Fig. 1.

Life cycles of Chlamydomonas reinhardtii (Chlamydomonas) and Volvox carteri (Volvox) (A and B) and phylogenomic screening of volvocine RWP-RK family genes expressed during sexual reproduction (C–E). (A) Chlamydomonas life cycle. During vegetative reproduction (green arrows) cells grow and divide by multiple fission. The sexual cycle (pink arrows) is triggered when nitrogen is absent (-N). Gametic differentiation occurs under the control of the MT ⁺ and MT ^– mating-type loci that specify plus or minus gametes. Ciliary agglutination is followed by fertilization and differentiation into a diploid zygospore. Upon germination, meiosis occurs yielding four recombinant haploid progeny. (B) Volvox life cycle. During vegetative reproduction of both sexes (green arrows), large stem cells in each spheroid called gonidia undergo embryogenesis and produce a new generation of spheroids. Exposure to sex inducer triggers the sexual cycle (pink arrows) leading to dimorphic embryogenesis in males and females under the control of the male (MTM) or female (MTF) sex-determining regions (SDRs). Postembryonic maturation of sexual germ cell precursors leads to their differentiation into sperm packets or eggs. Sperm packets produced by males swim to females, enter the female extracellular matrix (ECM), and fertilize the large immotile egg cells that then develop into dormant diploid zygospores. Upon germination, a single meiotic product survives and develops into a new haploid recombinant progeny. (C) Phylogenetic subtree of selected RWP-RK family proteins in volvocine algae. MID proteins (restricted to minus/male) are indicated at the top and autosomal RWP-RK proteins at the bottom. For the mRNA expression patterns, gray circles indicate weak or no induction (<twofold) in sexual versus vegetative phases; pink indicates 5 to 10-fold induction and red >10-fold induction in plus/female sexual phase; pale blue indicates 5 to 10-fold induction and dark blue indicate >10-fold induction in minus/male sexual phase. Colored rectangles highlight gene families of special interest. (D) Expression patterns of Chlamydomonas sexually expressed RWP-RK genes. FPKM values are extracted from (7). Protein domain schematics with RWP-RK domains in pale orange are shown next to the color key for mRNA expression of each gene. (E) Volvox RWP2 expression pattern across the vegetative and sexual development cycles. Arrows indicate time progression in the four sample types starting from precleavage spheroids for vegetative samples and from the addition of sex inducer for sexual samples: female sexual, female vegetative, male sexual, and male vegetative (see Fig. S1 for details).