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In a male mouse, meiosis markers of processed DNA double strand breaks (DSBs) such as DMC1 and RAD51 are regularly seen in the non-PAR region of the X chromosome; these disappear late in prophase prior to entry into the first meiotic metaphase. Marker evidence for DSBs occurring in the non-PAR region of the Y chromosome is limited. Nevertheless, historically it has been documented that recombination can occur within the mouse Y short arm (Yp) when an additional Yp segment is attached distal to the X and/or the Y pseudoautosomal region (PAR). A number of recombinants identified among offsprings involved unequal exchanges involving repeated DNA segments; however, equal exchanges will have frequently been missed because of the paucity of markers to differentiate between the two Yp segments. Here, we discuss this historical data and present extensive additional data obtained for two mouse models with Yp additions to the X PAR. PCR genotyping enabled identification of a wider range of potential recombinants; the proportions of Yp exchanges identified among the recombinants were 9.7 and 22.4 %. The frequency of these exchanges suggests that the Yp segment attached to the X PAR is subject to the elevated level of recombinational DSBs that characterizes the PAR.

A previous study indicated that genetic information encoded on the mouse Y chromosome short arm (Yp) is required for efficient completion of the second meiotic division (that generates haploid round spermatids), restructuring of the sperm head, and development of the sperm tail. Using mouse models lacking a Y chromosome but with varying Yp gene complements provided by Yp chromosomal derivatives or transgenes, we recently identified the Y-encoded zinc finger transcription factors Zfy1 and Zfy2 as the Yp genes promoting the second meiotic division. Using the same mouse models we here show that Zfy2 (but not Zfy1) contributes to the restructuring of the sperm head and is required for the development of the sperm tail. The preferential involvement of Zfy2 is consistent with the presence of an additional strong spermatid-specific promotor that has been acquired by this gene. This is further supported by the fact that promotion of sperm morphogenesis is also seen in one of the two markedly Yp gene deficient models in which a Yp deletion has created a Zfy2/1 fusion gene that is driven by the strong Zfy2 spermatid-specific promotor, but encodes a protein almost identical to that encoded by Zfy1. Our results point to there being further genetic information on Yp that also has a role in restructuring the sperm head.