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The organization and evolution of the human Y chromosome
Brian Charlesworth
Institute of Cell, Animal and Population Biology, University of Edinburgh, Ashworth Laboratories, Edinburgh EH9 2BR, UK
The recent sequencing of a large chunk of euchromatin from the human Y chromosome is a technical tour de force. It answers some evolutionary questions about this unusual chromosome while raising others.
Subject areas: Genetics, Evolution, Genome studies
Genome Biology 2003, 4:226.
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The Y chromosome is probably the most bizarre part of the human genome, reflecting its unique status as a huge block of largely non-recombining DNA, maintained in a permanently heterozygous state, and transmitted solely through males [1]. Early in the history of genetics, it was realized that the pattern of sex-linked inheritance, seen in species with chromosomal sex determination, implies that the Y chromosome lacks most of the genes carried on its pairing partner, the X chromosome. HJ Muller inferred that the X and Y were originally homologous chromosomes, and that the Y had lost most of the genes that it once contained, in response to its lack of recombinational exchange with the X and its permanent heterozygosity [2].
Later work has largely validated this inference, although the details of the evolutionary mechanisms leading to Y chromosome degeneration are now thought to differ substantially from that proposed by Muller [3,4]. It is also now known that Y chromosomes have evolved independently in many different groups of animals and plants that have sex chromosomes; the Y chromosomes of mammals share a common origin with each other, but have nothing in common with their counterparts in birds or Drosophila [1,5]. In addition, Y chromosomes tend to be unusually rich in repetitive DNA, which is contributed both by transposable elements and by tandem arrays of satellite DNA sequences, as is often the case for regions of the genome that have low frequencies of crossing over [1]. Much of the human Y chromosome consists of heterochromatin, made up entirely of such repeats (Figure 1) [6].
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