Molecular Biology and Evolution, Vol 8, 458-474, Copyright © 1991 by Society for Molecular Biology and Evolution
EM Lyckegaard and AG Clark
A diverse array of cellular and evolutionary forces--including unequal
crossing-over, magnification, compensation, and natural selection--is at
play modulating the number of copies of ribosomal RNA (rRNA) genes on the X
and Y chromosomes of Drosophila. Accurate estimates of naturally occurring
distributions of copy numbers on both the X and Y chromosomes are needed in
order to explore the evolutionary end result of these forces. Estimates of
relative copy numbers of the ribosomal DNA repeat, as well as of the type I
and type II inserts, were obtained for a series of 96 X chromosomes and 144
Y chromosomes by using densitometric measurements of slot blots of genomic
DNA from adult D. melanogaster bearing appropriate deficiencies that reveal
chromosome- specific copy numbers. Estimates of copy number were put on an
absolute scale with slot blots having serial dilutions both of the repeat
and of genomic DNA from nonpolytene larval brain and imaginal discs. The
distributions of rRNA copy number are decidedly skewed, with a long tail
toward higher copy numbers. These distributions were fitted by a population
genetic model that posits three different types of exchange
events--sister-chromatid exchange, intrachromatid exchange, and
interchromosomal crossing-over. In addition, the model incorporates natural
selection, because experimental evidence shows that there is a minimum
number of functional elements necessary for survival. Adequate fits of the
model were found, indicating that either natural selection also eliminates
chromosomes with high copy number or that the rate of intrachromatid
exchange exceeds the rate of interchromosomal exchange.
ORIGINAL ARTICLE
Evolution of ribosomal RNA gene copy number on the sex chromosomes of Drosophila melanogaster
Department of Biology, Pennsylvania State University, University Park 16802.
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