Abstract
Genome evolution and development of unicellular, multinucleate macroalgae (siphonous algae) are poorly known, although various multicellular organisms have been studied extensively. To understand macroalgal developmental evolution, we assembled the ∼26 Mb genome of a siphonous green alga, Caulerpa lentillifera, with high contiguity, containing 9,311 protein-coding genes. Molecular phylogeny using 107 nuclear genes indicates that the diversification of the class Ulvophyceae, including C. lentillifera, occurred before the split of the Chlorophyceae and Trebouxiophyceae. Compared with other green algae, the TALE superclass of homeobox genes, which expanded in land plants, shows a series of lineage-specific duplications in this siphonous macroalga. Plant hormone signalling components were also expanded in a lineage-specific manner. Expanded transport regulators, which show spatially different expression, suggest that the structural patterning strategy of a multinucleate cell depends on diversification of nuclear pore proteins. These results not only imply functional convergence of duplicated genes among green plants, but also provide insight into evolutionary roots of green plants. Based on the present results, we propose cellular and molecular mechanisms involved in the structural differentiation in the siphonous alga.
Highlights
The green alga, Caulerpa lentillifera, belongs to the family Caulerpaceae of the order Bryopsidales
Our analysis showed that the diversification of C. lentillifera occurred earlier than the split of the Chlorophyceae and Trebouxiophyceae with topological consistency among calculation methods
Based on results of present and previous studies, we hypothesize the following cellular and molecular mechanisms involved in the structural differentiation in the single cell (Fig. 4)
Summary
The green alga, Caulerpa lentillifera, belongs to the family Caulerpaceae of the order Bryopsidales. Macroscopic morphologies observed in the Bryopsidales are composed of a large, multinucleated, single cell (Fig. 1).[1] This type of body plan is termed siphonous, and multicellularity is not observed in this group. Some siphonous algae, including C. lentillifera, reach meters in size, likely being the largest single cells on earth. C. lentillifera possesses structures analogous to fronds (leaf-like), stolons (stem-like) and rhizoids (root-like) (Fig. 1). C. lentillifera resembles clusters of green grapes (Fig. 1), and is commonly known as sea grapes, or umi-budo in Japanese. This alga is one of the major edible
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