Changes in chromosome number during evolution

Post of the Month: January 1999

by rwaddle@worldnet.att.net

Newsgroups: talk.origins
Date:       January 12, 1999
Message-ID: 77h72e$q4p$1@nnrp1.dejanews.com

There are two common ways, polyploidy and chromosome fusion, by which chromosome numbers change during speciation. Ploidy refers to the numbers of sets of chromosomes an individual or gamete has. (A gamete is a sperm or ovum or their plant equivalents.) One set is haploid, 2 is diploid, 3 is triploid, 4 is tetraploid, 5 is pentaploid, 6 is hexaploid, etc. Any individual with more than 2 sets of chromosomes is said to be a polyploid. There are two types of polyploidy, autopolyploidy and allopolyploidy. Autopolyploids have 3 or more of the same set of chromosomes. They are common among plants. Those with even numbers of sets of chromosomes produce gametes with full sets of chromosomes and thus are capable of sexual reproduction. They are reproductively isolated from diploid members of the same species, however - mate a tetraploid watermelon with a diploid watermelon and the result is a triploid which produces seedless watermelons.

Alloploids are the result of hybridization between two related species of plant. The hybrid is sterile because the chromosomes from one parent do not pair with chromosomes of the other parent during meiosis. An accidental chromosome doubling in a growing shoot tip, however, can result in cells that have 2 of each chromosome, the total being 4 sets of chromosomes, i.e., tetraploid. In self pollinating plants, a tetraploid stem can produce diploid male and female gametes and thus tetraploid seed. The plants produced from these seeds are a new species.

In plants, polyploidization, particularly allopolyploidization, appears to be the major way - perhaps the only way - chromosome numbers increase in evolution. Allopolyploids behave as diploids during meiosis. Thus they are also called amphidiploids. The difference between a tetraploid amphidiploid and a diploid, however, is that a diploid has 1 pair of every gene, whereas the tetraploid amphidiploid has 2 pairs of every gene. What this means is that one pair of any gene can maintain the original function for that gene while the second pair is free to mutate. Out of these mutations potentially can come new genes with new functions. Thus over evolutionary time, what was once an amphidiploid may become simply a diploid. Moreover, indications of ancestral polyploidy may be obliterated by chromosomal rearrangements and by chromosome fusion.

In chromosome fusion, 2 chromosomes are fused together to make one chromosome. The classic example is the Robertsonian translocation. The chromosomes of most eukaryotic organisms have a single centromere with the genetic material in arms at either side. Acrocentric chromosomes have a short arm with very little genetic material and a long arm with most of the functioning genes. Metacentric chromosomes have centromeres in the middle with arms of nearly equal length. As described in at least some of the older textbooks, a Robertsonian translocation results when the short arm of one acrocentric is replaced by the long arm of another acrocentric chromosome. The resulting metacentric is nearly the same length of both acrocentrics combined and carries all the essential genes of both.

Ironically, Robertson never had the means of determining if chromosomes actually fused in the manner in which he described. The stain used for staining the chromosomes he examined did not reveal much detail in chromosome structure. In the late 70's and early 80's, some textbooks authors took to calling Robersonian fusions "centric fusions". But at least one auther has used "centric fusion" as a term for a process whereby the centromeres of each acrocentric were supposed to have broken in half. The resulting metacentric thus was produced by the fusion of the 2 centromere halves carrying the long arms of each acrocentric. When new staining techniques revealed the structure of human chromosome 2, however, what was revealed was a fusion that was neither Robertsonian nor centric. For human chromosome 2, the tips of the short arms of two acrocentric chromosomes were broken off. The 2 short arms then fused together. This resulted in a chromosome with 2 centromeres, one of which is suppressed.

Another known type of fusion is one in which the long arm of one acrocentric chromosome is broken off and added to the tip of the long arm of a second acrocentric. This appears to be the primary mode of chromosome reduction of Indian muntjacks. Their chromosomes are primarily the result of repeated tandom long arm fusions.

Do true Robertsonian translocations exist? I don't know. I would guess they do but I have not seen demonstration. Many writers use the term but it may be that they are using the term to mean any kind of chromosome fusion. If any of the many chromosomal fusions that are known to exist have been determined to be classic Robertsonian, I would appreciate being so informed. The same goes for centric fusions. The textbooks that describe them give no specific examples, nor do they list references that I might check. My suspicion is that centric fusion started as some textbook author's speculation which became a "fact" when repeated by other authors.

By whatever name, chromosome fusion nicely accounts for reduction in chromosome number in animals as well as in plants. But what about increases in chromosome number in animals? Polyploid animals don't exist, do they? Well yes, they do, though apparently not in mammals. It is possible in birds - a picture of a triploid rooster graced the cover of Science many years ago. It was rather sickly, and it is unlikely that viable, fertile tetraploids are possible. Triploid lizards do exist. There are several species of all female whiptails in the American Southwest, at least some of which are triploid. They reproduce by parthenogenesis. Some "Amazon" mollies are also triploid. They apparently arise by alloploidy. They reproduce by parthenogenesis but sperm from males of one of the parent species are necessary to stimulate egg development. I don't know that any tetraploid species of fish have been found but at least a few give evidence of being diploidised decendants of ancient tetraploids.

It is in the amphibians that polyploidization goes frog-wild. All amphibian polyploids are autopolyploids. A few salamanders are triploid, but a fairly large number of tailless amphibians, especially in South America, are tetraploid or even octoploid. In the US, the two species of treefrog, Hyla chrysocelis and Hyla versicolor, are identical in appearance and occupy the same range. They can be distinguished by their calls, however, with tetraploid H. versicolor having a slower trill than H. chrysocelis.

There are at least a few polyploid invertebrate species, some or all of which reproduce parthenogenically. Two polyploid insect lines have been produced in the laboratory. Triploid fruit flies are not hard to make. One needs only the appropriate diploid stocks and a bit of knowhow. Keeping triploids in stock, however, is a real chore. Triploid fruit flies are always female. They are very poorly fertile and, when mated to diploid males, produce a mix of triploid females, diploid females, diploid males, metafemales and intersexes. To keep the stock going, one selects out the few triploid females that are produced, puts them on fresh medium with their brothers and hopes they produce enough triploid progeny to keep the stock going.

In wasps and bees, females are diploid and males are usually haploid. Certain diploid gene combinations, however, produce males. These males produce diploid sperm in the same manner that haploid males produce haploid sperm. Many years ago, some geneticists mated diploid male parasitic wasps to normal diploid females. Fertilization of haploid eggs with diploid sperm produced triploid females. These were crossed back to diploid males and a few tetraploid females were produced. Voila, the start of a tetraploid line!

In mammals, birds, reptiles and a fair number of invertebrates, new sexually reproducing species cannot arise by chromosomal doubling. Does this mean that polyploidy has not been an important mechanism in animal evolution. Not necessarily. In his 1970 book Evolution by Gene Duplication (which apparently went unread by the textbook authors) Susumu Ohno argued that polyploidy was the mechanism that effected increases in chromosome number in our invertebrate, fish and amphibian ancestors. If so, then it was very important indeed.

Are there mechanisms other than polyploidy that can effect increased chromosome numbers in animals? Most genetics textbooks mention chromosome fission, usually along with chromosome fusion. But chromosome fusion is much easier for organisms to accomplish than fission. All chromosomes require a centromere and the specialized genetic material at the tips called telomeres. It's not hard to understand how two chromosomes can lose a centromere and two telomeres between them and combine into a single chromosome. But where might a single chromosome obtain the centromere and telomeres so as to split into two chromosomes? Many textbooks claim that fission occurs by the splitting of a metacentric chromosome's centromere in half so as to produce two acrocentrics. But they give neither references nor concrete example. Chromosomes so produced would each be missing an arm and the necessary telomere to cap it. Surely they are making a claim which is not true.

With some digging, one can find explanations that make sense. One is specific to species in which the Y chromosome determines male fertility but not sex, as in fruit flies. This involves translocation between autosome and Y chromosome followed in future generations by deletion of most of the Y's genetic material. Another explanation has a large metacentric chromosome excanging arms with a tiny metacentric chromosome - a "microchromosome" - which has hardly any functioning genes to speak of.

Microchromosomes are common in birds and their numbers appear to be inherited rather willy-nilly, i.e., they vary in number from individual to individual within species. In birds perhaps, the microchromosome mechanism might work. But what about mammals which generally don't have microchromosomes? How did the rhinoceros get its 94 mostly acrocentric chromosomes? If it was by fission of a smaller number of metacentrics, then by what mechanism? Or could the mammals have evolved from polyploid ancestors with an even larger number of chromosomes? Or by some mechanism not currently even imagined? It is a mystery.

Floyd

[Return to the 1999 Posts of the Month]