Associations between Variation in X Chromosome Male Reproductive Genes and Sperm Competitive Ability in Drosophila melanogaster

Abstract

Variation in reproductive success has long been thought to be mediated in part by genes encoding seminal proteins. Here we explore the effect on male reproductive phenotypes of X-linked polymorphisms, a chromosome that is depauperate in genes encoding seminal proteins. Using 57 X chromosome substitution lines, sperm competition was tested both when the males from the wild-extracted line were the first to mate ("defense" crosses), followed by a tester male, and when extracted-line males were the second to mate, after a tester male ("offfense" crosses). We scored the proportion of progeny sired by each male, the fecundity, the remating rate and refractoriness to remating, and tested the significance of variation among lines. Eleven candidate genes were chosen based on previous studies, and portions of these genes were sequenced in all 57 lines. A total of 131 polymorphisms were tested for associations with the reproductive phenotypes using linear models. Nine polymorphisms in 4 genes were found to show significant associations (at a 5% FDR). Overall, it appears that the X chromosomes harbor abundant variation in sperm competition, especially considering the paucity of seminal protein genes. This suggests that much of the male reproductive variation lies outside of genes that encode seminal proteins.

Figure 1

The distribution of P1 and P2 across 57 X substitution lines. P1 is the defense test, where the X-line experimental male was the first to mate (followed by a cn bw control male), and the P2 assay uses the reverse order. P2 tests the ability of the X experimental males to displace resident sperm. In both cases, arcsine transformation was applied before statistical testing.

Figure 2

Boxplot of the P1 scores of the 57 X replacement lines. The proportion of offspring sired by the X experimental male out of 57 naturally derived X chromosomes of D. melanogaster when the respective X-line male is the first to mate (followed by the cn bw control male). P1 scores are highly heterogeneous across lines (P = 1.38 × 10⁻⁶).

Figure 3

Boxplot of the P2 scores of the 57 X replacement lines. The proportion of offspring sired by the X experimental male out of 57 naturally derived X chromosomes of D. melanogaster when the respective X-line male is the second to mate (after the cn bw control male). P2 scores are highly heterogeneous across lines (P = 2.2 × 10⁻¹⁶).

Figure 4

SNP allelic associations with P1 and P2. (a): associations between a deletion found on the gene CG9156 and the arcsine square transformation of the proportion of progeny sired by the X experimental male when he was the first to mate. Gene CG9156 deletion 159-160 had 21 lines with a deletion in both positions, 4 lines with a deletion in just the second position, and 14 lines with the reference sequence AA. This deletion showed a statistically significant decrease in the P1′ statistic (q = 0.033). (b): for CG15208 position 1019, there were 7 lines with the rare allele C and 48 lines with the common allele T. Those lines containing the rare allele showed a significant increase in P2′ (q = 0.020). (c): for CG17450 position 210, 4 lines had the rare allele G, while 41 lines contained the common allele C. This rare allele also demonstrated a statistically significant increase in P2′ (q = 0.048).