Evolutionary crossroads in developmental biology: Hemichordates

Kewalo Marine Laboratory, Pacific Biosciences Research Center, University of Hawaii, Honolulu, HI 96734, USA.
Development (Impact Factor: 6.46). 07/2012; 139(14):2463-75. DOI: 10.1242/dev.066712
Source: PubMed


Hemichordates are a deuterostome phylum, the sister group to echinoderms, and closely related to chordates. They have thus been used to gain insights into the origins of deuterostome and chordate body plans. Developmental studies of this group have a long and distinguished history. Recent improvements in animal husbandry, functional tool development and genomic resources have resulted in novel developmental data from several species in this group. In this Primer, we introduce representative hemichordate species with contrasting modes of development and summarize recent findings that are beginning to yield important insights into deuterostome developmental mechanisms.

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Available from: Christopher Lowe, Jan 25, 2014
    • "Whereas the embryonic development of acorn worms is quite well known (see e.g. Van der Horst 1939) and Saccoglossus kowalevskii has even become a model organism for molecular studies of development (Röttinger and Lowe 2012), our knowledge of the embryology of pterobranchs is still fragmentary (dilly 2013, Sato et al. 2008, Stach 2013). in consequence, neurogenesis of enteropneusts is also well studied; whereas we know almost nothing about the origin of the nervous system in pterobranchs (Fig. 52 "

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    • "The relative ordering of these had been debated, with Cephalochordata traditionally grouped with Vertebrata to the exclusion of Tunicata. Molecular work (e.g., Eernisse and Peterson, 2004; Delsuc et al., 2006; Swalla and Smith, 2008; Cannon et al., 2009; Edgecombe et al., 2011; Röttinger and Lowe, 2012) places cephalochordates as the sister clade to Tunicata plus Vertebrata, as clade Olfactores. "
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    ABSTRACT: Dating the tree of life is a core endeavor in evolutionary biology. Rates of evolution are fundamental to nearly every evolutionary model and process. Rates need dates. There is much debate on the most appropriate and reasonable ways in which to date the tree of life, and recent work has highlighted some confusions and complexities that can be avoided. Whether phylogenetic trees are dated after they have been established , or as part of the process of tree finding, practitioners need to know which calibrations to use. We emphasize the importance of identifying crown (not stem) fossils, levels of confidence in their attribution to the crown, current chronostratigraphic precision , the primacy of the host geological formation and asymmetric confidence intervals. Here we present calibrations for 88 key nodes across the phylogeny of animals, ranging from the root of Metazoa to the last common ancestor of Homo sapiens. Close attention to detail is constantly required: for example, the classic bird-mammal date (base of crown Amniota) has often been given as 310-315 Ma; the 2014 international time scale indicates a minimum age of 318 Ma.
    Full-text · Article · Feb 2015 · Palaeontologia Electronica
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    • "The sister clade of Chordata, the Ambulacraria, comprises hemichordates plus echinoderms. The enigmatic, possibly primitive, possibly highly reduced, worm-like bilaterian Xenoturbella will not be further considered here, as its phylogenetic position as a basal deuterostome remains too controversial [20-22], and has recently been rigorously challenged [23]. "
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    ABSTRACT: Background The Hox gene cluster ranks among the greatest of biological discoveries of the past 30 years. Morphogenetic patterning genes are remarkable for the systems they regulate during major ontogenetic events, and for their expressions of molecular, temporal, and spatial colinearity. Recent descriptions of exceptions to these colinearities are suggesting deep phylogenetic signal that can be used to explore origins of entire deuterostome phyla. Among the most enigmatic of these deuterostomes in terms of unique body patterning are the echinoderms. However, there remains no overall synthesis of the correlation between this signal and the variations observable in the presence/absence and expression patterns of Hox genes. Results Recent data from Hox cluster analyses shed light on how the bizarre shift from bilateral larvae to radial adults during echinoderm ontogeny can be accomplished by equally radical modifications within the Hox cluster. In order to explore this more fully, a compilation of observations on the genetic patterns among deuterostomes is integrated with the body patterning trajectories seen across the deuterostome clade. Conclusions Synthesis of available data helps to explain morphogenesis along the anterior/posterior axis of echinoderms, delineating the origins and fate of that axis during ontogeny. From this, it is easy to distinguish between ‘seriality’ along echinoderm rays and true A/P axis phenomena such as colinearity within the somatocoels, and the ontogenetic outcomes of the unique translocation and inversion of the anterior Hox class found within the Echinodermata. An up-to-date summary and integration of the disparate lines of research so far produced on the relationship between Hox genes and pattern formation for all deuterostomes allows for development of a phylogeny and scenario for the evolution of deuterostomes in general, and the Echinodermata in particular.
    Full-text · Article · Jun 2014 · EvoDevo
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