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Meiosis and The Mule




The Same Information in Different Places

 Looking a bit more into the genetics involved, horses have 32 pairs of chromosomes while donkeys have only 31 pairs.  When a male donkey and a female horse mate, they produce a mule (a male horse with a female donkey produces a different creature called a hinny).  A mule has 31-paired chromosomes as well as an extra chromosome from its horse mother that is not paired.  Evidence suggest that this extra non-paired chromosome is what makes the mule infertile.  The suggested reason for this is that during meiosis, the extra chromosome would not have a partner to match up with.  When the paired chromosomes are split apart into separate haploid gametes, the split would be uneven, creating sterility.  This conclusion is biologically sound and most evidence supports this conclusion and thus it is assumed to be most likely the case. However, this conclusion is not the only reason. During meiosis all the chromosomes double themselves.  A cell undergoing gametogenesis in a diploid organism, such as horses and donkeys, will become tetraploid during the first part of meiosis.  At this point genetic recombination occurs between the four copies of a given chromosome.  After this, the four copies are eventually split apart to occupy four different cells called gametes.  Each gamete, containing only one copy of a given chromosome, is termed “haploid”.  However, if there is an odd chromosome out when a cell starts gametogenesis, this single chromosome with no sister chromosome can only double to make two chromosomes during meiosis instead of the usual four chromosome copies.  The second reason for sterility is because some genes of the donkey do not correspond to the horse. The matching of genes which are not homologous can only occure with a chromosomal shift or structural omission of some genes, deleting their effect. Thus an awkward match can achieve the same result. These two copies will also be sorted out into gametes.  But, since there are only half as many copies as "normal" diploid chromosomes, only half of the gametes will get the extra chromosome.  That means that 50% of the gametes will be viable, having the complete set of genetic information needed to code for that particular creature. So, since hybrid fertility is clearly possible for those having odd chromosome numbers, Horse-donkey hybrids (mules and hinnies), show a "block" during testicular meiosis at the primary spermatocyte stage which is caused by this incompatibility of synaptal pairing between paternal and maternal chromosomes - resulting in a total arrest of spermatogenesis. In order to understand this problem a bit more, it might be interesting to look into how interbreeding creatures can have variations in chromosome numbers and yet have the same information in those chromosomes.  As it turns out, many interbreeding creatures having different chromosome numbers have undergone chromosomal translocations.  What happens here is that a piece or "arm" of one chromosome becomes detached from its normal position on one particular chromosome and reattached to a different chromosome.  There are several different types of translocations and chromosomal fusions. Sometimes chromosomes can fuse with each other to form much longer chromosomes or they can split at the centromere to form two shorter chromosomes. One such rearrangement is known as a Robertsonian rearrangement and is the result of the fusion of two centromeres into one or the splitting of one centromere into two. A tandem fusion, on the other hand, is a fusion of two chromosomes in which one end of a chromosome fuses with the end or the centromere of another chromosome.

Comparisons between the chromosomal banding patterns of these rearranged chromosomes show that the information is still the same, only rearranged a bit.  Moreover, it appears that certain types of rearrangements are quite group-specific for a certain type of creature.   One type of rearrangement that occurs in one type of animal does not necessarily occur in another type of animal.  The problem with this is that during meiosis the various portions of translocated chromosomes must still match up with their normal counterpart on different chromosomes during meiosis in order for genetic recombination to proceed without lethal mistakes.  Of course, one can only imagine that some types of complicated chromosomal rearrangements would make correct lineups impossible.  Obviously then, if a matching lineup is impossible, interbreeding impossible. However, with some kinds of chromosomal rearrangements, the various segments are still able to pair up and crossover even though there might be more than two centromeres involved.  For example, two chromosomes might only match up half way while the rest of each of these two chromosomes, might match a third chromosome (see visual illustration).6  This would make it possible for various chromosome numbers to maintain the same information and still give rise to viable as well as fertile offspring.  And, various interbreeding creatures do in fact maintain a wide range of possible chromosome numbers in their respective gene pools.  Besides the wild and domestic horse mentioned above, there are several other fairly well known examples. Of particular interest are domestic dogs and wolves of the genus canis.  They have 78 chromosomes while foxes have a varied number from 38-78 chromosomes. The uniformity of chromosome number in canid dogs can be due to free interbreeding over a wide range, whereas foxes live in small family groups and smaller territories so that new arrangements will persist. If the "kind" is penned at the level of the family Canidae, then the implications in terms of the number of animals required to produce the present varieties are not as daunting as many fear. Indeed, numerous chromosome homologies have been identified in animals today.  The differences between many species can often be explained with chromosomal rearrangements, as in the case of kangaroos, where Robertsonian fusions can account for much of the variation that exists between the different kangaroo species.  Types of Chromosomal Alterations    Non-Disjunction:   When complex eukaryotes like humans replicate, specialized cells are created ("gametes", known as sperm and ovae) that have half as many chromosomes as the rest the cells. Instead of having two of each chromosome, these gametes only have one of each chromosome. Gametes from male and female partners fuse and recombine their single chromosomes into pairs of chromosomes again where half of each pair came from a sperm or male gamete and the other half from an ovum or female gamete. On very rare occasions, when new chromosomes are being made, the new copies will fail to separate (or "disjoin") from the original copy. When this happens, one gamete will get an extra copy of the chromosome that failed to disjoin, and the other gamete will get no copy. This is called a non-disjunction.  In such a case if a sperm that retained two copies of a given chromosome fused with an ovum that had one copy of that chromosome the resulting cell would have three copies of that chromosome instead of the normal two copies. Usually, the gain of an extra copy of a chromosome is fatal, and the cell dies, but not always. Another way to increase chromosome number is by duplicating all of the chromosomes in a sort of whole genome non-disjunction resulting in polyploidy. This seems to work alright for plants, but for animals, this is very rare.  However, there are many examples of tetraploid (4n) and hexaploid (6n) insects, triploid and tetraploid fish (Catfish, Cyprinids and Carp), and even a hexaploid fish (a subspecies of the tetraploid Carp). There are also several tetraploid species of claw-toed frogs, and even a tetraploid rat (the red viscacha rat, or Tympanoctomys barrerae).  In the case of this rat species, the number of chromosomes (102) is much lower than one might expect by looking at closely related rat species. Based on these relatives, one would expect the tetraploid rat to have 112 chromosomes, which brings us to the other mechanisms for changing chromosome number.      Robertsonian Fusion:   Robertsonian fusion changes the chromosome number, but not the arm number. When chromosomes line up during meiosis I, a metacentric chromosome lines up with two acrocentric chromosomes (see illustration). For example, the house mouse Mus Musculis has 40 chromosomes, and a population of mice form the Italian Alps was found to have only 22 chromosomes. This population differs slightly from the normal house mouse in morphology as well and is classified as a different species named Mus poschiavanus. Other populations have been discovered with chromosome numbers varying between 22 and 40. For example, over 40 Robertsonian "races" of Mus musculus domesticus have been found in Europe and North Africa.1  The number of chromosome arms are the same and banding studies reveal the genes to be homologous. Obviously, in terms of their relationship, these different species are all one group.  Another example of a Robertsonian polymorphism is the house musk shrew that lives in the central region of West Malaysia and has a variation in chromosome numbers from 36 to 40.  Also, in Southern India and Sri Lanka musk shrews can be found with having between 30 and 32 chromosomes due to Robertsonian-type changes.2 A new Robertsonian translocation has been found in cattle. A bull from Marchigiana breed (central Italy) was found to be a heterozygous carrier of a centric fusion translocation involving chromosomes 13 and 19 according to RBA-banding and cattle standard nomenclatures. CBC-banding revealed the dicentric nature of this new translocation, underlining the recent origin of this fusion. In fact, both the bull's parents and relatives had normal karyotypes. In vitro fertilization tests were also performed in the bull carrying the new translocation, in two bulls with normal karyotypes (control) and in four other bulls carrying four different translocations.3 The significance of centric fusions (Robertsonian translocations) in domestic animals, with special reference to sheep, is also interesting. A study was done that considered 856 ewes with either a normal chromosome number 2n = 54 or carrying one or more of the three different translocations (centric fusions) t1, t2 and t3 in various heterozygous and homozygous arrangements. Rams, which were used in the matings, were homozygous for one of the translocation chromosomes (2n = 52), double heterozygotes (2n = 52), triple heterozygotes (2n = 51), or were carriers of four translocation chromosomes (2n = 50) and five translocation chromosomes (2n = 49). A remarkably even distribution of segregation products was recorded in the progeny of all combinations of translocation ewes plus translocation rams in those groups in which sufficient animals were available for statistical analysis. Forty-eight chromosomally different groups of animals were mated. Further, the overall fertility of the translocation sheep (measured by conception rate to first service via the lambing percentage and number of ewes that did not breed a lamb) was not significantly different from the New Zealand national sheep breeding data where this study was done. In some groups the poorer reproductive performance could be explained by the age structure of the flock and inbreeding depression, which probably affected the performance of some animals. Sheep with progressively decreasing chromosome numbers, due to centric fusion, 2n = 50, 2n = 49 and 2n = 48, were reported. The 2n = 48 category represents a triple homozygous ewe and a triple homozygous ram and is the first report of the viable evolution of such domestic animals. Less than 1% of phenotypically abnormal lambs were recorded in a total of 1,995 progeny born over 10 years. It is now considered that 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.4 A detailed investigation of testicular meiosis in a mule, a hinny and a Przewalski horse/domestic horse hybrid were made. Abnormalities of pairing were observed in the mule and hinny in most germ cells at the pachytene stage of meiotic prophase, and spermatogenesis was almost totally arrested. A few mature spermatozoa were recovered from the ejaculate and epididymal flushings of the hinny. The Przewalski horse/domestic horse hybrid was fertile and showed normal spermatogenesis. Chromosome banding studies showed a close homology between the karyotypes of the Prezwalski horse (Equus przewalskii, 2n = 66) and the domestic horse (E. caballus, 2n = 64), and it is evident that a single Robertsonian translocation occurred that transformed four acrocentric chromosomes of E. przewalskii into two metacentric chromosomes in E. caballus. The investigations showed that a trivalent is formed at meiosis in the hybrid (2n = 65), segregation which gives two classes of genetically balanced spermatozoa. Both of these are capable of producing normal offspring if they fertilize the eggs of a domestic mare.    Tandem Fusion:   Tandem fusion changes arm number and chromosome number. Tandem fusions have been found in some antelope species where a sex chromosome is fused with an autosome. Although rare, one can assume that the organisms probably had a common forerunner. The antelope displaying this fusion range in size from the eland (the largest of all the antelopes) to smaller species such as the sitatunge and the bushbuck. However, they all share common features, such as similar shapes of the horns and stripes on the body which may be prominent as in the case of the Bongo or less prominent as in the case of the eland. Species with this type of fusion are: the eland, bongo, lesser and greater kudu, bushbuck, sitatunge and nilgai (Indian antelope) where the y-chromosome is fused to an autosome. Tandem fusions are also found in Malaysian swamp buffalo and Asian river buffalo. Another very interesting example of this type of fusion is also found in the Asian deer. In the species Muntiacus muntjac, the females have only 6 chromosomes and the males have 7 chromosomes (this is the smallest chromosome number in mammals). However, in a different "species" of the group, Muntiacus reevesi, both the males and the females have 46 chromosomes.  However, banding studies show that the same genetic material is present in both species, the chromosomes in M. muntjac are just fused together to form very long chromosomes. Once again no new information is added, it is just reshuffled, thus providing differential expressions and increased variety.   Translocations:    . A translocation is what happens when two chromosomes that are not part of a pair get stuck together as if they were a pair, and exchange segments. If the segments that get exchanged are large enough most of both chromosomes can move onto one single chromosome.  In this figure, the blue and red figures are cartoons of two different chromosomes. The indentions in their centers represent the "centromeres" where the cell attaches filaments that drag the chromosomes around. When these chromosomes cross-over, the result is two chromosomes of very different sizes. One larger chromosome, contains almost all of the genetic material of the two chromosomes, while the other smaller chromosome contains almost none. In this example, the material from one chromosome has been moved or "translocated" onto another chromosome. In dramatic cases where one chromosome is significantly smaller after the translocation than the other chromosome the smaller chromosome may be lost resulting in a reduced chromosome count Translocations can also lead to reduced fertility.  In humans a well-known example of a translocation of part of chromosome 21 results in Down Syndrome.  Humans with such translocations generally experience many problems as well as reduced fertility depending upon when the translocation occurred and how much genetic information was involved.  However, in some insects and plants that have meiotic drive, healthy, viable, and even fertile offspring can be produced via such translocations.6     Pericentric Inversions:   These provide changes in arm number but not chromosome number. The number of arms depends on the position of the centromere. If it is located at the end, then there is one arm and if in the middle there are two arms. The inversion can change acrocentric chromosomes to metacentric chromosomes. The rodents Neotoma and Peromyscus differ by this inversion.     Paracentric Inversions:   In this type of inversion the centromere is not included. This inversion is relatively uncommon, but has been proposed for some bats, hares and apes.     Drastic Rearrangements:     Under certain circumstances of severe environmental stress, drastic rearrangements can produce greater varieties, which could enhance survival. These changes can be rapid when new adaptive zones are entered (canalization model). Such rearrangements have been proposed for the Spalax mole rat.    Discussion   So, the potential problems with chromosomal rearrangements are obvious.  There can be multiple translocations involving the same chromosome as well as chromosomal inversions and a number of other interesting cuttings and splicings of chromosomes.  These can result in a difficult or even impossible situation where chromosomes in such involved hybrids simply cannot match up properly during meiosis.  This lack of matching capability results in sterility.  For example, if a piece of donkey chromosome is inverted relative to its counterpart in horses, then gene-by-gene pairing cannot occur without elaborate looping and twisting. The chance of a successful cell division is very much reduced. So, the mule cannot make egg or sperm cells. Thus, the mule is sterile.  However, a mule can still be born healthy because such growth of non-meiotic cells occurs via a different process called mitosis.  Mitosis does not have to match things up; it only has to make copies. So, inversions and translocations do not prevent the mule from growing up to be an adult. Rearrangements can and do account for differences in insectivores, bats, primates, marine mammals, rodents, rabbits and hares, ungulates etc. The potential for change certainly exists.  Nevertheless, there are certain barriers that cannot be transgressed.  Such changes, as occur with reshuffling of genetic information, do not add to that information.  The information itself is still the same.  True, there are a few documented cases where mutations do actually result in unique or novel functions that are beneficial to the host, such as the galactosidase evolution experiments performed by B.G. Hall, the nylonase evolution experiments performed by Kinoshita, et. al. and of course the all to familiar everyday examples of antibiotic resistance by bacteria.  Surprisingly though, these very examples of evolution in action are the ones that make the boundaries of evolutionary processes most clearly delineated.     The Key Human-Ape Differences: It is becoming more and more clear that the key functional differences between living things, like humans and apes, are not so much found in protein-coding genes, but in the non-coding regions of DNA once thought to be functionless "junk-DNA" - evolutionary remnants of past mistakes that are shared between various creatures.  This notion is starting to be shed with more and more discoveries that show that many of these same regions are not just functional, they carry the vast majority of the genetic information.  The "genes" that were once thought to be so important for genetic function are turning out to be equivalent to the most low-level basic building blocks within the genome, like bricks and mortar.  Surprisingly, it is the non-coding regions of DNA control what is done with these building blocks - that determine what kind of "house" to build so to speak