Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha

Abstract

Xenoturbella bocki, a marine animal with a simple body plan, has recently been suggested to be sister group to the Acoelomorpha, together forming the new phylum Xenacoelomorpha. The phylogenetic position of the phylum is still under debate, either as an early branching bilaterian or as a sister group to the Ambulacraria (hemichordates and echinoderms) within the deuterostomes. Although development has been described for several species of Acoelomorpha, little is known about the life cycle of Xenoturbella. Here we report the embryonic stages of Xenoturbella, and show that it is a direct developer without a feeding larval stage. This mode of development is similar to that of the acoelomorphs, supporting the newly proposed phylum Xenacoelomorpha and suggesting that the last common ancestor of the phylum might have been a direct developer.

Evolution, divergence and loss of the Nodal signalling pathway: new data and a synthesis across the Bilateria

“…However, a newer phylogenomic analysis could not recover the result of Philippe et al, 2011, instead it confirms the Acoelomorpha as earliest split from the remaining Bilateria (Srivastava et al, 2014). Acoelomorphs and X. bocki share some ultrastructural features, including morphological characters in the digestive and nervous systems as well as a similar mode of development Nakano et al, 2013). The fact that so far no LR morphological asymmetries have been described in either of these two groups and that no nodal ortholog has been identified in any of them may suggest that the last common ancestor of all bilaterians was morphologically symmetrical in the left-right axis.…”

What Explains Patterns of Diversification and Richness among Animal Phyla?

“…The three character states are well established in the literature and known for most phyla (Nielsen 2001; Hickman et al 2012; Nakano et al 2013). Coelomates represent the most derived form, in which organs are attached to each other in a particular order while still being able to move freely within the cavity (Hickman et al 2012).…”

“…Bryozoa are mostly hermaphroditic (Thomsen and Hakansson 1995), so we considered them to be 0% dioecious. The reproduction of Xenoturbellida is poorly understood but they are likely monoecious (Nakano et al 2013) so we also considered them to be 0% dioecious. Six phyla exhibit both monoecy and dioecy. Their relative proportions were estimated as follows.…”

“…It is unclear how prevalent direct development is in this phylum. Therefore, metamorphosis was tenatively considered to be present in all species in this phylum. The phyla Acoela, Gastrotricha, Gnathostomulida, Onychophora, Placozoa, Tardigrada, and Xenoturbellida are considered to undergo direct development (Wald 1981; Hickman et al 2012; Nakano et al 2013). The presence of metamorphosis in Nematoda is difficult to assess, as the juveniles resemble parents more than what is usually expected under metamorphosis (Wald 1981).…”

Acoelomorpha and Xenoturbellida

“…A recent publication on the sole species described, Xenoturbella bocki , indicates direct development because the hatchling is a completely ciliated juvenile worm (Nakano et al 2013 ). The hatchling has no digestive tract or mouth opening and has basiepidermal nerve cells and a musculature that is not yet organized into longitudinal and ring musculature as in the adult (Nakano et al 2013 ).…”