Organ formation without cellular boundaries: development of the single-celled giant alga Caulerpa

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The project seeks to understand the genome and development of an unusual and little-studied plant, the green alga Caulerpa prolifera. Most plants and animals are composed of very large numbers of small cells, each serving a specific function in the organism as a whole. Caulerpa is as large as ordinary plants, and has the equivalents of leaves, stems and roots – but it is, exceptionally, one giant cell that can be up to a meter across. The single algal cell partitions into organs, rather than, as in other plants, the organs forming from the division and differentiation of many cells. To understand how organs develop without cell division and cellular specialization, experiments are planned to characterize the expression of genes in different organs when mature and as they develop, and to study the structure of the functionally different parts of this giant cell using electron microscopy. Additional experiments are planned to localize different gene transcripts to different locations within the cell; and to develop methods for introducing new genetic material to the alga to test mechanisms of gene expression control. The planned work will fill in knowledge of a little-known part of the ocean ecosystem, and will also inform about the growth and development, and potentially control, of Caulerpa, some species of which are invasive pests. Algae are also sustainable sources of industrial products. The knowledge gained has the potential to lead to novel products from a currently unused source. Undergraduate and graduate students, along with postdoctoral reasearchers, will be trained in state-of-the-art genomics methods while learning to address questions that span disciplines of development, cell biology and genomics.The chlorophyte alga Caulerpa prolifera poses a challenge to the cell theory, which considers organisms with differentiated parts to be social organizations of specialized cells. Caulerpa has fronds, stolons and rhizoids as differentiated organs, but consists of a single, giant, multinucleate cell. Preliminary work shows it to have a 35Mb genome containing around 11,000 genes. The goal of the proposed work is to learn how organs differentiate without cell division or cellular differentiation. There are five specific sets of experiments, whose aims are: complete the analysis of the genome with determination of long contigs; test differential RNA expression and differential presence of proteins in mature and developing organs; develop methods for in situ hybridization and FISH-seq for RNA localization in early developing organs; describe the internal structure and development of the different parts of the giant cell using serial block-face scanning electron microscopy; and attempt genetic transformation of the alga by biolistic delivery of fluorescent reporter constructs composed of promoter, gene and 3’ untranslated regions derived from endogenous genes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.