Genomics of Volvocine Algae

Abstract

Volvocine algae are a group of chlorophytes that together comprise a unique model for evolutionary and developmental biology. The species Chlamydomonas reinhardtii and Volvox carteri represent extremes in morphological diversity within the Volvocine clade. Chlamydomonas is unicellular and reflects the ancestral state of the group, while Volvox is multicellular and has evolved numerous innovations including germ-soma differentiation, sexual dimorphism, and complex morphogenetic patterning. The Chlamydomonas genome sequence has shed light on several areas of eukaryotic cell biology, metabolism and evolution, while the Volvox genome sequence has enabled a comparison with Chlamydomonas that reveals some of the underlying changes that enabled its transition to multicellularity, but also underscores the subtlety of this transition. Many of the tools and resources are in place to further develop Volvocine algae as a model for evolutionary genomics.

Figure 1. The phylogenetic relationship of the Volvocales to other algae, plants and eukaryotes

The Chlorophyceae, with Chlamydomonas and Volvox indicated, are members Chlorophyta, or green algae, one of the five major groups of Archaeoplastidia. The unrooted cladogram is adapted from (Baldauf, 2003).

Figure 2. The morphology and phylogeny of the Volvocales

(A) Micrographs of Volvocine species and their relationship to each of David Kirk’s twelve steps of multicellular evolution (Kirk, 2005). (B) Phylogeny of selected Volvocales adapted from (Nozaki, 2003; Kirk, 2005). Species are color-coded based on morphology as indicated in panel (A). Asterisks indicate two isolates of Eudorina elegans that subsequently were reclassified as distinct species (Nozaki, 2003; Yamada et al., 2008).

Figure 3. Cell structure of Chlamydomonas reinhardtii

Schematic representation of the subcellular components of a typical Chlamydomonas cell; flagella (F), basal body (B), nucleus (N), eye spot (E), golgi apparatus (G), mitochondria (M), chloroplast (C), thylakoids (T), and the pyrenoid (P).

Figure 4. The multiple fission cell cycle and sexual cycles of Chlamydomonas and Gonium

During vegetative growth (A), Chlamydomonas cells may grow many fold size with the extent of growth somewhat indeterminate. Cells then divide multiple times to produce uniform-sized daughters. Two rounds of cell division are shown in this panel, with division numbers ranging from one to four in a typical culture. Gonium colonies (B) follow a similar growth and division pattern as Chlamydomonas, but the daughter cells remain attached to each other through cytoplasmic bridges and ECM (see Fig. 6). The Chlamydomonas sexual cycle (C) is induced by lack of nitrogen (-N) that causes cells to differentiate into gametes. Gametes from each mating type (plus and minus) are similar in size. Flagellar adhesion between gametes of opposite mating type precedes cell fusion to form a diploid zygote. Upon germination four meiotic progeny are produced that can reenter the vegetative cycle. In Gonium –N also triggers gametogenesis that involves colony dissolution into unicellular gametes. Post-meiotic Gonium progeny are single cells that produce a new vegetative colony after their first passage through the cell cycle.

Fig. 5. Vegetative and sexual developmental in Volvox

(A) The Volvox vegetative life cycle for males and females is identical and begins with a mature adult (upper right) whose gonidia (large green cells) undergo cleavage to begin embryogenesis. During the sixth cleavage cycle asymmetric division occurs and leads to production of 16 large anterior cells that are destined to form the germ cells in the next generation. After a total of 12 cleavage cycles the ~2000-celled embryo undergoes inversion to place the gonidial precursors inside the spheroid and the flagella of the somatic cells pointing outside in their final adult configuration. After a period of growth the daughter spheroids hatch and continue to grow and mature into adults that can restart the vegetative cycle. (B) Sexual development begins when immature gonidia are exposed to sex-inducer protein. Their subsequent embryogenesis is then altered to produce egg-bearing females or sperm-packet-bearing males (Hallmann et al., 1998; Ferris et al., 2010). Females contain 32–48 eggs and ~2000 somatic cells, while males contain 128 somatic cells and 128 packets of 64–128 sperm. Sperm packets are released whereupon they swim as a single unit until they encounter a sexual female. Upon interacting with a sexual female the sperm packet dissolves into individual cells that enter the female ECM through a fertilization pore and swim until an individual sperm encounters and fuses with an egg to form a diploid zygote. When a Volvox zygospore germinates, only one of the four meiotic products survives and differentiates into a new vegetative spheroid.

Figure 6. The mating-type loci of Chlamydomonas and Volvox

(A) The mating-type locus for both species is near the telomere of a syntenic chromosome (chromosome 6 in Chlamydomonas, linkage group I of Volvox). (B) MT+ and MT− mating haplotypes of Chlamydomonas. Rearrangements between the two haplotypes are indicated by gray shading. Locations of the sex determining genes MID and MTD1 and the sex limited gene gamete fusion gene FUS1 are shown. Also indicated are the EZY2/OTU2 regions that may be important for uniparental chloroplast inheritance (Ferris et al., 2002; Goodenough et al., 2007). (C) The Volvox male and female mating-type loci are about six times larger than Chlamydomonas MT and contain very little syntenic gene order between haplotypes. Several novel genes are shown as well as genes encoding homologs MID, MTD, and the retinoblastoma tumor suppressor homolog, MAT3. Figure adapted from (Ferris et al., 2010; Umen, 2011).

Figure 7. Comparison of the ECM structure of Chlamydomonas, Gonium and Volvox

The Chlamydomonas cell wall (A) is composed of an inner layer and an outer tripartite layer that is conserved with other Volvocales (indicated as a black layer around the cell). Individual Gonium cells (B) have an identical tripartite layer and the entire colony is surrounded by an additional outer capsule layer of ECM. Cells are held together by specialized attachments at their wall junctions. (C) Volvox cells are completely embedded in an expanded and compartmentalized ECM with a conserved tripartite boundary layer surrounding the entire colony (instead of individual cells). Inside the tripartite layer of the somatic cells are the cellular zones of ECM and the deep zone of the interior.