Nataliia Iakovenko

Platyzoan Paraphyly Based on Phylogenomic Data Supports a Noncoelomate Ancestry of Spiralia

Based on molecular data three major clades have been recognized within Bilateria: Deuterostomia, Ecdysozoa, and Spiralia. Within Spiralia, small-sized and simply organized animals such as flatworms, gastrotrichs, and gnathostomulids have recently been grouped together as Platyzoa. However, the representation of putative platyzoans was low in the respective molecular phylogenetic studies, in terms of both, taxon number and sequence data. Furthermore, increased substitution rates in platyzoan taxa raised the possibility that monophyletic Platyzoa represents an artifact due to long-branch attraction. In order to overcome such problems, we employed a phylogenomic approach, thereby substantially increasing 1) the number of sampled species within Platyzoa and 2) species-specific sequence coverage in data sets of up to 82,162 amino acid positions. Using established and new measures (long-branch score), we disentangled phylogenetic signal from misleading effects such as long-branch attraction. In doing so, our phylogenomic analyses did not recover a monophyletic origin of platyzoan taxa that, instead, appeared paraphyletic with respect to the other spiralians. Platyhelminthes and Gastrotricha formed a monophylum, which we name Rouphozoa. To the exclusion of Gnathifera, Rouphozoa and all other spiralians represent a monophyletic group, which we name Platytrochozoa. Platyzoan paraphyly suggests that the last common ancestor of Spiralia was a simple-bodied organism lacking coelomic cavities, segmentation, and complex brain structures, and that more complex animals such as annelids evolved from such a simply organized ancestor. This conclusion contradicts alternative evolutionary scenarios proposing an annelid-like ancestor of Bilateria and Spiralia and several independent events of secondary reduction.

Diversity and distribution of tardigrades in soils of Edmonson Point (Northern Victoria Land, continental Antarctica)

This work contributes to the knowledge on distribution, diversity and ecology of the Antarctic soil biota. Different soil habitats from several ice-free coastal sites were sampled along the Victoria Land across 7° of latitude from 71° to 78°S during five austral summer seasons between 2003/04 and 2011/12. In this paper we report preliminary data on soil tardigrades (water bears) from Edmondson Point, Northern Victoria Land. Tardigrades were found to be present in 23 of the 41 examined soil samples (56%). Their presence was associated exclusively with soil samples collected from bryophytes communities and under cyanobacterial mats, whereas they were completely absent in fellfield and ornithogenic soils. Tardigrades were least numerous among all soil micrometazoans, their abundance in the positive samples was very variable and ranged from 3 to 1824 individuals per 100 g of soil DW. High water content seemed to be the major factor determining occurrence of tardigrades in the soils investigated. On the other hand low water content and toxic compounds from penguin guano seemed to act as a strong constraint on their existence in the Antarctic soils. Taxonomic evaluation of the extracted tardigrades revealed presence of only two species belonging to class Eutardigrada: Acutuncus antarcticus (Richters, 1904) and Milnesium antarcticum Tumanov, 2006. While A. antarcticus has already been reported previously as the most widespread and abundant tardigrade across the Victoria Land, the information on M. antarcticum is novel, both for Victoria Land and the continental Antarctica.

Antarctic bdelloid rotifers: diversity, endemism and evolution

Antarctica is an isolated continent whose conditions challenge the survival of living organisms. High levels of endemism are now known in many Antarctic organisms, including algae, tardigrades, nematodes and microarthropods. Bdelloid rotifers are a key, widespread and abundant group of Antarctic microscopic invertebrates. However, their diversity, regional distribution and endemism have received little attention until recently. We provide the first authoritative review on Antarctic Bdelloidea, based on published data and new collections. Our analysis reveals the extreme levels of bdelloid endemism in Antarctica. Sixty-six bdelloid morphospecies are now confirmed from the continent, and 83–91 putative species are identified using molecular approaches (depending on the delimitation method used). Twelve previously unknown species are described based on both morphology and molecular analyses. Molecular analyses indicate that only two putative species found in Antarctica proved to be truly cosmopolitan. The level of endemism based on the available data set (95%) is higher than that in any other continent, with many bdelloid species occurring only in maritime or continental Antarctica. These findings are consistent with the long-term presence of Bdelloidea in Antarctica, with their considerable isolation facilitating intraregional radiation, providing further evidence that does not support the microbial global ubiquity hypothesis that “everything is everywhere.”

Diversity gradients of rotifer species richness in Antarctica

We gathered taxonomic information regarding the occurrence of rotifers in Antarctica and Subantarctica, producing a database of more than 1100 records from all 93 papers published on the region since the start of research expeditions in the far South. From this literature review, we outline a history of rotifer research in Antarctica. Then, using the database, we address specific questions on biogeographic patterns in species richness in rotifers in Antarctica and Subantarctica. We highlight a complex scenario of differences between areas and latitudinal gradients, differentially affected by problems in sampling bias. The number of species of monogonont rotifers seems to decrease with increasing absolute latitudes, whereas the number of species of bdelloid rotifers generally increases with increasing absolute latitudes.

Otostephanos (Rotifera, Bdelloidea, Habrotrochidae) with the description of two new species

Abstract We present a morphological feature-based key for the genus Otostephanos and describe two new species, O. jolantae sp. nov. and O. ukrainicus sp. nov. For O. jolantae sp. nov. we analysed the intraspecific morphological variability and provided the barcodes of mtCOX1 mitochondrial gene. Further, we developed a method to standardize measurements for the Philodina type of corona and trophi measurements of bdelloids. Otostephanos jolantae sp. nov. is a large rotifer with a smooth cuticle and bright red-orange gut; it can be distinguished from the known species by a high triangular upper lip with a tongue-like tip not divided into lobes, spade-shaped swollen rump, and 6/6 dental formula. It is found in Sphagnum collected in Poland, Ukraine and the Czech Republic. Otostephanos ukrainicus sp. nov. has a long body covered with coarse-grained cuticle on the last two neck segments, trunk and rump; it is distinguished by long narrow head and neck, saccular-like swollen trunk, spade-like rump, and a tiny foot. Unlike the other species of this genus, it has band-like upper lip with two narrow, sharp protrusions separated by an interspace, unusually small spurs and trophi with 5/5 major teeth. Thus far, it is only found in Ukraine, in pine and oak forest litter. http://zoobank.org/urn:lsid:zoobank.org:pub:9FC484E3-67F5-43A9-8478-A7BE14FBE5E33

Hybrid asexuality as a primary postzygotic barrier between nascent species: On the interconnection between asexuality, hybridization and speciation

Although sexual reproduction is ubiquitous throughout nature, the molecular machinery behind it has been repeatedly disrupted during evolution, leading to the emergence of asexual lineages in all eukaryotic phyla. Despite intensive research, little is known about what causes the switch from sexual reproduction to asexuality. Interspecific hybridization is one of the candidate explanations, but the reasons for the apparent association between hybridization and asexuality remain unclear. In this study, we combined cross‐breeding experiments with population genetic and phylogenomic approaches to reveal the history of speciation and asexuality evolution in European spined loaches (Cobitis). Contemporary species readily hybridize in hybrid zones, but produce infertile males and fertile but clonally reproducing females that cannot mediate introgressions. However, our analysis of exome data indicates that intensive gene flow between species has occurred in the past. Crossings among species with various genetic distances showed that, while distantly related species produced asexual females and sterile males, closely related species produce sexually reproducing hybrids of both sexes. Our results suggest that hybridization leads to sexual hybrids at the initial stages of speciation, but as the species diverge further, the gradual accumulation of reproductive incompatibilities between species could distort their gametogenesis towards asexuality. Interestingly, comparative analysis of published data revealed that hybrid asexuality generally evolves at lower genetic divergences than hybrid sterility or inviability. Given that hybrid asexuality effectively restricts gene flow, it may establish a primary reproductive barrier earlier during diversification than other “classical” forms of postzygotic incompatibilities. Hybrid asexuality may thus indirectly contribute to the speciation process.

Hybrid asexuality as a primary reproductive barrier: on the interconnection between asexuality and speciation

Speciation usually proceeds in a continuum from intensively hybridizing populations until the formation of irreversibly isolated species. Restriction of interspecific gene flow may often be achieved by gradual accumulation of intrinsic postzygotic incompatibilities with hybrid infertility typically evolving more rapidly than inviability. A reconstructed history of speciation in European loaches (Cobitis) reveals that accumulation of postzygotic reproductive incompatibilities may take an alternative, in the literature largely neglected, pathway through initiation of hybrids’ asexuality rather than through a decrease in hybrids’ fitness. Combined evidence shows that contemporary Cobitis species readily hybridize in hybrid zones, but their gene pools are isolated as hybridization produces infertile males and fertile but clonally reproducing females that cannot mediate introgressions. Nevertheless, coalescent analyses indicated intensive historical gene flow during earlier stages of Cobitis diversification, suggesting that non-clonal hybrids must have existed in the past. The revealed patterns imply that during the initial stages of speciation, hybridization between little diverged species produced recombinant hybrids mediating gene flow, but growing divergence among species caused disrupted meiosis in hybrids resulting in their clonality, which acts as a barrier to gene flow. Comparative analysis of published data on other fish hybrids corroborated the generality of our findings; the species pairs producing asexual hybrids were more genetically diverged than those pairs producing fertile sexual hybrids but less diverged than species pairs producing infertile hybrids. Hybrid asexuality therefore appears to evolve at lower divergence than other types of postzygotic barriers and might thus represent a primary reproductive barrier in many taxa.