DEGENERATION OF A NONRECOMBINING CHROMOSOME

Comparative studies suggest that sex chromosomes begin as ordinary aut osomes that happen to carry a major sex determining locus. Over evolut ionary time the Y chromosome is selected to stop recombining with the X chromosome, perhaps in response to accumulation of alleles beneficia l to the heterogametic but harmful to the homogametic sex. Population genetic theory predicts that a nonrecombining Y chromosome should dege nerate. Here this prediction is tested by application of specific sele ction pressures to Drosophila melanogaster populations. Results demons trate the decay of a nonrecombining, nascent Y chromosome and the capa city for recombination to ameliorate such decay.