Fig 1.
Life cycle diagrams for two related species of Volvox.
Based on Nozaki et al.[3]. (A) V. reticuliferus (heterothallic, dioecious type). Germination of a diploid zygote yields a single meiotic product: either a female or a male depending on which MT locus is inherited. The algae reproduce asexually and can undergo sexual induction to produce sperm packets (in male lineages) or eggs (in female lineages). (B) V. africanus (homothallic, monoecious with males type). Germination of a diploid zygote yields a single meiotic product that reproduces asexually. Upon sexual induction, a clonal population can produce both spheroids containing sperm packets only (male spheroids) and spheroids containing both eggs and sperm packets (monoecious spheroids).
Table 1.
Strains of the three Volvox species used in the present study.
Fig 2.
Exon-intron structures of three MID orthologs: VaMID (Volvox africanus), VrMID (V. reticuliferus), and VfMID (V. ferrisii).
Gray bar represents the VrMID probe for Southern blotting (S5 Fig).
Fig 3.
Maximum-likelihood (ML) tree (based on LG model) of 16 full-length MID proteins from colonial volvocine species and two species of Chlamydomonas.
Branch lengths are proportional to the estimated amino acid substitutions, which are indicated by the scale bar above the tree. Numbers above and below branch points indicate bootstrap values (50% or more) of the ML and neighbor-joining (based on the JTT model), analyses, respectively. The sequences of MID orthologs with asterisks (*) were determined in this study; filled circles (●) indicate homothallic strains.
Fig 4.
Synonymous (dS) and nonsynonymous (dN) substitutions of MID genes in the Volvocaceae of volvocine algae (Fig 2).
Analyses were conducted with the outgroup Gonium pectorale MID gene (AB353340) using the modified Nei-Gojobori (assumed transition/transversion bias = 1.55) model ([23,24] by MEGA6[21]. All positions containing gaps and missing data were eliminated. There were a total of 162 positions in the final dataset. Filled circles (●) indicate the homothallic strain.
Fig 5.
Semi-quantitative RT-PCR of MID orthologs in three species of Volvox (V.).
(A-C) The products of RT-PCR reactions are resolved by agarose gel electrophoresis. The loading volume for each lane was normalized to the quantity of EF1-like (internal control) product. The number of PCR cycles are indicated separately for MID and EF1-like. (D-F) Gel band quantification analyses by ImageJ. Bars show means and standard deviations of three individual experiments.