Centromeres were identified by ACA foci. MLH1 signals were scored only if they were localised on the SC. In the parental species the length of the SC of all bivalents was measured in micrometers using MicroMeasure 3.367. To estimate the total SC length in the hybrids, whose pachytene cells contained partially or completely unpaired chromosomes, we measured the lateral elements of SC and then divided the sum by two.