Monday, October 17, 2011

Chromosome fun

Someone asked me the other day how our ancestors made the transition from having 24 pairs of chromosomes to 23 pairs of chromosomes. I didn't have a great answer then. What I presume would have happened is this sequence of events: One individual in the population has a fusion between two chromosomes (just one of the two pairs probably) in a germ line cell. This single mutation then spreads through the entire population. Either by drift, or by selection, if it was favored. I would guess that while this was happening, the progenitor chromosomes 2p and 2q, were probably present in most individuals initially as well. My guess is that the fused chromosome 2 rose in frequency in the population to fairly high levels, and that 2p and 2q dropped out of the population after that.

Note that chromosome fusion is not unprecedented in other species. The poster child of chromosomal fusion is the muntjac, particularly the Indian Muntjac (Muntiacus muntjak). The female of this species has the lowest chromosome number of any mammal (6) while the male has 7. This extreme reduction in chromosome numbers has resulted from several serial fusions of chromosomes. Take a look at this phylogeny by Wang and Lan (2000), which shows the diploid number of several Muntjac species, going from the probable ancestral state of 2n = 46 to the current number of 2n = 6 in female Indian Muntjacs:

Amazingly, Indian Muntjacs can produce viable hybrids with Chinese Muntjacs, which have the ancestral 46 chromosomes, and that partial spermatogenesis followed, indicating that they may be partially fertile. In my brief foray into the internet, I was unable to find anyone who had posited why these species have had such dramatic reductions in chromosome number.

Besides these mammalian representatives, even weirder games with chromosomes are played by certain Middle Eastern Toads, which turn out to be triploid rather than diploid. Triploidy isn't actually that uncommon in nature. In fact, some fisheries will induce triploidy in trout by using high pressure to produce sterile fish that they can then stock in waterways without worrying about them spreading. A common means by which triploid species are made is by hybridization. Improper meiosis results in a normal haploid gamete meeting an abnormal diploid gamete. The resulting individual may grow up fine, but it is mostly out of luck when it comes to sexual reproduction, because there is difficulty in the chromosomes lining up and dividing properly. Yet all is not lost. These species could simply just clone themselves by a process called parthenogenesis. A particular remarkable example of this is found in Ambystoma salamanders of the Jefferson-Laterale complex. Oftentimes these species are "sexual parasites" of their parental species. They still require mating as a stimulus, but do not incorporate their partners sperm into their offspring. Sometimes they sexually reproduce with a normal individual, or another hybrid, but the result is an even greater increase in ploidy. Consequently, you can find triploid, pentaploid, heptaploid, and nonaploid salamanders.

But what about the toads? It turns out they've gotten over the problem sexually reproducing. These triploid toads are sexually reproduce and have both males and females. Essentially, one of the chromosomes becomes like a mitochondria, it's only inherited on the maternal line, while the other pair are passed on normally. Therefore the egg is 2n, while the sperm is only n. Here's a recent story on this system.

It's true that a lot of diseases can be caused by chromosomal mutations such as this, but occasionally it appears it isn't that big a deal. It's also certainly possible that some of these mutations were at some point adaptive, and that facilitated their spread through the population. Pretty amazing stuff really.


  1. I thought this was a very interesting post, and Im glad someone wrote about it, as we spent little time discussing this in class. I had no idea that chromosome fusion occurred in other species than human, not to mention to the extent that it has in the Indian Muntjac. Even more interesting than that was the possibility of these creatures producing viable offspring with the Chinese Muntjac, which has such a different chromosome makeup. How does crossover occur during meiosis? This would be a very interesting diagram to study, and much could be learned from this process. I am aware of the process that led to human chromosome fusion, and I wasn't to surprised to find out that it occurs in other mammals, but it seems extreme that so much fusion could occur in one species as has occurred in the Muntjac. I also found the information in the latter half of the post to hold some interesting information. Amphibians really do seem to be a diverse and extraordinary group, especially when studying evolution. It seems there is no limit to the possible size, shape, coloration, habitat, life cycle, and gene composition of this diverse group.

  2. Wow this is really fascinating. I, like Dan had no idea that chromosome fusion could take place in animals other then humans but it definitely makes sense seeing as any species with chromosomes should have some chance of this occurring. My question is how would it be possible that the Indian Muntjacs and Chinese Muntjacs can produce partially viable hybrids if the Indian munjacs have 6 or 7 chromosomes and the chinese munjacs have 46 chromosomes. Would it be similar to the case of breeding between a horse (64 chromosomes)and a donkey (62 chromosomes), in which the offspring has a combination of the two and ends up with 63 chromosomes? And even then how is there a possibility for viability?

    Thanks again for the post it was very intriguing!

    -Lauren Wolf

  3. After reading your post the first question that came to my mind was, what is the benefit of animals decreasing their number of chromosomes by chromosomal fusion? Is it favored by natural selection and does it increase their fitness?

    I also did some research and found an article about different types of animals, mostly focusing on sheep, which are decreasing their total chromosome amount through centric fusions. The article states, “This is the first report of viable evolution of such domestic animals.” What amazes me is that these animals overall are not sterile and can reproduce just like the Muntjacs and the Middle Eastern Toads discussed. This directly goes against the theory that when two species of animals with different amounts of chromosomes mate, then they create a sterile offspring as seen with the production of the mule from a horse and a donkey. Also when comparing this to the human population, humans often miscarry if the developing embryo has a different amount of chromosomes than the “normal” amount and those that survive usually only have abnormal amounts in respect mostly to sex chromosomes (an extra X or Y may be present or the absence of a second sex chromosome). Those who survive with abnormal autosomal chromosomal amounts usually show dramatic effects; for example an additional 21 chromosome causes Down Syndrome. It is very interesting how different species are affected by the addition or subtraction of their total chromosomal amount.

    The article concludes, “There is little or no evidence to suggest that centric fusions in a variety of combinations affect the total productive fitness of domestic sheep. It is suggested that future research should be more actively directed to understanding their genetic significance.” I think it would be very fascinating to explore and research why this chromosomal change has such a positive effect on certain species yet a negative effect on others.

  4. The article I obtained my information from is :