Abstract
Irradiation of male germ cells in newborn guppies, Lebistes reticulatus Peters, causes an increase in the exchange frequency between the X and Y chromosomes in comparison to unirradiated controls among the offspring of the irradiated males, provided the offspring are produced during an interval between 161 and 283 days after irradiation. No significant increase of crossing-over frequency was obtained among offspring produced during the interval between 59 and 145 days after irradiation. It is concluded that irradiation of early spermatogonia (59 to 145 days postirradiation) does not increase the exchange frequency between sex chromosomes whereas the irradiation of stem cells (161 to 283 days) provides a significantly higher incidence of crossing-over events in the irradiated series than in the controls. The excess of crossovers in the irradiated stem cells of spermatogenesis is due only to clusters of crossovers in the irradiated series, since on excluding the clusters no difference between irradiated and control series could be found.
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