The effects of Robertsonian fusions on chiasma frequency and distribution in the house mouse (Mus musculus domesticus) from a hybrid zone in northernScotland

Chiasma frequency and distribution were studied in male Mus musculus domest icus from the John O'Groats-standard chromosomal hybrid zone in northern Sc otland. Individuals of the John O'Groats race (2n=32; homozygous for the Ro bertsonian fusions 4.10, 6.13, 9.12 and 11.14) and the standard race (2n=40 , all telocentric), and hybrids with various karyotypes, were examined. Chi asma frequency was significantly negatively correlated with the number of R obertsonian configurations in the meiotic cell. The decrease of chiasma fre quency can be attributed to intrachromosomal effects that reduce the number of chiasmata in Robertsonian bivalents (formed in homozygotes for Robertso nian fusions) and trivalents (formed in heterozygotes). However, the reduct ion is more pronounced in Robertsonian bivalents and is related to a shift of chiasmata to the distal ends of the chromosome arms. A different type of repatterning occurs in trivalents where there is a significant increase in proximal and interstitial chiasmata.