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VARIATIONS IN CHROMOSOME NUMBER
The phylum Ciliophora is a clade of protists united by the possession of cilia and dimorphic nuclei (macronuclei and micronuclei). There are considerable differences in how chromosomes are processed in ciliates (Katz, 2003). In higher eukaryotes, chromosome number often varies between closely related organisms. This may occur through chromosome fission and fusion and the duplication of parts of the genome. In general, there are models in which chromosomes can fuse to reduce diploid numbers and can fission to increase them (Todd, ).
Diploid numbers of chromosomes in the ant genus Myrmecia vary from 3 to 76 and includes ant species with diploid counts of 3, 4, 8, 10, 18, 19, 22, 23, 27, 38, 47, 48, 50, 52, 53, 55, 56, 57, 59, 60, 64, 66, 70, 74, and 76. The number of chromosomes possessing 28S rDNA regions varies from 2 to 19 (Hirae, 1996). The individuals of the marine snail Nucella lapillus can vary in chromosome number between 26-36 and the chromosomes of the Hymenopteran genus Diprion varies in number from 14-28 (Kolnicki, 2000)
The number of chromosomes in the families of ascidians (the most primitive chordates) can vary: 18 and 24 in Clavelinidae; 18, 28, and 40 in Polyclinidae; 8, 30, and 40 in Didemnidae (with 8 and 30 known in the genus Trididemnum), 12, 18, and 28 in Cionidae (with all 3 variations known in the genus Cina); and 4, 16, and 32 in Styelidae (with 16 or 32 known in the genus Styela). It seems as if the ancestral chordates possessed a diploid count of about 24. Hagfish chromosome counts can be 46, 48, 50, or 52. Lampreys of the genus Lampetra can possess 60, 94-6, 142, 146, 156, and 165-74 chromosomes while other lampreys can possess 76, 164, and 168 chromosomes (Chiarelli, 1973).
In cartilaginous fish, chromosome counts of 24, 28-40, 60-72, and 62 are known. In the genus Raja, chromosome counts of 98 and 104 are known. Diploid numbers can vary within families of bony fish. The chromosome numbers of members of family Nototeniidae (Antarctic ice fish), subfamily Nototheninae can be 22, 24, 26, 46, 48, and 50 while those of subfamily Trematominae can be 24 or 48. (Eastman, 1993, p. 118-9). Species in the family Salmonidae can vary in their chromosome number from 36 to 104. Chromosome counts vary from 44 to 104 in Cyprinidae, 18 to 50 in Cyprinodontidae, and 36 to 48 in Poecilidae (Chiarelli, 1973).
In bony fish, chromosome numbers can vary within a genus. Coregonus nasus possesses 58-60 chromosomes (2n), C. peled 74, C. muksun 78, C. zenithicus 80, and C. chadary 80-84 (Phillips, 2001). Prosopium gemmiferum possesses 64 chromosomes (2n), P. abyssicola 72, P. spitonotus 74, P. cylindraceum 78, and P. ooulteri 82. Thymallus arcticus pallasi possesses 98 chromosomes and T. thymallus possesses 102 (Phillips, 2001). In the genus Aphyosemion, diploid counts of 18-30, 20, 22, 28, 32, 34, 36, 38, and 40 are known. In the genus Salmo, counts of 56, 58-64, 60, 64, 70, 80, 84, 96, and 104 are known; counts of 52, 56, 58, 60, and 74 are known in Oncorhynchus; and 36, 48, 72, 80, and 96 in Coregonus (Chiarelli, 1973).
The South American lungfish, Lepidosiren paradoxa, possesses a diploid karyotype of 38 chromosomes and the Australian lungfish, Neoceratodus forsteri, possesses 54 chromosomes. There are four species of African lungfish of the genus Protopterus. P. annectens and P. aethiopicus possess 34 chromosomes and P. dolloi is a tetraploid species with 68 chromosomes.(Morescalchi, 2002; Rock, 1996)
Diploid chromosome numbers can vary in families of amphibians. In caecilians, diploid numbers of 24, 36, and 38 are known in Caeciliidae. In salamanders, chromosome numbers can vary 40-62 in Hynobiidae and 62-64 in Cryptobranchidae. In frogs, chromosome numbers vary 24-38 in Discoglossidae, 22-36 in Pipidae, 24-48 in Hylidae, 20-22 in Bufonidae, 24-26 in Microhylidae, and 14-26 in Ranidae (Chiarelli, 1973). In the frog genus Eleutherodactylus, chromosomes counts range from 18 to 36 (Chiarelli, 1973).
Diploid chromosome numbers can vary in families and genera of reptiles. In the lizard genus Anolis different species vary in chromosome number from 26 to 48 and individuals of the same species may vary with counts of 32, 34 and 36 known in A. latifrons, 34, 36, and 48 known in A. carolinensis, 30 and 40 known in A. chrysolepis, and 36 and 40 known in A. usocoauratus. Chromosome numbers vary from 26-40 in Iguanidae, 34-54 in Agamidae, 24-32 in Scincidae, 38-54 in Teiidae, 30-48 in Anguidae, 30-50 in Amphisbaenidae, 36-44 in Boidae, 38-44 in Elapidae, 24-50 in Colubridae, 52-66 in Trionychidae, 26-34 in Pelomedusidae, 58-68 in Chelidae, and 30-42 in crocodilians. The iguana species Lioiaemus monticola can be divided into a number of races whose chromosome counts vary; the four described races possess diploid counts of 32, 34, 38 to 40, and 42 to 44 (Lamborot, 1998). The diploid number of pleurodire turtle chromosomes ranges from 28 to more than 60 and that of cryptodire turtles ranges from 50 to 60 (Solari, 1994).
Chromosome numbers can vary within a species of bird such as variations of 62-77 in Porphyrio poliocephalus viridis, 56-72 in Larus argentatus, 77-81 in Streptopelia risoria, 70-8 in Otus scops japonicus, 82-4 in Bubo v. virginianus, 72-74 in Coracina melanoptera, 74-82 in Turdus sibiricus davisoni, 82-84 in Zonotrichia albicollis, and 76-78 in Passer domesticus and Passer montanus. These differences occur in the number of minichromosomes found in bird karyotypes which complicate the analysis of karyotypes (Chiarelli, 1973). When comparing all birds, the number of chromosomes varies from 52 to 126 (Solari, 1994).
Marsupials have fewer chromosomes, on average, than placental mammals. More than 90% marsupials have between 14 and 22 chromosomes. The highest number of chromosomes in marsupials is 32 compared to 78 in placental mammals. In the family Didelphidae, 11 species have 14 chromosomes, 2 species have 18 chromosomes, and 6 species have 22 chromosomes. Species of Phalangeridae can have either 14 or 20 chromosomes. Species of Petauridae can have 10, 16, 18, 20, or 22 chromosomes. Species of Macropodidae can have 10, 12, 14, 16, 18, 20 or 22 chromosomes (Stonehouse, 1977; Rofe, 1985). In macropodids, tammar wallabies possess a diploid count of 16 while three rock wallaby species possess diploid counts of 22 (Waugh, 1999).
It is thought that the ancestral diploid count for mammals is 14 from which chromosomal fission created a greater number of smaller chromosomes. Modern mammals possess between 6 and 92 chromosomes. In the insectivore genus Sorex, diploid counts of 42, 52, 58, 66, and 20-31 are known. The insectivore genus Crocidura includes species with 24, 40, and 42 chromosomes (Todd, ; Avivi, 2001).
The four species of blind mole rat of the
have chromosome numbers of 52, 54, 58, and 60 (Avivi,
2001). The diploid count in the
pocket gopher Thomomys talpoides can
be 40, 44, 46, 48, 56, 58, or 60. Species
of mice in the genus Acomys can possess diploid counts of 36, 38, 50, 60, 64, or
66. Species of the South American
(tuco-tucos) vary in diploid counts from 22 to 68. The rodent Reithrodontomys megalotis may
possess 42, 43, 44, 45, or 46 chromsomes. The rodent Sigmodon fulviventer may possess 28, 29, or 30 chromosomes.
Other variations occur in rodents such as the Leggada minutoides-musculoides
species complex (18-34), the ground squirrel Spermophilus townsendi (36-46), Meriones (42-72), Ellobuis (17-56),
and Sigmodon (22-52). In rodents, chromosome counts vary from 32-54
in Sciuridae, 40-78 in Geomyidae,
26 and 46 in the one genus of Echimyidae, 38-72
in Heteromyidae, 48-62 in Gliridae,
32 to 72 in Dipodidae, 62-74 in Dasyproctidae,
18-66 in Cricetidae, and 26-78 in Muridae. The two families of rabbits may possess 42-52
and 38-62 chromosomes. (Chiarelli,
1973). Different races of
the house mouse in a small geographic area are known which have 22, 24,
26, and 40 chromosomes. Forty
five chromosomal races are known in
In Canids, 78 chromosomes represents the diploid count in Canis, 76 in Chrysocyon, 74-6, in Atelocynus, 74 in Speothos, 38 in Vulpes vulpes, 40 in Vulpes rupelli, and 60 in V. bengalensis (Todd, 1970; Kolnicki, 2000). Mustelid chromosomes vary from 38-64 One bear possesses 52 chromosomes while the others all share 74 (Chiarelli, 1973; Todd, 1970). The diploid number of chromosomes in the family Viverridae can be 34, 36, 38, 40, 42, 44, and 52. In the family Herestidae, the diploid chromosome number can be 36, 40, and 44. In Asian species of Herpestes it is 36 while in African species the number varies from 40 to 44. The diploid chromosome number in cats varies from 36 to 38. The five Latin American cats with 36 chromosomes have one unique chromosome that other cats lack (Hunt, from Szalay, 1993). The number of acrocentric and subacrocentric chromosomes can vary within a family: 0-4 in Felidae, 0-22 in Herpestidae, and 2-20 in Viverridae (Hunt, from Szalay, 1993), Most whales and dolphins have 44 chromosomes while sperm, beaked, and right whales have 42 chromosomes (Arnason, from Szalay, 1993). In the family Phocidae of true seals, chromosome number varies from 32 to 34 (Arnason, from Szalay, 1993).
It seems as if the earliest artiodactyls possessed diploid counts of about 14 and higher numbers resulted from chromosomal fissioning. Modern artiodactyl chromosome counts range from 14-74. Within the deer subfamily Cervinae, Cervus nippon can possess 64, 65, 66, 67, or 68 chromosomes; other members of the genus Cervus can possess 58 and 68 chromosomes; and other members of the subfamily possess 56, 68, and 70 chromosomes. Chromosome counts of 32/31, 38, 46, 52, 54, and 60 are known in the subfamily Bovinae. Chromosome counts of 30, 30/31, 34/35, 38, 56, 58, and 60 in the subfamily Antilopinae. Diploid chromosome counts of 38, 39-40, 50, 52, 56, 58, and 60 in Hippotragininae. Diploid chromosome counts of 42, 48, 50, 54, 56, 58, and 60 are known in Caprinae including chromosome counts of 54, 56, and 58 in 4 species of sheep. Rhino counts vary from 82-84. Two species of llama may have both 72 and 74 chromosomes. The horses Equus caballus, E. przewalski, E. asinus, E. onager, E. grevyi, E. burchelli, and E. zebra possess 64, 66, 62, 56, 46, 44 and 42 chromosomes, respectively (Chiarelli, 1973; Kolnicki, 2000; Todd, 1975).
Chromosome number in the bat families vary from 32-42 min Emballonuridae, 40-48 in Molossidae, 56-58 in Rhinolophidae, 16-46 in Phyllosomatidae, 34-42 in Pteropidae, 56-62 in Rhinolophidae, 20-52 in Rhinopomatidae, and 36-44 in Vespertilionidae. Only two species of tree shrew in the genus Tupaia possess the same numbers of chromosomes; the others can vary from 52 to 68 and the closely related Urogale everetti possesses 44. Hapalemur grisaceus can possess 54 or 58 chromosomes. Lemur fulvus fulvus can possess 60 or 48 (Chiarelli, 1973). There are variations in chromosome counts in the four families of lemurs: Lemuridae 44-62, Cheirogaleidae 46-66, Indridae 40-70 and Lepilemuridae 20-38 (Kolnicki, 2000; Pastorini, 2001).
. In the
Chromosomal changes have been implicated in the recent speciation of fruit fly and mosquito species groups. Chromosomal differences between humans and chimps, which include the fusion of two ape chromosomes to form human chromosome 2 and pericentric inversions forming on nine chromosomes to differentiate human and chimps, may have been a factor in the speciation of ancestral hominins (Ayala, 2005).
Chromosome fusions have led to varying numbers of chromosomes within a genus of plant species (Schubert, 2007).
One of the most significant conclusions from the analysis of genomes is the great role played by gene duplications to create gene families (Danielson, 1999). As has been discussed in other chapters, a large percentage of modern genomes appear to be homologous derivatives of ancestral genes which have resulted from multiple duplications and subsequent modification. This process continues today and individuals can vary in their possession of duplicates of individual genes (such as the globins and opsins). These duplications offer an evolutionary opportunity to gain new functions. For example, duplications of ancestral invertebrate immune cascades resulted in the vertebrate lectin and coagulation cascades and duplications of Hox clusters preceded greater specialization of the vertebrate head and limbs. Selection may be accelerated in these duplicates. The evolution of coding sequences in duplicated teleost Hox genes occurred at an increased rate compared to unduplicated sequences, seemingly through directional selection (Prohaska, 2004). Carp are tatraploid and possess two copies of the myc gene. The nucleotide sequence of one is evolving faster than the other, suggesting that it is adapting to a new function (Futami, 2001). It seems that extensive gene duplication occurred before the origin of animals which gave rise to new domains and domain shuffling (Muller, 2001a).
Duplications of the same ancestral genes can occur separately in different lineages. Although both tunicates and vertebrates both have duplicate gonadotropin releasing hormone receptors, they have arisen from independent duplications (Kusakabe, 2002). A number of genes have been duplicated in amphioxus since its separation from other chordate lineages. These genes include a 14th gene in the Hox cluster, a duplicate Evx (whose function has diverged from the standard function of Evx), and a duplicate Emx gene (Minguillon, 2002).
Not only do many genes seem to be present in multiple copies resulting from duplications, entire chromosome segments seem to be duplicated. For example, human chromosomes 7p15-12, 17.q11-22, 12q12-13, and 2q31-34 not only contain homologous Hox clusters, but also EGFR homologs. Human chromosomes 6p21, 9q33-34, 1q22-31, and 19p13 possess homologs of nuclear receptors, vav-like oncogenes, notch-like receptors, pbx genes, tenascin homologs, complement proteins, abl-like kinases, TNF homologs, and MHC I related clusters (Spring, 1997).
Although it is possible that entire regions of chromosomes be duplicated, this can also result from polyploid events which duplicate the entire genome. A great deal of evidence suggests that two such genome duplications occurred before the evolution of the jawed vertebrates from primitive chordate ancestors. Tunicates have 6% DNA content as mammals (Lundin, 1993). The genome of tunicates, about 160 million base pairs, is about 20 times smaller than that of humans. The predicted 15,000-16,000 genes is a number similar to that found in other nonvertebrates (such as fruit flies, 14,000, and nematodes, 19,000). The tunicate genome is intermediate in many ways between invertebrates and vertebrates (Dehal, 2002). . A few hundred tunicate genes are more similar to those of protostomes than to those of vertebrates. Tunicates have 6 FGF genes (compared to 1-2 in protostomes and 22 in mammals), 5 Smad genes (which include TGF-β and bone morphogenetic proteins; 8 genes of this family are known in mammals), and 10 T-box genes (mammals have 18) (Dehal, 2002).
From the analysis of Hox sequences in hagfish, it appears that at least one of the genome duplications early in vertebrate evolution occurred before craniates evolved and it seems that additional duplications occurred in hagfish after this. The second genome duplication event occurred in the gnathostome lineage after its divergence from that of lampreys, although additional duplications may have susequently occurred in lampreys(Stadler, 2004; Hahn, 1998; Escriva, 2002; Hoyle, 1998). The following pattern is frequently observed in comparative genomics in which vertebrates have four homologs of single invertebrate genes.
There are a number of cases in which a single
invertebrate gene is homolgous to four vertebrate
genes. This is observed in Hox clusters, syndecan, myc, BMP (5-8), EGFR/ERBB2-4, ENGR,
The comparative analysis of the genomes of
teleosts and other vertebrates indicate that
genome duplications have occurred at the base of the teleost
lineage. The teleost
lineage was subsequently characterized by substantial gene loss from this
polyploidy event and, at least in the teleost
Fugu, there are relatively few new genes which have evolved
from duplications since then. In
contrast, the lineage which led to humans developed many new genes from
duplications and thus many human genes are younger than those found in
Fugu (Vandepoele, 2004). Additional copies of a number of genes (such
as bHLH, interleukin, and Sox genes) are located
in chromosome regions which seem to have undergone duplication in teleosts (Chiang, 2001; Ledent,
2001; Galay-Burgos, 2004, Bowles, 2000; Magor, 2001).
Some have even proposed that a tetraploidy event occurred in the early mammalian lineage given that reptiles only possess 60-80% DNA found in mammals (Lundin, 1993). Humans possess 46 chromosomes, depicted below.
Is polyploidy possible? There are many modern species which have arisen through more recent polyploidy events. Yeast contains 55 duplicated regions and is also thought to be tetraploid (Ohno from Muller, 1998). The number and arrangement of multiple copies of genes indicate that a tetraploid genome duplication resulted in yeast after the divergence of the genus Saccharomyces from Kluyveromyces (Wolfe, 1997). Polyploidy seems to occur fairly frequently in plants. It is estimated that changes in ploidy are responsible for 2-4% of the speciation events in flowering plants and 7% in ferns (Otto, 2000).
The fish family Salmonidae seems to have arisen after an autotetraploid event and considerable chromosome variation has occurred since. In the family, chromosome numbers vary from 74 to 170 (Phillips, 2001). The common carp (Cyprinus carpio, 2n=104) is a tetraploid species compared to the grass carp (Ctenopharyngodon, 2n=52) (Ohno from Muller, 1998). Most duplicate genes typically lost. Polyploidization is estimated to have occurred 50 million years ago in catostomid and salmonid fish and half the duplicated genes have been lost since (Lundin, 1993). Salmon and catastomids are tetraploid, unlike other fish in their order. (Hoyle, 1998).
In amphibians, Xenopus laevis possesses 36 chromosomes while X. ruwensoriensis possesses 108. Virtually all of the species in the subfamily Xenopodinae are polyploid (Flajnik, 1991; Wu, 2003). In the subfamily Xenopodinae (family Pipidae), the genus Silurana is composed of 1 diploid species (2n=20) and 1 tetraploid (2n=40) species while the genus Xenopus is composed of 10 tetraploid species (2n=36), 5 octoploid species (2n=72), and 2 dodecaploid species (2n=108) (Evans, 2004). In the frog genus Ceratophyrs, C. calcarata possesses 26 chromosomes, C. ornata is known with both diploid (26) and octoploid (104) individuals, and C. dorsata possesses 104 chromosomes. In the genus Odontophrynus, diploid (2n=22) and tetraploid (2n= 44) are known. The same diploid and tetraploid counts are known in the geuns Pleurodema. Diploid and tetraploid species are known in the genus Hyla and Phyllomedusa (Chiarelli, 1973). Triploid chickens are not uncommon and there is one line of chickens which often produces triploid offspring (20% of embryos and 8-12% of hatchlings) (Solari, 1994).
Plant species can vary with respect to their chromosome number within a genus. Haploid number in Chrysanthemum can be 9, 18, 27, 36, and 45; meadow rue 7, 14, 21, 28, 35, and 42 ; roses 14, 21, 28, and 35; solanum (nightshade) 12, 18, 24, 30, 48, 54, 60, and 72 (Marsh, 1991).
When comparing the chromosome numbers of species in the same family, there are a number of cases in which one species has double the chromosome number observed in another. Tetraploidy is one of the mechanisms which could produce this variation.
THE AMOUNT OF DNA
The amount of DNA in a haploid set (measured in picograms) can vary within a family. In fish families it can vary from .77 to 1.4 in Clupeidae, 1.6 to 4.4 in Callichthyidae, 1.7 to 3.0 in Batrachoididae, and .7 to 1.6 in Cyprinodontidae (Hinegardner, 1972). African lungfish have about 80 times more DNA than zebrafish (Sato, 2000) and picograms of DNA vary from 100 to 178 in lungfish (Chiarelli, 1973). In amphibians there can also be considerable variation within a family: 7.4-27.9 in Caeciliidae, 43.7-105.0 in Ambystomidae, 20-62 in Plethodontidae, 28-98 in Salamandridae, 48-189 in Proteidae, 6-13 in Ascaphidae, 10-21 in Discoglossidae, 1.6-8.9 in Pelobatidae, 3-13 in Leptodactylidae, 2-11 in Hylidae, 5-14 in Bufonidae, and 7-16 in Ranidae. In amniotes, the amount of DNA per cell varies from 1.7 to 10.5 picograms (Chiarelli, 1973). The amount of DNA in nuclei ranges from 2.0 to 3.8 picograms in birds (Solari, 1994).
As the size of the genome increases, the amount of time needed for cell division increases as does the volume of the cell. The amount of DNA in the genome varies greatly among frogs of the family Myobatrachidae, ranging from .95 pg in Limnodynastes ornatus to 19 pg in Areophryne rotunda. In salamanders, genome size varies from 13 to 120 pg and in caecilians it varies from 3.7 to 14 pg. Australian lungfish possess a genome of 50 pg while that of the African lungfish is 130 pg. Lamprey genomes vary from 1.3 to 2.1 pg and those of hagfish vary from 2.3 to 4.6 pg. In grasshoppers, genome size can vary between 1.5 and 16 pg and in hemipterans it varies between .2 and 6.2 pg (Gregory, 2002). The difference in genome size in protists can vary by 300,000 times. The genome of snakes and lizards can vary from 1.1 pg to 5.4 pg (Ryan, 2001).
The average primate has a slightly larger quantity of DNA than that found in the human genome, about 108% percent. Tarsiers have more DNA than anthropoid primates or lemurs. Chimps and orangutans possess a greater amount of DNA than do humans. Using the amount of human DNA as a reference (of 100%), artiodactyls possess 99.5% this quantity of DNA, rodents 96%, bats 56%, birds 50%, reptiles 71%, frogs 139%, salamanders 1,542%, teleosts 33%, jawless fish 60%, and the lungfish Lepidosiren 3540%. Vertebrates possess the greatest amount of DNA among animals. The chordate relatives of vertebrates possess significantly less DNA than vertebrates: lancets possess 17% the human quantity of DNA and tunicates possess 6% (Chiarelli, 1973).
The amount of DNA contained in mitochondria can vary. In higher plants, the mitochondrial can vary from 200-2400 kb and plants in the same family can possess mitochondria whose genomes differ in size by a factor of 8 (Lonsdale, 1984).
About 300 chromosomal rearrangements seem to have occurred since the divergence of the human and mouse genomes (Nimura, 2003).
In ancestral anthropoid primates, a segmental duplication occurred in the chromosomal region 1q22 region. Two new genes unique to anthropoid primates resulted from this duplication (Kuryshev, 2006).
Unlike therian mammals, the monotreme genome does not seem to undergo imprinting and there is no evidence of X inactivation. In marsupials, some imprinting is known and the X inactivation does occur, although its mechanism is not as complex as that observed in placental mammals (Grutzner, 2003; Grutzner, 2004).