The Genome of the Western Clawed Frog Xenopus tropicalis Page: 7 of 86
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stretches of tens of Mb. Human chromosome 1 is seen to have grown by three lineage-
specific mammalian fusions. In contrast, there are several mammalian-specific
breakpoints (Fig. 2B). The genomic material on the entire q arm of chicken shows
linkage conservation to frog LG VI while the human counterparts are scattered over
regions of chromosomes 2, 3, 11, 13, 21, and X. The p arm indicates two mammalian
breaks, suggesting that regions of chromosomes 7, 12, and 22 were once part of the same
chromosome.
By extending this analysis to all human and chicken chromosomes we identified 22
human fusion and 21 fission events, versus only four fusions and one break in chicken.
Clearly, the mammalian lineage has undergone considerably more rearrangement than the
sauropsids, although the total chromosome count appears to have remained fairly
constant. The segments analyzed here are distributed on 23 human and 22 chicken
chromosomes, consistent with a derivation from 24 or 25 ancestral amniote
chromosomes. Note that the chicken microchromosomes are unresolved by this analysis,
preventing determination of the exact ancestral chromosome number. Both the vertebrate
and eumetazoan ancestors have been suggested to have had about a dozen large
chromosomes (13, 15). The current analysis indicates that the amniote ancestor had twice
as many, suggesting substantial chromosome breakage on the amniotic stem.
The extensive conserved synteny among tetrapods allows us to provisionally place frog
scaffolds without genetic markers onto the linkage map. These are shown in Fig 2 as
black bars within the blocks of conserved linkage with frog. A total of 170 large scaffolds
containing about 200 Mb of sequence were assigned a linkage group in this manner. Such
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Hellsten, Uffe; Harland, Richard M.; Gilchrist, Michael J.; Hendrix, David; Jurka, Jerzy; Kapitonov, Vladimir et al. The Genome of the Western Clawed Frog Xenopus tropicalis, article, October 1, 2009; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc831346/m1/7/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.