STRUCTURAL EVOLUTION OF MODULAR ORGANISMS: THE SPECIFICS OF THE MODES AND RESULTS OF SEMOPHILIA

Modular and unitary organisms are two absolutely different types of living beings. They can be seen in all components of the biota, including plants, animals, mushrooms and prokaryotes. Due to the peculiarities of the organization of modular objects, their structural evolution had a pseudo cyclic character. Homeotic transformations became usual. Significant channeling of semophilia revealed itself. Analysis of the results of biological evolution on a planetary scale is current from the perspective of the theory of modular organization.

High fusibility and chimera prevalence in an invasive colonial ascidian

The formation of chimeric entities through colony fusion has been hypothesized to favour colonisation success and resilience in modular organisms. In particular, it can play an important role in promoting the invasiveness of introduced species. We studied prevalence of chimerism and performed fusion experiments in Mediterranean populations of the worldwide invasive colonial ascidian Didemnum vexillum. We analysed single zooids by whole genome amplification and genotyping-by-sequencing and obtained genotypic information for more than 2,000 loci per individual. In the prevalence study, we analysed nine colonies and identified that 44% of them were chimeric, composed of 2–3 different genotypes. In the fusion experiment 15 intra- and 30 intercolony pairs were assayed but one or both fragments regressed and died in ~45% of the pairs. Among those that survived for the length of the experiment (30 d), 100% isogeneic and 31% allogeneic pairs fused. Fusion was unlinked to global genetic relatedness since the genetic distance between fused or non-fused intercolony pairs did not differ significantly. We could not detect any locus directly involved in allorecognition, but we cannot preclude the existence of a histocompatibility mechanism. We conclude that chimerism occurs frequently in D. vexillum and may be an important factor to enhance genetic diversity and promote its successful expansion.

A new modular palaeopascichnid fossil Curviacus ediacaranus new genus and species from the Ediacaran Dengying Formation in the Yangtze Gorges area of South China

Non-biomineralizing Ediacaran macrofossils are rare in carbonate facies, but they offer valuable information about their three-dimensional internal anatomy and can broaden our view about their taphonomy and palaeoecology. In this study, we report a new Ediacaran fossil, Curviacus ediacaranus new genus and species, from bituminous limestone of the Shibantan Member of the Dengying Formation in the Yangtze Gorges area of South China. Curviacus is reconstructed as a benthic modular organism consisting of serially arranged and crescent-shaped chambers. The chambers are confined by chamber walls that are replicated by calcispars, and are filled by micritic sediments. Such modular body construction is broadly similar to the co-occurring Yangtziramulus zhangii and other Ediacaran modular fossils, such as Palaeopascichnus. The preservation style of Curviacus is similar to Yangtziramulus, although the phylogenetic affinities of both genera remain unresolved. The new fossil adds to the diversity of Ediacaran modular organisms.

Intracolonial variation in Siderastrea de Blainville, 1830 (Anthozoa, Scleractinia): taxonomy under challenging morphological constraints

The morphology of sessile invertebrates, such as scleractinian corals, can be controlled by environmental and genetic mechanisms and, consequently, it is highly variable. Morphological variation has puzzled taxonomists by posing challenges to species classification within cryptic species complexes. The 'Atlantic Siderastrea Complex' is a suitable example. Because of overlapping diagnostic traits, the morphological interspecific limits of this group remain controversial and often resulted in doubtful synonyms. In addition, the recent identification of the Caribbean S. radians in the Southwestern Atlantic has revealed that intraspecific variation has been equivocally assessed. Traditionally, hierarchical categories of variation have been the criteria used to investigate the patterns of modular organisms as corals. However, despite its taxonomic and ecological implications, the category 'intracolonial' has been largely neglected. To evaluate the influences of intracolonial morphological variation in the identification of Atlantic siderastreids, colonies from Bahia State, northeastern Brazil, were collected and measured. Six characters were selected in S. radians and S. stellata, and the variation in these characters was analyzed with Discriminant Canonical Analysis. The columellar depth and diameter varied consistently within S. stellata and S. radians, but the septal number was the most important for differentiating the two species. The results of the study also represent the first report of S. radians on the northern coast of Bahia.