Dorid nudibranchs (Gastropoda: Nudipleura) are a key taxon for studying the evolution and interaction of chemical defence, colour patterns and feeding specialization, but we lack a robust phylogeny for hypothesis testing. To provide new morphological characters, we investigated the extensive interior spicule networks of several dorid taxa. We compared traditional staining methods versus micro‐computed tomography (μCT) of 31 specimens representing 10 species of Aldisa, Cadlina and Onchidoris. We found that μCT offered a nondestructive view of sufficient resolution to study the gross morphology of networks, although images of fine structures were too grainy to allow robust comparisons. Network form did not vary within species and was consistent within genera. The three genera varied in several obvious characters, such as the presence of a pleural sinus channel, large dorsal spicules and multispicular tracts, as well as in relative size, shape and orientation of spicules. These characters, combined with those from the literature, supported recent molecular phylogenies that group Cadlina with Aldisa, and that question the monophyly of the Cryptobranchia. This suggests that these network forms will prove a fruitful source of phylogenetic characters with at least genus‐level resolution.
Some taxa occupy our imaginations as “living fossils” because they were known from the fossil record before being discovered alive today. Other taxa are considered “living fossils” because modern relatives bear a strong morphological resemblance to fossil relatives, or because they occupy a contracted geographic range or have less diversity now than in the past, or because they represent phylogenetic diversity that requires conservation. A characterizing feature of living fossils–and thus an implicit assumption of all criteria–is that the “living fossil” of interest is extant. However, the general research questions that “living fossils” inspire–Why do rates of evolution vary across organisms, across traits, and across time? Why do some clades decline in diversity over extended periods?–may be applied to any clade, including completely extinct clades. We propose that there is nothing special about “now” when it comes to pursuing these questions and that it is unnecessarily limiting to restrict research programs to clades for which an extant member meets some conception of the “living fossil” moniker. To this end, we investigate the extent to which Permian trilobites might resemble “living fossils,” albeit from the perspective of 253 million years ago, when the last trilobites were still alive. We do so by comparing the taxonomic diversity, geographic range, and morphological disparity of trilobites living in the Permian to earlier time periods. We find that Permian trilobites meet most definitions of living fossils, although our assessment of morphological change and character retention depend on taxonomic scale.
Streptomyces griseus protease B (SGPB), a Pronase enzyme, has been shown to be stable and active in the presence of 6.0 M guanidinium chloride (Siegel, S. et al. (1972) J. Biol. Chem.247, 4155–4159). In order to determine the cleavage specificity of this unusual enzyme under denaturing conditions, 12 peptides of known amino acid sequence were hydrolyzed by SGPB in the absence and presence of 6.0 M guanidinium chloride. The new N‐terminal amino acids produced by the action of SGPB were dansylated and quantitatively identified by reverse phase HPLC. The results indicate that SGPB retained its cleavage specificity for phenylalanyl, tyrosyl, tryptophanyl, and leucyl peptide bonds in the presence of 6.0 M guanidinium chloride. Of these peptide bonds, SGPB exhibited a greater cleavage preference for phenylalanyl and tryptophanyl bonds, which was relatively unaffected by the presence of the denaturant. The SGPB‐catalyzed cleavages of the leucyl peptide bonds examined (Leu‐Met, Leu‐Arg, Leu‐Val, Leu‐Thr, and Leu‐Ile) were substantially decreased under denaturing conditions, while Leu‐Gly bond cleavage by SGPB was virtually unaffected by denaturant. The demonstrated predictability of the catalytic preference of this unusual protease for phenylalanyl, tyrosyl, tryptophanyl, and leucyl‐glycine peptide bonds under denaturing conditions enhances its utility in the site‐specific proteolysis of insoluble or otherwise proteolysis‐resistant polypeptide substrates.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.