The Y-encoded gene zfy2 acts to remove cells with unpaired chromosomes at the first meiotic metaphase in male mice

Abstract

During male but not female mammalian meiosis, there is efficient apoptotic elimination of cells with unpaired (univalent) chromosomes at the first meiotic metaphase (MI) [1]. Apoptotic elimination of MI spermatocytes is seen in response to the univalent X chromosome of XSxr(a)O male mice [2], in which the X chromosome carries Sxr(a) [3, 4], the Y-chromosome-derived sex-reversal factor that includes the testis determinant Sry. Sxr(b) is an Sxr(a)-derived variant in which a deletion has removed six Y short-arm genes and created a Zfy2/Zfy1 fusion gene spanning the deletion breakpoint [4, 5]. XSxr(b)O males have spermatogonial arrest that can be overcome by the re-addition of Eif2s3y from the deletion as a transgene; however, XSxr(b)OEif2s3y transgenic males do not show the expected elimination of MI spermatocytes in response to the univalent [6]. Here we show that these XSxr(b)OEif2s3y males have an impaired apoptotic response with completion of the first meiotic division, but there is no second meiotic division. We then show that Zfy2 (but not the closely related Zfy1) is sufficient to reinstate the apoptotic response to the X univalent. These findings provide further insight into the basis for the much lower transmission of chromosomal errors originating at the first meiotic division in men than in women [7].

Figure 1

Information Pertaining to the Mouse Models (A) The mouse Y short arm (Yp; represented to scale in the magnified view) has seven single-copy genes, two duplicated genes, and one multi-copy gene. (Note that the mouse Y chromosome has a single distal pseudoautosomal region [PAR] on the long arm.) (B) The Y chromosome short-arm-derived Sxr^a sex-reversal factor, here attached distal to the PAR on the X chromosome, includes most of the Yp genes. (C) The Sxr^a-derived deletion variant Sxr^b has a >900 kb deletion removing six single-copy genes (Δ^Sxr-b) and creating a Zfy2/1 fusion gene spanning the deletion breakpoint (†). (D) H & E-stained stage XII-I testis tubule section from 30-day-old XSxr^aO and X^ESxr^bO mice. In XSxr^aO there are many arrested, darkly stained (dying; stars), MI spermatocytes. In X^ESxr^bO most of the MI spermatocytes have already divided to form interphasic secondary spermatocytes (arrow), and those that remain are lightly stained (healthy; arrowheads). (E) Tables describing the transgenes and the mouse models analyzed in this study. The scale bar for magnified views in (A), (B), and (C) represents 150 Kb; it represents 40 μm in (D). See also Figure S1.

Figure 2

The Secondary-Spermatocyte Stage Is Reached in XO Eif2s3y Rescue Mice (A) H & E-stained stage XII and V–VI testis tubule sections from 30-day-old XY, X^EOSry, and X^ESxr^bO mice. In XY males the two meiotic divisions occur within stage XII. Metaphase plates (arrowheads) and a pair of interphasic secondary spermatocytes (SS) are indicated. At stage V–VI, small, round haploid spermatids (St) are seen adjacent to the tubule lumen. In X^EOSry and X^ESxr^bO males metaphase plates (arrowheads) are again present at stage XII; there are also some darkly stained cells or groups of cells that are indicative of cell death. Haploid spermatids are not found at stage V–VI; instead, the cells adjacent to the lumen are larger and have the morphology of interphasic secondary spermatocytes (SS). An example of spermatids or secondary spermatocytes is represented at a higher magnification in the left bottom part of each panel. The scale bar represents 20 μm. (B) Spread spermatogenic cells from 28- to 30-day-old XY^ΔSrySry and X^EOSry males stained with DAPI (blue) and an antibody against SYCP3 (green). The pachytene cells serve as a positive control for SYCP3 staining. There is no SYCP3 staining in spermatogonia. Interphasic secondary spermatocytes with bar-shaped SYCP3 staining remaining at the majority of chromocenters are found in both genotypes. An SYCP3-negative round spermatid nucleus is shown for the control male (these were not found in X^EOSry males). Cells with the DAPI nuclear morphology of interphasic secondary spermatocytes but with very weak (SS) or absent (SS^∗) SYCP3 staining (lower right panel) are an additional abundant cell type in X^EOSry males. (C) Nuclear DNA content quantitated by integrated intensity measurement of DAPI fluorescence on spread spermatogenic cells from 28- to 30-day-old X^EOSry, X^ESxr^bO, and XY^ΔSrySry males. The values are adjusted to give an arbitrary value of 4 for pachytene nuclei. In XY^ΔSrySry males the spermatogonia (Sg), pachytene spermatocytes (P), secondary spermatocytes (SS), and spermatids (St) have the expected DNA content of 2C, 4C, 2C, and 1C, respectively. For X^EOSry and X^ESxr^bO, “SS” includes cells with strong and weak SCP3 staining, and “SS^∗” includes those with no SYCP3 staining; both have a 2C DNA content, and no cells with a 1C DNA content were identified. Two mice were analyzed per genotype, and the number of cells quantified for each group is indicated. ^†XY^ΔSrySry is a fertile control male genotype that is generated by the cross that produces X^EOSry males.

Figure 3

Markedly Reduced MI Apoptosis in XO Eif2s3y Rescue Mice (A) TUNEL-positive (green) first-meiotic metaphases (MIs) and healthy phosphor histone H3 (pH3, red) MIs were identified by their position away from the cell basal layers. DAPI (blue) was used as a nuclear stain. The upper panels show the paucity of apoptotic cells in 30 dpp X^EOSry compared to XSxr^aO mice. Below, at higher magnification, the boxed areas from the upper panels show the marked reduction of apoptotic MIs in stage XII tubules from X^EOSry mice. Healthy meiotic divisions are present in both genotypes. The scale bar represents 325 μm for the upper panels and 40 μm for the lower panels. (B) Quantitation of MI apoptosis was carried out on testis sections from 30-day-old XSxr^aO, X^EOSry, X^ESxr^bO, and XY^ΔSrySry mice. The latter three genotypes have significantly lower levels of apoptosis than XSxr^aO. ^∗p ≤ 0.05, ^∗∗p ≤ 0.01, and ^∗∗∗p ≤ 0.001.

Figure 4

Zfy2 Is the Y Gene that Maps to Δ^Sxr-b and Reinstates the Apoptotic Elimination at MI (A) The ratio of apoptotic (TUNEL-positive) to healthy (pH3-positive) MI spermatocytes is plotted for each mouse, and the average ratio for each genotype is shown as a bar. Left panel: ratios for XSxr^aO, X^EOSry, and X^ESxr^bO “controls” and for the X^EOSry mice with Δ^Sxr-b genes added as transgenes. Usp9y, Ube1y1, Ddx3y, Kdmd25, and Uty did not reinstate the apoptotic elimination at MI. The right panel presents the results for the Zfy transgene addition: only the Zfy2 transgene reinstates the apoptotic response. Importantly, a nonfunctional Zfy2 transgene integrated at the same locus (X^Z2(nf)) is unable to reinstate the MI apoptosis. ^†The XSxr^aO males also carry the nonfunctional Zfy2 transgene as a control for possible effects of the disruption of the Hprt locus. ^††The apoptotic/healthy ratio for X^E,Z2OSry is significantly greater (p < 0.00001) than that for X^EOSry and not significantly different from that for XSxr^aO. (B) H & E-stained stage XII and stage IV testis tubule sections from 30-day-old X^EOSry and X^E,^Z2OSry mice. Early and late stage XII testes were differentiated by the presence of diplotene (D) spermatocytes in the former and the lack of diplotene spermatocytes with the presence of secondary spermatocytes (SS) in the latter. In X^E,Z2OSry testes at stage XII there is a marked increase in MI spermatocytes that are heavily stained with eosin (starred), indicative of apoptosis; at stage IV there is a dramatic reduction in the number of secondary spermatocytes in comparison to X^EOSry testes. This reduction is due to the increased apoptotic elimination at MI in X^E,Z2OSry testes. The scale bar represents 40 μm. (C) Comparison by qRT-PCR of Zfy transcript levels in testes of different genotypes and between Zfy1 and Zfy2; the primers used were common to both Zfy transcripts (a primer list is available in Table S1). Testes were collected when mice were 17.5 days old, when no overt differences in cell population were observed between genotypes; the germ-cell-specific transcript Dazl was used for normalization. XY^ΔSrySry, XSxr^aO, X^EZ1/UOSry, and X^EZ2OSry provide estimates of the levels deriving from Zfy1+Zfy2 on the Y chromosome, Zfy1+Zfy2 in Sxr^a, Zfy1 in the X^Z1/U transgenic line, and Zfy2 level in X^Z2 transgenic line. Relative transcript levels are expressed as the n-fold change ± SEM. The comparison of the two transgenic lines shows that Zfy1 levels were significantly higher (^∗∗∗p = 0.0001) than Zfy2 levels. See also Figure S2.