Abstract
Volvox is a very interesting oogamous organism that exhibits various types of sexuality and/or sexual spheroids depending upon species or strains. However, molecular bases of such sexual reproduction characteristics have not been studied in this genus. In the model species V. carteri, an ortholog of the minus mating type-determining or minus dominance gene (MID) of isogamous Chlamydomonas reinhardtii is male-specific and determines the sperm formation. Male and female genders are genetically determined (heterothallism) in V. carteri, whereas in several other species of Volvox both male and female gametes (sperm and eggs) are formed within the same clonal culture (homothallism). To resolve the molecular basis of the evolution of Volvox species with monoecious spheroids, we here describe a MID ortholog in the homothallic species V. africanus that produces both monoecious and male spheroids within a single clonal culture. Comparison of synonymous and nonsynonymous nucleotide substitutions in MID genes between V. africanus and heterothallic volvocacean species suggests that the MID gene of V. africanus evolved under the same degree of functional constraint as those of the heterothallic species. Based on semi quantitative reverse transcription polymerase chain reaction analyses using the asexual, male and monoecious spheroids isolated from a sexually induced V. africanus culture, the MID mRNA level was significantly upregulated in the male spheroids, but suppressed in the monoecious spheroids. These results suggest that the monoecious spheroid-specific down regulation of gene expression of the MID homolog correlates with the formation of both eggs and sperm in the same spheroid in V. africanus.
Highlights
Volvox is a genus of spheroidal, multicellular green algae with a surface layer of hundreds to thousands of biflagellated somatic cells, and a much smaller number of non-flagellated germ cells that develop into asexual progeny
Starr [4] recognized four types of sexuality in several strains identified as Volvox africanus originating from locations around the world (S1 Fig); 1) heterothallic, dioecious type: male spheroids or female spheroids are formed in the male or female strain, respectively; 2) homothallic, dioecious type: separate male and female spheroids are formed in the same strain; 3) homothallic, monoecious type: monoecious spheroids are formed; and 4) homothallic, monoecious with males type: monoecious spheroids and male spheroids are both formed in the same strain
We identified full-length cDNA sequences and intron-exon structures of V. africanus MID (VaMID), V. reticuliferus MID (VrMID) and V. ferrisii MID (VfMID) (Fig 2)
Summary
Volvox is a genus of spheroidal, multicellular green algae with a surface layer of hundreds to thousands of biflagellated somatic cells, and a much smaller number of non-flagellated germ cells (gonidia) that develop into asexual progeny. Spheroids that contain eggs or sperm packets (bundles of male gametes or sperm) or both are produced This genus exhibits various types of sexuality and/or sexual spheroids that have been used to define separate taxa within Volvox [1,2,3]. Coleman [5] resolved a small clade composed of these four sexual types of V. africanus based on the internal transcribed spacer-2 (ITS-2) of nuclear ribosomal DNA (rDNA) sequences. These related strains may be very useful for studying the diversity and evolution of monoecy and/or homothallism in Volvox. The other was re-identified as V. africanus, and produces both monoecious and male spheroids in a single strain (sexual type 4 of Starr [4], Fig 1B and S2 Fig)
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