Wilko H. Ahlrichs

Novel implications for the basal internal relationships of Gastrotricha revealed by an analysis of morphological characters

A cladistic analysis of Gastrotricha based on morphological characters is presented. Unlike previous morphological analyses, our study uses species rather than higher level taxa, for which the ground pattern is often unknown. The analysis comprises 79 ingroup taxa, 4 outgroup taxa and 135 binary and multistate characters in total. Character coding is based on a careful assessment of original species descriptions. Characters included cover general body organization, internal and external features as, for example, data on the adhesive tubes, digestive tract or cuticle armament. Character systems such as many ultrastructural findings, for which it was problematic to obtain data for a large set of the included taxa, were not considered. To minimize a priori assumptions, all characters were treated with equal weight and left unordered. The four outgroup representatives were chosen in accordance with the current sister group hypotheses for Gastrotricha. Two search strategies, a heuristic search (maximum parsimony) and a parsimony ratchet search, reveal a comparable scenario. Gastrotricha split into two sister taxa. One group comprises genus Neodasys only, the sister group N.N.1 (Eutubulata nom. nov.) consists of all remaining Gastrotricha. Within Eutubulata, monophyletic Macrodasyida s. str. and N.N.2 (Abursata nom. nov.) are sister taxa of highest rank. Abursata consists of the ‘freshwater macrodasyids’Marinellina and Redudasys as sister group of monophyletic Paucitubulatina. Some traditional families are supported by this analysis. We evaluate possible apomorphies for the most basal stem lineages and track the evolution of selected organs. Our findings reveal that secondary character loss may play an important role in the stem lineage of Abursata and further in Paucitubulatina. Moreover, according to this analysis there might have been a single invasion of the freshwater environment in the stem lineage of Abursata followed by several independent returns to marine habitats within the monophylum Paucitubulatina.

How does the ‘ancient’ asexual Philodina roseola (Rotifera: Bdelloidea) handle potential UVB-induced mutations?

SUMMARY Like other obligate asexuals, bdelloid rotifers are expected to suffer from degradation of their genomes through processes including the accumulation of deleterious mutations. However, sequence-based analyses in this regard remain inconclusive. Instead of looking for historical footprints of mutations in these ancient asexuals, we directly examined the susceptibility and ability to repair point mutations by the bdelloid Philodina roseola by inducing cyclobutane-pyrimidine dimers (CPDs) via exposure to UVB radiation (280–320 nm). For comparison, we performed analogous experiments with the facultative asexual monogonont rotifer Brachionus rubens. Different strategies were found for the two species. Philodina roseola appeared to shield itself from CPD induction through uncharacterized UV-absorbing compounds and, except for the genome reconstruction that occurs after desiccation, was largely unable to repair UVB-induced damage. By contrast, B. rubens was more susceptible to UVB irradiation, but could repair all induced damage in ~2 h. In addition, whereas UV irradiation had a significant negative impact on the reproductive output of P. roseola, and especially so after desiccation, that of B. rubens was unaffected. Although the strategy of P. roseola might suffice under natural conditions where UVB irradiation is less intense, the lack of any immediate CPD repair mechanisms in this species remains perplexing. It remains to be investigated how typical these results are for bdelloids as a group and therefore how reliant these animals are on desiccation-dependent genome repair to correct potential DNA damage given their obligate asexual lifestyle.

Revisiting the Cephalodella trophi types

The six trophi types proposed by Wulfert (Archiv für Hydrobiologie 31:592–636, 1937) are used as one main character to identify Cephalodella species, although these trophi types were just based on the trophi of 31 species described after light microscopy findings. Given the 160 species nowadays valid and the possibility of examining trophi with scanning electron microscopy it is questionable if the definitions of the six types might or rather have to be improved in order to facilitate species identification. Here, a new even simpler definition scheme is proposed to identify the six trophi types.

Evidence Supporting the Uptake and Genomic Incorporation of Environmental DNA in the "Ancient Asexual" Bdelloid Rotifer Philodina roseola.

Increasing evidence suggests that bdelloid rotifers regularly undergo horizontal gene transfer, apparently as a surrogate mechanism of genetic exchange in the absence of true sexual reproduction, in part because of their ability to withstand desiccation. We provide empirical support for this latter hypothesis using the bdelloid Philodina roseola, which we demonstrate to readily internalize environmental DNA in contrast to a representative monogonont rotifer (Brachionus rubens), which, like other monogononts, is facultative sexual and cannot withstand desiccation. In addition, environmental DNA that was more similar to the host DNA was retained more often and for a longer period of time. Indirect evidence (increased variance in the reproductive output of the untreated F1 generation) suggests that environmental DNA can be incorporated into the genome during desiccation and is thus heritable. Our observed fitness effects agree with sexual theory and also occurred when the animals were desiccated in groups (thereby acting as DNA donors), but not individually, indicating the mechanism could occur in nature. Thus, although DNA uptake and its genomic incorporation appears proximally related to anhydrobiosis in bdelloids, it might also facilitate accidental genetic exchange with closely related taxa, thereby maintaining higher levels of genetic diversity than is otherwise expected for this group of “ancient asexuals”.

A comprehensive and integrative re-description of Synchaeta oblonga and its relationship to Synchaeta tremula, Synchaeta rufina and Synchaeta littoralis (Rotifera: Monogononta)

A comprehensive re-description of the monogonont rotifer Synchaeta oblonga Ehrenberg, 1832 is presented with the aim of creating a specific and robust suite of characters to identify this species that takes account of its morphological intraspecific variability. To accomplish this, we used an integrative approach that combined morphological data of the habitus and trophi (light and scanning electron microscopy) together with ecological and molecular data to generate a data set that clearly delineates S. oblonga from the morphologically similar Synchaeta tremula (Müller, 1786) with which it is often confused. In addition, by comparing S. oblonga to all remaining members of the genus Synchaeta, we found that the literature descriptions of Synchaeta rufina Kutikova and Vasiljeva, 1982, a Lake Baikal endemic species, and Synchaeta littoralis Rousselet, 1902, a species whose taxonomic status with respect to S. oblonga had already been called into question, to both lie entirely within the range of intraspecific morphological variability present in S. oblonga. Thus, we conclude that S. oblonga can be clearly demarcated from S. tremula, but that no unambiguous morphological differences exist to delineate it from either S. rufina or S. littoralis. Because our molecular data indicate S. oblonga to be a single species despite the morphological intraspecific variability that is present, we recommend that thorough re-examinations of the taxonomic statuses of both S. rufina and S. littoralis should be performed on topotypes sampled from their type localities (Lake Baikal, Russia and Dundee, Scotland, respectively).

Phenotypic influences on the reproductive strategy of the facultative sexual rotifer Brachionus rubens (Monogononta)

Facultative sexual species employ a dual reproductive strategy (heterogony) comprising primarily asexual reproduction with intermittent sexual reproduction. Given the higher relative costs of sexual reproduction, elucidating the triggers underlying these transitions might help our understanding of the evolution of (obligate) sex in general. Existing hypotheses into how and when facultative sexuals invest into sex focus largely either on environmental (habitat-deterioration and resource-demanding hypotheses) or genetic factors (condition-dependent hypothesis), but tend to lack experimental evidence, especially with respect to within-population variation. To address this deficit, we examined the influence of several variables that potentially affect fitness (food quality, water temperature, physiological acclimation, and all combinations thereof) on both the lifetime reproduction (total number of offspring) and investment into sexual offspring per female in a clonal population of the monogonont rotifer Brachionus rubens. Investment into sex, both absolutely and relative to lifetime reproduction, was tied most closely to and positively correlated with individual fitness (i.e., lifetime reproduction): individuals with higher fitness invested more into sexual reproduction. These results run contra to the condition-dependent hypothesis and indicate an energy-budget analogue of the resource-demanding hypothesis. Furthermore, investment into sex increased after a period of physiological acclimation to the new conditions, probably because of the amelioration of short-term stress effects or clonal selection. Our results underscore that life history and general phenotypic considerations—here, energetic provisioning of offspring, the presence of a sexual resting stage, and the relative timing of sexual versus asexual reproduction—can modify existing hypotheses based either on environmental or genetic factors alone.