(A) Schematic of the interspecific spretus hybrid system. A Mus musculus strain with larger centromeres (L, CF-1 or C57BL/6J) is crossed to a Mus spretus strain (sp, SPRET/EiJ). In the hybrid offspring, chromosomes with musculus and spretus centromeres are paired in meiotic bivalents. (B) C57BL/6J x SPRET/EiJ (L x sp) hybrid oocytes, or C57BL/6J x C57BL/6J (L x L) as controls, were fixed at metaphase I and stained for the indicated centromere proteins. Graph shows centromere signal ratios, calculated as the brighter divided by the dimmer signal for each bivalent (n > 36 bivalents for each condition). (C) C57BL/6J x SPRET/EiJ (L x sp) oocytes expressing Major Sat. TALE-mClover were stained for MCAK. Graph shows centromere signal ratios, calculated as the C57BL/6J centromere divided by the spretus centromere signal for each bivalent (n = 24 bivalents). Images (B, C) are maximum intensity z-projections showing all chromosomes (left), or optical slices magnified to show single bivalents (right); scale bars, 10 μm. In the graphs, each dot represents a single bivalent; red line, mean; *P < 0.001, indicating significant deviation from 1 in (C). (D) Schematic of relative MT destabilizer levels in both intraspecific and interspecific hybrid models.