Origin of reciprocal translocations and their effect in Clarkia speciosa

Reciprocal translocations occur in high frequencies in Clarkia speciosa and closely related species. Observations from C. speciosa suggest this species is predisposed to translocations involving breaks in or adjacent to the centrochromatin (centromeric chromatin) due to the characteristic association of all nonhomologous centrochromatin in the genome during early meiotic prophase. Translocation heterozygote multiples involving six different breaks were examined for homologous pairing and in each case the euchromatic arms were completely paired, the change in homologous pairing occuring within the nonhomologous centrochromatic association. Such a proximal exchange point precludes the possibility of a structurally determined interstitial or differential region and, therefore, any genetically differential regions that might exist must be maintained solely by means of distal localization of crossing over. — The frequency of chromosomal nondisjunction (adjacent segregation) was found to be positively correlated with the number of chromosomes in the translocation multiple. Rings of four chromosomes had an average disjunction of over 99% and therefore had little affect on fertility whereas the largest multiples of 16 chromosomes had an average disjunction of about 10% and correspondingly low fertility.