Question

14. Describe or draw/label what is inside the fleshy lobe of a sarcopterygian fish.

15. What single characteristic of the Arthropoda appears to account for their success, diversity and abundance?

16. How many times did chitin evolve?

Answer 1

15. The success of the arthropods can mainly be attributed to the presence of exoskeleton, which makes them versatile, is protective in nature and also allows flexibility and mobility. You can see arthropods living on land and in water.

16. The distribution and evolutionary history of chitin synthases was studied most extensively in fungi. It was shown that most of fungi have not one, but several different chitin synthase genes per genome. According to phylogenetic analysis and studies of domain structure (Roncero 2002; Ruiz-Herrera and Ruiz-Medrano 2004), fungal chitin synthases form seven monophyletic classes, which in turn form two divisions. These classes do not correspond directly to organismal phylogeny: any given fungal genome may contain one chitin synthase from any class or several ones from different classes and divisions. Monophyly and patchy distribution of the various classes leads to the conclusion that they have diverged before divergence of fungi themselves and chitin synthase genes have been duplicated and lost many times since then. While there seem to be some differences in expression patterns and domain organization of chitin synthases from different classes, exact nature of these differences is yet understudied (see Merzendofer 2011 for review).

The distribution of diatom chitin synthases is somewhat similar (Durkin et al. 2009): there are four classes (called clades A–D by Durkin and coauthors, but any monophyletic paralogous chitin synthase groups are referred to as classes in this paper for clarity's sake). Classes A–C are found in pennate and multipolar centric diatoms and follow the same distribution as their fungal analogues: their number varies quickly and significantly (e.g. from 2 to 10 chitin synthase genes in different species of Thalassiosira genus) and any combination of classes may be present in a genome. Class D, on the other hand, has been found only in the genomic sequence of raphid pennate Phaeodactylum tricornutum. It was noted by Durkin and coauthors that domain structure of classes A–C and D is reminiscent of fungal Divisions 2 and 1, respectively, but they attributed it to domain shuffling sometime in diatom evolution.

Animal chitin synthase tree (Zakzewski et al. 2014) is even more complex. On one hand, there are monophyletic chitin synthase groups for sponge/Cnidaria/Choanoflagellates, lopotrochozoans, ecdysozoans and deuterostomes. On the other, inside lopotrochozoan and ecdysozoan clades there are patterns of paraphyly similar to fungal and classes A–C diatom trees; genes of the same species are present in several sublades and there may be from 1 to 6 different genes per genome. The Metazoa group as a whole, according to analysis that included fungi and classes A–C diatom sequences, is monophyletic with 1.0 posterior probability/100% BS support and belongs to fungal Division 2.

Such a placement of metazoan sequences within fungal Division 2 implies that divisions are not, strictly speaking, fungal; their divergence should date back at least as far as early Opistokonta, and possibly even earlier. All of the works above do not address the broader chitin synthase distribution: they focus on one of the major eukaryotic taxa, including only a few proteins from other organisms as an outgroup. Phylogenetic position of chitin synthases from several groups, such as Oomycetes, has not been studied at all despite the availability of sequences. The aim of this paper, thus, was to reconstruct chitin synthase history on as taxonomically broad dataset as possible. Analysis was restricted to the chitin synthase domain, leaving the history of other domains that may be present in chitin synthases beyond the scope of this paper.