Variation of relative chromosome lengths in mammalian karyotypes: A hypothesis of equalizing selection

A hypothesis on the selective neutrality of relative lengths of karyotype c hromosomes was tested. Idiograms expected based on an assumption of selecti ve neutrality of chromosome lengths were compared with actual idiograms in more than a hundred mammalian species. The observed idiograms differed from those expected in a similar manner: in the observed idiograms, the longest chromosomes were shorter, and the shortest were longer than expected. It i s suggested that karyotype chromosome variation is limited by selection aga inst chromosome rearrangements that produce very long or very short chromos omes. An analysis of reciprocal translocations in the mouse and Drosophila showed that translocations generating chromosomes of extreme lengths were m ore deleterious than those generating normal-sized chromosomes. A working h ypothesis was advanced stating that within-karyotype variation of chromosom e lengths is accounted for by two factors: chromosome rearrangements and na tural selection. Chromosome rearrangements tend to randomize relative chrom osome lengths in a karyotype, whereas natural selection acts to equalize th em.