Fig. 5: Ssk-FB4s represent a young rapidly evolving protein family.

a Unique peptides of Ssk and Ssk-FB4s. Three Ssk-FB4s are color-coded as in Fig. 3c. The top row shows the sequence of Ssk, whereas the distinct residues between Ssk and individual Ssk-FB4s are marked in gray. b Abundance (intensity-based absolute quantification, iBAQ) of Ssk and X-linked Ssk-FB4 in the RAL-399 line. Dots indicate 2 technically independent replicates. c Numbers of nonsynonymous and synonymous changes across three functional regions in Ssk-FB4s and Ssk. The upper polymorphism data are based on the codon-level alignment of Ssk-FB4s (Supplementary Fig. 14a). The lower divergence data of Ssk are based on the whole Drosophila genus (Supplementary Fig. 14e). Each cell is shown as the “number of changed sites (observed/expected proportion of changed sites)”. *P-values were calculated for the regions of interest with the one-sided proportion test. d Allele frequency spectrum of FB4s and Ssk-FB4s in 83 DGRP lines. e Transcription of Ssk, mesh, and Tsp2A in the gut across lines with (n = 15 samples) or without Ssk-FB4s (n = 23 samples). The distribution is shown as violin plots in Fig. 2f. Because the expression profiles of Ssk and Ssk-FB4s are largely similar, we simply merged the expression of Ssk and Ssk-FB4s. The one-sided Wilcoxon rank-sum test was used to calculate the P-value. f Relative abundance of Ssk and Ssk-FB4 in RAL-399 cells before and after co-immunoprecipitation with an antibody against Mesh. Dots indicate two biologically independent replicates.