A cis-regulatory change underlying the motor neuron-specific loss of terminal selector gene expression in immotile tunicate larvae
The evolutionary history of animal body plans cannot be fully reconstructed without considering the roles of both novelties and losses. Some of the more remarkable examples of massively parallel evolutionary losses in animals comes from many species in the tunicate genus Molgula that have independently lost the swimming larva and instead develop as tail-less, immotile larvae that bypass the period of swimming and dispersal observed in other tunicates, marine invertebrate chordates that alternate between motile larval and sessile adult life cycle stages. The larvae of Molgula occulta and other tail-less species do not fully develop structures that are essential for swimming behavior, including notochord, tail muscles, and otolith, and loss-of-function mutations have been identified in various genes required for the differentiation of these tissues. However, little is known about the extent of development of the larval nervous system in M. occulta. While differentiated neurons might in principle be entirely dispensable to the non-swimming larva, the adult has a fully functional nervous system like any other tunicate. To further investigate this conundrum, we studied the specification and patterning of the M. occulta Motor Ganglion, which is the key central nervous system compartment that drives the motor movements of swimming tunicate larvae. We found that the expression patterns of important regulators of MG neuron subtype specification are highly conserved during the development of the non-swimming larvae of M. occulta, suggesting that the gene networks regulating their expression are largely intact in this species, despite the loss of swimming ability. However, we identified a M. occulta-specific reduction in expression of the important motor neuron terminal selector gene Ebf (Collier/Olf/EBF or COE) in the Motor Ganglion. Although M. occulta Ebf is predicted to encode a fully functional protein, its expression was reduced in developing motor neurons when compared to species with swimming larvae, which was corroborated by measuring allele-specific expression of Ebf in interspecific hybrid embryos produced by crossing M. occulta with the closely related swimming species M. oculata. Comparative reporter construct experiments also revealed a specific cis-regulatory sequence change that underlies the reduced expression of M. occulta Ebf in motor neurons, but not in other tissues and cell types. This points to a potential mechanism for arresting larval motor neuron differentiation in the non-swimming larvae of this species.
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