VARIATIONS IN CHROMOSOME NUMBER

 

EVOLUTIONARY MODEL

If evolution is correct, then the same types of changes which separate biological groups could also occur within those groups. For example, significant changes in chromosome number could exist between closely related organisms.

CREATIONISM MODEL

Given that the creationist model would contend that evolution could not produce the genetic differences between humans and chimps (which include a chromosomal fusion and thus a difference of two chromsomes in their diploid genomes), it is not expected that an even greater genetic differences exist within modern groups which are clearly descended from a recent common ancestor.

INTELLIGENT DESIGN

If intelligent design is correct, the genome of an organism is too complex to modify significantly without supernatural influence.


Another possible form of mutation is the change of chromosome number through the fission of one ancestral chromosome into multiple daughter chromosomes or the fusion of multiple ancestral chromosomes in to a new single chromosome. One of the most obvious genetic changes which occurred in human ancestors is the fusion of two smaller chromosomes in apes to produce the second largest chromosome in humans, chromosome 2 (which changed the diploid chromosome number in humans from the ancestral ape count of 48).


Changes in chromosome number far greater than that which separates humans from other apes are known in closely related organisms. 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, ).

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). Among angiosperms, the stonecrop Sedum suaveolens possesses a diploid chromosome count of 640 while that of the fern Ophioglossum pycnostichum is 1260 (Otto, 2000).
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 genus Spalax 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 rodent Ctenomys (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 Europe and North Africa (Corti, 2001).

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 New World monkey family Cebidae, species are known with 20, 34, 44, 46, 48, 50, 52, 53, 53, and 62 chromosomes. Chromosome numbers in the New World monkey family Callithricidae very from 44-46. Species of the Old World monkey family Cercopithecidae may possess 42, 44, 48, 50, 54, 58, 60, 64, 66, 70, and 72 (and counts of 42, 48, 54, 58, 60, 64, and 66 are known in the genus Cercopithecus). Species of the subgenus Cercopithecus can possess 58, 58-60, 58-62, 60, 64, 66, 66-70, 72 chromosomes; other subgenera possess 48 and 54 chromosomes. Chromosome fissioning is believed to have produced the higher diploid counts in Old World monkeys compared to New World monkeys and apes. The two genera of gibbon vary in chromosome number with 44 and 52 (with counts of 44 and 52 known in the two subgenera of Hylobates) (Chiarelli, 1973). Pongo, Gorilla, and Pan all have 48 chromosomes but these chromosomes can vary in structure; these genera possess 20, 16, and 12 acrocentric chromosomes respectively (Chiarelli, 1973; Giusto, 1981).
Although the karyotype of humans (46 chromosomes) differs from that of other higher apes (with 48 chromosomes), that is not an unusual amount of variation in a family. Comparison of chromosome banding indicates that this reduction resulted when 2 smaller ape chromosomes fused to form a large chromosome in humans.

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).