Onset of feeding in juvenile sea urchins and its relation to nutrient signalling

Hormonal Regulation of Programmed Cell Death in Sea Urchin Metamorphosis

Programmed cell death (PCD) has been identified as a key process in the metamorphic transition of indirectly developing organisms such as frogs and insects. Many marine invertebrate species with indirect development and biphasic life cycles face the challenge of completing the metamorphic transition of the larval body into a juvenile when they settle into the benthic habitat. Some key characteristics stand out during this transition in comparison to frogs and insects: (1) the transition is often remarkably fast and (2) the larval body is largely abandoned and few structures transition into the juvenile stage. In sea urchins, a group with a drastic and fast metamorphosis, development and destruction of the larval body is regulated by endocrine signals. Here we provide a brief review of the basic regulatory mechanisms of PCD in animals. We then narrow our discussion to metamorphosis with a specific emphasis on sea urchins with indirect life histories and discuss the function of thyroid hormones and histamine in larval development, metamorphosis and settlement of the sea urchin Strongylocentrotus purpuratus. We were able to annotate the large majority of PCD related genes in the sea urchin S. purpuratus and ongoing studies on sea urchin metamorphosis will shed light on the regulatory architecture underlying this dramatic life history transition. While we find overwhelming evidence for hormonal regulation of PCD in animals, especially in the context of metamorphosis, the mechanisms in many marine invertebrate groups with indirect life histories requires more work. Hence, we propose that studies of PCD in animals requires functional studies in whole organisms rather than isolated cells. We predict that future work, targeting a broader array of organisms will not only help to reveal important new functions of PCD but provide a fundamentally new perspective on its use in a diversity of taxonomic, developmental, and ecological contexts.

Adenosine Triggers Larval Settlement and Metamorphosis in the Mussel Mytilopsis sallei through the ADK-AMPK-FoxO Pathway

Settlement and metamorphosis of planktonic larvae into benthic adults are critical components of a diverse range of marine invertebrate-mediated processes such as the formation of mussel beds and coral reefs, the recruitment of marine shellfisheries, and the initiation of macrobiofouling. Although larval settlement and metamorphosis induced by natural chemical cues is widespread among marine invertebrates, the mechanisms of action remain poorly understood. Here, we identified that the molecular target of adenosine (an inducer of larval settlement and metamorphosis from conspecific adults in the invasive biofouling mussel Mytilopsis sallei) is adenosine kinase (ADK). The results of transcriptomic analyses, pharmacological assays, temporal and spatial gene expression analyses, and siRNA interference, suggest that ATP-dependent phosphorylation of adenosine catalyzed by ADK activates the downstream AMPK-FoxO signaling pathway, inducing larval settlement and metamorphosis in M. sallei. This study not only reveals the role of the ADK-AMPK-FoxO pathway in larval settlement and metamorphosis of marine invertebrates but it also deepens our understanding of the functions and evolution of adenosine signaling, a process that is widespread in biology and important in medicine.