Abstract
Green Ulvophyte macroalgae represent attractive model systems for understanding growth, development, and evolution. They are untapped resources for food, fuel, and high-value compounds, but can also form nuisance blooms. To fully analyze green seaweed morphogenesis, controlled laboratory-based culture of these organisms is required. To date, only a single Ulvophyte species, Ulva mutabilis Føyn, has been manipulated to complete its whole life cycle in laboratory culture and to grow continuously under axenic conditions. Such cultures are essential to address multiple key questions in Ulva development and in algal–bacterial interactions. Here we show that another Ulva species, U. linza, with a broad geographical distribution, has the potential to be grown in axenic culture similarly to U. mutabilis. U. linza can be reliably induced to sporulate (form gametes and zoospores) in the laboratory, by cutting the relevant thallus tissue into small pieces and removing extracellular inhibitors (sporulation and swarming inhibitors). The germ cells work as an ideal feed stock for standardized algae cultures. The requirement of U. linza for bacterial signals to induce its normal morphology (particularly of the rhizoids) appears to have a species-specific component. The axenic cultures of these two species pave the way for future comparative studies of algal–microbial interactions.
Highlights
The growth and development of land plants has been extensively studied and representative model systems have been developed for molecular genetic studies in several major clades, for example, Arabidopsis for dicots, Oryza/Brachypodium for monocots, Selaginella for lycophytes, and Physcomitrella for early evolving Bryophytes (The Arabidopsis Genome Initiative, 2000; Goff et al, 2002; Rensing et al, 2008; Banks et al, 2011; Girin et al, 2014)
The greatest progress has been made with one species of Chlorophyte macroalga, Ulva mutabilis, [which is native to Southern Cost of Portugal and originally collected by Føyn (1958)], with (i) an established laboratory culture protocol (Stratmann et al, 1996; Wichard and Oertel, 2010), (ii) a collection of developmental mutants (Løvlie, 1968), (iii) definition of bacterial species and partially purified substances that are required for proper morphogenesis (Spoerner et al, 2012) and (iv) the isolation of factors that prevent the breakdown of leafy thallus tissue into unicellular spores/gametes
If gametogenesis was induced in a small volume of Ulva culture medium (UCM), the fully developed gametes were not released in the morning, despite illumination, until the medium was changed again, which implies the accumulation of a swarming inhibitor’ (SWI) as reported in U. mutabilis (Wichard and Oertel, 2010; Figure 1C)
Summary
The growth and development of land plants has been extensively studied and representative model systems have been developed for molecular genetic studies in several major clades, for example, Arabidopsis for dicots, Oryza/Brachypodium for monocots, Selaginella for lycophytes, and Physcomitrella for early evolving Bryophytes (The Arabidopsis Genome Initiative, 2000; Goff et al, 2002; Rensing et al, 2008; Banks et al, 2011; Girin et al, 2014). The greatest progress has been made with one species of Chlorophyte macroalga, Ulva mutabilis, [which is native to Southern Cost of Portugal and originally collected by Føyn (1958)], with (i) an established laboratory culture protocol (Stratmann et al, 1996; Wichard and Oertel, 2010), (ii) a collection of developmental mutants (Løvlie, 1968), (iii) definition of bacterial species and partially purified substances that are required for proper morphogenesis (Spoerner et al, 2012) and (iv) the isolation of factors that prevent the breakdown of leafy thallus tissue into unicellular spores/gametes (zoosporogenesis and gametogenesis, respectively, collectively “sporulation”; Nilsen and Nordby, 1975; Stratmann et al, 1996). A third substance, the ‘swarming inhibitor’ (SWI) prevents gamete www.frontiersin.org
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