The Gonium pectorale genome demonstrates co-option of cell cycle regulation during the evolution of multicellularity

Alternative evolution of a spheroidal colony in volvocine algae: developmental analysis of embryogenesis in Astrephomene (Volvocales, Chlorophyta)

BACKGROUND

Volvocine algae, which range from the unicellular Chlamydomonas to the multicellular Volvox with a germ-soma division of labor, are a model for the evolution of multicellularity. Within this group, the spheroidal colony might have evolved in two independent lineages: Volvocaceae and the goniacean Astrephomene. Astrephomene produces spheroidal colonies with posterior somatic cells. The feature that distinguishes Astrephomene from the volvocacean algae is lack of inversion during embryogenesis; the volvocacean embryo undergoes inversion after successive divisions to orient flagella toward the outside. The mechanisms of inversion at the molecular and cellular levels in volvocacean algae have been assessed in detail, particularly in Volvox carteri. However, embryogenesis in Astrephomene has not been subjected to such investigations.

RESULTS

This study relied on light microscopy time-lapse imaging using an actively growing culture of a newly established strain to conduct a developmental analysis of Astrephomene as well as to perform a comparison with the similar spheroidal volvocacean Eudorina. During the successive divisions involved in Astrephomene embryogenesis, gradual rotation of daughter protoplasts resulted in movement of their apical portions toward the embryonic posterior, forming a convex-to-spheroidal cell sheet with the apical ends of protoplasts on the outside. Differentiation of the posterior somatic cells from the embryo periphery was traced based on cell lineages during embryogenesis. In contrast, in Eudorina, the rotation of daughter protoplasts did not occur during successive cell divisions; however, inversion occurred after such divisions, and a spheroidal embryo was formed. Indirect immunofluorescence microscopy of basal bodies and nuclei verified this difference between Astrephomene and Eudorina in the movement of embryonic protoplasts.

CONCLUSIONS

These results suggest different tactics for spheroidal colony formation between the two lineages: rotation of daughter protoplasts during successive cell divisions in Astrephomene, and inversion after cell divisions in Eudorina. This study will facilitate further research into the molecular and genetic mechanisms of the parallel evolution of the spheroidal colony in volvocine algae.

Volvox: A simple algal model for embryogenesis, morphogenesis and cellular differentiation

Patterning of a multicellular body plan involves a coordinated set of developmental processes that includes cell division, morphogenesis, and cellular differentiation. These processes have been most intensively studied in animals and land plants; however, deep insight can also be gained by studying development in simpler multicellular organisms. The multicellular green alga Volvox carteri (Volvox) is an excellent model for the investigation of developmental mechanisms and their evolutionary origins. Volvox has a streamlined body plan that contains only a few thousand cells and two distinct cell types: reproductive germ cells and terminally differentiated somatic cells. Patterning of the Volvox body plan is achieved through a stereotyped developmental program that includes embryonic cleavage with asymmetric cell division, morphogenesis, and cell-type differentiation. In this review we provide an overview of how these three developmental processes give rise to the adult form in Volvox and how developmental mutants have provided insights into the mechanisms behind these events. We highlight the accessibility and tractability of Volvox and its relatives that provide a unique opportunity for studying development.

Distinct shape-shifting regimes of bowl-shaped cell sheets - embryonic inversion in the multicellular green alga Pleodorina

BACKGROUND

The multicellular volvocine alga Pleodorina is intermediate in organismal complexity between its unicellular relative, Chlamydomonas, and its multicellular relative, Volvox, which shows complete division of labor between different cell types. The volvocine green microalgae form a group of genera closely related to the genus Volvox within the order Volvocales (Chlorophyta). Embryos of multicellular volvocine algae consist of a cellular monolayer that, depending on the species, is either bowl-shaped or comprises a sphere. During embryogenesis, multicellular volvocine embryos turn their cellular monolayer right-side out to expose their flagella. This process is called 'inversion' and serves as simple model for epithelial folding in metazoa. While the development of spherical Volvox embryos has been the subject of detailed studies, the inversion process of bowl-shaped embryos is less well understood. Therefore, it has been unclear how the inversion of a sphere might have evolved from less complicated processes.

RESULTS

In this study we characterized the inversion of initially bowl-shaped embryos of the 64- to 128-celled volvocine species Pleodorina californica. We focused on the movement patterns of the cell sheet, cell shape changes and changes in the localization of cytoplasmic bridges (CBs) connecting the cells. The development of living embryos was recorded using time-lapse light microscopy. Moreover, fixed and sectioned embryos throughout inversion and at successive stages of development were analyzed by light and transmission electron microscopy. We generated three-dimensional models of the identified cell shapes including the localization of CBs.

CONCLUSIONS

In contrast to descriptions concerning volvocine embryos with lower cell numbers, the embryonic cells of P. californica undergo non-simultaneous and non-uniform cell shape changes. In P. californica, cell wedging in combination with a relocation of the CBs to the basal cell tips explains the curling of the cell sheet during inversion. In volvocine genera with lower organismal complexity, the cell shape changes and relocation of CBs are less pronounced in comparison to P. californica, while they are more pronounced in all members of the genus Volvox. This finding supports an increasing significance of the temporal and spatial regulation of cell shape changes and CB relocations with both increasing cell number and organismal complexity during evolution of differentiated multicellularity.

Co-option during the evolution of multicellular and developmental complexity in the volvocine green algae

Despite its major impact on the evolution of Life on Earth, the transition to multicellularity remains poorly understood, especially in terms of its genetic basis. The volvocine algae are a group of closely related species that range in morphology from unicellular individuals (Chlamydomonas) to undifferentiated multicellular forms (Gonium) and complex organisms with distinct developmental programs and one (Pleodorina) or two (Volvox) specialized cell types. Modern genetic approaches, complemented by the recent sequencing of genomes from several key species, revealed that co-option of existing genes and pathways is the primary driving force for the evolution of multicellularity in this lineage. The initial transition to undifferentiated multicellularity, as typified by the extant Gonium, was driven primarily by the co-option of cell cycle regulation. Further morphological and developmental innovations in the lineage leading to Volvox resulted from additional co-option events involving genes important for embryonic inversion, asymmetric cell division, somatic and germ cell differentiation and the structure and function of the extracellular matrix. Because of their relatively low but variable levels of morphological and developmental complexity, simple underlying genetics and recent evolutionary history, the volvocine algae are providing significant insight into our understanding of the genetics and evolution of major developmental and morphological traits.

Sequence of the Gonium pectorale Mating Locus Reveals a Complex and Dynamic History of Changes in Volvocine Algal Mating Haplotypes

Sex-determining regions (SDRs) or mating-type (MT) loci in two sequenced volvocine algal species, Chlamydomonas reinhardtii and Volvox carteri, exhibit major differences in size, structure, gene content, and gametolog differentiation. Understanding the origin of these differences requires investigation of MT loci from related species. Here, we determined the sequences of the minus and plus MT haplotypes of the isogamous 16-celled volvocine alga, Gonium pectorale, which is more closely related to the multicellular V. carteri than to C. reinhardtii. Compared to C. reinhardtii MT, G. pectorale MT is moderately larger in size, and has a less complex structure, with only two major syntenic blocs of collinear gametologs. However, the gametolog content of G. pectorale MT has more overlap with that of V. carteri MT than with C. reinhardtii MT, while the allelic divergence between gametologs in G. pectorale is even lower than that in C. reinhardtii. Three key sex-related genes are conserved in G. pectorale MT: GpMID and GpMTD1 in MT–, and GpFUS1 in MT+. GpFUS1 protein exhibited specific localization at the plus-gametic mating structure, indicating a conserved function in fertilization. Our results suggest that the G. pectorale–V. carteri common ancestral MT experienced at least one major reformation after the split from C. reinhardtii, and that the V. carteri ancestral MT underwent a subsequent expansion and loss of recombination after the divergence from G. pectorale. These data begin to polarize important changes that occurred in volvocine MT loci, and highlight the potential for discontinuous and dynamic evolution in SDRs.