The unique sex chromosome system in platypus and echidna |
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Authors: | M A Ferguson-Smith W Rens |
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Institution: | 1.Department of Veterinary Medicine,Cambridge Resource Centre for Comparative Genomics,Cambridge,UK |
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Abstract: | A striking example of the power of chromosome painting has been the resolution of the male platypus karyotype and the pairing
relationships of the chain of ten sex chromosomes. We have extended our analysis to the nine sex chromosomes of the male echidna.
Cross-species painting with platypus shows that the first five chromosomes in the chain are identical in both, but the order
of the remainder are different and, in each species, a different autosome replaces one of the five X chromosomes. As the therian
X is homologous mainly to platypus autosome 6 and echidna 16, and as SRY is absent in both, the sex determination mechanism
in monotremes is currently unknown. Several of the X and Y chromosomes contain genes orthologous to those in the avian Z but
the significance of this is also unknown. It seems likely that a novel testis determinant is carried by a Y chromosome common
to platypus and echidna. We have searched for candidates for this determinant among the many genes known to be involved in
vertebrate sex differentiation. So far fourteen such genes have been mapped, eleven are autosomal in platypus, two map to
the differential regions of X chromosomes, and one maps to a pairing segment and is likewise excluded. Search for the platypus
testis-determining gene continues, and the extension of comparative mapping between platypus and birds and reptiles may shed
light on the ancestral origin of monotreme sex chromosomes. |
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