you have, by any chances, some direct references I can look at? What I would like to know is if it is solely random recombination that drove those changes which would indicate that the number of chromosomes in an organism would change randomly within some size constrains. – cagliari2005 Jun 26 '15 at 00:05
  • I've added a few refs that may serve as starting points. When you say "recombination", do you mean recombination (the normal pairing and crossover of homoologous chromosomes during meisois) or chromosome-rearrangement (mutation leading to novel chromosomal structure)? – Dave Gerrard Jun 29 '15 at 09:33