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+--Forum: Cabbages and Kings
+---Topic: Chromosome evolution started by Art

Posted by: Art on Mar. 09 2003,09:44

The discussion on ARN about chromosome evolution, and the supposed difficulties that karyotype differences pose for the evolution of, say, humans from a common ancetsor with other primates, brings to mind several similar discussions on AOL that have dealt with the same subject.  I won't begin to pretend that the following are an exhaustive list, or that they are the first or last word on the subject, but they are among the abstracts that address some specific issues.

The first one describes a great deal of karyotpic variability in populations of rats on Mauritius, all of which most likely arose from a small ancestral population.  The ramifications vis-a-vis the possibilities of chromosome evolution are obvious.

Chromosoma 1979 Oct 2;75(1):51-62

Mauritius type black rats with peculiar karyotypes derived from Robertsonian fission of small metacentrics.

Yosida TH, Kato H, Tsuchiya K, Moriwaki K, Ochiai Y, Monty J

All seventeen black rats collected from Mauritius Island were characterized by having many extra small acrocentric autosomes. Their basic karyotype was of Oceanian type, because of the presence of the large metacentric M1 and M2 pairs, but chromosome numbers in 13 specimens among them were 42, those of 3 specimens 43, and those of the remaining one specimen 44. Although the Oceanian type rat had 2 small acrocentric autosomes (pair no. 13), 16 Mauritius rats had 10 small acrocentrics, and the remaining one had 8 small acrocentrics. Comparative karyotype analysis between Oceanian and Mauritius type rats showed that the extra small acrocentrics found in Mauritius rats were due to Robertsonian fission of small metacentric pairs no. 14 and 18 of the original Oceanian type rat. Only one rat with 8 small acrocentrics showed the heteromorphic pair no. 18 consisting of one metacentric and two acrocentrics. The large metacentric M1 chromosome in 13 of 17 rats examined showed homologous pair, but two of them were heteromorphic by involving one metacentric M1 and two acrocentrics. In the remaining two rats M1 chromosome was not observed, but acrocentric pairs no. 4 and 7 were included. These acrocentrics were also suggested to be originated from Robertsonian fission of the large metacentric M1 chromosome. Robertsonian fission seemed to be one of the important mechanism found in karyotype evolution.

The second abstract describes variations in the karyotypes of a primate.  Of particular note is the occurrence of heterozygotes that from matings of parents with different karyotypes - this demonstrates conclusively that, in primates, changes in chromosome morphology are not always, invariably detrimental with respect to fertility or health.

Am J Phys Anthropol 1999 Oct;110(2):129-42

Complex, compound inversion/translocation polymorphism in an ape: presumptive intermediate stage in the karyotypic evolution of the agile gibbon Hylobates agilis.

Van Tuinen P, Mootnick AR, Kingswood SC, Hale DW, Kumamoto AT

Medical College of Wisconsin, Milwaukee, Wisconsin 53226, USA.

Karyotypic variation in five gibbon species of the subgenus Hylobates (2n = 44) was assessed in 63 animals, 23 of them wild born. Acquisition of key specimens of Hylobates agilis (agile gibbon), whose karyotype had been problematic due to unresolved structural polymorphisms, led to disclosure of a compound inversion/translocation polymorphism. A polymorphic region of chromosome 8 harboring two pericentric inversions, one nested within the other, was in turn bissected by one breakpoint of a reciprocal translocation. In double-inversion + translocation heterozygotes, the theoretical meiotic pairing configuration is a double inversion loop, with four arms of a translocation quadrivalent radiating from the loop. Electron-microscopic analysis of synaptonemal complex configurations consistently revealed translocation quadrivalents but no inversion loops. Rather, nonhomologous pairing was evident in the inverted region, a condition that should preclude crossing over and the subsequent production of duplication-deficiency gametes. This is corroborated by the existence of normal offspring of compound heterozygotes, indicating that fertility may not be reduced despite the topological complexity of this polymorphic system. The distribution of inversion and translocation morphs in these taxa suggests application of cytogenetics in identifying gibbon specimens and avoiding undesirable hybridization in captive breeding efforts.

The last abstract makes mention of a variant chromosomal race in  Australian aphids.  As I read the abstract, it seems as if this race arose while the aphids were "cultured" - e.g., there is not much question as to the ancestry of the race, nor of the fact that such a chromosomal variation was, in this instance, not an insurmountable barrier to overall karyotpic evolution.  If others see this differently or are more familiar with these things, please feel free to comment.

Genetics (1996) 144, 747-756

Microsatellite and chromosome evolution of parthenogenetic sitobion aphids in Australia.

Sunnucks P, England PR, Taylor AC, Hales DF

School of Biological Sciences, Macquarie University, Sydney, New South Wales, Australia.

Abstract: Single-locus microsatellite variation correlated perfectly with chromosome number in Sitobion  miscanthi aphids. The microsatellites were highly heterozygous, with up to 10 alleles per locus in this species. Despite this considerable allelic variation, only seven different S. miscanthi genotypes were discovered in 555 individuals collected from a wide range of locations, hosts and sampling periods.  Relatedness between genotypes suggests only two successful colonizations of Australia. There was no evidence for genetic recombination in 555 S. miscanthi so the occurrence of recent sexual reproduction must be near zero. Thus diversification is by mutation and chromosomal rearrangement alone. Since the aphids showed no sexual recombination, microsatellites can mutate without meiosis. Five of seven microsatellite differences were a single repeat unit, and one larger jump is likely. The minimum numbers of changes between karyotypes corresponded roughly one-to-one with microsatellite allele changes, which suggests very rapid chromosomal evolution. A chromosomal fission occurred in a cultured line, and a previously unknown chromosomal race was detected. All 121 diverse S. near fragariae were heterozygous but revealed only one genotype. This species too must have a low rate of sexual reproduction and few colonizations of Australia.



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