Third chromosome candidate genes for conspecific sperm precedence between D. simulans and D. mauritiana

Abstract

Background: Male - female incompatibilities can be critical in keeping species as separate and discrete units. Premating incompatibilities and postzygotic hybrid sterility/inviability have been widely studied as isolating barriers between species. In recent years, a number of studies have brought attention to postmating prezygotic barriers arising from male - male competition and male - female interactions. Yet little is known about the genetic basis of postmating prezygotic isolation barriers between species.

Results: Using D. simulans lines with mapped introgressions of D. mauritiana into their third chromosome, we find at least two D. mauritiana introgressions causing male breakdown in competitive paternity success. Eighty one genes within the mapped introgressed regions were identified as broad-sense candidates on the basis of male reproductive tract expression and male-related function. The list of candidates was narrowed down to five genes based on differences in male reproductive tract expression between D. simulans and D. mauritiana. Another ten genes were confirmed as candidates using evidence of adaptive gene coding sequence diversification in the D. simulans and/or D. mauritiana lineage. Our results show a complex genetic basis for conspecific sperm precedence, with evidence of gene interactions between at least two third chromosome loci. Pleiotropy is also evident from correlation between conspecific sperm precedence and female induced fecundity and the identification of candidate genes that might exert an effect through genetic conflict and immunity.

Conclusions: We identified at least two loci responsible for conspecific sperm precedence. A third of candidate genes within these two loci are located in the 89B cytogenetic position, highlighting a possible major role for this chromosome position during the evolution of species specific adaptations to postmating prezygotic reproductive challenges.

Figure 1

Average second male paternity success (P2) for males from 60 different D. simulans introgressed (IG) lines. For each line we show averages and standard errors. The average P2 score of D. mauritiana is shown as a black circle. The upper bound of the 95% confidence interval of D. mauritiana average P2 is shown as a dotted line.

Figure 2

Map position for the 60 D. mauritiana introgressions within the D. simulans third chromosome tested in this study. Highlighted in grey are the minimum two loci introgressions causing a breakdown in second male paternity success and chromosome sections are divided by the two loci into five regions. Different shapes are used to box introgressions as spanning only regions 1 or 5 (rectangles), 2 and 3 or 3 and 4 (round edge rectangle), 2 and 3 or 3 and 4 (circle), at least four regions (round edge thick rectangle), and 2, 3 and 4 (thick rectangle). Average P2 values are given besides the line denoting the position of the introgression. Dashed lines are used for IG males with average P2 not significantly higher than D. mauritiana males. The thinner dashed line is used for one IG line that is not significantly different than D. mauritiana only when the reduced data set is used for analysis (see results). Asteriks identified two IG lines for which data is lost when the reduced data set is used. One P2 value significantly higher than D. mauritiana (underlined) containing the two candidate loci for CSP is suggestive of the possible existence of suppressor somewhere between 73A10 and 77B map position. Two other introgressions (within region 5) outside the mapped loci with average P2 nonsignificantly different than D. mauritiana are suggestive of other loci responsible for second male paternity breakdown. An inversion (relative to D. melanogaster map) is shown in the X-axis.

Figure 3

Average fold difference in expression from male reproductive tract RNA extractions for 81 candidate genes between D. simulans and D. mauritiana. The differences in gene expression are shown as D. mauritiana relative to D. simulans (ma/si). The data is plotted with the X axis representing the cytogenetic map position. Experiment-wise statistical threshold at P < 0.05 and P < 0.1 are shown by solid and dotted lines respectively. Notice that 3 out of 5 genes showing significant differences in gene expression (P < 0.05) are located in map position 89B (CG14891, CG10317 and Mst89B).