Intrinsically weak sex chromosome drive through sequential asymmetric meiosis

Abstract

Meiotic drivers are selfish genetic elements that bias their own transmission, violating Mendel's Law of Equal Segregation. It has long been recognized that sex chromosome-linked drivers present a paradox: Their success in transmission can severely distort populations' sex ratio and lead to extinction. This paradox is typically solved by the presence of suppressors or fitness costs associated with the driver, limiting the propagation of the driver. Here, we show that Stellate (Ste) in Drosophila melanogaster represents a novel class of X chromosome-linked driver that operates with an inherent mechanism that weakens its drive strength. Ste protein asymmetrically segregates into Y-bearing cells during meiosis I, subsequently causing their death. Unexpectedly, Ste segregates asymmetrically again during meiosis II, sparing half of the Y-bearing spermatids from Ste-induced defects, thereby weakening the drive strength. Our findings reveal a mechanism by which sex chromosome drivers avoid suicidal success.

Fig. 1.. Moderately expressed Ste leads to defective sperm nuclear compaction.

(A) Schematic showing the locations of Ste and Su(Ste)/cry on the X and Y chromosomes. Ste is typically silenced by piRNAs produced from Su(Ste). (B) Schematic of D. melanogaster male germ cell differentiation. (C) Western blotting of whole testis lysates from the indicated genotypes, probed with anti-α-Tubulin (loading control) and anti-Ste antibodies. (D to G) Immunofluorescence staining for Ste (green) in needle-stage spermatid cysts of X^Ste40/Y (D), X^Ste200/Y (E), X^Ste200/Y^cry- (F), and X^Ste200/Y; bam > ste^RNAi (G). Yellow arrowheads indicate sperm nuclei with DNA compaction defects. Gray, DAPI. Scale bars, 10 μm. (H) Diagram showing phenotypes under different Ste-expressing conditions.

Fig. 2.. Ste preferentially segregates to Y-bearing spermatids through asymmetric segregation during meiosis I.

(A and B) Immunofluorescence staining for Ste (green) combined with DNA FISH for X- and Y-chromosome–specific satellite DNA sequences (X: Cy5-TCTA, blue; Y: Cy3-AATAAAC, magenta) in early (A) and late (B) spermatid cysts of X^Ste200/Y males. Nuclei of Ste-containing spermatids are indicated by yellow dotted lines. Gray, DAPI. Scale bars, 10 μm. (C) Percentage of X- or Y-bearing spermatids among Ste-containing spermatids in the indicated genotypes. The number of scored Ste-containing spermatids is shown above the bar graph. Statistical analysis was performed using two-sided Fisher’s exact tests (null hypothesis: Ste-containing spermatids have equal chances of carrying the X or Y chromosome). ****P < 0.0001. (D) Immunofluorescence staining for Ste (green) combined with phalloidin staining (F-actin, blue) in a telophase I cell (indicated by white dotted lines) of X^Ste200/Y; Ubi-GFP-Pav males (GFP-Pav, contractile ring, magenta). Gray, DAPI. Scale bar, 10 μm. (E) Percentage of telophase I cells displaying asymmetric segregation of Ste protein in the indicated genotypes. The number of scored telophase I cells is shown above the bar graph. (F) Immunofluorescence staining for Ste (green) and GFP-Pav (blue), combined with DNA FISH for the Y chromosome–specific satellite DNA sequence (Cy3-AATAAAC, magenta) in a telophase I cell (indicated by white dotted lines) of X^Ste200/Y; Ubi-GFP-Pav males. Gray, DAPI. Scale bar, 10 μm. (G) Percentage of telophase I cells with Ste cosegregating with the X or Y chromosome among cells with asymmetric Ste segregation in the indicated genotypes. The number of scored telophase I cells is shown above the bar graph. Statistical analysis was performed using two-sided Fisher’s exact tests (null hypothesis: Ste has equal chances of cosegregating with the X or Y chromosome during meiosis I). *P = 0.0121; **P = 0.0018 ****P < 0.0001.

Fig. 3.. Ste exhibits asymmetric segregation during meiosis II.

(A) Immunofluorescence staining for Ste (green) combined with phalloidin staining (F-actin, blue) in a telophase II cell (indicated by white dotted lines) of X^Ste200/Y; Ubi-GFP-Pav males (GFP-Pav, magenta). Gray, DAPI. Scale bar, 10 μm. (B) Percentage of Ste-containing telophase II cells displaying asymmetric segregation of Ste protein in the indicated genotypes. The number of scored telophase II cells is shown above the bar graph. (C) Immunofluorescence staining for Ste (green) and GFP-Pav (blue), combined with DNA FISH for the Y chromosome–specific satellite DNA sequence (Cy3-AATAAAC, magenta) in a telophase II cell (indicated by white dotted lines) of X^Ste200/Y; Ubi-GFP-Pav males. Gray, DAPI. Scale bar, 10 μm. (D) Model of asymmetric segregation of Ste during meiosis I and II, producing two X-bearing sperm and one Y-bearing sperm. Note that the frequency of asymmetry is not 100% and this model represents the scenario with the highest probability. (E) Ratio of X- to Y-bearing sperm produced by males of the indicated genotypes (calculated by the ratio of female to male progeny). Each dot in the graph represents a single male. Ten males were assayed for each genotype. Data are means ± SD. Dashed line indicates the expected 1:1 ratio. Statistical analysis was performed using two-sided unpaired t tests. **P = 0.0086; ***P = 0.0001.

Fig. 4.. Weak sex chromosome drive avoids population extinction.

(A) Population size across generations with varying degrees of drive strength (defined as the frequency of X-sperm produced by males). (B) (a) Population change rate over generations with varying degrees of drive strength. The transition line, where the population change rate is one (no population growth), is indicated by the white arrow. (b) Two-dimensional projection of (a), showing that the transition line separates the positive growth phase (rate > 1) from the negative growth phase (rate < 1). A driver with a strength of 75% will reach a steady state (no population growth) after a certain number of generations. (C to E) Simulation showing the male/female ratio (black line), number of females (blue line), and number of males (red line) across generations when the drive strength is 100% (C), 75% (D), and 60% (E).