EM studies of female meiosis in wood lemmings with different sex chromosome constitutions

The chromosomes were studied throughout meiotic prophase by electron microscopy of surface-spread oocytes from one XX, four X*X, and three X*Y female wood lemmings, Myopus schisticolor. The X* chromosome had originated from X by a deletion and an inversion in the short arm. The deletion was confirmed in pachytene cells from X*X females; a D-loop was present in the sex bivalent in 16.8% of the cells, and asynapsis of unequal ends was seen in 9.1% of other cells. At late pachytene the D-loop underwent synaptic adjustment. The breakpoints of the deletion are in G-light bands. No inversion loop was seen, which also is in agreement with Ashley's ('88) hypothesis; at least one of the presumed breakpoints of the inversion is in G-dark chromatin. Various types of synaptic abnormalities, such as nonhomologous pairing (triple pairing, interchange, self-synapsis), univalents, foldbacks, and broken lateral elements, were encountered in all types of female. X*Y females showed a high frequency of abnormal oocytes (70.7%), which significantly exceeded that of X*X (23.1%) and XX (8.1%). Univalents were particularly common in the X*Y females. J. Exp. Zool. 290:504-516, 2001. (C) 2001 Wiley-Liss, Inc.